Historic Structures
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Historic structures significant structures of the past ® The superb lenticular spans and wide piers, seen from the west. It is the oldest and longest through-truss extant bridge in the U.S. HAER PA-Pitbu,58-9. Jack. E. Boucher, Photographer. 1974. either a fire, a worker falling 80 feet site, and efficientlyCopyright reused into the river (he was back to work here. Thus, source materi- the next morning), nor an ice floe als were conserved. tearing away part of the false work The striking lenticular To Engineer is to Sustain Nprevented or slowed the construction of one of (meaning a “lens” form of this country’s most significant bridges. the truss-frames) through- Gustav Lindenthal had enormous energy and truss bridge constructed from 1881-1883 contains Gustav Lindenthal’s drive. He received education to about age 14. 1070 tons of iron in the superstructure and 322 Smithfield St. Bridge Born in Brün, Austria in 1850, he worked in tons in the foundation. The original one-bay-wide carpentry, railroad engineering, masonry, and carriageway is two 360-foot spans with trusses Pittsburgh, PA, 1883 other construction, rising quickly and eventu- comprised of 13 panels, each 27 feet, 71/2 inches. ally immigrating to America. A self-actualized magazineWind and sun exposures are the same on all parts By Alice Oviatt-Lawrence learner, he absorbed every Sbit of knowledgeT inR of the Usuperstructure: C Lindenthal T thusU equalized R E his midst. Nineteenth century Europeans had the thermal expansion and contraction. an inherent, sustainable approach to construc- The original bridge spans were 48 feet wide tion, emphasizing the use and reuse of durable in 1883, leaving an eight-foot overage on the materials (acquired locally if possible), efficiency, upstream side of the channel piers. Lindenthal adaptability and elegant aesthetics. designed the original bridge to accommodate Lindenthal worked as a stonemason for the future expansion up to 65 feet. He designed and Alice Oviatt-Lawrence is 1876 Philadelphia World’s Fair Centennial installed reusable, detachable sidewalks to flank principal of Preservation International Exhibition Memorial building’s the carriageways, another example of sustain- Enterprises – an international granite foundation, and then rose quickly ability evident in the project. architectural-engineering research to a three-year stint as assistant engineer of continued on next page and historic-building analysis construction for Philadelphia’s permanent cen- organization. Alice may be reached tennial buildings. He then worked on bridge at [email protected]. and rail projects, and by 1879 he called himself a bridge engineer. The iron and steel Smithfield Street Bridge replaced a weak suspension bridge – Roebling’s first highway bridge of 1845 – over the Monongahela River. Remarkably, Gustav Lindenthal constructed his new bridge with the old twenty-foot lower-clearance bridge remain- ing in service. False work was erected under the northern section of the bridge that both enabled one lane of the old bridge to stay in service during Interior through-truss with the third set of lenticular construction, and assisted in the erection of the trusses of 1891 (left) first for horse-drawn trolleys, later six wrought iron plate girders for the new bridge. for the 1898 electric trolley track. Lindenthal extended Some iron and machinery were retrieved from the bridge’s service life via planned, sustainable design. a proposed, but withdrawn, bridge at the same HAER, PA-2 Pitbu, 58-18. Jack E. Boucher, 1974. STRUCTURE magazine 11 The Masonry firing temperatures, hardness and smoothness of finish. However, there was not yet an abso- To raise the old bridge the required twenty feet lute scientific method to determine if there for river traffic, Lindenthal integrated and added were flaws in metals. onto the substantial existing sandstone-faced piers partially constructed for the previously pro- The Structure posed but abandoned bridge. The pier centers are filled with concrete, followed by alternate The structure can be visualized via two design header and stretcher stone-courses, of which concepts (parti): as an arch and cable structure, each is iron clamped. Lindenthal adapted the and/or as a schematic of the distributed loads. 56 foot wide piers not only to elevate the grade In the first, the upward thrusts on the upper of the original 1883 bridge, but also to prepare chord ends (arch) are resisted by the pull of lower them for future superstructure expansion with chord (cable). Lindenthal referred to the lower associated loading, which he foresaw. chord as a cable in 1883. In the second parti, the The existing south abutment of concrete with maximum strength is directly expressed at the sandstone facing was reused, but Lindenthal truss mid-span, reflecting the greatest bending Close up of steel plate® top and bottom chords, built a new north abutment. The north abut- moment: The truss tapered-end form displays acting as arch and cable. The wrought iron vertical ment has a stone backing sized about two or a diminishment of bending moment to near members are seen, with the horizontal intermediate three feet wide by two feet thick by four-to- self-equilibrium at the end posts. bracing at midpoint running the length of the seven foot-long stones, also laid in a header The wrought iron vertical web-members trans- bridge. HAER, PA-2 Pitbu, 58-18. Jack E. and stretcher pattern. All voids were filled and fer loading from the deck to the upper and lower Boucher, 1974. compacted with concrete, with the finished- chords, acting in tension vis-à-vis the dead load Copyright to prevent vibration and to create an 18-inch face sealed with Portland cement. and the evenly distributed live loads. The ver- camber to the floor assembly of each 360-foot ticals act in compression only when the load is span. Each steel end-post rests on a six-inch The Steel unevenly distributed. Lindenthal selected iron pin, bearing on a cast iron pedestal. This here, as the weight was no different than that of Just before this era, a new, locally manufac- allows some friction-restricted rocking. No the steel web members of the same dimensions. tured “open hearth” carbon steel emerged, roller bearings were used at the pier posts. The adjustable steel eyebar and turnbuckle of a smoother and stronger quality than The bottom chords, consisting of eight to ten diagonal web members stabilize the arch and the equally available, but less-uniform, eyebars of varying thicknesses, were placed and cable system under a variety of distributed Bessemer steel or wrought iron. Metals were the iron vertical web members connected in loadings, and contribute to the forming of not standardized chemically or for mechani- sequence from the ends. Then the steel plate the shape. Each diagonal increases in cross cal properties until nearer 1900. Lindenthal top chords, weighing seven to nine tons each, sectional area from the panels nearest the end tested all the materials going into his bridge, magazinewere placed on the verticals working similarly in supports to the mid-span panels. including the cements, the concretesS andT new R U C T Ufrom R each truss Eend. A block and tackle pulled While Lindenthal and others describe the steel. When most of the Bessemer steel failed the top and bottom chord-ends together into bridge as a triangulated Pauli-truss, Thomas in compression testing, the new steel was the lenticular form. The top chord is comprised Boothby and others mathematically prove integrated into the bridge for the top and of a box section made up of ten and twelve inch that it is a “parabolic lenticular truss bridge”. bottom chords, pier-posts, diagonal ties and steel-riveted plates and angles, and increases in Lindenthal himself compares a “sine-curve” pins. Of the total 1810 tons weight of the cross sectional area toward the end posts. arc to a circle arc and, after studying circle arcs original superstructure, 740 tons are steel. and parabolic arcs, he states that the difference In addition to the increased strength of the Post Construction between the two is a negligible 21/2 inches. new steel, its price dropped from $166 per The two truss-ends of the 13 irregularly The iron rusted immediately. Even after ton in 1867 to about $30 per ton in the early shaped panels of the lenticular span trusses, scraping the rusty areas with lime pastes, 1880s. Suddenly, steel-use meant economy where they converge, are supported by a subsequent ample applications of iron oxide both in terms of expenditure and for effi- middle pedestal on the pier. The entire primer and lead paint mixed with raw lin- cient selection of materials. Lindenthal saved assembly is rigid and is fixed cable-to-floor seed oil did little to protect the bridge from $21,600 in construction costs by specifying via non-adjustable, stiffened, iron suspenders Pittsburgh’s harsh environment. the thinner, lighter, and stronger steel. The specified steel tested for compression at 50,000 to 55,000 pounds per square inch elastic limit, with ultimate strength of 80,000 to 90,000 pounds per square inch. Tests for members in tension proved to be 45,000 to 50,000 pounds per square inch elastic limit, with 70,000 to 80,000 pounds per square inch ultimate strength. Elongation, reduc- tion of area at fracture and cold bending were analyzed via early U. S. testing methods and machines. The sounds generated by punching A portion of Lindenthal office ‘stress sheet’ or technical drawing: “ Elevation and Plan of the Channel holes in steel plates and angles were scruti- Spans”. In Transactions of the ASCE, Vol. XII, Plate XXIV. Sept. 1883. nized; an experienced ear could determine the STRUCTURE magazine12 October 2012 STRUCTURE magazine Lindenthal 1881 office illustration: “View of the Two Channel Spans”, showing the massive one-bay-wide, original steel-supported and embellished with iron portals, under a three-bay-wide arcade supporting a mansard roof.