BRIDGES AND
TUNNELS OF
ALLEGHENY COUNTY,
PENNSYLVANIA
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HAER
Smithfield Street Bridge, Pittsburgh, PA
01 Cover Page
02 Foreword
03 Ferries
05 Monongahela
Bridge and
Fire
12 Masonry
17 Removal
of Old and
Erection of
New Bridge
18 Flooring
19 Ornamental
Towers and
Painting
22 Alterations
23 Footnotes
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Smithfield Street Bridge, Pittsburgh, PA Historic American Engineering Record PA-2 page 14
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Channel Spans "It was found that the use of steel, in the trusses at least, would prove economical as compared with wrought iron. The saving based on the prices at that time was over $21,600. "The Pauli trusses were designed with an uneven number of panels, namely 13, in order to get two tangential points of attachment for each truss to the floor-construction, thereby securing greater longitudinal and transverse rigidity of the entire bridge frame. Roller bearings for pier posts were avoided; the middle posts, supporting two truss ends each, have a fixed and square bearing on heavy pedestal castings on the pier. Each end post has a bearing on a 6 inch steel pin in a cast-iron pedestal on which it can rock. It is probable that very little movement takes place on account of friction on the pin, and that the posts would bend or spring. The resulting bending moments on the end posts have been considered in proportioning them. "The projected length, 27 feet 7-5/8 inches, of all panels being alike, it follows that the lengths of chords in a curved line are unlike, and if the curve were a circle or a parabola, then the angles formed by the straight chord sections would also all be unlike. "For practical reasons it is desirable to have these angles all alike, so as to have only one template for the beveled joints. This condition would prescribe the character of the curve, in this instance a sine-curve. The difference in curvature between a sine-curve and arc of a circle was found to be small (2-1/2 inches). The difference in the bevel points was inappreciable (3/64 inch). Therefore a true arc line was then assumed for the chords to facilitate other calculations. "The vertical web-members are in tension from the dead load or from a uniformly distributed live load. They will sustain compression strains only from an uneven distributed load. Near the centre of truss they are long and slender, requiring intermediate bracing, which was placed at half the truss height for the entire length of trusses. "The suspenders from trusses to floor, which were all stiffened to prevent vibration, were not made adjustable; their exact lengths were calculated to give the required camber of 18 inches to the floor construction. The truss camber was obtained by shortening the lower and lengthening the upper chord members 3/16 inch, so that after erection it amounted to 2 inches. "All diagonal bars were made adjustable and single; they are strained from partial loads only. The trusses were adjusted to their proper shape by means of these ties, which received a slight initial strain. "The top and bottom chords, pier-posts, diagonal-ties, and pins are of steel; all other parts are of wrought-iron with steel rivets. The calculated sections of the vertical web-members for steel were so light that for practical reasons they were all made of wrought-iron and of the same section.
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Introduction Last modified on
30-Sep-99
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