it. 122. BRIDGE TRUSSES NOW OUT OF DATE. 



221 



Many bridges with bowstring trusses of short span were 

 built. In modern bridges, the curved top chord is used only for 

 long spans and only the intermediate panel points are located on 

 the curve, the end posts being given a steeper inclination than 

 in the bowstring truss. 



A few double-bowstring or lenticular trusses have been built 

 in this country. In this type, both the upper and lower panel 

 points are on curves. A notable example is the Smithfield Street 

 bridge over the Monongahela River at Pittsburgh, consisting of 

 two spans of 360 feet each. 



POST TRUSS 

 Fig. 158. 



Mr. S. S. Post invented a bridge in which the trusses were 

 of the style shown in Fig. 158. In 1865 he built the first iron 

 bridge of this type, the top chord being of cast iron. 



The Post truss has a double system of web bracing. The 

 braces have a horizontal projection of one half panel length, and 

 the main ties (except the end ones) of a panel and a half. It 

 wr.s built with a counter tie in each panel. The stresses are stati- 

 cally indeterminate. For full load, the two systems may be x as- 

 sumed to act independently, but for partial loads, the counter 

 tirs on one side of the middle net ns a single system, while the 

 main ties on the other side act as a double system. 



WHIPPLE TRUSS 



Fig. 159. 



In 1852-3 Mr. Squire Whipple built the first Whipple-truss 

 bridge on the Renssolaer and Saratoga Railroad. It was replaced 

 in 1883 because it was designed for a much smaller load than it 

 was then carrying. 



