3.1 Hagia Sophia, Istanbul, 532-537: Interiot, North Wall
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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.