As the zone that normally carries th~ decora tions of church or civic architecture-frescoes, tapestries, carved capitals. and friezes, or stained glass panels­ the wall prominently announces the narrati ve and sy mbolic meaning of a building. Yet any decorative or symbolic program must exist within the functional requirements of the wall, including su pport, access, and lighting. This chapter, dealing mainly with walls, will also trea t more compl ex systems of vertical structural elemems: piers, arcades, and buttresses, as well as systems contained within walls, including galleri es and passages. All of these clements consti­ tute the supporting connection bcrween the foun­ dations below and the vaults. domes, and roofs above. Walls serve two mam functional roles: to form an envelope providing security and sheller from sight, wind, rain, and temperature, and 10 suppon the weight o f the building superstructure. Load­ bearing walls combine both of these fu nctions, acting as a continuous support to carry roof loads all along their tOp and to transfer them down dir«tly to foun­ dations. Such walls tend to be equall y strong al ong every po int of thei r length and arc therefore usually characterized by planar surfaces and substantial thicknesses. Openings for windows and doors gen­ erally remain modest so as not to disrupt the struc­ tural continuity of the system. Walls constructed of stone, brick, and adobe normally fall into the clas­ sification of continuous, load-bearing walls. In non-load-hearing wall systems, roof and floor loads are suppOrted on ve rtical shafts and, typ­ ically, a lighter material fills the openings between

3. 1 Hagia Sophia, Istanbul, 532-537: interiOT, north wall. Walls Q,w Otha Vert/CD/ Ekmntts " •

above. The surface of wall in between the "strong" shafts needs only to suppOrt il5 own deadweight as well as rel atively low loadings from the roof. And as the wall thickness between the bunresses decreascs, this system, tOO, approaches the skeletal support so evident in mature Gothic design. Building loadings may be classified as dead or /il/e, depending on whether they change wi~h rime. Dead loads derive from the fixed mass of a building'S Structure, while live loads are caused by time-depen­ dent external factors such as wind, ea rthquake, and the motion of people and furniture within the build­ ing (figure 3.4). In a simple wall, the action of the dead loads alone usually resul ts in a state of pure compression. as illustrated in figure 3.5. When any material is compressed, including stone, it compacts in a similar manner to a squeezed sponge. Stone, of 3.:2. Imperial Roman Blui/ica, Trier, early fourth vaults to points on the piers and exterior burtresses, course, is much stiffer than sponge, and such changes century: south flank. but rarely to the walls that arc then opened up to cannot be observed by the naked eye. In fact, no great windows and wide arcades. building material is absolutely rigid, but some ma­ Where windows occur at regular intervals terials behave relatively rigidly compared to others. the shafts. Two materials are usually employed for in an otherwise load-bearing wall, an intermediate A structural element composed o f iron or of steel, these walls because of the very different physica l structural system results. In these, the ve rtical wall for example, is some ten times stiffer (i.e., it will requirements for structural support and fo r environ­ between the wi ndows acts similarly to an isolated 3.3 Abbey Church, Jumieges ca. 1067: wall but­ defl ect only one tenth as much) than the same cle­ mental control. In half-timbered wall construction, structural shah; thc larger the wi ndows, the more tresses Oil the north waff of the nave. ment made o f stone, and about twenty to thirty times for example, the load is transferred from the roof to the system approaches a fully non-load-bearing wall. stiffer than the same element made of construction­ the foundation through heavy timber posts, while This wall type is exemplified in the imperial Roman grade timber. the wall berween the timbers is composed of brick basilica at Trier, where the recession of the: masonry Under ordinary, short-time loading condi· or stone nogging, stuccoed over to provide a spandrels behind the wall plane clearly expresses ti ons, most building materials can be considered elas­ weather-tight surface. A non-load-bearing wall can their nonsupportive role (figure 3.:2.). tic, that is, when the loading is removed, they return be relatively thin and pierced with large windows as Another hybrid system closer in spirit to to their original form, as does a rubber band. are , for example, the great lateral walls, or tympani the non-load-bearing wall resull5 where the wall be­ Extreme loadings, and loadings of long duration, (figu re 3.1), beneath the massive arches sup porting comes thicker at intervals to accept concentrated produce additional permanent deformation, called the dome of the Hagia Sophia in Constantinople loads from vaults or roofs. These projecting ele­ creep. In modern engineering practice, when two (modem Istanbul). More common examples of non­ ments, known as wall buttresses (figure 3.3), gener­ materials with widely differing stiffness, such as load-bearing walls are fo und in Gothic churches, ally coincide with the bay system of interior vaulting stone and timber, are used together in construction, where loads are directed from both the roof and the or the spacing of the principal trusses of the roof Willis m.d Orllt, Vntical E.ltmtnl$ " " I

3.4 Strucruralloadings. can usually be obtained from gOllernmental me­ 3·5 Wa ll forces (rom deadweight. To detennine the deadweight gravIty loadings act· teorological sources. Maximum winds normally For a loaded structure to maintain its integrity ing Itlithin a structure, one needs first to calculate occur Qvcr a fairly wide azimuth, so the full wind (equilibrium), resistmg forces withi'l the structure the I/O/urnes of material and the IQcations of the loadings are considered to acl ;n their most crit· must counteract tI,e applied loadillgs. Pulling on centers of gravity of the indIVidual building ele­ ical direction, usually transverse to a building's a sapling, for example, subjeets tbe sapling to ments-usually from detailed drawings of the longitudinal axis. Wind-pressure distrlhutions tension (a stretcbmg force) of the same magnitude bllliding, but often supplemented by on-site mea­ (and suction on the lecward side of a building) as the applIed forGe. Simtlarly, the illustrated wall w sure,nenrs. The magnitudes of the loadings are are then calculated from these data and from undergoes compression (a pushing force) from its then (ound by multiplying the I/O/umes by a sUm­ wind-tunnel test data for the particular configu­ own weigM At tile top of the wall the eompres­

dard unit weight for the part;cuwr material; for ration of the building by means of the equation sian foree IS zero; at the base, the eompressivc example, the unit weight of construction stone is P = '/,l3xV2 xCxG, foree equals the total weight of the wall. generally taken as 2.JOO kglml. For estimating the wind loading on a where p is the wind pressure at any point on the w tall building. one must first rollsult local meteo­ bUIlding surface, 13 is the mass density of air; V rological records for the general w;"d speeds and is the wind speed; C is a dimensionless coefficient directions over extcllded periods of time, as well related to building form, usually established from as theoretical wind-velocity profiles (llelocity liS. wind-tunnel tests; and G is a gust factor to ac­ heIght abolle ground level) for the particular ter­ count for tl,e dynamie action of impinging air rain of the building site. \Vind'lIelocity data over (Mark 1981., 21.-1..5). A typical middle European long periods of time. ellen as long as a century, preindlfstTlal townscape wind speed and pressure distribution (with C = G = 1.0) is illustrated. .. Note the high sensitivity of pressure to '(lind 'UmJ speed; doubling the speed giues four tImes the vt/(I<:Il), W".d Iuhld pressure. P,~uu,£ ,0 Earthquakes are essentially Ilibratio'IS of

~ the earth's crust accompanying dynamic adjust­ ,• "- " ment to subterranean ground faults. During an ~ earthquake. seismic loadings are induced as the .. ground surface moves in att directions, and inertia causes the building to resist these motions. Usu­ ally tbe most perilous ground mot;on for build­ o • • • o '0 .0 '" ings is horhontal, along tbe ground surface, p,tuurt (Kgfm') " which generates lateral forces, similar to those caused by wind, throughout the structure. " ChapIn 3 Wlllh Ilnd Othn Vatu:,,1 EionOfU "

one usually assumes [hat the slOne remains rigid and Walls ex perience bending, or in extreme 3.6 Wall forces from combined deadweight and height of the force above the base, y; and a shear­ that the deformation takes place mainly in the cases, ovenurning, when they are subject to lateral applied loading; reactions; ovenuming. ing force of magnitude H. timber. loadings. The mni n sources o f such loadings arc The inclined, applied {oru F aamg on the top of Overtuming occurs after the base sec­ Before the Scientific Revolution, almoSt all wind, earthquake. and the lateral thrusts of internal the wall (dashed line) which, for example, rep­ tion is cracked and the applied bending force western monumemal buildings used stone for th ei r arches or vaults (see fi gure 3.4). For a wall subject resents thrust applied by a grDined vault. can be (H x y) exceeds the "righting moment" (W + structural walls and, even in even the tallest of these to only light vertical loads bur relatively heavy lateral resolved into vertical and "arizonal components; V) X (tl1) set into play by the downward forces buildings, the high compressive stress levels almost loads, as might be experienced by an infilled wall that is, the inclined force can be replaced by two tending to rotate the wall oppositely about its never resulted in failure. Part of this history of suc­ supported by timber pOSts in a high wind, overturn­ imaginary torus that together have the same ef­ outside edge. Hence raising the wall (to increase cess is due to the layer of mortar between the stones ing is usuall y averted by some sort of lateral brac­ fect as the single force. These are found geomet­ its weight, W) or splaying out the wall base (to that helps to distribute compressive forces over the ing-furnished in many buildings by internal cross­ rically: If the length of /ine F represents the mag­ increase its thickness, t) helps to stabifhe it. full contact su rface, rather than allowing the SIrUC­ walls. In some large masonry buildings without nitude and direction of the indined force, the ture above to rest on a few high poims in the CUt cross- walls, however, stability against ovenurning lengths of the vertical and horizontal legs of the stone. Moreover, stone is very St ro ng in compression. £rom lateral loads depends on the massive dead­ right triangle formed with F as hypotenuse give Medium sn ndstOne, for example, which IS nOI par­ weight of the wall itself (sec figure 3.6) or upon the magnitudes of these components. The vertical , ticularly Strong and weighs aboul l,}OO kgfmJ , external braces, such as the flying bunresses illus­ l force component V adds to the compression from boasts a crushing strength of about 415 kglc.rn . The trated in figure 3.7. the wall's weight, and the horizontal force com­ Washington Monument, the tallest unreinforced ma­ Structural elements subject to bending ex­ ponent H sub;ects the wall to internal bending 7 sonry tower in the world, is essentially a hollow shaft perience far more complex distributions of stresses and shear forces. Ullder bending, the side of the H I with walls increasing in thickness toward the base. than those from pure compression, as illustrated in wall meeting the load stretches; and tlu opposite I Yet even if the shaft had no taper and was instead figure J.8. Bending of a pier or wall causes one face side experiences additional compression from built of solid masonry. its 17I-meter heighl would to shonen and the other to elongate. The strCl ched y bending. A combination of all three interfUll generate bUI 40 kycmZ of compressive stress at its material then experiences tensil e stress that can lead forces is usually present in any structure. base due m vertical deadweight- less than 10 per· to cracking, especially in the layers of mortar. As Reactions are the internal forces that cem of sandstone's crushing strength. already noted, it would be a mistake to consider provide support to a structure. For example, the On the other hand, tension, occurring when masonry walls to be securely "cemented together." supporting reaction at the base of the wall iI/us· ---- materials are pulled apart or stretched, is consider­ Mortars serve mainly to avoid stress concentrations trated in figure 1.5 is a vertical compressive force ably more perilous for masonry. While some con· at stone and brick interfaces and provide sealing for equal to its total ~ight. With the addition of the struction materials, such as wood and iron, have the joints. indined fora, F, there must be three reactions: a nearly equal strength in resisting tension and Fortunately, a state of pure bending is v+w vertiCIJi compression equal to the sum of the compression, stone and brick arc relatively weak in rarely encountered in masonry construction. The weight of the wall and the vertical force compo< tension compared to their strength in compression. bending caused by lateral loadings is usually accom­ nent V; a bending force (or "moment," similar to Even slight levels of tension in a masonry wall can panied by compression from the deadweight of ma­ torque exerted by a wrench) equal to the product result in cracking. A single block of stone will usua lly sonry in the structure above; and in almost all in- of the horiwntal force component H and the display appreciable tensile strength, but mortared joints cannot bc depended on, over time, m trnnsmit 3·7 (Overleaf} Bourges Cathedral, choir, "95- tensile forces reliably. 1214: flying bllttresses. Walls and Oth~ T Vertical ElementJ 6,

J.8 Wall stresses. A I,orhontal beam in bending behaves (a) From axial forces alone (axial stress): Stress in exactly the same manner as the illustrated ver­ is a measure of the local intensity of force acting ticalwall. Like the wall, bendillK stress is inversely within a structure. For a simple. cOll ce",rically proportional to the product of the beam depth loaded structure such as the sapling described in mId cross-section area. Hence, doubling the beam the caption of figure 3.5, the axial, tensile stress depth reduces bending stress to only '/4 of its is found by dividing the rot.(li force. applied to the initial /lalue. sapling by its cross-sectional area. For example, a 10 kg force applied to a sap/mg having a cross­ 2 section area of o. I ,," produces a tensile stress 2 2 of 10 kglo. l cm - 100 kglcm • In the same manner, axial compression forces gives riSt' to ,'1 uniform compression stress. The compressive stress at the base of the wall of figure (a) is sim­ ilarly fOlmd by dividing the total co-axial vertical load (W + V) by the cross-sectiOtl area (A) of the wall bose. (b) From be"ding alolle bending stress: The magnitllde of the bending stress accompany· ing moment (caused by the lateralloadmg, H ) IS inversely proportiollal to the product of the wall thick"ess t and its cross·section area (see Gordon, 377-379). Since any Increase in wall thIckness results also ill an im;rease ill tf,e wall areo, the bending stress lIories IIlllersely with the square of the wall thickness (Ilr ). In other words, doubling the wall thickness reduces bending stress to only " b. '/4 of its initial /lalue. Maximum tenSIle stress oc­ curs at the surface all the side of the wall meeting the load, and it is here that cracking tends to develop in masollry. Bending stress is effectively zero at the wall center, while maximum compres­ sive stress occurs on the opposite wall surface, as j/Justrated. Chapter 3 Walls and OlhtT Vlrlieal Elements 'J

3·9 Wall Stresses from combined axial and lat­ 3.10 Wall stresses from offset axial force; thrust extreme cases, where the effective thrust line falls eral forces (combined compression and bending lines. outside the wall or pier face (i.e., e is larger than stresses). Force V, applied at eccentricity (e) from the cen­ thY, failure results as in figure J'9d because of (a) Low bending compared to vertica/loads. ter-lille of tiJe wall is equilibrated by a cOllcentric the inability of most mortars to reliably "cement" (b) Moderate bending compared to veTtica/loads. force (V) and a bending moment (equQf to V x the stones together. A more common, and less (c) High bendmg compared to /lerticalloads. e). Hence the stress distribution corresponding to dire, problem is experienced when the thrust line (d) Extreme bending compared to vertical loads; an eccentric force is the same as those given in falls within the face of the wolf, but outside of wall overturns. figures 3.80 and).9 for axial, and combilled axial tlte so-calJed middle Ihird (i.e., e is greater than Note: I" additIon to the illustrated, axial stresses alld bending forces. tJ6 but leu than liz). For this range of loadings, arising from axial and lateral forces, the internal In the absence of lateral forces, the so­ highest t:ompressive stresses o"ur on the fau of shearing force (H ) gives rise to shear Stresses that rulled, thrust line, or resultant ofaXIal forces such the wall, or pier. closest to the thrust /ine, and can also cause a wall to fail through stones sliding as Wand V (in figure J.8a), runs vertically down tension, which general/y results in masonry crack­ over One allother. This problem, (I,ollgh rarely the wall center (i.e., e =- 0) and the resulting stress ing, is displayed on the for side. With the thrust encountered in historic buildings, can be amelio­ distribution is given by figure ).8a. When lateral line within the boundary of the middle third of rated by raising the wall height, thereby increas­ loads such as those due to vaulting or wmd push the wall or p~r (i.e., e equals or is less than t/6), ing the sliding friction between the stones. against the wall, the thrust line is displaced off stresses throughout the section aTe compTessive center, unth its eccentricity (e) incrtasin8 in pro­ (figures ).90 and b) and cracking wil/lIOt develop portion to the magnitude of the {atera/loads. {n (see Schadek, ::88-::90).

V

I , I I I 11, I , 1!, 1 y I , I I , I f f

MzV'e ( tenSion)

V b. •• <. d. L,j Wulls lind Other VerI/ali Ekme .. t~ "

, stances, both surfaces of a wall will experience ,. , , • , compression, as illustrated in figures 3-9a and 3-9b­ - ,. , When tension does devdop, as illustrated in figure • .. • 3-9C, the ensuing cracking may not in itself prove - " destructive to the integrity of a structure. Yet cracks •, can allow the entry of water, which can wash away -' ,, ~ lime mo rtar, or in the presence of freezing tempera­ , 'J tures, expand and further break up the masonry. • ' .. ...1 Although both phenomena can pret:ipitate great re­ i" .:' ...,... ductions in wall strength, master masons could make design corrtctions by examining thdr structures for the presence of such cracks, even during the process of construction. The thickness of a wall or pier can 3.11 Untel support {rom column capitals. ).11 A deep lintel, cracked at three points, re­ 3. I 3 Corbel-arch figuration. be increased, or they might be made taller to increase mains stable if if.S supports are unyielding. compression forces that tend to dose up the cracks, an especiall y attractive option when fou ndations have alrtady been completed and the width of the springing of a vau lt to a free-standing pier. The Hying formance in bending is to reduce the effective dear use of correspondingly large and unwieldy mono­ wa ll fixed. Heavy parapets or cornices were some­ buttresses of the cathedral of Bourges (figure 3-7) span of an opening by employing wide capitals on lithic limels, corbeling can be constructed from rel­ times been used to solidify wall s, as have pinnacles stunningly i11ustrate how slender and lightweight top of columns (figu re 3.11). Such a small reduction atively sma ll clements, usuaUy of cut stone or brick, and even statues (Mark 1990, 118-12.0). Even so, it such a structure can be. in dear span may seem inconsequential, bur since each of which projects into the opening sligh tl y paSt is highly unlikely that master masons understood the The simplest method for creating the open­ bending stresses in a lintel vary with the square of the element beneath it, as illustrated in figure 3.13. theory of the rule of the "middle third" defined in mgs In walls nccessary for doors and windows is its unsupported length, even small increments in span And unlike true arches, corbels require no supporting figure 3.10. through the usc of trabeated supportS, that is, post can have a major effect on lintel Stresses. Further­ centering in the construction process; the stability of Although many problems of structure could and lintel construction. In this system the jambs, or more, cracking of a stone lintel does not necessa rily the individual elements is assured by the mass of be avoided simply by constructing thick, heavy walls sides of the openings, act as supporting posts, and lead to its destruction. If the ends of:1 cracked, deep wall placed above. But because it does nor act as a or piers, costs escalate with the greater volume of the lintel over the top acts as a beam in bending. lintel are prevented from spreading apart, it will not true arch, an important limitation of a corbeled stone to be quarried and transported, not only for Wood makes for an efficient lintel because of its good be able to collapse. as ill ustrated in figure ).12.. A opening is that its height must be fa r greater than its the wall itself but also for the foundation providing tensile strength and light weight; but because of the cracked structure is. of course, more susceptible to base width. As with the lintel, this limitation effec­ its support. When lateral loads are applied at local­ greater relative durability of stone, wooden lintels seismic movements; and freezing water in colder cli­ tively restricts practical span lengths. ized regions of the wall, such as at the supports of were rarely used in monumental buildings. On the mates can COler the cracks and force the segments of True arches, however, circumvent the in­ groined vaults or below domes supported by arches other hand, Stone, being weak in tension, makes for the lintel apart. Yet many ancient, cracked lintels trinsic span limitation of the trabeated system and and pendentives, it is more economical to th icken a most inefficient lintel. Openings framed with have survived for centuries in southern Europe and the corbel arch by securing all of the constituent the walls loca ll y to form a wall buttress. Another monolithic stones arc therefore limited in span. northern Africa (Heyman). clements, known as lIoussoirs, in a state of compres­ viable structural solution involves indining the wall Ancient architects were well aware of this Though not so practical fo r providing sion. In effeer, the curvature of an arch, as opposed or pier ro match the angle of the resulting force, as limiration and developed schemes to circumvent it. openings in wa ll s as the trabeated system, the so­ to the linear fo rm of a lintel, engenders hori zontal wi th a steeply pitched Hyi ng buttress connecting the One method of compensating for stone's poor per- called corbel arch averts many of its problems. While as well as vertical reactions (figure 3.1 4a). The forces large spans in a trabeated building necessitate the generated within the arch by these reactions then act "

3.14 Tru~ arch behavior. (a) Form, loading, and reactiO'IS. I (b) Abutment {ailure. With the {irst slJreading of an abutment. a masonry arch will likely acquire three "hinges" due to cracks forming at both abutments as well as at the crown. Nonetheless, a three-hinged arch is stable and willlike/y endure unless the motion of the abutment becomes grNUr. This characteristic of arches ofteu allows '. for the reinforcement of an Insubstantial abut. ment before major damage ensues. (c) Four-hinge failure mode (after Coulomb),

-- 3.[5 Segovia Aqueduct, (irst century A.D. Detad practical arch applications) andlor it is thick enough of the upper arcade with c/)Qracteristic sllrcharge. to maintain the resulting thrust line within the con­ fines of irs extrados and intrados. (The concept of thrust lines is discussed in figure }.w). More typi­ to confine the voussoirs. Masonry arches afford great cally in early structures, the haunches are surcharged

interio r spans (reachi ng 33 meters by the sixth cen­ (provided with heavy fill) 10 prevent large displace­ b tury; see comments on Hagia Sophia, below), a po­ ments of these regions of the arch (sec figure }.IS). tential that establi shed the arch as the structural When an arch is inserted into a wall, the system o f preference for large-scale monumental region of the wall above ,he arch acts as surcharge. buildings. True arches also share a major construc­ Moreover, when an arcade of arches is placed in a tional advantage of the corbel system: assembly from wall, the horizontal thrust from each arch counters --. relatively small , easily managed dements. thai of its neighbor, so thai the supponing pier below Yet despite all of these intrinsic advantages, experiences only vertical compression. It only reo arches demand special treatment in construction. Be· mains to securely anchor the ends of the arcade. For cause they produce horizontal reactions from verti­ this reason, in Rom:m aqueductS, massive piers were cal, gravitational loadings, arches require either rigid positioned at any point where the aqueduct changed <. abutments or tension ties across their bases to pre­ direction (figure 3.16). In large medieval churches, vent spreading and possible collapse, as described in the relatively heavy crossing piers and the semicir­ figure 3.14b. Moreoyer, an arch may proye unstable cular (in plan) apse usually buttress the ends of choir (and f:lil in the mode illustrated in figure 3.14C) arcades, while the arcades of the nave are buttressed unless its form approaches a singular, optimal shape: by the remaining crossing piers and me twin lOwers (a catenary for a un iform arch supporti ng its own of the facade. deadweight, but closely enough approximated by a One of the more important questions in parabola or even a shallow circular arc for most arch (as well as vaul! and dome) construction con· 6, Walls and Other Vertical £lffllllllt5

sonry construction. Precision of execution. rigidity of form. and the case of removal of th is temporary structure played a key role in the building process. The centering, which determines the profile of the underside of an arch (or a vault. as discussed in chapter 4) remains in place until the completed arch can stand on its own. While in use, though. the ).17 "Flying centering~ used in Koman bridge centering must resiST deformation as incremental coustruction.

loadings are applied, to keep the desired final shape of the arch. Hence, the centering used for large-scale construction, such as in imperial Roman architec­ 3.18 Pont du Card aqueduct. Nimes, {irst cen­ ture, was of necessity powerful1y built. wry. A.D. Centering falls into twO basic types: (I) that supported dir«dy on the ground, by means of vcr­ ri cal o r radial wooden Struts, and (l. ), that springing from a masonry pier, wall, or vertical support at the end of an arch (figure 3.17). The second method is especially expedient for bridge construction, as the river flow might wash out temporary columns placed 3.16 Segovia Aqlledllct: buttressing at ;unction. in the main channel. It also saves timber whenever the arch is being constructed high above the ground, and so was employed in the majority of these cases. The technique was used, for example, in the con­ structi on of the Roman aqueduct at Nimes, the Pont du Gard (figure 3.18), where one can still observe cerns the temporary centering used to suppOrt vous­ three levels of masonry projections from which the soirs until the placement of the final VOUSSOlr, or centering was sprung (Adam. 191). Note that build­ keystone. Since it had to be designed to be taken ing up the haunches of each side of the arch and down after construction, often for reuse, we have "flying" the centering from both the column capital little first-hand knowledge of the centering that was and uppermost part of the springing, as in figure used to su pport both workman and StOnework dur­ 3.19, reduces the actual arc to be built to a segment ing the erection process. Although in its most prim­ smaller than a half-cirde, thereby saving labor and itive form, centering involved the use of tamped, materials, thanks to the reduction in centering span. mounded earth (Fitchen, 30-)1 ), timmr was me pri­ In addition, this order of construction ensures that mary medium employed for formwork in hi storic the surcharge has already been placed prior 10 de­ buildings and therefore, both structurally and eco­ cemering, so that the haunches of the arch have no nomically, it constituted an essential clement of ma- opportunity to rise and deform. 7' ChaplU J

dure that required close coordination in the removal of wedges, as uneven dccenrering might result in distortion or collapse o ( the structure. Many approaches were employed to reduce the expense of centering, the most common of which was reuse. Wide masonry arches were often built up over a seri es of parallel arches. In figure 3.1.0, for example, three similar SlOne arches are clearly seen from below the intermediate arcade of the Roman llonr du Gard. With the keystone of the fi rst arch in place, the opposing wedges were driven out and the , frttd ce nteri ng was slid sideways and raised with 3.10 Pont du Card; intermediate arcade from wedges for the construction of the next parallel arch. be/ow. As four similar arches were also used for the wider, lower arcade of the aqueduct, a single, thin centering _" seems 10 have served fo r the construction of all seven. Where large stone voussoirs were used, the lagging }.19 Timber centering slIpporting ribbed-vault would be Stoutly proportioned and widely spaced, construction (after Fitchen). wi th each voussoir bridging the gap between the lagging, as at the Roman aqueduct at Segovia (figure }.IS ). Closer spacings were required in Roman con· In irs simplest fo rm, when used to erect a crete construction where rubble stone or brick was single, se micircular masonry arch, the centering tim­ laid in thick beds of hydraulic ponola" mortar. berwork consists of at least twO parallel armes When the centering was stru ck, the impression of braced by triangulated framing. These arches, made the closely spaced lagging remained, just as do form­ up of shorr, joined timbers, carry between them work patterns on modern concrete surfaces. planks, known as laggings (illustrated in figure }.19), Centering costs could also be reduced by upon which the stones arc sct. To provide for safe first building a relatively light arch on correspond­ decentering, builders inserted pa irs of opposing ingly light centering, and then using the completed wedges beneath the wooden centering ei ther on tem­ arch to support additional concentric rings of ma­ porary foundations or, in the case of flying centering, sonry. The Romans used brick and tile arches in this on masonry projections. Upon complerion of the manner, building up much heavier forms on what arch the centering was struck by driving out the became, in esst:nce, permanent centering (Viollet-Ie­ wedges, which in turn dropped the centering a few Due, IX: 465-467). This practice was continued in centimeters and allowed the voussoirs to wedge Romanesque construction, especially in church por­ themselves into pl ace:. In large arches with many tals in southwest France, where the concentric arches wedged supports, decentering was a difficu lt proce- became an aesthetic focal point (figure 3. 1.1 ). 3.1. I Au/nay, I" 9-1 1 J5: facade.

I 7' ChllP11!T J

J.n N:lluraJ lighl illuminance Assummg the same source intensity of light {or Vi. building scale. two similar buildmgs of differenl scale. both will The level of natural-light illuminance (surface experience the S:lme levels of illuminance because brightness) at an interior point in a hui/ding is the longer light paths of the farger building serve direc.tly related to window area, the inverse to reduce light transmission by an inverse-square square of the light-path-lengths from the windows relationship while the area of its window open­

(light levels at twia the distance from Q source ings, and hence its sources of lighting, are pro­ wjfl be but '/4 as strong), and the orientation of portional to the square of its scale. The two effects the axes of the light paths (Mark 1990, 43-47). thus cancel each other.

3.13 Bourges Cathedral: cross-section of the choir t('rpreted symbolically, as Abbot Suger's £welfth­ showing ligl"paths from wall openings to a region century writings on the windows of St. Denis reveal. of the nalle {loor. Ught was considered a force for the elevation of the spirit toward God, and Suger believed that the lu­ Interior lighting needs also played an im­ minous interior of his church brought the worshipper portant role in the development of wall forms. The out of a purely physical realm into a higher state of desi re for large windows, for example, was central comemplation (Panofsky, 13)· to the evolution of the non-load-bearing wall. What­ The creati on of light-filled interiors, espe­ ever the form of the wall openings, the materials cially in the Middle Ages, may well have been meta­ adopted by different societies to enclose them have physically motivated; yet in lighting, physics plays a ranged from glass and fabric to mica-like minerals. more dirta role than metaphysics. Since historic In the Romanesque and Gothic periods, particularly, buildings were dependent mainly upon sunlight for the wall became a surface activated both visulllly illumination, describing the eHective source intensity and structurally by a series of arched and vaulted at a window proves complex: it varies with the time openings, illuminated by panels of stained glass. of day, weather, season, orientation of the windows, liglu admined through these openings could be in· external obstructions such as surrounding buildings

I Chapl~ J

and, of course, the light transmission permitted by walls and other vertical elements in general terms, window fittings, especially by srained glass. Although we may now consider the application of these prin­ one must interpret with caution any purely quanti­ ci ples to speci fic buildings constructed before the tative data on architectural lighting, pa ying due at­ scientific revolution. tention to aesthetic and symbolic concerns, analysis of the interior lighting can in some cases help 10 AN CIENT clarify dc:sign intentions in historic buildings. Illuminance, or surfa ce brightness, provides Greek monumental buildings were almost exclu­ the most readily accessible measure of interior light­ sively based on trabeated wall construction. True ing. As shown in figure ).1.1., illuminance remains arches were ce rtainly known to the Hellenistic unaffected by building scale, since for larger build· Greeks, who did in fact use barrel vaulting based on ings the windows are both greater in size and farther the arch, but not often, and then usually in subter­ from Ih(' observer. Gwmetry, on the other hand, ranean, utilitarian structures or tombs. Ear lier Greek 3-1.4 The LIon Gate, Mycenae, mid-thirteenth exploiting the rich marble quarries on Mount Pen­ pla ys a major role in d(,fermining lighting levels. As cultures em ployed corbel arches, with perhaps the century B.C. telicus (Bruno, 31.7). These new quarries became the the viewing angle becomes increasingly oblique, for beSt known of these being the partially destroyed primary source of building stone for Athenian reli­ exampl(', the window appea rs more foreshort('ned Lion Gate at Mycenae, constructed in the mid­ construction, however, dates only to the Roman era, gious and public buildings, and their stone was and provides less light to the observer for a given thineenth century R.C. (figure ).1.4 ). In addition to in the first century .... 0. In most Greek monumental widely exported as well (Dinsmoor, 188). When surface brightness of the window. Conseq uently, exhibiting the corbeling technique, the gateway buildings of this later period, walls would have been le sser-qualiry stone was used, wall surfaces were windows SCt high in the wall lose their effectiveness incorporates a heavy stone lintel deepened at its mid· covered with a thin revetmcnt of marble. often covered by stucco. in providing intense levels of illuminance tovi('wers section (where bending is greatest-benefit from in­ Many clements of the Doric order (figure As the volume of stone use increased, build· at ground level, as d('monstrafed in figure ).2.} com­ creasing beam depth is discussed in figure ).9b} to ).2.5) probably represent the styli sti c continuation in ers devised techniques for lessening the cost of nans· paring the light reaching the floor from the upper span a }.2 meter-wide opening. stone of formerly wooden dements: architraves portation. Frequently blocks were hollowed out be-­ and lower clerestories of Bourges Cathedral. Until the mid-eighth century 8.C., Greece evolved from the wooden beams spanning the col­ fore leaving the quarry, espe:ciall y during the Archaic Choices of materials also affect architec­ possessed no truly monumental architecture. Build­ umns, triglyphs and metopes arose respe:ctively from period when building stone often was shipped by sea tural lighting conditions. Above and beyond the ing activity seems to have increased in both scale and the beam ends and from the infill between those from distant sou rces (Co ulton, 146 ). An exam pl e of choice of window covering, the reflectivity of interior intensity during the following two centuries as the beams, and cornices derived from the supporting this strategy, probably because of its remote location, surfaces and the presence of artificial light sources basic forms of the Doric temple were developed. First member for the rafters. Yet in their newly manu' is found at the Temple of Apollo at Bassai (ca. 4)0- can contribute significantly to both the amount and constructed of timber, early Doric temples were mental stone buildings, Greek architects confronted 400 B.C.). The llarian marble ceiling beams of the the qualiry of light. Many Greek temples, for ex­ adapted to stone construction, most likely because a new set of problems concerning materials. trans­ temple: are formed with U-shaped sections as illus­ ample, depended on candles and (Qrches to light their of stone's greater durability. Even so, Greek builders PO", and of course, structural suppo". trated in figure ).2.6. The weight of the beams before interior cellas. Despite all of these complications, had employed many different construction materials As early as the seventh ttntury 8 .C., CUt voiding, about 2..4 tons, should have presc:med no however, satisfying a parron's taste for lighting con­ before settling on stone for monumental architecture. blocks of easily worked limestone were used (or unusual difficulties for lifting with a contemporary ditions must have always constituted an important Low-cost walls composed of mud-brick or field monumental buildings and by the sixth century hand-powered crane (Landels, 84-85 ), but the ap­ design goal for the early builder, one that also mo­ stones, laid dry or in day and reinforced with timber, Athens had begun to import fine white marble from proximately 40 percent reduction in weight would tivated notable structural innovation in wall forms. appeared very early and were common even in clas­ the islands of Paros and Naxos for use in both build­ have been advantageous for transportation as well Having examined aspects of design, con­ sical times for modest buildings. The introduction of ing and sculpture. By the early fifth century, the as in reducing the deadweight loading of the finished struction, and the structural behavior of historic fired brick together with mortar and concrete in wall Athenians turned 10 sources closer at hand and began beam itself. The U·shape is also structurally logical

I Walls a"d Ofha VatICal ElfflIfflts 77

COrM& === mu'~I. r '''illyph because ollly the less critical compression side of the beam was reduced; the beam section remains undi­ minished on the critical tension side. Indeed, the fact , " that both beams and ashlar blocks of stone were

hollowed out III this way suggcsts that the architects carne to realize that the stone in temple walls is substantially understresscd in compression. A surprising technique using iron to en­ hanee the reliability of stone structural members is found at the Propylaea in Athens, built between ca. 437 and 432 D.C. by the architect Mnesikles. The 3.2.7 l'ropylea, Athens, ca. 4Jz-.H7 B.c. lron­ }.28 Hieron, Samot/lfaCt!, late fourth century B.C. ]·2.5 Doric temple nomenclature. marble ceiling of the Propylaea is supported by an bar insert ill ceiling beam (after Coulton). Deepened ceiling beam (after COI/ftoll ). array of beams that in turn res ted on Ionic archi­ traves, as illustrated in figure 3.27. Those beams coinciding with columns below the architrave merely transmit forces from above to below, engenderi ng Another struCtural device, which we have compression; but those over the midspan of the ar­ already encountered in the Lion Gate at Mycenae, chitrave produce significant bending and associated was used for suppOrt of the coffered cei ling of the tension (as in figure 3.8b). To reduce this bending­ late-fourth-century Hieron at Samothrace. The eeil­ by transferring loading away from the center of the ing rests on beams that span more than six meters. architraves toward the supporting columns-iron This considerable span is achieved by having the bars were set into the top faces of the architravcs, invisible fOp fa ces of the beams c:xu~nd upward to with a 2.·5 em-deep gap cu t below the hars 10 allow form a tapered rib (figure 3.2.8), dfectively deepening their centers to deflect under load without coming the beams at their centers. inro contact with the architrave (Coulton, 14 8-149). In lieu of mortar, iron cramps and dowels [n effect, the iron bars acted as independent "reliev­ were generally used fO fasten together the blocks of ].2.6 Temple of Apol/o, Bossa;, ca. 400 /l.C. Ceil­ ing heams." Even so, the bars should not be inter­ stone in Greek monumental construction. Perhaps ing detail showing hollowed-ollt marble beams. preted as behaving in any way si milar to reinforcing this use comributed to the Greek pracriee of building steel in modern concrete. Modern steel reinforcement with large stones, effectively reduong the number of does not undergo significant bending; rather the re­ cramps Ilc¢ded as well as the total area of carefully }.2.9 "Bow-tie" cramps employed in Creeks tem­ inforcement functions by accepting di rect tension in dressed stone facing, to eliminate the danger o f Stress ple COnstmction (after COl//ton). the beam, thereby relieving the concretc itself from concentrations from rough protrusions. A typical having to resist this pernicious fo rce. cramp was a bow-tic-shaped iron piece that fit into channels carved into the ends of the stone blocks, stitching them together (figu re ].2.9). Similarly, iron dowels were I.l!ied 10 fasten the drums of columns, preventing them from sliding under shear. The chan-

, is with the second col umns out from the center. This modification not only allowed more space for the cult starue, but it also solved the problem of a large roof span by providi ng support at shorrer, more nearl y equal intervals (see chapter 5). Despite the la rger scale of the Parthenon, the intercolumniarion remains at just over tWO meters, not unlike that of the earlier temples. More radical strucrural innovation was pursued at the giant temple of Zeus Olympios at Akragas (present-day Agrigento) where the great scale of the temple reqUired modification of the basic Greek temple format. Instead of using the more-or­ • , '0'" less standard intercolumniation of between two and }.}O Parthenon. Athens, 44 7-4)1 B.C. West has been long noted for its visual refinements and three meters, the span of the li ntels at Akragas would 3.31 Comparatille sections: Temple of Zeus, Facade. the blending of Doric and Joni e elements (figu re have needed to be extended to a full four meters. Olympia, 470- 457 R.C. (above) and Parthenon 3·30). Its also marks a structural transformation in The architect compensated for this large span by (after Coulton). nels and holes for both cramps and dowels were cut the orga nization of the interior of the traditional inserting great columns carved as standing male fig­ larger than the iron fittings, so that molten lead Doric temple. Coulton has delineated this change by ures that appear to help support stone lintels at their poured around the fitting would seal it firmly and comparing the cross-section of the Parthenon with center (figure J. jl.). In fact, the architrave at Akragas help prevent air and moisture from rusting and ex­ that of the Temple of Zeus, Olympia (figure j.jl), does not act a.~ a beam, since the spaces between the panding the metal. an important and prestigious mainland tempi.: begun columns and the Atl anti s statues are filled With The practice of us ing sueh cramps and in ca. 46S R.C. (Couhon, '1 3-1 17). Although the coursed masonry walls. But even with these walls, dowels to resist sliding might seem surpri singly con­ two temples are of virtually the same height, the there must have been some lingering concern on the servative, since the frictional forces between the Parthenon is wider by more than three meters. This part of the architect for the overhanging portion of blocks of stone resulting from the deadweight of the dissimilarity in width reflects differences in the in­ the architraves. As shown in section in figure 3.31., wall would already have been substantial. More terior span of their ce ll as: that of the Parthenon is iron bars were also positioned in these regions (as in likely, the cramps were placed in an attempt to guard close to ten meters, significantly wider than any other the Propylaea, di scussed above). In su m, the classic against potential stone shihs due to foundation set­ roofed, Doric Greek temple, including the Temple of structural system was eschewed at Zeus Olympios. tlements, or possibly, earthquakes. Ironically, the Zeus whose cella span was less than 6'1. meters. O nl y the basic exterior form of the Greek temple presence of metals probably caused more harm than Responding to the greater width, Iktinos and Kalli­ was retained and adllpted to the new and large r scale. good: once the buiJdings were abandoned, scaven­ krates altered the six-column portico pattern of the Beams of marble or stone, carrying the , • I D '" gers searching for lead and iron gouged holes into Temple of Zeus to an eigh t-column pattern for the • combined loads of cntablarure and roof, could never the walls of many Greek buildings, weakening them Parthenon. In both temples, the inner colonnades of 3.31. Temple of Zeus Olympio5 at Akragas (Agrj. be counted on to safely span great distances. The and contributing to their collapse. the cella are aligned with exterior columns, but at gento), ca. 500-460 Il.C. Pacade with male figures: inherent weakness of stone in tension, more than Constructed between 447 and 431 B.C. by Olympia, the alignment was made with the two cen­ Ilote iron reinforcement a/Jove figure (afte r any other single 3trribute of the classic design, eHcc­ the architects Ihinos and Kallikrates, the Parthenon tral columns, while at the Parthenon, the alignment Coulton). tive1y limited the r3nge of varia nts that Greek archi-

• So Chaptu J

teetS could devise for the rempl(' (orm. The limitation allowing it to be poured into forms. In contrast, was overcome only when a new aesthetic for mon­ Roman concrete was hand-layered together with umental architecture was introduced inro the Roman chunks of aggregate that often consisted of rubble world. from earlier buildings. Modern concrete also gains great tensile strength from integral reinforcing steel, whereas Roman concrete, dependent only upon IMPERIAL ROM E weak cement bonding in tension, could not be relied upon to accept appreciable tensile forces. Because it J.33 Roman concrete facing: (a) opus incertum; century; by the beginning of the fourth century, it Even before the advem of imptrial Roman architec­ shared the same weakness under tension, Roman (b) opus reticulatulIl; and (c) opus testaceum (note had escalated to some 6 em, the thickness of the ture in the first centu ry A.U., the use of voussoir construction in concrete did not really differ from use of "/elJeling course'" in opus testaceum brick itself (Dodge, 111.). arches-known but rarely exploited visibly in Greek construction in stone or brick masonry. Indeed, the constmction). Roman concrete walls also incorporated so­ architecture--came into its own. Roman walls tend use of pOlzolan concrete seems to have bccn moti­ called leveling courses: through-The-wall hotizontal 10 be massive and marked by deep recesses. Primary vated primarily by its economic advantages in con­ As ea rl y as the second century B.C., baked layers, usually composed of large bipedales placed at building materials were brick, stone, and by the tum struction (Mark J990, 71). By the beginning of the bricks and tiles were occasionally used as building vertical interva ls of about '/~· me t er. These strata (il­ of the first century for monumental Roman build­ second century A.D., panolan concrete had become ma terial, for example in the Basilica of Pompeii (ca. lustrated in figure j.33), not keyed into the concrete ings, polZolan concrete. the material of choice for large-scale building. ll-O B.C.). lbeir application became increasingly fre­ core above or below, created planes of "potential Rome and its surrounding region were not Unlike modem practice, which employs quent in the first century B.C., but it was not until cleavage ... a source of weakness" (Ward-Perkins, favored, as was Athens, with nearby marble quarries. temporary wooden or metal forms to suppon poured the reign of Nero that o pus ,erieu/alum was effec­ 99) that could have been troublesome, were it not Commonly available stone included pepcrino, trav­ concrete until it hardens, Roman walls, as well as ti vely supplanted by opus testaceum. Brick facing for the great mass of the wall itself. In addition to ertine, and a soft volcanic tufa that conveniently piers and columns, most often used permanent forms was at first a characteristic of buildings in and ncar providing a level surface at the end of each stage of hardened when exposed to the atmosphere (Sear, of brick or stone, classified according 10 the pattern the city of Rome, but the technique was quickly wall construction, inserts of this type (and similarly, 7j). In the provinces, brick and local stone, ranging of facing used. The three main facing types are: opus disseminated to the rest of the empire. Brick was of brick ~ribs" in domes) may have played two prin­ from the light-colored, fine-grained limestone of incertum, an irregular facing of variously shaped especially favored by the Romans because it could cipal structural roles: the first, to act as a cover for southern France to the rough, dark-gray native stone small Stones, opus reticu/atum, square stones set di­ be produced quickly and cheaply under industrial the rece ntly laid concrete during construction, help­ of the Val d'Aosta, were used. POZlOran concrete, agonally, and OPIIS testaceum, coursed brick or tile conditions, yie ldi ng building units of standard size ing to keep it (rom drying out too rap id ly and mal­ used commonly during the Imperial era, "cures, nor facing. As illustrated in figure j.33, facing stones and shape. Roman brick was not configured like its forming; the second, to provide deliberate "planes sets, chemica lly in a simi lar manner to modern Port­ and bricks were usually triangular, serving to in­ modem equivalent; rather. it had the shape of large, of weakness" (similar to modern expansion joints) la nd ce ment (see chapter I on building materials). crease the surface area betwccn the facings and the square tiles, typica ll y about 6 cm thick. It was pro­ that could accommodate structural deformations as­ This allowed it to be used for underwater construc­ concrete core. Found almost universally in early con­ duced in standard Siles up to about 60 em on a side. sociated with powerful, thermally induced forces (see tion at h:lTbor sites as well as for constructing aque­ crcte construction, the use of opus incertum declined When cut along their diagonals, the larger-sized Mark 1990,66) and thereby avert unplanned cra ck­ ductS and sewers, without having the mortar dis­ in the last quarter of the second century B.C., though bricks, known as bipedales (two Roman feet), pro­ ing in regions of the wall remote from the inserted solved by efOuent. Indeed, this property has it continued to be used for precinct walls and for duced fou r triangular facing bricks. Interestingly, brick layer. contributed to the preservation of many Roman rough construction. In finer work, it gave way to typical Roman monar-joint thickness did nOt remain The amphitheater was one of the most im­ ruins. Even so, Roman concrete differs from modern opus retit;u!atum. that provided a new standardiza­ constant over time; the standard, J-cm joint thick­ portant public buildings of a Roman city_ From the concrete in that the consistency of modern concrete tion of stone blocks, as well as the greater possibility ness of first-century brickwork in the capital in­ first century B.C., Roman architects were able to mIXes, composed of water, Ponland cement, sand. for effects of polychromy, particularly popular until creased to about" em by the beginning of the second reproduce- in built form the geometry and substantial and fine rock aggregates, is fluid and homogeneous, the second century A.D.

, Chapter J

seating capacities of "natural" amphitheaters carved arena by a squad of sailors ro shield the audience of bedrock. In the amphitheaters at Aries, Nimes, from the sun, are recalled by the stone corbels that Rome, and Verona, masonry walls converging to­ once held thei r supporting mastS. ward the interior and sepa rated by vaulted passage­ The materials needed to build this massive ways provided a surface for the seating ramps. Pas­ arena included 100,000 cubic meters of travertine sages also opened onto circulation corridors from and 300 tons of iron cramps to hold the blocks which further slopes or stairs could be accessed. This together (Cozro, 2.9-30). Most of the Colosseum ingenious interconnection of conceorric corridors was originally covered with a veneer of travertine, and inclines facilitated the handling of large audi­ scavenged for reuse in the fifteenth century. Too big ences. The passageways not only tapered and sloped to be easily carted off, the marble ashlar blocks of in a conical form, but were placed at varying angles the perimeter wall remain, as do brick facings of because the arenas had more than one geometric ponolan concrete-filled walls_ In some cases, ex­ center_ The concentric corridors also required a com­ posed pozrolan concrete surfaces remain with the plex system of vaulting; but our interest here lies imprin t of the formwork still clea rl y Vis ible. Con­ primarily in the large curving walls that support the struction of the Colosseum was completed over such 3.34 C%ssellm, Rome: bllttressing of severed upper wall, but the tremendous weight of the large vaults and seating. a short period of time that it has been suggested that wall. travertine blocks comprising the wall itself plays a The Colosseum in Rome, the largest of Ro­ conventional methods of building (story upon com­ far more significant role in ensuring its stability. As

man amphitheaters, measures liS )( I S6 x 4S 1. pleted story) would have been tOO slow (Sear, '39). discussed in the introduction, the horizontal thrust meters high and provided seating for up to 55,000 Rather, it appears that construction began with the ically with water. When poured in large masses, even of each arch in the arcade is effectively countered by spectators_ Begun under Vespasian in 75 A.D., the raising of the outer travenine walls up to the top of in cooler temperatures, modem concrete becomes adjacent arches; and because of the dosed, oval amphitheater was placed on the si te of the lake of the first order, together with the two concentric walls warm to the touch. Roman pozzolana exhibited a form, no additional supports were nceded to termi­ Nero's Golden House (see chapter 1.), du:reby trans­ behind them. Pi ers were then constructed up to the similar reaction, and this probably allowed builders nate the arcade. (Because of the curvature, the op­ forming part of a palace site into public space. In­ point of the va ul ts, leavi ng only a concealed spring­ to continue construction ri ght through the relatively posing thrusts of the arcades are not exactly aligned, deed, the weight of the lake water probably helped ing for the later vault constru ction. Finally, brick mild Roman winters. but the resultant small outward forces are easily to preconsolidate the subsoil prior to the constru c­ arches were built to support the sloping barrel vaults, The extremely tall perimeter wall of the countered by the building's mass. ) After the removal ti on of this enormously heavy structure in a low, which in turn supported the banks of seats. Virtually Colosseum btnefits from secure bracing at each level of a portion of the outer wall of the Colosseum,

marshy area (Sea r, I 3S). the entire skeleton of the Colosseum could in this of the three superimposed sets of concentric pas­ however, a large inclined buttress had to be placed Of all the imperial amphitheaters, only the way have been erected in a short time, permitting sageways, so that it acts not as a single 48 '1'~ meter­ to secure the arcade arches (figure 3.34). Colosseum possessed twO passageways encircling the large gangs of skilled and unskilled workers to fill in high wall, but rather like fou r individual walls, each Although used extensively in buildings to converging ramps of seats on the same level (figure the spaces between the piers latcr on. one only a quarter of the total height. The upper introduce large openings for light and access, the 1..13). This second concentric passageway permitted Another aspect of the rapid construction wall proves least stable, not onl y because its greater voussoi r arch is perhaps most dearly studied in the the addition of extra banks of seats above, in turn concerns the behavior of fresh masonry in cold elevation subjects it to higher wind pressure (see great aqueducts erected by imperial Roman engi· making a third order of arches necessary in the outer weather. Freezing temperatures may damage uncured figure }.4), but also because it receives bra cing only neers. One of the best examples is the 1.75·meter wall. Domitian added a fourth story in A.D. SI-S1., mortar because expanding ice can literally pull mor­ along its lower perimeter; the other three Tiers are long Pom du Gard near Nimes, dating from the early but all trace of the upper seats has been lost, sug­ tar apart. While this problem is endemic to lime securely held in place, at both top and bottom, by first century II.D. Built to carry water across the gesting that they were constructed of wood. Canvas mortars, ponolan concrete cures exothermically; in the internal vaults. The curve of the arena also con­ valley of the river Gard, the bridge is effectively a awnings, known to have been stretched across the other words, it produces heat as it combines chem- tributes a small measure of additional stiffness to the wall that ri ses 49 meters above the river surface,

I with large openings to permit ri ve r flow through the in figure 3.36) joined by relieving arches, as illus­ base and to reduce the total volume of quarried trated in figure 3.37. Eight deep niches, one forming masonry on the upper levels. the entrance and seven others that sheltered statues The construction of the Pont du Gard can of the sevcn ma;or Roman gods, helped to lighten be described in terms of a successful formula: large the massive wall and lessen the total load on the cUNtone blocks laid without mortar to eliminate the foundations. The mu ltiple tiers of arches ;oining the danger of water leaching mortar out from the joints piers further demonstrate that Roman architects

and possibly leading to collapse; projecting stones at wisel y distrusted trabeated support systems for large the springing to provide a ledge on which the flying loads. Even where the decorative articulation on the centeri ng was supported during construction (visible Pantheon interior appears to be trabeated, relieving in figure 3.18) and si milar stones on the outer faces arches hidden behind the ma rble veneer of the of the piers and spandrel s helped support the scaf­ walls show the building·s dependence on arcuatcd folding on wh ich workmen stood and the materials struCf\lre. were raised; and the extensive reuse of centering as 8ccaust' it had been thought that the step described above. rings su rroun di ng the dome acted to reinforce il and Irs great width (originally 6'1, meters at the thereby remove much of the horizontal th rust that lower arcade, -4 '1, at the intermediate, and 3 meters would OI herwise be rransferred from the dome base for the uppermost arcade) and deadweight of its piers against the upper supporting wall, the thickn ess of and arches more than adequately coumer potential 3.35 Pantheon, ROII/e, ca. A.D. 11 8-1:8: interior. the Pantheon's cylindrical wall drum has been bending stresses from lateral wind loadings (as de­ deemed excessive (see, fo r example, Middleton, I: scribed in figure 3.9). The tiered arches are stabilized 66, II : 131, and Robertson, 233-234). However, by stone surcharges, as discussed in the introduction, recent studies have demonstrated the way in which through which horizontal thrust is transmitted from the dome actually behaves as an arra y of arches that one arch to the next, with a ze ro-resu ltant net lateral exert immense horizontal forces on the drum wall thrust on the su pporting piers. At the ends of the (see chaptcr 4). No doubt from earlier experience arcades, SlOne cliffs resist the reactions of the outer with arch behavior (as well as with smaller, similar arches, creating, as the nearly 2,ooo-year endurance domes), the designer of the Pantheon understood of the Pont du Gard demonstrates, an extremely that the supporting structure would need to resist stable structure. these forces against bending, and he wisely speci fi ed The Roman Pantheon, built between ca. Its great rhi ckness-in effect, making his wall per­ n 8 and 128, presents a different problem in wall form in a manner similar to that ill ustrated in figure structure and design. A massive circular drum wall, ).9. The long life o f the Pantheon owc5 much to Ihis composed of a pozzolan concrete core faced with crucial design decision. 3.37 Pantheon: relieving arches in the wall brick and marble, supportS the colossal H-meter Like the Colosseum, the Markets ofTrajan, (MacDonald). • J 10 ... diameter dome above (figure 3.Jj). Although 6 me­ constructed in Rome between A. D . 1 00 and Il l., ters thick, the drum is not actually solid throughout_ demonstrate the readiness of the Roman emperors Rather, it consists o f eight great piers {shown in pla n 3.36 l)allt}Jeon; plan (after MacDonald). to build structures used by the community at latge. 3.38 Markets of Tra;all, Rome, A.D. lOa-lIZ ".- *" "Flying buttresses" of the market hall.

3·39 Basilica Nova (Basilica of Maxientius and Constantine), Rome, begun 307: interior recon­ struction (after Ward·Perkins).

3.40 Basilica Nova: walls of the north exedra. in J07 and completed by Constantine following his

victory over Maxentius in 3 I z. The so·call ed Basilica

Nova enclosed more than I Y. acres divided into three

The markcts housed more than 150 shops and offices high. groin-vaulted bays, flanked on both sides by in a large comple)( similar to a modern mall. To lower barrel-vaulted bays (figure J.39). To counter­ buttress the high groin vaults of the main arcade (see act the thrust of the huge groin vaults, massive lateral chapter 4). the designer employed external braces bunressing walls, some 4 meters thick, were can· supported by masonry arches (figure 3.38) not unlike structed between the lower flanking bays (figure thc flying bunresses created a millennium later by 3.40). These structures then emerge above what in French Gothic masons. This innovative structure, effect are the side aisle roofs to help support the however. does not appear to have greatly influenced clerestory walls (figures 3.4' and 3.41.). One may subsequent Roman builders. also perceive in these projections, with thei r promi· A similar need to provide support for high nent arched openings, a proto-flying buttress; but groin vaults, but at a substantially larger scale than structurally, the projections read as more solid spur at the Market, was encountered in the construction walls, providing lateral suppon to the great central of the late Roman baths and basilicas. Of these. vaults. Vertical suppon for the high vaults was at perhaps the greatest achievement was the imperial least partially provided by the great engaged columns basilica begun in Rome by the emperor Ma)(entius (shown in figure 3.40), which effectively reduce the Will/, lind Other Vert'cal Elements 8,

}.4 I Basilica Nova: {ragment stnutural support for high vallit above the north exedra.

• '" }.42. Basilica Nova: exterior reconstruction (after Dunn). ).4} St. Peter's Basilica, begun ca. 333: C'O$$­ ral in scale: St. Peter's boasted a nave clear span of ~ction through the nove (after Letarouilfy). 1) meters! By eliminating the great weight and as­ sociated outward thrust components of heavy rna· sonry vau lting (see chapter 4), the walls of timber· clear span of the vaults, while the flanking barrel roofed basilicas could be much reduced in thickness. vaults were supported by the massive laleral W3I1 S. As long as the lengthy botlom chords of the timber It is worth noting 100 [hat because neither [he high trusses retained their imegrity, wind, seismic loads, groined vaults nor the lower barrel vaulrs of the and foundation movement would be the only pos­ basilica receive lateral su ppOrt from adjacent vault­ sible sources of lateral forces acting on the walls. ing af the building's ends, the end walls were built Lik (' ('arlier imperial basilicas, St. Peter's appreciably thicker than the inner ones. displayed long parallel walls without buttresses; in­ In contrast to monumental imperial vaulted tersecting cross-walls provided bracing only at the halls, St. Peter's BasiliC3, li ke other early Christi an facade and in the region of the transept. The n3ve basilican churches sponsored by the emperor Con· clerestory walls were supported on colonnades while stantine, was timber-roofed (figure 3.43 ). This choice the walls between the inner and outer aisles rose was no doubt influenced by the rela tive speed af· above arcades. Lateral stability, therefore, would forded by such construction, and it may also have have needed to be maintained mainly by the exterior been related to the ch3nncling of vast economic re­ w311s , with the roof trusses over the side 3isles help­ sources toward the construction of a new imperi al ing to secure the raised clerestory waUs to these more capital at Constantinople. Yet the more economical firmly rooted structures. By the fifteenth cemury, it timber roof construction was hardly less monumen· was observed that the southern clerestory wall s were " Chapter J

inclined as much as two meters out of plumb in to occupy the heirs of the Roman designers, as de­ ce rtain locations (Alberti, J. 10.). Si nce the width of velopments in Constantinople were to demonstrate. Ihese waUs could be no greater than 1. V. meters (see chapler 2.), the resuhant force from Ihe deadweight of the wall itself was well outSide the wall " middle BYZANTINE third" (see figure 3. 10). Hence Alberti's inference that the collapse of the nave was prevented only by The transfer of the imperial capital from Rome to the propping up of the roof trusses was probably Constantinople in the fourth century meant that correct; but his placing the blame for the wall dis­ there were no nearby sources of ponolan. Builders 3.44 ~Temple uf Minerva Medica" (Pal/ilion in tortion on the prevailing northeast wind is less so. in the new capital were thus forced to adapt tech­ the Licinian Gardens), Rome, ca. p.o: wall detail. As discussed in chapter 2. , substantial settlement of niques of Roman pozzolan construction to local the soumern foundations under the nave had taken building materials. One example can be seen in the place by that time, providing a far more likely ex­ massive fortification walls of Ihe city (figure .3-... 5)­ 3 .... 5 Consum/i/lople, fifth-century city wall. planation for the existence of Ihe gross wall defor­ Begun in the fifth century but rebuilt and enl arged mation. Nor would it have been surprising if me peri odically, Ihey are composed of a mortar and tension ti es of the roof trusses, the most vulnerable rubble core faced with alternating bands of brick­ element of a timber truss, had paned due to rot or wo rk and stone (a treatment that conti nued to be insect damage by the fifteenth century, placing still used well into the Renaissance era throughout much additional lateral loads on the top of the wall. of Ihe Mediterranean region). Dating from the same decades as the Basil­ Walls of monumental Byzanti ne buildings ica Nova, the so-called Temple of Minerva Medica were normally composed of ahernating layers of in Rome, also known as the Pa vilion in Ihe Licinian brick and mortar, with the morrar consisting o f brick Gardens, succeeded in incorporating generous win­ fragmentS and dust, including bits of charcoal, that dow openings at the base of a large dome. The sup­ impart semi-hydrauli c properties (i.e., the abili ty to porting structure for the dome actuall y has the form cure chemically and develop good suength-but at of a decagon, 2.5 meters across its nat si des, that was a slowcr rate than mortars based on truc hydraulic composed of ten piers, visually "tied" at several lev­ cements) to its li mestone sand mortar base (Penelis, els by arches. Large, tall windows were set high in 136-1)9). Extremely thick mortar joints, often ex­ Ihe wall between the piers (figure 3. 44) above rune ceeding the Ihickness o f the bri cks themselves, today projecting apses and the entrance. Apparently the display a hard monolithic mass si milar to Roman piers exhibited strucrural distress earl y o n because concrete. Such wide mortar beds would have allowed they were reinforced by increasing thei r depth (di­ very rapid bricklaying, perh aps even approaching the mension t in figure 3.8b), even before the building speed possible with Roman concrete construction, was completed. Despite the structural problems of but their uneven appearance, compared wirh tightly the pavilion, however, the pu rsuit of domed build­ pointed brickwork, called for a covering with thin ings with extensive, high fenestration would continue veneer. Almost all Byzantine walls displayed stucco, marble veneer, or decorative mosaics. " ChapIn- J

Wh~r('as Roman walls into which niches have been CUt appear massive and monumental, most BYlantine interior walls, even when load-bearing, S(:c m visually thin and ptanar thanks to the vtined marbl(' veneer and gold mosaic coverings. The By­ zantine sensibility modified even the architectonic forms of the Gruk orders, with column capitals drilled to produce fragile, lacy forms that seem to provide little means of supporting the loads above (figure )-46). In some ways, the church of 55. Sergi us and Bacchus, begun ca. 51.7 in Constantinople, provides , . , " a transition (if 3t a much reduced scale) between the - fourth-century Roman Pavilion in the Licinian Gar­ 3.48 S5. Sergius and Bacchus, ground plan. dens and Hagia Sophia in Constantinople, under­ taken some five years later. Recalling the pavilion, Given the dose correspondence in scale be­ support for the sixreen-sidcd "pumpk in domc" with tween the original dome of Hagia Sophia and that windows at irs base is provided by eight (rather than of the Pantheon (sec ch3pter 4), it is likely that the ten) piers that form an octagon 16 merers across rh e Pantheon provided the principal strUCtural model for flat sides (figure 3. 47). Unlike the pavilion, however, Justinian and his architects as they translated Roman the church is largely recrangular in plan except for concrete into Byzantine brick. Although the behav­ Its apse (figure 3.48), resembling morc closely the io ra l implications of thIS translati on from concrete 6 exterior form of Hagia Sophia. Its exte ri or walls do to brick are less significant than might be supposed, 3.4 Hagia Sophia, Constantinople; {ealling south 3,47 SS. Sergius and Bacchus, Constantinople, ca. not reflect the imemal buttressing providing lateral bttause the walls and dome of the Pantheon arc gallery pier UJith characteristic capital. 527: interior. suppon to the piers under the dome. unreinforad and have little snength in tension, Justinian 's great domed chu rch of Hagia Hagia Sophia was a precedent-setting building in Sophia (Holy Wisdom), constru cted in Constanti­ many other ways. Where the vast dome of the. Pan­ nople between the yea rs 531. and 537 (figure 3·49), theon restS on continuous massive., niched walls, four also looked back 10 the Roman tradition, particu­ great arches and a like number of pendcntives direct larly the Basilica Nova. the Pavilion in the Licinian the weight of Hagia Sophia's superstructure to four Gardens, and the Pantheon. Its innovative design, by prodigious supporting piers (see figure 3. 1) so the. the architects Amhemius of Tralles and Isidorus of tympanum walls need merely to smtai n their own Milerus. combined Ihe traditional longitudinal bas­ de3dweighr. The tympani arc pierced with many ilican plan with an enormous central dome. By com­ windows (whose original size was larger than the bining the two building forms, the architects suc­ present openings), bringing light di rectly into the ceeded in fusing ecclesiastical and imperial liturgies. central sp3ce of the church. The combination of such large glazed surfaces with a dome of monumental ,.

... ,. "

scale had never before been attempted and stands as 3.50 Hagia Sophia: analytical drawing of the one of the greatest architectural achievements of the structure. age. Compounding the physical and structural s[mcrnre, the aisle and gall ery colonnades also dis­ chall enges of the design was political instability play the effects of the deformations. As illustra ted in wimin the empire that required Hagia Sophia, as the figure 3-46, their outward inclination is startling. most visible symbol of Justinian'S power and prestige Historians have pointed to the speed of in the capital, to be completed as swiftly as possible. Hagia Sophia's construcrion as contributing to the Construction proceeded in more or less horizontal great distortions, reasoning that lime mortar would layers until the erection, in ca. mid-nSf of the main not have had su ffi cient time to set in the massiv e

arches to support the dome. These are J I meters In piers before they were loaded laterally by the great span and spring some :z.S meters above the floor arches_ Yet the main piers are not o f brick and mor­ 3-49 Hagia Sophia, Constantinople: air view. (figure j.50). "Flying centering" of the ty pe iII us­ tar, but rather of CUI Stone, at least up to the level rrated in figure J. 17 was probably used fo r assembly of the gallery floor. It is, rather, the four great pier of these arches. In atllikelihood, the ccntering would buttresses intended to br2ce the main piers to the nor have been adequately tied to prevent immense north and south that are constructed of brick with horizontal forces from impinging on the upper por­ mortar joints appreciably thicker than the individual tions of the main piers, which then proceeded to tilt bricks_ (The main piers, because of their form with outward. Though they are essentially sC(:ondary greater depth in the east-west than in the north-south WallJ atld Olhn Vnl;cal ElnnCflIJ "

directions, as well as Ihe bracing eventually received from the east and west semidomes, are effectively more rigid In the eas l ~west direction than to the north-south.) The problems of the buttresses became evident to the builders, and before proceeding further with the erection of the main dome, the exterior pier buttresses were reinforced and raised to their present height (see figure j.50). Raising the buttresses and thereby iTlcreasing their weight provided additional stability by effectively reducing the ratio of bending to direct loading (in the manner described in figure ).9). The buttresses must have then seemed secure, because the dome was raised in time to allow the vast building project to be completed in five years. The great central dome fell in 558 after being subjected to two major earthquakes: one in August 55), and the second in December 557. A second dome, having a higher profil e than its pre­ decessor, was then erected in 558-561. Despite par­ ti:lI collapses after an earthquake in the tenth cen­ ).52. Old cathedral (the "Basse Oeuvre"), Beau­ tury, and again after another in the founeenth, the vais, eleventh century: west {ace. general fonn of the second dome today remains un­ changed (rom that of 562.. But structural repairs associated with these incidents, as well as other ad­ versities, involved the placement of much additional buttressing around the entire structure. The Justinianic buildings of Ravenna se rve to bridge the gap between the Byzantine and western medieval traditions. The most famous of these, the church of San Vitale, resembles SS. Sergius and Bac­ chus in its general fo rm (figure J.S I). San Vitale, however, took advanrage of a western European technique for lighrweight vaulting (sec chapter of ).

).p San Vitale, Ravenna, aJ. HO-H8: interior. - ,s Willis lind Olhtr V~"'CIlI ElcnnJ1S "

Thus freed from the need to buttress :I heavy dome, , the upper walls were pierced with large clerestory windows, whil e the lower levels of San Vitale shim­ mer wIth mosaics. Yet despite its adopting westem­ style construction, bmh the mosaic surfaces and the use of long thin bricks at San Vitale fo llow the style of Constantinople.

EARLY M eDIEVAL

Next TO the massive scale and advanctd technology of the Byzantine achievement 31 Hagia Sophi a, de­ velopments in the treatment of wall s during the early medieval period in the West seem comparatively in­ significant. Several notable innovations, however, were com bined with the we ll -preserved Roman ar­ chitectural heritage. Ru bble core wall s were gener­ ally used with facings composed of stonework, reused ashlar, pillaged hrick, or STUCCO. In the ab­ 3.53 Abbey Gale House, Lorsch, late eighth sence o f polzolan concrete, the rubble cores em­ century. ployed lime mortars o f varying qU::Jlities. Few earl y medicv::J1 walls remain that have nOt been contitlu­ ously maintained w prevent rainwater from washing decorative surface, but the structural core of the wall out those cores. as well (figure 3· S» ). Decorative appli cations analogous to those Classicized wall construction was com mis­ used in Roman practice were common, but in at least sioned by ambi tious dynasties attempting to invokt." some early medieval buildings, this decorative appli­ imperial Roman tradition. Although Merovingian que also formed an important element in the wall and Ottoni an rulers also adopted this strategy, Char­ structure. The eleventh-century Basse-Oeuvre in lemagne himself made the most direct architectural Beauvais, for example, preserves much of its original all us ion to antiquity in his Paiatltlc Chapd at Aachen decorati ve facing of octagonal Stones. These stones, (figure ).54) which fr:mkly recalls, among other an­ which appear to be planar facing, actually penetrate cient and classical monumentS, Justinian's San Vi­ 15 em inw the rubble core, fo rm ing a thick stone tale. Begun in 791 under the direction of Odo of formwork, much like Roman opus rttjcu/alum (fig­ Metz, the character of the Carolingian bULlding dif­ ure 3.51). Similar octagonal blocks penetrate the fers radIcally from that of the prototype. To a large entire thickness of the walls of the Caroli ngian extent , these aesthetic di fferences stem from the ) ·54 Palatine Chapel, Aachen, begun 791: eleva ­ Louch Abbey gatehouse forming not only a striking choice of malerials. The walls of Charl emagne's tion reconstrltction (after Schneider). •••

church were executed in cur Stone rather than the mosaic-encrusted brick seen a l Ravenna. Moreover, the heavy stone dome a l Aachen requires th ick, small-windowed walls for support. These walls arc further buttressed by stone galleries about the perim­ Clef of the central domed sp3ce. In pan, the U~ of stone as the pri ncipal building material in western early medieval archirec­ ture reflects the availability of good building stone in much of northern Europe as well as economic and demographic changes in the post-Roman world, wh ich supported labor-intensive stone cutting over 3.SS Romanesqlle I'eader-stretcher stotlc-laying the industrial production methods of the empire. tcchniqll ~ (after Adam). Although the aesthetics and structural behavior of Charlemagne's chapel remain similar to Roman an­ tecedents, the shift in the north away from the pre· to be relatively massive, layered, and articulated with dominam brick and rubble and mOrIar consrruchon round·headed openings at all levels, much like Ro­ to construction with large Cut stones marks a decisive man prccedents. change in primary constructi on of the monumental Fortuitous natural conditions near a build· buildings of this region for the balance of the Middle ing site sometimes led 10 bui lding techniques such as Ages. "integral coursing," like that fou nd at Aul nay (figure J.l.I), where each course can be followed around the entire building, across openings, inside and out. In R OMANES QUE other instances. which may denve from Carolingian precedent, the outer facin gs are built integrally into The Romanesquc period ushered in a new era of the ru bble core, as in the so-called opus monspelien­ complexity in wall structurc. Increasingly taller Ro­ sis at Maguelone Cathedral and other Romanesque manesque wall s supported massive stone vaults as buildings in southern France. At Maguelone, a uni­ well as heavy timber roofs. It is im portant to rec­ form ashlar block is laid in a header-stretcher fashion ognize, however, that during this period walls also (figure J.SS) so that a positive ti e is created with the beca me more complex even when the spaces they rubble core, and at the same rime a deco ra tive sur­ framed we re un vaulted. face is achieved with the alternation of their long Coined in the nineteenth century, the term and short sides. In regions where geology did not Romanesque itself arose in recognition of the simi­ permit such stonework, masonry coursing was less lari ty to Roman fonns. Ordinarily constructed of regular. In some ma jor buildings. such as 51. Semin rubble cores with brick or ashlar facings, related to in Toulouse and St. Alban's in England, reused Ro­ the earlier Roman opera, Romanesque walls tended man brick replaced slO ne as the primary building 3.56 St. Mark 's Venice, 106}-

Romanesque builders had co make do with­ galleries, as illustrated by the cross-section (figure out the fine pozzolan mortars found in Roman build­ 3.57) of St. Foy at Conques (ca. 1050-<:a. I I1.S). ing. Higher-strength mortars from naru ra l cemcm This scheme was adopted in all the so-called major sour~s were ava il able in a few localities. as al the pilgrimage road churches that included, in addition trass deposits on Ihe Rhine River, or the "scaly, to St. Foy and St. Sern in, Sa ntiago de Compostda in rugged" stone mentioned by Palladia near Padua Spain and two others now destroyed, St. Manial at (Pall adi a, I. 5.), but such deposits we re probably Limoges, and St. Marrin at Tours. The galleries in uncommon ::and did not influence the main line of all of these buildings functioned liturgically as well architectural development until well into the as structurall y, sinee they could be used for ci rcula­ Renaissance. tion by pilgrims. But because the galleries permitted Although ru bble co re walls bound with rel­ only indirect: lighting of the main vessel, the high atively weak lime mortars perfo rmed well when ini­ vaults and the upper regions of the nave walls were tially built, water inevitably seeps through them over left in shadow (figure 3.SS). ti me and rends to leach out the mortar, leaving sub­ In other buildings like the unvauhed abbey stantial voids that allow the two outer faces 10 sep­ church of Jumieges, nea r Rouen, and the third arate and bulge outward and, sometimes, evcn to church at the abbey of Cluny, the cl erestory walls collapse. Although this phenomenon probably did were opened to permit light to enter the upper zone not develop as a major problem during the first sev­ of the main vessel. At J umieges (ca. r 067) the walls, eral generations after construction, structural prob­ reaching up 1.3 meters, arc comparatively fla t and lems and continual repair are typical for Roman­ , , • .om appear to be qui te thin (figure 3.59). To sustain esque buildings, as the example o f the eleventh­ lateT31 loadings from wind and the weight of the century Chu rch of St. Mark's in Venice (figure 3.S6) roof, the tall, thin walls were thickened appreciably dcmonSlTatcs. Al though adopting the five-domed 3.57 St. Foy, Conquest begun ca. 1050: cross­ jusr below the clerestory windo ws and were but­ Greek cross layout from the (long' 3go destroyed) section of the nalle (after Mark ). tressed from beh ind by vaulted tribunes built above Jus tini3nic church of the Holy Apostl es in Constan­ the side aisles. The walls were also rei nforced on the tinople, the solidity of the walls 3t St. Mark's does exterior by wa ll buttreSses and on the interior by not compare with that of typical Jusrini3nic walls, appli ed colonnettes on alternating piers. It is prob­ and this has led to continuing structural and main­ able too that the nave was di vided by diaphragm tenance difficulties (Krautheimer. 41 ). A comparison arches crossing the major (reinforct'd) piers. Dia­ 3.S8 St. Fay, Conques; nallt! mterior. of Romanesque walls with their Roman antecedents phragm arches act li ke transverse walls, helping to shows how the use of pazzolan mOrtars res ult in far stabilize the thin nave wa ll along its more than -1 0- better preservation despite thei r greater antiquity. meter length as well as to provide additional support The tall walls of Romancsque churches also resulted from the superposition of bands of 3r· to the roof. someti mes stemmed from a giant-order nave arcade, cades as at St. SeTnin in Toulouse or St. Alb:I.O's in Where masonry vaul ts ri se above the cleres­ as in the Auvergnat churches like Notre-Dame-du­ England. In either case, 131eTa i stability of the high tory, as in the now la rgely destroyed, giant third Port in Clermont-Ferrand or in west French hall nave wall was assured, following Roman precedent, abbey church at Cluny ( lo88-ca. tIll ), the high chu rches like 5t-5avin-sur-Gartempe near Poi tiers. It by flanking, vaulted side aisles and, often, vaulted walls, in addition to supporting wind loadings and 'OJ

• J' IQ'"

).59 (Opposite page) Abbey Church, jumieges, north rnterior wolf of tl,e naUi!.

).60 (Left) Third abbey church, Cluny, 1088- ca. 111.1: interior reconstnlction (Conant).

).61 (A bove) Third abbey church. Cluny: recon­ structiOlI of flu cross-section through the nove (after Col/alii). ,.6 Chaplf!.r J

the weighr of the roof, must also resist the substantial etrati on. Intense direct light in Romanesque churches of equal weight. In th is regard, it ma y be well to add outward thrusts of the Yaults (figure 3.60). Wall but­ was interm ittem and related to function. Tht." nave that the original nonthrusting timber-trussed roof tresses, usually coinciding w1th the transverse arches and choir, when long enough, wert." wmparativcly over the nave of St. Etienne was replaced hy ribbed on the underside of [he high barrel vaults, help to dark and punctuated by intervals of more imense vaults in ca. 11 30-1 135; and although wall marion stabilize such walls at regular intervals. Cluny and lighting, which signaled the liturgically important has brought somc distress to the vaults over the its early-twelfth-century cousins at Autun and Paray­ areas of the crossing bay and the apse, the usual loci yea rs, it does not appear to be vcry serious (Mark le-Monial, relied mainly on wall buttresses for the of the two main altars. 19 82, 111). security of their clerestOry walls. But because of the With the new height of huildings in the While it could he difficult to argue that the magnitude of the bending forces in the pierced upper cleventh ce!\lury, upper chapels and rooms also he­ widely dispersed Romanesque building projects pre­ walls, there were inevitable design limitations. Win­ came more common. Mural passages and internal figured the devclopmem of the Gothic style, the era dows needed to be small and few in number. The stairs, located mOSt often in crossing towers and in certainly did signal an increased level of structural derestory of Aunm is pierced by only one window, west facades, began to be used to gain access to these and srylisric experimentation that, with hindsight, which occupies less than a third of the bay wall upper areas. The passages and galleries nor only dOt."S indeed point to many of the events that length . At Paray-Ie- Moni al, there are three windows fa cilitated circulation; they also provided a structural followed. per ba y, bUI the height of these is but a small fraction respon~ to the problem of stiffening railer walls. • J ~ J '" of the height of the full clerestory. The almost 17- Comparing the nave wall of Jumieges with that of meter-tall clerestory walls of Clun y, pierced by thccc the originall y un va ultcd contemporary nave of the 3.61 Comparatiw: nave sectimls: (right) GOTH IC more generous windows per bay (figure 3.6r), wl­ ahbey church of Saint Elienne at Caen (figurt." 3.61), St. Etienne, Caen. begun ca. 1065. and (left) Abbey lapsed within five years after theIr erection. Lime is we can appreciate this innovalion, the so·called mllr ChlffCh, jumieges (modified after Dehio and Structural, spatial, and aesthetic innovation evolved known of the disaster, which might ha ve been caused ipais (or "thick wall," by Bony 1939). The total Bezold). together to an extraordinary degree in Gothic archi­ by weakness of the walls or by problems with me thickness of the "walls-with-voids" at St. Etienne tecture, spawning unprecedented wall configurations foundations (see chapter 1). It is known, though, exceeds thar of the high walls at Jumieges, but consist having gready enlarged fenestration set within in­ that the clerestory walls were quite heavy, almost of thinner wall sections joined by narrow harrel­ creasingly light and tall structural systems. Although 2'1.-meters thick, and that lightweight materials had vaulted passages. As discussed in the introduction, much of the impetus for the new Gothic style came been used for the high vaulting to reduce thrust walls depend primarily on their thickness for lateral from incorporating ribhed cross-vaulting (discussed (Conant, 203). stability, so thar a wider wall overturns far less easily. in chapter 4) that focused supporting forces at dis­ Consequently, in barrel-vaulted churches, In essence the mllr epais accomplishes the effect of crete points on the wall and thus fostered a more even with clerestories, the lighting of their central increasing the width of the wall without requiring a skeletal system of support, the Gothic adoption of vessel tended to be diffused and limited. Only in corresponding increase in the amo unt of quarried the planar, pointed arch was equally crucial. The special regions, as at an octagonal crossing tower or and transported swne. Bur b«ause the two walls advantage of the pointed arch is more spatial than in the curving wall of an apse which was usually tend to shear, or slide rel ative to one another when structural. Like its semicircular predecessor, it is substantially lower than the central vessel, could the subjected to lateral force, to be effective the connec­ composed of circular arcs that could be easily laid walls be opened with larger windows. In these re­ tions between the inner and outer wall sections must out by the early builders (see chapter I on design). gions, the curving form of the wall itself (tending to be secure, constraining them to act as a single unit. But unlike Ihe semicircular arch, whose fixed center develop a more effeaive cross-section in plan than If the barrel vaults of the passages are well placed to demanded that its rise be always hali the span, the that of a linear wall for limiting stress and defor­ resist these shearing forces, the mur epais will be far locati on of the cenlers of the pointed arch segments mation) serves to stiffen the wall and allow for pen- stiffer under lateral loading than a convt."ntional wall was flu id (see figure 3.63). The floibiliry of the ><

.---' L--___-'--' pointed arch alloww its adaptation to almost any spatial need. There were structural benefits as well, bur these have betn exaggerated by commentators who have not taken into account that the longer, pointed arch employs more stone than a semicircular arch of the same span, and hence that the weight of the pointed arch is greater. But it d(Xs benefit from the fact that the horizontal component of thrust of any arch against its abutments varies inversely with its ri se. And since pointed arches risc higher than L-.._.__ semicircular arches, they tend to generate less thrust, even if this reduction is somewhat offset by thcir 3.6} Laying-olfl a Gothic poil1ted arch (after greater weight. ViolJet-le-Dllc). The flying buttress, the third principal at­ tribute of Gothic structure, relates to both the adop­ tion of the grolned vault and the pointed arch. It functions simply as a linear brace in the manner of the timber prop illustrated in figure 3.64-10 resist the focused thrust of a vault or of wind loading on a great roof. The brace itself is normally composed of one or more rows of ashlar masonry all acting in compression, which in turn are usually supported from below by a segmental arch (figure 3.7). Thc effect of all of these devices was to pare down the supporting structure of Gothic walls, leaving the in­ terstices almost entirely free to be opened up to fields of brilliant stained glass-the aftermath best judged by a comparil>OTl of the interior walls of the choir of the early-twelfth-century giant Romanesque abbey of Cluny (figure 3.61) with the remodeled abbey church of St. Denis (figure 3.65) realil.ed a little over a century later. Below Cluny Ill 's pointed barrel vauh were relatively small windows that emphasized char­ actcristic Romanesque wall thickness and mass. On

3.64 Timher props supporting the weakened wall 3.65 St. Denis, choir alld northeast transept, of a budding In Leon. ca. 1240: wafl elevation; cf figure ).60. .. , Chapl", J

the other hand, the tall , open clerestory and triforillm set below the high qlladripanitc ribbed vaults of St. Denis impartS a sens:lIion of weightlessness. While there were earlier buildings of the l1e-de-France th:lt anticipated later Gothic develop­ ment, the birth of the Gothic style is generally dated to T T 44, when Abbot Suger consecrated the new choir at St. Denis. Although the relatively small spans and only moderate elevation of the twelfth-century choir do not display anything like the structural pyr­ otechnics of later Gothic structure (for example, the later thirteenth-century building at St. Denis itsclf), the use of supple, ribbed vaulting and large stained­ glass windows set within a highly voided non-load­ bearing wall was (and still is) most dramati c (figu re }.66). The Cathedral of Sens. contem porary with St. Denis but planned on a much larger scale, was begun in the Romanesque style and thm altered to bring it more up-to-dine. Notwithstanding its Gothic elementS (figure }.67), it retains the Romanesque 3.66 St. Denu amb,,/atory, "40-"44. elevation that typically comprised no more than two or three Stories. During the second half of the twelfth century, masons of the lle-de-France vigorously pur­ 3.67 Sens Cathedral, north wail of the naue, ca. sued development of wall design. This so-called early Il45-1164· Gothic period saw an increasing emphasis on height, generally achieved through the use of four-story elevations. In addition to the three stories found at Sens, the greal cathedrals of the early Gothic period incorporated fully vaulted galleries. These buildings, largely constructed between 1160 and 1 '90, include the cathedrals of Laon (figure 3.68) and Noyon, and the delicate south transept of Soissons Cathedral. Begun around " 55, the early GOlhic ca­ thedral of Notre Dame in Paris was planned 10 be the tallest building in France. Compared to irs nexl­ tallest Gothic predecessors, Sens and Laon, itS na ve Wails IIIIJ Otlln Vtrtlcal fltmt"rs "J

was one-third again as high, although its construc­ tion was generally lighter. With a keystone height at JJ meters and a steeper timber roof than found in earlier buildings, the wind pressures impinging againSi its upper walls and roof are significantly greater than in the lower buildings with which its designers would have h:ad experience (see figure }.4). The combination of higher pressures with the pres· entation of l:arger areas of resistance to wind called for a new and radical solution. But unfortunately, substantial reconstructions made to Notre-D:arne after , :u.S have obscured exactly how this design problem was originally solved. From archaeological clues in the structure of Notre Dame and in contemporaneous buildings in the l'aris region, as well as from drawings and photographs made before another m:ajor campaign of restoration in the mid·nineteenth-century, a new reconstruction of the original structural configura­ tion of the nave was determined that incorporated probably the first medieval use of Aying buttresses (Clark and Mark,s I ). The horizontal thrust caused by the dead loading of the nave vaults was mainly resisted by the masonry arches that also supported J.68 (Opposite page) Laon Cathedral, ca. r [70.­ the roof above the gallery (figure ).69). Whereas the north wall of the nave. walls of earlier churches had been adequately braced against live wind loadings by the structure suppOrt· ing the vault, the considerable wind loadings on the ).69 (Above) Paris, Notre Dame CAthedral, rt· unprecedented tall clerestory and roof at Paris could construction of tl,e nQve, CQ . 1 J 80 (Clark mId no longer be dfecth'ely counrered at this level. l'er­ Mark). haps because of observing cracking during construc· [ion, or possibly, movemem of [he clerestory and parapet walls, flying buttresses were added to in­ creasc the building'S resistance to wind. Model analysis (figures 3.70 and 3.71) of Notre Dame's recoll5truclcd original nave structure suggests chat local cracking would still have been '"

detected following heavy storms, such as modern records indicate could have raken place from time to time during the forty-odd-year life of the original structure. The tensil e cracks would have required continual, expensive maintenance, which probably led to the decision to rebuild the cathedral structure. But the experience was nO[ a complete loss; its mean· ing was applied by marons to the taller cathedrals begun at Bourges and Chartres near the close of the twelfth century, and then brought around full ci rcle to the thirteenth·century rebuilding of Notre Dame itself. The flying buttress was quickly recognized 3.70 Photodastic modeling. modeling assumption is that the heavily loaded as an important device to li berate tall Gothic walls AnalysIs of tbe 10llg vessels of Gothic churches IS foundations give complete fixity to tlJe bases of from the earlier constraint of stacked galleries. facilitated by tlmr repeating modlliar bay desIgn, the piers and b'lttress walls. Among the first buildings to take full advantage of as iIIllstrated ill figure ).69. The structllral sup· In tlJe mode/bIg process, stress-free di­ the new technology was the cathedral of Bourges port of each nave bay is provIded by a series of mensionally similar (to the prototype) models of (figu re 3.7), a remarkably efficient and elegant stru c­ paraflel transverse frames comprismg piers, b,lt· epoxy plastIC are loaded by scaled arrays of ture. A close relative to Notre Dame, Bourges incor· tresses, latera/wal/s, and ribbed valllts. Moreover, weights representing the distributions of wind and porates light, steeply sloped flying buttresses which, structural forus within the masonry are hypoth. deadweight on the actual stfllcture (see figure recent studies have revealed, were apparently super­ esized to be distributed as tlley would be in a 1.4). Test are performed in a rontrolfed·temper­ 3.7 1 Photoelastic interferenu pattern in wind­ imposed on an initial flying buttress configuration homogeneous, monoluhic materIal; but for tl,is ature enVIronment where the loaded model is first ward buttressillg of the reconstructed stru cture of much like the conjectured original design of Notre to be so, it must also be assumed that the elltire heated to 140· C. and then slowly cooled to room the nave of Notre-Dame de Paris under simulated Dame (Mark and Clark, 181 ). Sim ilarly, on the in­ "frame" is in compression. Thu assumption co· temperature. Model deformations at the higher wind loading. Arrows designate regions of local terio r Bourges adopts much of the Parisian spatial incides with criteria for successful/ong-term mao temperature are klocked 11/" after cooling so the tension. scheme except for the omi§sion of transepts and an­ sonry performance because medieval mortars loadings Call be removed with lIegligible effect. other, more crucial change: the gallery floors have cannot withstand tensile stresS/!s over {otiC peri­ The deformed modeJ, now viewed through polar­ been eliminated. creating extraordinarily tall nave ods of time (see chapter I , Bulldmg Materla/s). iung filters In the iffustrated polariscope, displays arcades and inner aisles. Most notably, this disposi­ l)revious studies indicate that compression does patterns IClhich, with cahbration Qlld scaling the­ tion of internal structure allows far more direct in­ indee'/ prevail throughout Gothic buildings, and ory (see Mark '9Hz, 2.4-2.6), call predict the foru terior lighting than the configuration of Notre Dame, that regions of tellSion, whert. they do eXISt, are distriblltions III a {tlff·scale structure. where the light from the gallery windows has to pass highly Jocaliud (Mark 1!JB2, u ). A final maJOr over gallery Roors. The direct light from the lower clerestOry windows at Bourges proves most effective at floor levd si nce these windows appear less fore· shortened than those of the upper clerestory (figure 3.2.3), as well as their being closer to the floor. The Walls I1nd O,her Ver,ical Elements "7

structure of Bourges is also exceptional, displaying the lowest stress levels found in any of the great Gothic cathedrals, while at the same time using a relatively small quantity of stone for its structure (Mark 1982., 36). Despite its successes, however, the influence of Bourges on the future course of Gothic design was to be less than that of Charnes Cathedral, which displays far less efficient structure (d. figures 3.7 and 3.72.). Scholars have speculated that this influence may derive from the greater flexi bility of the Chartres formal and structural scheme (Branner 1950, 169). In any event, it was Chartres, along with its smaller contemporary, the nave of Soissons, that pointed the 3·73 Comparative seetim,s: (left) Bourges Cathe­ way to Gothic architectural development. The era inaugurated by the work at dral, choir, and (right) Chartres Cathedral: nave (Mark). Chartres, usually referred to as the High Gothic, is generally regarded as the golden age of the Gothic style. The High Gothic elevarion typically consisted of a tall arcade and sim ilarly tall clerestory, separated by a narrow, dark triforium. The dark triforium, embodying a stone wall behind a screen of columns, may have been seen as desirable for aesthetic reasons, since it provided a visual balance point between the brightly lit zones above and below. It corresponds to the "attic" below the side aisle roofs, and originally it hid the transverse, triangular spur walls that pro­ vide lateral support to the piers as well as to the side aisle roofs at each bay. With the advent of the flying buttress, the heavy spur was no longer needed, but the designer of Chartres still retained it in his design (sec figure 3.73; note that the spur had been aban­ doned at Bourges). [n fact, the buttressing of the Chartres nave, which uses the equivalent of three

3.72. Chartres Cathedral, begun II94: bllltressing 3.74 Reims Cathedral, begun 1210: cross·section of the nave. of the nave (Mark). 3.75 Reims Cathedral: bar tracery in the upper 11emicycle (Villard de HOllnecourt).

3.76 Exaggerated deformation of Gothic building frame under the action of lateral, wmd loading.

3.77 Amiens Cathedral, begun 1220: flying but­ tresses and pinnacles atop nave buttress.

sepa rate fl yers and the spur wall, has been charac­ stituted bar tracery for plate tracery in the upper terized as being technologically clumsy (Mark 1990, portions of the clerestory zone. While earlier rosenes I Ij). Even so, Chartres seems to have pointed the consisted of large ca rv ed stone frames, the fi rst ar­ way to the far more efficient bunressing systems of chitect of Rei ms, probably Jean d'Orbais, redefined Reims and Amiens that support their nave walls with these stone surfaces as a se ries of thinner stone but twO specialized buttresses, as illustrated in figure "lines" or bars erected within the window vo id (fig­ j.74: a lower flyer to receive the outward thrusts of ure 3.7S). This formal treatment underlines the non­ the high vauJrs, and an up~ r flyer, strategically load-bearing nature of the wall, with the windows placed to receive the thrust of high wind loadings on functioning as curtain walls, hung between the far the tall superstructures. heavier supporting Structures of piers and exterior

The cathedral of Reim5, begun in I 2.1 I, flyers. adhered to the High Gothic elevational system of Generally conside red as the apogee of Chanres, generally repeating the nave arcade/trifor­ French High Gothic achievement, the cathedral of iumlcleres tory composition, but on a large r scale. As Amiens demonstrates, at least in its 42-meter-tall the coronation church of France, Reims was also na ve (figu re ~.~3 ) . kten anennon to details of struc­ executed with lav ish decoration and sculptural tu ral per(ormam:e. In high winds, [he waUs of framed adornment. In particular, the Reims workshop sub- buildi ngs tend to bend into an S-shape (as ill ustrated Chapler J

in figure 3.76) be1:ause of the resm)int of the ties cou ld be given over to the display of elaborate tra­ between the walls, which in a Gothic church consist cery; yet an unforsten problem set into motion by of the high crossing ribs, vaulting, and the heavy the presence of a pyramidal side·aisle roof might principal framing of its timber roof. This character­ have led to the collapse of the tallest of all the Gothic istic structural response was fo und, from model test­ cathedrals. ing, to produce tension in the upper region of the The original choir of the Cathedral of Beau­ leeward pier buttresses of the Amiens nave during vais, begun in 1 us and whose vaults reached up to exceptional storms (Mark 1981, 55). Furthermore, 48 meters (figure 3.78 ), stood for only twelve years it was found that the weight of a pinnacle placed on before the vaults collapsed. Too easily interpreted as the outside corner of the pier buttress compensates a Gothic Babel Tower doomed to failure by its au­ for this effect (figure 3.77); that is, the pinnacle en­ dacity, a less simplistic view of Beauvais and its place genders enough compression just beneath it to over­ in history reveals much about the Gothi c design power the effect o f the tension. This positioning of process and demonstrates the importance o f eco­ the pinnacle does not contribute to the overall su­ nomic factors in the patronage of large architectural bility of the buttress (although the pinnacles are rel­ projects. atively light compared to the great weight of the Model analysis corroborated by archeolog­ buttresses, and could not affect stability very much), ical investigation pointed to a problem in the Beau­ but mUSt instead have becn motivated by observation vais design at the juncti on of the intermediate pier of tensile cracking in this zone, likely in one of the buttress with the aisles (figure 3.79). Because of first bays of the nave during erection. Clearly, the Beauvais's open rriforium and the accompanying Gothic master mason was intimately involved with covering of this crucial region of the structure by a the building process and receptive to incorporating pyramidal timber roof, the masons may have fail ed design changes in response to on·site empirical to make adequate inspection and take corrective ac­ observations. tion (Mark 1982., 131). Inslead of succumbing sim­ By the mid-thirteenth century, the High ply 10 fare, then, Beauvais appears to have been a Gothic was succeeded by the Rayonnant style, which victim of identifiable design flaws which, undetected, brought the voiding of the wall to its natural con­ proved fatal to its tall, delicate structure. elusion. The glazing of the rriforium, initiated mO~1 The notable decline in northern French probably at the Cathedral of T royes or in the re­ building after 1284 is often attributed to a lack of modeled parts of St. Denis (figure 3.65), and applied confidence in tall- buil ding safety, set off by the Beau­ in the eastern transept walls and choir of Amiens, vais collapse. In fact, the inactivity was caused eliminated the last blank stone surface in the Gothic m3inly by a downturn in the local e1:onomy (Strayer. wall. Spur walls had already been cast off, and the 57-58), and soon thereafter, also by the outbreak of timber shed roofs that had formerly blocked the light to this zont were replaced with flat or pyramidal

roofs. And with this gesture the structure had 3.78 Beauvais Cathedral, begun 1225: choir evolved fully into an armature, where the glass walls interior. Walls aNd Other Vertical ElemeNts

the Hundred Years War between England and France. Enormous late Gothic churches, in Men, Narbonne, Palma (on the island of Majorca; figure 3.80), Milan, Seville, Ulm, and others, scarcely smaller, at Bologna, Gerona, and Prague, attest to the continuing vitality of the Gothic style throughout the rest of Europe (although the southern churches, generally topped by much less steep roofs than those of the north and subjected to gentler winds, did not present similar critical problems of wind loading; Mark 1982, 98). Indeed, the Gothic still flourished in Germany aher the fourteenth century when a modified type of basilica-church interior elevation became more common. [n these late Gothic so-called hall churches, which include the Cathedral of Vienna (ca. [370--[ 433 ) and the Lorenzkirche in Nuremberg (begun [439), the heights of all three aisles were made almost equal. By eliminating the high central clerestory of the nave, a large floor plan could be created without the expense of carrying the building interior to extreme height. Yet a great timber-framed roof covering all three aisles, as illustrated in figure 3.81, would display a tall exterior. Flying buttresses were replaced by wall buttresses, or simply by rein­ forcing the relatively low exterior walls by a pro­ jecting masonry "leg" between the windows. 3.79 Beauvais Cathedral: choir seelioll (Mark). More is known about the design techniques 3.80 Palma Maiorca Cathedral, nave illferiar, be­ 3.8 [ Cross section through a typical/ate Gothic used for these buildings than about High Gothic gun r357. German hall church (Mark). planning, for several of the architects' notebooks from this later period have been preserved (Shelby 1977). We are also in possession of certain structural design rules, as set out in the Instmcfions written in [5 [6 by a master mason at Heidelberg, Lorenz Lech­ ler, for the benefit of his son (Shelby and Mark). In

addition to presenting geometric schemes for plan­ ning the configuration of hall churches, Lechler gives specific advice on structural details such as wall thickness, window-opening sizes, and buttress and the only prudent course for assuring safe structural vault-rib dimensions. design would have been to follow the text in detail. Although Lechler docs not clearly state And as discussed in chapter 6, this would eventuall y their interdependence, the dimensions of all the serve to stifle structural innovation. building elements are related to the interior span of Although the cathedrals and other major the central aisle, which he recommends keeping be.­ churches of the era provided the most visible and tween 6 and 9 meters. For example. he advises that spectacular embodiments of the Gothic achievement, the height of the central-aisle vault keystone be made secular architecture also evolved dramatically_ Mili­ equal to one and one-half or two times the span of tary, civic, and domestic structures all responded to the central aisle_ Wall thickness is essentially one­ the same economic and political forces as did the tenth of the span, as is the breadth of the outstanding Church, and incorporated many formal features of leg of the wall buttress at ground level. The buttress ecclesiastical buildings. extends from the outside edge of the wall about two­ Fortification wall development was marked tenths of the span, giving a total walllbuttress depth by continuity with the Romanesque tradition. For at ground level of three-tenths of the span of the obvious reasons, fu nction takes precedence over style central aisle. Thus, for a church with a 9 meter in a structure whose form might determine life and central-aisle span, the vault keystone elevation may death_ Fortifications were often larger, better funded, 3_82 Carcas!;OIll:: thirteenth-century precinct walls be 13'1, or 18 meters, wh ile the total depth of but­ and more widespread than other public construction (pre-restoration Ilhoto). tressing in both cases is about ~>/J meters. Holding projects, yet they were often designed, like walls in the buttress depth constant for a range of building most structures, to carry the loads of masonry vaults heights makes sense only with regard to resisting the or timber roofs. Others, li ke the massive precinct J.83 Medieval battering ram (Viollet-Ie-Dllc). outward thrust of the vault, where stability against walls of the town of Carcassone built in the thir­ overturning is a fUllction of the total walllbuttress teenth century, were designed to be virtually free­ weight. As discussed in figure 3-6, the higher vault standing structures in themselves (figure 3.8 2) . These would produce greater bending at the buttress base, were crowned with wall-walks and shooting plat­ but this in turn is resisted by the greater weight of a forms, and contained gallery passages along their correspondingly higher buttress. Evidently, Lechler's length as well as garrison rooms in [he towers. "Live rule does not take into account the effects of wind, loads" experi enced by military walls included trOOps but since the buildings with which he is concerned and munitions, as well as the pressures of assault. are relatively small, the omission is not serious. The arsenal of attack included a number of features Lech ler's criteria seem to have been based that exploited the potential structural instability of on his observations of preexisting buildings. Al­ the wall to advantage: battering rams to encourage though he did advise his son to use his own judgment shearing (figure 3.83), and sappers to tunnel under and not necessarily to "follow [the text] in all the wall or, more often, to undermine one of its things," the very existence of the instructions must corners. In turn, military designers answered with have had the effecr of standard izing building fo rm. thickened walls to protect against shear failure, With no theory offered to guide experimentation, deeper and thicker foundations to avoid tunnelling, ,,' CJ.apti'!r J

and round comers TO prevent the prying Out of cor­ ner stones. Round towers also gave the added ad­ • • vantage of increased stiffness TO the wall. The talus, or sloped base (figure ].84), similarl y had a dual advantage. It was developed as a defensive measure against sapping and overturning, but it could also be used as a ri cochet to distribute morc widely missiles • dropped from openings in the wall above. Fortifications also developed features sel­ dom seen in nonmilitary walls: round profiles and unusual thickness, as well as the additional projec­ tions of fighting pl:uforms, garrison stations, mach­ icol:lIion5, and crenekltions. All these traits were de­ veloped primarily as responses 10 military auack and _...... siege. On the other hand, castles and churches some­ .- times shared a common vocabulary or construction techniques. As in Romanesque and Gothic church construction, castles witnessed a new and wide­ spread use of wall passages and galleries executed in ashlar. In an ecclesiastical COntext, these mural pas­ sages were meant primarily to lighten the wall and to carry more effectively the loads of heavy timber 3.84 Fortified-wall talus (Wollet-Ie-Due). roofs and Stone vaults. In castles, these passages also lighten the upper stories of extremely thick walls. At Dover, for example, the base of the wall is nearly 4 meters thick. If carried solid to roof height, this wall would require massive foundations. Instead, the walls were hollowed with gallery passages. These galleries not only reduced the load of the wall; they also provided a system of communication and pro­ tection for the garrison within the skin of the strUeNre. Civic pride, in the late Middle Ages, also motivated the consrruction of many la rge parish churches, town hall s, and towers. In Germany, parish churches such as those at Dinklesbuhl and Nordlin­ gen have dramatic towers. Even larger were the (3- 3.85 Strasbourg Cathedral: spire. 1439 (1851 photo). ,,8 a.aPt"~ J w"JIJ and Other V~rflU/1 DmrarlS n.

thffiral spi res of Vienna, VIm, and Strasbourg, the sance architectural ideas. Although the technical of cornices in securing all the walls to each other also has the advantage over equally economical ir­ latter, at I.p. meters, the tallest Structure of the Mid~ content of De architectura is not completely valid, it (Alberti, III. 8.). regular shaped stones because of its superior perfor­ die Ages (figure 3.85). Other significant monuments served as the model for architects o f the fifteenth and Alberti was concerned with the quali ty of mance under loading. Its parallelepiped fo rm insures of morc explicitly civic flallor include the Palais de sixteenth centuries to follow in their own wri ting. building matenals and their application. He recog­ the transfer o f compressive forces perpendicular to Justice in Rouen and the [Own halls of Bruges and This is not to say that Renaissance writers neglected nized the need to select stone for exterior walls based the interface joint, whereas in a strongly compressed Brussels. Indeed, the Gothic era brought new ele­ contemporary building practice. In fact, much of the on its capacity to resist weathering as well as wall, weakly bound stones with inclined faces, such gance to even utilitarian secular structures. The high technological exposition in their treatises simply re­ strength, and he noted that stone must be ori ented as those constituting the earl ier Roman opus incer­ quality of workmanship in stone and timber that capitulates the traditional "rules of thumb" pre­ with itS bedding planes parallel to the ground in lum and opus retiCillatum. tend to work their way had been established in the workshops of the great served previously in the guilds. order to avoid splitting due to compressive loads. out of a wall. The Romans could depend on pozzolan cathedmls carried over into the construction of Renaissance writers understood enough He al so understood that mortar docs nOl ce ment mortars to affix such stone, but with less reliable houses, hospitals, and guild halls. There can be little about stability of walls, for example, to recommend walls together into a monolith, but rather acts pri ­ mort3TS at their di sposal, Renaissance builders pre­ doubt th:u at least the upper classes in nonhcrn lessening their thickness as they rose, saving the ex­ marily to di stribute forces evenly across the surfaces ferred the predictable squared surfaces of brick. Europe enjoyed, by the fifteenth century, a fairly high pense of extra m:uerial in the upper portions and of the stones. Such understanding o f architectural The few large-scal e projects of the era that standard of living, enriched by the crafr-oriented tra­ providing a sufficiently wide base to prevent over­ technology was probably common among master did require outStanding structural and constructional di ti ons of their culture. But meanwhile in Italy, the tu rning (see figure j.6}. They also took care to rec­ builders o f earlier eras, but it was only after Alberti's innovation were great domed buildings, including seeds were being sown fo r a revolution in architec­ ommend a symmetrical arrangement in freestanding and later Ireati ses that there was wider dissemination the cathedrals o f Florence and London, and the new tural design that would fundamentally alter the re­ walls and supports: "'the wall's thickness should di· of such knowledge. basil ica of Rome. The earliest of these projects, at lation of craftSmanship and design. mi nish equall y on each si de of the centerli ne, col· Perhaps because o f the import attached to Aorence, call ed for an immense dome over earlier umns should be placed exactly above one another, the writing down of design rules, struCtural innova­ Gothic construction. Hence this project is bcsf de­ RE NAISSANCE and windows should likewise be placed over one ti on became less of an issue in the architecture o f the scribed in chapter 4 ; background on the design of another so as to leave a solid, unbroken vertical Renaissance, an o utlook that can be gleaned even the supporting walls for the other two major buildi ng Traditional practice no longer served as the we ll ­ support wall" (palladio, I. I.}, from the Renaissance architect's treatment of walls. projects follows. spring of architectural knowledge in the Renaissance. Architects clea rl y appreciated that not all Unlike the diaphanous northern medieval wall, most The building site fo r the new basilica of It came to be replaced by reliance on the authority sections of a wall support equal loadings. Alberti, Renaissance walls were built in imitation of the mas· St. Peter's remained active for almost tbe entire six­ of ancient architecture. Filippo Brunell eschi (1377- for example, divides the wall into the separate parts sive works found in the ruins of imperial Rome. teen th century. The first proposal to replace old ba­ 1446), Leon Battista Alberti (ca. 140 4- 1472.) , of os et complement'lm, or bones and paneling. By Niches and applied pilasters were intended to impart silica came in the 1450S during the pontificate of

Donato Bramame (1444-151 4), and Andrea Palla­ "bones," Alberti indicates "corners and inherent or a similar monumental quality. Even 50, the lise of Nicholas V. But although thc idea was di scussed di o (I S I I- I 59 2.) developed architectural sk ill s not additional elements such as piers, colu mns, and any­ Slone-faced rubble core walls tapered off, especially throughout the second half of the fifteenth century, through exclusive training at the building site but thing else that actS as a column and supportS the in the more modestly scaled palaces and villas. Part nothing was actually done until the election of Julius rather by the study of ancient ruins. Alberti perhaps trusses and roof arches" (Alberti, 111. 6.). In this, he of the reason fo r this is geographical: the cultural II in 1503. Hi s pl3ns for St. Peter's began with his best iIlusrrates this new phenomenon: a humanist by is aClU all y referring to wall buttresses; corners are and artistic movement we know as the Renaissance tomb in a chapel appended to the old Constantinian rraiOlng who studied Roman ruins while working as gcnerally reinforced, either by using larger blocks of was pri marily an Italian phenomenon, and brick was basilica, but the dilapida ted condition of the ancient secretary at the Vatican, he later employed crafts­ stone, sometimes accentuated by thei r projection the common building material throughout much of structure, together with a good deal of ambition on men-architects on sire who directed the construction andlor contrasting color, or by pilasters and engaged Italy. Un li ke ashlar, whose hi gh quarrying and shap­ the part of both the pope and his architect, Bra · of his designs. Moreover, the "discovery" of the trea· columns. He also noted the Roman practice of using ing COSts proved economically unattractive, brick mante, encouraged the decision to tear down the old tise of Vi truvius by Poggio in the St. Gall mo nastery periodic leveling courses that "acted as li gatures, or was rel atively inexpensive and was therefore found building and replace it with an edifice worthy of the in t 4 I S dramaticall y altered the course of Renais- muscles, girding the structu re together," and the role prudent to use for consrruction of sol id walls. Brick seat of the western church. ' JO Chapter J Wall. and Othtr VvtiPJl Fllmnll$

Given the enormous Pantheon-sized dome move them, at least until he was compelled to Stop they would eventually have 10 support (see chapter by nearby property owners. 4), vertical elements were a vital part of the structural Wren drew on Roman precedent for the system of the new basilica. Yet Scrlio accused Bra­ new design of St. Paul's, but even with the backing mante of cuning corners and using inferior materials of Charl es II , he could not prevail against the insis­ in the construction of the four principal piers that tcnce of the high church clergy that the layout of the were 10 support the dome (figure }.86), in order 10 new building follow "cathedral fashion"'-that it

complete 3S much work as possible while the enthu­ have the cruciform, Latin-cross plan of a medieval siastic Julius II still lived (Scrlio, 111. 6. fol. 16). It is church. This issue, probably more than any other, likely, however, that BTamante simply underesti­ required Wren to produce and submit a series of mated the strucrural forces accompanying such a designs and to make major concessions before work monumentally scaled dome. He had never before on the new St. Paul's finally began in (675. undertaken a project of this scale, and later project In the cathedral as it finally materialized, architectS, nOiably the craft·trained Antonio da San­ Wren effectively created two buildings. The interior, gallo the Younger (ca. 148j-I546), would spend modeled on a medieval basilican plan and elevation much rime and cHon increasing the size of the piers. with a high center aisle flanked by lower side aisles Owing to the large scale of the building, (figure 3.87), even reflects the exterior fl ying but­ the walls of St. Peter's arc extremely thick. Many tresses intended to help support the domical vaults. passages within the walls provided access for work­ Nothing of this form, however, is suggested to a men and help lighten them, as with the mur ~pai5 viewer on the street to whom the great central dome of the nonhem Romanesque bulldings. In fact, appears to ri se from a massive tWO-story base (figure 3.86 St. Peter's Basilica, Rome: Bralllllllle'$ great Michelangelo (1 475- 1 564) was able to build a spiral 3.88). The cross-section of the nave (figu re 3.89) crossing piers in constmcrion (Van Heemskerck. stair within the walls large enough to be used by reveals that this effect was achieved by raising the ca. rno). small draft animals, to aid in carrying building ma­ perimeter walls so that they conceal both the inner terials for the dome up to the roof. Strucrurally, the featu res of the building and the fl ying buttresses. The thickness of these walls helps to prevent ovenurning walls, replete with false "windows," are nothing from the thrusts by the I:Jrge vaults above the nave more than "screens" calculated to hide the GOIhic­ and aisles. derived clerestory and flying buttresses of which Sir Christopher Wren's St. Paul 's Cathedral Wren disapproved. Yet the extreme weight of these reflects an unusual combination of Gothic building walls has also given rise to the thought that they tradition and Renaissance aesthetic. Although Wren were intended to playa structural role, perhaps 10 (J6p.- 172.j) was a scientist by training, he became resist the thrust of the flying buttresses. familiar with medieval building techniques from To try to answer the question of Wren's projects of restoration and while removing the burnt­ purposes, with an eye toward the possibility that he out remains of the Norman/Gothic St. Paul's after employed scientific insights, the structure of a typical the Great Fire of 1666. So solid were the piers of the bay of St. Paul's was analyzed using photoe!astic

old cathedral that Wren resorted 10 blasting to re- modeling in the same manner as that used to study Willis lI"d Olbo VotlC41 Elements 'JJ

).87 (Opposite page) St. Paul's Cathedral. Lon­ don, 1675-1710: interior. looking east.

3.88 (Above) St. Paul's, J..Qndon: view from the southeast. W"I/sllnd Other Vtrtlafl Elnnents 'H '"

Gothic structures (see figure 3-70). The model was stonecutters cur back the inside edges so mat a pre­ cise surface would not ~ required. The additional tCSted first under simulated loadings scaled (0 actual deadweight, and then a second tcst of the model was space W3S then filled in with a thick bed of mortar, performed by applying scaled wind loadings. After leaving a relatively crude joint inside but 3 precisely the first series of tests was completed, the flying detailed joint on the outside. The mort3r between buttresses were Tl'm oved from the model and it was the blocks h3s sh runk slightly, causing the outside tested again under the scaled deadweight and wind face of the block to carry a disproportion3te portion loads. Without the flying buttresses, stresses in the of the compressive force. The resulting high stresses piers were higher, bu r it was ascertained that well­ have c3used the stone to spall 3t the joints, weak­ constructed piers of solid masonry (o r at least piers ening the wall and providing an entry point for dam­ having a firm oUler shell composed of scvcrallaycrs aging moisture. Nor was this just a long-term prob­ of coursed masonry) CQuid successfully resist both lem: the building records of St. Paul's indicate similar the thrusts of the vaulting and the effects of high problems of spalling in rubble-filled piers as early as winds. Wren's flying buttresses appear therefore 1690 (Furst, 114). It m3y be wcll to remark that unnecessary. during the constru ction of St. Paul's, Wren was con­ In view of his earlier warning that ~eausc cerned 31so with the progress of a great many other of their "~ing so much exposed to air and weather, building projects-leading to the observ3 tion that (flying buttresses] arc the first thing that occasion the problems of th is rype were more likely to be diag­ ruin of Cathedrals" (Wren, 2.98), it seems ironic that nosed and corrected early on wi th me more intimate they were employed at all in St. Paul's. Of course, connection between the architect and the building Wren may have believed that both the flyers and the process of earlier er3S. heavy perimeter walls were necessary for the build­ ing's stru ctural integrity. Yet if he was thinking only of structural necessity, he could have provided dis­ 3.89 St. Palll's, LOl/do,,; comparative cross~ crete piers placed to re ceive each flying buttress bUI sections: left: Gothic choir of the old cathedral; still hidden behind light walls along the perimeter of rig/)t: Wren desipl (Dorn and Mark). the cathedral. The height to which he carried these massive and expensive walls suggests that he was using them primarily for visual effect in the classic tradition. Even more ironic is the fact that the addi­ tional weight of the parapets produces high enough compressive forces to cause spalling in the masonry. The outer Stones of the walls of St. Paul's were precisely cut on the outside surface to give only a very thin mortar joint. Because o f the e}(pense of finishing the entire surface o f a stone block evenly, 'J' Chaptn J Walls alfd Othn Vntlcal Elcments

BJ8LJOGltAPHY Dinsmoor, W. B. "The Architecture of the Par­ Mark, Robert. Light, Wind, and Structure. Cam­ Serlio, Sebastiana. Tufte opere c/'architettura e pros· thenon." In Bruno, ed., The Parthenon, 1974. bridge, MAINew York, 1990. pettiva di Sebastim,o Serlio. 1619; reprint, Venice, Adam, jean·Pierre. La Construction Romaine: Ma­ 1964. teriaux et Techniques. Paris, 1984. Dodge, Hazel. "Brick Construction in Roman Greece Mark, Robert and Willi3m W. Clark. "Gothic StruC­ and Asia Minor." In S. Thompson, cd., Roman Ar­ tural Experimentation." Scientific American, 1.S I Strayer, joseph. 0" the Medieual Origins of the Alberti, Leon BaTtisTa. On the Art of Building, trans. chiUcture in the Greek World. London, 1987. (November 1984) pp. 176-185. Modem State. Princeton, 1970. j. RykwerT, N. Leach, and R. Tavenor. Cambridge, MA, 1988. Dorn, Harold, and R. Mark. "The Architecture of Middleton, John Henry. The Remains of Ancient Viollet-le-Duc, Eugene. Dictiomlaire Raisonni de Christopher Wren." Scientific American, 2.45 (July Rome, London, 1892.. L'A"hitecture Fram;aise du Xle 0/1 XVie Siccle, 10 Bony, Jean. "La technique normandc du mur epais 1981), pp. ,60-173. voh. I'aris, 1854-1868. a l'ipoque romane." Bulletirr Monumental, 96 Palladio, Andrea. The Four Rooks of Architecture, ( 1939), pp. 15)-188. Fitchen, John. The Construction of Gothic Cathe­ trans. Isaac Ware, London 1738; reprint, New York, Ward-Perkins, J. B. Roman Imperial Architecture. drals. Chicago, 1961. 1965 New York, 1981. Branner, Robert. The Cathedral of Bourges. Cam­ bridge, MA, 1989. Furst, Viktor. The Architecture. of Christopher Wren. Panofsky, Erwin. Abbot Sliger: On the Abbey Wren, Christopher. Parelltalia, London, 1965; re­ London, 19S6. Church of St. Denis and its Art Treasures, 2d cd. pnnt, 17 So. Bruno, V. j ., cd. The Parthenon. New York, 1974. Princeton, 1979. Heyman, Jacques. "Gothic Construction in Ancient Clark, William W., and R. Mark. "The First Flying Greece. ~ Journal of the Society of Architectural His­ Penelis, George. et al. "'Ille Rotunda of Thessaloniki: Buttresses: A New Reconstruction of the Nave of toriarrs, XXXI (March 1971), pp. )-9. Seismic Behavior o( Roman and Byzamine Struc­ Notre·Dame de Paris." Art Bulletin, 66 (March tures." In R. Mark and A. S. Cakmak, eds., Hagia

1984), pp. 47-6S. Krautheimer, Richard. Rome: Profile of a City J 11- Sophia from the Age ofjllstinian to the Present. New 1)08. Princeton. 1980. York, 1992, pp. 1)2.-157. Conant, K. J. Carolingian and Romanesque Archi· tecture 800-HOO. New York, 1959. Landels, John G. Engineering in the Ancient World. Robertson, D. S. Greek a"d Roma" Architecture, 2.d Berkeley, 19 78. ed. Cambridge, 194). CoulTon, J. J. Greek Architects at Work; Problems of Structure and Design. London, 1977. MacDonald, William L. The Pal/thean; Design, Shelby, Lon R. Gothic Design Techniques; The Fif­ Mearring and Progeny. Cambridge, MA, 1976. teenth-Centrlry Design Booklets of Mathes Roricler Couo, Guiscppe. 11 Colosseo: I'An/iteatro Flavio ans Hanns Schmuttermayer. Carbondale, 1977. nella Techica E.dilizia, nella Stana delle Strutture, nel Mango, Cyril. Byzantine Architecture. New York, Concetto EsecutillO de; Lavari. Rome, 1971. 1976. Shelby, Lon R. and R. Mark. " Late Gothic Structural Design in the ' Instructions' of Lorenz Lechler." Ar­

Dehio. Georg Gortfried, and G. von Bezold. Die Mark, Robert. Experiments In Gothic Structure. chitectura, 9, 2., (1979). pp. 11)-1)1. Kirchliche Baukunst des Abendlandes, 7 vols. Stutt· Cambridge, MA, 1982.. gart, 1884-1901. Sear, Frank B. Roman Architecture. London, 1982..