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An Anthropomorphic Model

Henry Petroski

eing able to feel the forces in represents a cantilever beam whose Ban unusual structure, even if If seeing is believing, self-weight must be supported at the only by proxy through a model, is a shoulder alone. great advantage for engineers, students Railroad bridges employing the and laypersons alike. If we can actually feeling may be cantilever principle had been built in be part of the model, and feel the forces North America during the 1880s, but directly, all the better. Probably the most understanding the form was new to Britain, so Baker famous anthropomorphic model of a prepared a public lecture to explain major engineering work is that of the the nature of a cantilever bridge, trac- Forth Bridge, located near Edinburgh, rience would believe me. Where no ing its roots to centuries-old Oriental Scotland. This structure had its origins precedent exists, the successful engi- examples but not mentioning any in the need for fixed crossings of the neer is he who makes the fewest mis- contemporary American examples. firths (or estuaries) of the Forth and takes.” Fortunately for engineers, it is His 1887 lecture is an exemplar of the Tay rivers on the country’s east coast. precisely where no precedent exists form, and its highlight was an anthro- Without such crossings, rail traffic was that they tend to be the most careful pomorphic model—variously referred interrupted by the need to transfer roll- and hence successful. to also as a living model and a human ing stock to and from ferries at each cantilever—of one full span of the river’s shores. The railway engineer The Cantilever Comes to Britain structure that was being built across Thomas Bouch was commissioned to Baker’s design for the bridge was quite the Firth of Forth. design the two bridges, and construc- unusual for its time. It was based on the The apparatus for the model con- tion of the one across the shallower cantilever principle, which had roots in sisted of a pair of chairs, four wooden Firth of Tay was completed in 1878. It corbeled arches and vaults. Galileo was struts, two pallets of bricks, a swing- was not a particularly daring design, the first to provide a rational analysis of like seat and rope to connect some of and the bridge was remarkable only for a cantilever structure, understanding the components to each other. Three its length of almost two miles across the that if he could successfully determine men were required to complete the wide estuary. Unfortunately, the lon- a relationship between the geometry, tableau of the model. Two of the men gest and highest girders of the bridge material strength and load carried at sat upright in the chairs, and each of collapsed during a storm in December the end of a generic cantilever beam, these men grasped the tops of a pair 1879, killing 75 people who were on the then he could predict the behavior of of struts, the bottoms of which were train making the crossing at the time. beams of more complex design and notched to bear against the edge of A court of inquiry found that the thereby shed light on the hitherto in- the chair seat. The triangular arrange- structure was “badly designed, badly explicable spontaneous failure of mas- ment so formed on either side of each constructed and badly maintained,” sive structures like obelisks and ships. man represented the portions of the and its engineer was discredited. Con- Galileo’s analysis of the cantilever beam bridge structure cantilevered out from struction was halted on the suspen- was correct in methodology but flawed the bridge towers, which were rep- sion bridge that Bouch had designed in detail; still, it provided the basis for a resented by the torsos of the seated to cross the deeper Firth of Forth, and rational method of structural analysis men and their chairs. To the tops of subsequently an entirely new design that is taught to engineering students the inside struts were attached the was commissioned from the firm of to this day. The classic illustration for sides of the swing seat, which repre- the distinguished engineer Sir John what has come to be known as Galileo’s sented the suspended central portion Fowler and his young partner Benja- Problem has been widely reproduced of the bridge span across which the min Baker, who would be knighted and, although not strictly speaking an railroad trains would run. Each out- on the completion of the project. Bak- anthropomorphic model, provides a side cantilever, consisting of a man’s er would be the lead designer of the feel for the gross forces involved. A true arm and associated strut, was tied by bridge, and he would also confess to anthropomorphic version of the canti- rope to a pallet of bricks, which pro- the enormity of the task: “If I were to lever is readily produced by stretching pretend that the designing and build- one’s arm out horizontally and holding Henry Petroski is Aleksandar S. Vesic Professor ing of the Forth Bridge was not a a briefcase, backpack or similar load in of Civil Engineering and a professor of history at source of present and future anxiety one’s hand. Even without such a load Duke University. Address: Box 90287, Durham, to all concerned, no engineer of expe- at its extremity, the outstretched arm NC 27708-0287 www.americanscientist.org Copyright © 2013 by Henry Petroski. Requests for permission to reprint or 2013 March–April 103 reproduce this article should be directed to the author at [email protected]. When engineer Benjamin Baker was faced with the need to convince a skeptical public that a new type of bridge, the cantilever, was the right choice for spanning the Firth of Forth, near Edinburgh, Scotland, he prepared a model for a lecture presented at the Royal Institution in Lon- don. Using three men, chairs, sticks, rope and bricks for counterweights, he demonstrated the principles that would make the record-length span possible. The model was likely photographed at the construction site and shown by use of a photographic lantern slide. (Photograph by Evelyn George Carey from Mackay, The Forth Bridge: A Picture History.) vided a counterweight to balance half legs into compression. In the world. The North British Railway, no the weight of the suspended central Forth Bridge you have to imag- doubt at least in part to allay fears that span plus half the weight of the third ine the chairs placed a third of a might have kept paying passengers man, who occupied the suspended mile apart and the men’s heads from booking travel across the soon-to- seat. In Baker’s lecture demonstration, to be 300 ft. above the ground. be-completed bridge, surely would not a large drawing of the bridge span was Their arms are represented by have discouraged Baker from giving displayed behind the human model, huge steel lattice members, and his lecture, in which he would empha- thereby making it self-evident—if it the sticks or props by steel tubes size that, in spite of the public’s con- was not already—what parts of the 12 ft. in diameter and 1¼ in. thick. fusion of the two, the Tay and Forth model corresponded to what parts of bridges were quite distinct. the bridge. In Baker’s own words: Baker’s lecture incorporating the The lecture was delivered on May human model was prompted by sev- 20, 1887, at a weekly evening meeting Two men sitting on chairs ex- eral factors. One was that the Tay of the Royal Institution in London. In tended their arms and supported Bridge that collapsed (and was rebuilt, addition to clarifying the location of the same by grasping sticks butt- reopening in 1887, the year of the lec- the Forth Bridge, which is about 35 ing against the chairs. This repre- ture) was on the same rail line that the miles south of the Tay, Baker gave a sented the two double cantilevers. new-style bridge would carry over the sense of its enormity and thereby the The central girder was represent- Forth. There was thus public anxiety difficulty of the task of erecting it. His ed by a short stick slung from one about the safety of the line generally. challenge in preparing his lecture, he arm of each man and the anchor- The other factor prompting the lecture admitted, was that he “had to consider ages by ropes extending from the was the unfamiliarity of the bridge how best to make a general audience other arms to a couple of piles of form that was being erected over the appreciate the true nature and direc- brick. When stresses are brought Forth. Although steel cantilever rail tion of the stresses on the Forth Bridge, on this system by a load on the bridges were not absolutely new, this and after consultation with some engi- central girder, the men’s arms and one under construction near Edin- neers on the spot, a living model was the anchorage ropes come into burgh was novel to Britain and was arranged.” Although Baker did not tension and the sticks and chair to have the longest spans of any in the claim that the idea was wholly his, the

104 American Scientist, Volume 101 Copyright © 2013 by Henry Petroski. Requests for permission to reprint or reproduce this article should be directed to the author at [email protected]. model has been closely associated with him ever since.

Anthropomorphic Evolution Baker may have consulted with some engineers on the spot, but as with most things designed, the details of the model that was to become so famous involved fine-tuning, some of which was captured in photographs, a collection of which now reside in the National Archives of Scotland. The photos were taken by Evelyn George Carey, an as- sistant engineer on the project who also became its official photographer. The bridge under construction was also documented by Philip Phillips, son of the resident engineer, Joseph Phil- lips. The younger Phillips published contemporary with the project several books capturing its progress in sketches and photographs. One of the books was Like the engineering project itself, Baker’s model underwent considerable modification and subtitled The Giant’s Anatomy, reinforc- evolution before reaching the finished state shown on the opposite page. In the second version ing the tendency to speak of the bridge (above) the bridge drawing has appropriately weighted lines, but other elements, such as the in anthropomorphic terms. Still another size of the counterweight support lines and the slope of the ground (requiring shims for the left example was provided by reference to chair) are less than ideal. (Photograph from Gray & Maggin, Evelyn George Carey: Forth Bridge.) the bridge towers as having a “Holbein straddle,” an allusion to the character- be seen in the precariousness of the cen- but the effect is to make the ground istic spread-legged stance in which the tral figure. It was later suggested that if look cluttered. It is easy to see why this Northern Renaissance artist Hans Hol- someone stumbled over an anchorage, version of the model also was not used bein painted his male subjects. To give the gentleman in the swing seat would in Baker’s lecture. the Forth Bridge stability in the wind— have “an ignominious tumble.” Had The third of Carey’s photographs is and not incidentally distinguish it from that happened, it might have but fur- the well-known one, and its composi- the Tay that collapsed—the structure ther demonstrated the effectiveness of tion and that of the human model itself narrowed from about 120 feet wide at the model in simulating the behavior of correct the faults found in the previ- the base to only 33 feet wide at the top. the real structure. ous two representations. The ground Many of Carey’s striking images of In a subsequent version of the mod- and background are simple and un- the bridge under construction have be- el, the background drawing of the cluttered; the size of the drawing is no come very familiar, but photos he took bridge is much improved and looks larger than it has to be; the struts are of early efforts at a human cantilever more like the one in the iconic image. of an appropriate size; the ropes are have remained relatively obscure, no It differs from that in having all mem- heavier; and the photo itself clearly doubt because their inferior composi- bers of the cantilever—diagonals and shows what the model is intended to tion and execution makes them pale in chords alike—drawn with the same demonstrate. Once again, we have a comparison to the one that has become weight line, thus failing to emphasize dramatic illustration of how human- iconic. In one of the alternative mod- the upper and lower chords to which made things evolve by improving on els, the chair seats are so high that the the model’s struts and arms holding their predecessors through elimination feet of the men sitting in them do not them correspond. The suspended na- of their faults. touch the ground and so make these ture of the swing seat is more clearly men appear suspended like the one on visible, but the struts remain longer Varying the Anthropomorphology the swing seat. And, by an unfortunate than necessary. Also, the cords sup- Different combinations of people ap- positioning of the swing seat before porting the swing seat and those con- pear in each of the three versions of what appears to be a stone pillar or nected to the piles of bricks remain the human cantilever. The three in the fence post, the elevated seat appears lightweight and do not show up well first do not reappear as a group in the not to be supported by the cantilevers against the background. Furthermore, later two, but the men occupying the but by a pier beneath it. In addition, the the model was photographed before a chairs in the second version appear to struts employed are longer than they building with a distracting off-center be the same ones who occupy them in need to be, thus confusing the geom- window, destroying the symmetry that the iconic version. (Even though they etry of the structure. And, finally, the could be better emphasized. Finally, bear no resemblance to the famous lines of the bridge in the drawing hung the ground on which the building is engineers, the anonymous chair sit- above the human model do not show erected appears to slope slightly down ters have been incorrectly identified as up clearly. It is no wonder that this ver- to the left, which seems to have neces- Fowler and Baker on occasion, includ- sion of the model had to be improved. sitated placing the left chair on some ing in a 2007 BBC news story about the Another reason for emending it might boards to make it level with the right, issuance by the Bank of Scotland of a www.americanscientist.org Copyright © 2013 by Henry Petroski. Requests for permission to reprint or 2013 March–April 105 reproduce this article should be directed to the author at [email protected]. to have been invited to participate in the famous model “to remind audienc- es of the debt the designers owed to the Far East.” His relatively slight stat- ure may also have made him a likely choice to occupy the suspended seat. An illustration of the iconic version of the model appeared in the engineering press first in the United States, in the issue of Engineering News dated June 11, 1887, which was only about three weeks after the model’s debut at the Royal Institution in London, and a full seven weeks before it appeared in the British Engineering. The Ameri- can “journal of civil engineering and construction” called the model “novel” and “ingenious,” captioning the engraving of it “A Japanese Illustration of the Cantilever Principle,” and refer- ring to it as “a Japanese idea, as may be suspected from the central figure,” perhaps inferring too much from Wata- A re-creation of the anthropomorphic model was on display at the Forth Bridges Visitor nabe’s position in the model. Who- Centre for many years. In 2012, the Forth Bridges Visitor Centre Trust, members of which are ever or whatever culture should be shown here, donated it to the town of South Queensferry. (Photograph courtesy of Roland credited, the Engineering News article Paxton.) reported that during Baker’s lecture new ₤20 note featuring an image of the new challenges in bridge building, the the human cantilever was “received bridge and the living model of it.) greatest projects became magnets for with loud and general applause.” It The person in the suspended seat visitors from afar. The construction at does not seem likely that the model does change from the second to the midcentury of the Britannia Tubular was demonstrated live at the lecture last version of the model, and his iden- Bridge across the Menai Strait in north- but rather that it was projected from tity is known because of the special west Wales was one such project, and a lantern slide, for the Royal Institu- circumstances that caused him to be the construction of the bridge across tion, where the lecture was given, has in Scotland at the time. Engineering the Firth of Forth was another. At 1,710 occupied a formal becolumned stone has always been an activity of wide- feet long, each of its two spans was building on Albemarle Street since ranging geographical interest, with to be the longest in the world when shortly after its founding in 1799. The practitioners from far-away lands visit- the bridge was completed. Among setting of the demonstration must have ing the sites of innovative construction those who traveled to Britain in the been the construction site of the bridge, projects to learn firsthand how some- latter third of the 19th century, when where Baker conferred with colleagues, thing new was being done. This was the railroad was being introduced in probably including Watanabe, and the true in ancient Rome, when the likes Japan, was the young engineer Kaichi model was created “on the spot,” with of Vitruvius and Frontinus traveled Watanabe. He came to Scotland with its various stages captured by on-site around the empire to study temples a degree from the University of Tokyo photographer Carey. The photograph and aqueducts and talk to their engi- and continued his studies at Glasgow of the final demonstration, at least, was neers about successful practices and University. He became Baker’s assis- converted into one of the lantern slides failed efforts. In the 19th century, when tant and a construction foreman on the with which the lecture was illustrated. the nascent railroad was presenting Forth Bridge project. Watanabe is said Engineering News acknowledged the American bridge engineer Thomas C. Clarke, who visited the construction site in 1887, for the use of the photograph from which the journal made the engraving appearing in its pages, at- testing the speed with which engineering news, knowledge and documents traveled across the Atlantic in the late 19th century.

An Enduring Model However much misinformation and anonymity remains associated with The replica model has certain details that make it less than true to the physical principles the original human cantilever model, demonstrated in the original. (Photograph by Catherine Petroski.) it has become an unforgettable image

106 American Scientist, Volume 101 Copyright © 2013 by Henry Petroski. Requests for permission to reprint or reproduce this article should be directed to the author at [email protected]. that has often been reproduced and his arms appear to be crooked a bit, ture under the chair seat, the long part recreated. When I was at the bridge further suggesting that they are not be- representing the lower chord of the site in 2003, there was on the grounds ing fully extended in tension. The men cantilever assumes the proper angle. of the Visitor Centre a pair of chairs are posing with the model apparatus, The effect can be seen in the in-place and all the rest of the apparatus need- but those in the chairs may not have anchorage struts, which clearly do not ed for a trio of visitors to participate been experiencing the forces involved need any human arm to hold them at in animating the model while look- to the extent that the original model a proper angle. The angle of the bend ing out at the real bridge. Today, that was designed to demonstrate. in the tube and the length of rope are apparatus has been relocated across In the original model, Watanabe such that the rope is kept taught even the Forth to South Queensferry, where holds his hands close to his side, grasp- when the model is not in use. With the it sits in the back court of the Orocco ing the swing seat, perhaps to secure steel chairs so bolted to the concrete Pier Restaurant. According to Roland his balance. In the 2012 re-creation, slab, not only is the distance between Paxton, engineer, scholar of British however, Paxton is resting his hands them fixed, but they are also capable of engineering, bibliophile and lover of on the struts, perhaps because the other resisting sideways forces. bridges, “the replica model is on loan gentlemen have left room for him to When the middle two steel struts to the restaurant from the town’s Forth do so, but the potential effect is to in- are properly installed beneath the steel Bridge Memorial Committee to whom troduce confusion among observers of chairs, the middle seat is locked in it was donated by the Forth Bridges this human model reenactment about place by an arch-like action, and the Visitor Centre Trust in 2012.” The final how the real structure works. In fact, mechanism is effectively a rigid struc- meeting of the trust was commemo- given that the gentlemen in the chairs ture. When the chairs are occupied, the rated in a photograph of the trustees do not appear to be holding the struts men on them can steady—but need posing with a re-creation of the hu- in place with any tension in their arms, not hold up—the struts supporting the man cantilever model, with Chairman the actual structural action of the model middle seat, which explains why the Paxton sitting in the catbird seat, albeit appears to be more of an arch than a members of the Trust did not need to not nearly as high off the ground as cantilever. What holds the swing seat pull up with their arms to make the Watanabe in the original model. and its living load in place is the chairs model work in the same way the hu- There are some other interesting and their occupants serving as abut- man participants did in the original differences in the two demonstrations ments rather than towers. The same enactment. Regardless of these quib- posed 125 years apart. In the 1887 was not the case in the living model bles about the false structural action of Baker-lecture tableau, the human par- used in Baker’s lecture, for the second this reconstruction of the living canti- ticipants representing the towers are version photographed by Carey shows lever apparatus, the anthropomorphic grasping the struts with their palms especially clearly that the suspended model of the Forth Bridge remains a facing the camera, suggesting that their seat was hung by ropes from the struts. brilliant concept and one that deserves arms are indeed in tension, as they Finally, in the Baker demonstration to be re-created, photographed and would be expected to be to hold the the notched lower end of each strut reflected upon. It just cannot always struts at the angle necessary to sup- rests on the edge of a chair. However, be relied on to give the participants port the swing seat as high above the in the Visitor Centre Trust model this sitting on it a true feel for the forces ground as it is. In the 2012 re-creation, does not seem to be the case. Extrapo- involved in the real structure. however, the gentlemen in the chairs lating the visible portion of any one of are grasping the struts with their palms the struts suggests that its lower end Bibliography facing away from the camera, a less does not rest on the edge of the chair Baker, B. 1887. Bridging the Firth of Forth. En- natural and efficient way to hold up the seat but rather intersects the chair gineering, July 29, pp. 114, 116. struts, which are less steeply inclined. structure just under the seat. Closer Baker, Benjamin. 1887. Bridging the Firth of Furthermore, in contrast to the origi- inspection of the juncture of the upper Forth. Proceedings of the Royal Institution, 12 nal living model, in which the chair end of a strut with the suspended seat (No. 81): 142–149. occupants grasp the struts as close to shows it to look like a welded joint. Engineering News. 1887. A novel illustration of the cantilever principle. June 11, p. 385. their far end as possible, thereby exert- This indicates that the struts are metal Gray, Michael, and Angelo Maggi. 2009. Forth ing minimum force to form isosceles- tubes and suggests that such connec- Bridge: Evelyn George Carey. Milan: Federico triangle cantilevers, the trust members tions may have been employed at least Motta Editore. are holding the struts at about mid- in part to keep the apparatus complete Mackay, Sheila. 1993. The Forth Bridge: A Pic- point, which does not give the model and so always at the ready to be used. ture History. Edinburgh: HMSO. the verisimilitude it should have. In In fact, this is the case. The nature of Paxton, Roland, ed. 1990. 100 Years of the Forth addition, the way they are holding the the apparatus without people partially Bridge. London: Thomas Telford. struts suggests that rather than sup- obscuring some of its parts is shown in Phillips, Philip. 1888. Sketches of the Forth porting them, they are leaning on them, a 2003 photograph taken when it was Bridge; or, The Giant’s Anatomy, from Various thus putting their arms not in tension still located at the Visitors’ Centre. The Points of View. Edinburgh: R. Grant & Son. but in compression. This is especially struts are indeed made of steel tub- Phillips, Philip. 1890. The Forth Railway Bridge; Being the Expanded Edition of The Giant’s evident in the open right hand of the ing and are not entirely straight and Anatomy. Edinburgh: R. Grant & Son. gentleman in the left chair: He does not separate like simple lengths of “loose Westhofen, W. 1890. The Forth Bridge. Engi- even have his fingers wrapped around sticks,” as Engineering News described neering, Feb. 28, pp. 213–283. the strut and appears to be pushing their counterparts. Rather, the steel Wills, Elspeth. 2009. The Briggers: The Story of down on it with the heel of his hand. struts contain an elbow, so when the the Men Who Built the Forth Bridge. Edin- As for the gentleman in the right chair, short part is fixed to or fitted over a fix- burgh: Birlinn. www.americanscientist.org Copyright © 2013 by Henry Petroski. Requests for permission to reprint or 2013 March–April 107 reproduce this article should be directed to the author at [email protected].