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Field Notes:
Three Sisters: Sixth, Seventh, and Ninth Street Bridges, Pgh, PA

These notes add to the information on the following pages:
view page - Sixth Street Bridge
view page - Seventh Street Bridge
view page - Ninth Street Bridge

Typical suspension bridges use cables made of many individual wire strands. Hundreds of strands may be used to create the main cable which stretches from the anchorages at each end of the bridge and passes over the top of the towers. From this main cable, suspenders hang to carry the deck.

The "Three Sisters" in Pittsburgh do not use wire rope; their catenaries (from the Latin word for "chain," this term is used to describe the curve of the main cable of a suspension bridge) and suspenders are constructed using eyebars. An eyebar looks something like a giant needle with holes at each end. A round metal bar -- a pin -- is passed through the eye hole to link the eyebars together.

Eyebars were once more common in bridge construction. They may be found on older truss bridges as tension members. A similar component is the Tension Rod, which has eyed end pieces threaded onto a metal rod.

Many older bridges constructed using eyebars used one, two, or three parallel eyebars to form the catenaries. The "Three Sisters" are much more substantial. Each catenary is constructed with eight eyebars on the anchor span and twenty eyebars on the channel span. Each link in the catenary chain is made of eight (or nine, they alternate) parallel eyebars. Where they are pinned together, there are seventeen thicknesses of eyebar, plus two pair of suspender eyebars.

The center shaft of each catenary eyebar is 13.5" wide x 1.625" thick. The suspender eyebars are 4" wide x 0.75" thick. The deck is riveted plate girder, marked in many places "Carnegie USA."

The suspenders are pinned with 3.5" dia. threaded rods closed with 6.5" hex nuts. This pin connects the bottom of the suspenders to the girder assembly of the deck.

The size of the pins connecting the catenary eyebars are more difficult to judge. There is a 3" nut holding an 18.5" diameter, 1.625" thick plate (like a solid steel hubcap), then the 21 thicknesses of steel described above, another "hubcap" and nut. The outside diameter of the eye on the catenary eyebar is approximately 30", so that may imply the pin is 12"-24" diameter and over 30" long.

(The Smithfield St Bridge has eight parallel eyebars forming the catenaries in its lenticular truss configuration.)

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Last modified: 09-Jul-1999