page_title: Three Sisters (Pittsburgh)
section_title: Engineering background
hierarchical_section_title: Three Sisters (Pittsburgh) / History / Engineering background
caption_attribution_description: English: Erection Sequence and Force Diagram, Three Sisters Bridges, from HAER. Reduced from original TIF file found here [1] and then edited to remove non erection sequence/force diagram material, rearrange some text and convert to .png which I believe, does not change the public domain status of the image (but in case my changes to it require license I hereby release back to PD, see below) Original Image Drawing 2 of 8, Reuse or reproduction requests credit to original draftsmen, to wit: "Delineated by Julie A. Maleeva, James P. Hanley, &amp; Susan H. Gordon, 1998, Thomas M. Behrens, 1999"
context_page_description: The Three Sisters are three very similar self-anchored suspension bridges spanning the Allegheny River in downtown Pittsburgh, Pennsylvania at 6th, 7th, and 9th streets, generally running north/south. The bridges have been given formal names to honor important Pittsburgh residents:
  Roberto Clemente
  Andy Warhol
  Rachel Carson
  Designed by the Allegheny County Department of Public Works, they were all built in a four-year period, from 1924 to 1928, by the American Bridge Company, replacing earlier bridges of various designs at the same sites. Their construction was mandated by the War Department, citing navigable river clearance concerns. They are constructed of steel, and use steel eyebars in lieu of cables.
  The Three Sisters are historically significant because they are the only trio of nearly identical bridges, as well as the first self-anchored suspension spans, built in the United States. They are among the only surviving examples of large eyebar chain suspension bridges in America, and furthermore, unusual for their self-anchoring designs. The bridges’ design was viewed as a creative response to the political, commercial, and aesthetic concerns of Pittsburgh in the 1920s.
context_section_description: A suspension bridge works by hanging a roadway from cables or chains under tension. Though a few unstiffened suspension bridges exist, a longitudinal stiffening truss or girder is usually added to prevent excessive movement of the deck. The cables pass over towers and are anchored at both ends. Conventional suspension bridges use massive concrete or rock anchorages to resist the cable’s tension. In self-anchored suspension bridges, however, the cables are fastened to both ends of the longitudinal girders. These girders are  therefore compression struts in addition to stiffening the roadway.
  Because each of Pittsburgh's Three Sisters appears to be a self-contained unit not dependent on the river banks for anchorage, a debate ensued among engineers whether these structures were cantilevers rather than suspension bridges. Current wisdom holds them to be the latter. Each bridge was built in halves toward the center, with temporary diagonal struts placed between chain and deck to provide shear resistance. These struts temporarily turned each incomplete half into a trussed cantilever arm. The struts 'freed themselves' when the tension on them was removed by jacking each halve and connecting them together to form a suspension bridge.
caption: A portion of the Erection Sequence and Force Diagram for the bridges, produced by the Historic American Engineering Record in the 1990s. The full diagram includes the following text:
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