Hell Gate Bridge
Hinged spandrel-braced steel through-arch railway bridge at Hell Gate, Queens and The Bronx, New York.
Completed in 1917.
Designers: Gustav Lindenthal and Othmar Hermann Ammann
The arch bridge and adjacent viaducts were erected by the American Bridge Co.
Span: 977 ft between pin centres. Long approach spans.
The bridge inspired the design of the Sydney Harbour Bridge, the Bayonne Bridge, and Birchenough Bridge. Aspects of the design and construction of Hell Gate Bridge itself followed many precedents, but at the time of construction, the arch of Hell Gate Bridge was the longest and heaviest. See here [1] for a list of the longest iron and steel arch bridges with spans of 400 ft or more, built up to 1907.
Some aspects of construction were described in The Engineer, with the emphasis on stress measurement [2]. The bridge was erected on the cantilever principle, with the use of temporary back stays. As natural anchorages were not available in solid rock at a reasonable depth below the surface, huge counterweights (5,300 US tons per side) were used, to which the back stays could be attached. Struts were provided along the ground surface between to take thrust from the counterweights to the base of the towers. In order to economize, parts of the back stays were designed to be later incorporated in the permanent bridge (in the main span and in the approach spans). Consequently many of the back stay members comprised webbed plate girders. The total weight of the back stays was 15,500 tons,of which all but about 2,300 tons was finally incorporated in the bridge. Such girders formed boxes, which, on the Long Island side, were partly filled with spoil from the excavations. Less spoil was available on the Wards Island side, so additional steel members were utilised to provide weight.
Adjustment of the arch trusses in height was required at various erection stages. For this purpose a 3000-ton hydraulic jack was placed at the top of each of the four erection posts E F (see Fig. l). The jacks had a diameter of 39in., a stroke of 26in., and a water pressure of 5000 psi.
When the two half arches met, it was necessary to join them such that the top chord was put into compression rather than in the state of tension which prevailed when the half arches were cantilevered. When in compression, reaction forces would be transmitted through the back stays and increase the downward force on the counterweights, possibly causing sinking. Therefore it was necessary to de-load the counterweights quickly, and to be prepered to react with the 3000-ton jacks if necessary.
See here[3] for a 1918 Paper presenting extensive information and discussion on the design and construction of the arch bridge and the approach viaducts.
Unlike most pinned-arch bridges, which have distinctive pins or cylindrical surfaces at the hinge points, at Hell Gate the portion of hinge attached to the lower chord is a steel casting with a plane surface, which bears on a convex-faced steel casting having a radius of 1150". The aim was to allow rocking motion with very little friction. The maximum angular motion under live load was calculated at +/- 1.5 deg. Each bearing was designed to transmit a thrust of 15,131 US tons to its granite skewback. The calculated tangential force was 1785 tons, 'easily resisted by friction. However, to prevent displacement of the upper shoe, four steel dowels 5 1/2 in. diameter, are set into the lower shoe and engage holes in the upper shoe.'
The subject of extensometer measurements was covered in detail in the 1918 Paper. The measurements were taken to determine the strains at critical points in the arch ribs and back stays during erection. Another application of the extensometer was to provide a check in some of the critical operations of erection. After seven panels had been erected the hydraulic jacks at the tops of the towers were operated to put the forward or upper stay in tension and release the lower stay.
The extensometer, designed by Mr. James F. Howard, which was essentially a 20" micrometer calliper. The fixed and moving anvils had hard steel conical points attached to the barrel and the rod respectively, pointing downwards. The distance between the two points was measured by a micrometer contact screw at one end of the barrel, reading by a circumferential vernier to 1/10000 in. A rectangular steel bar, provided with two centre holes 20in. apart, accompanied the instrument, and when a reading was made a comparison reading was made on the bar, the difference between the two readings being the measurement. The bar was to be kept in contact with the measured member with a view to equalizing temperatures. Temperatures were also monitored during the exercise. It was said to be a most difficult matter to get a satisfactory micrometer "feel" with conical points bearing against the inclined sides of the holes, and in the confined space and disadvantageous conditions in which the measurements were taken. The holes for the datum points were drilled with what appears to have been a Slocomb centre drill, using a hand brace.