Engineering background Three Sisters (Pittsburgh)

a portion of erection sequence , force diagram bridges, produced historic american engineering record in 1990s. full diagram includes following text:

a suspension bridge works hanging roadway cables or chains under tension. though few unstiffened suspension bridges exist, longitudinal stiffening truss or girder added prevent excessive movement of deck. cables pass on towers , anchored @ both ends. conventional suspension bridges use massive concrete or rock anchorages resist cable’s tension. in self-anchored suspension bridges, however, cables fastened both ends of longitudinal girders. these girders therefore compression struts in addition stiffening roadway.

because each of pittsburgh s 3 sisters appears self-contained unit not dependent on river banks anchorage, debate ensued among engineers whether these structures cantilevers rather suspension bridges. while current wisdom holds them latter, erection procedure shows how each bridge built in halves toward center. temporary diagonal struts between chain , deck provided shear resistance turning each incomplete half trussed cantilever arm. these struts freed when halves jacked , connected form suspension bridge.