Pin and hanger assembly
|This article does not cite any references or sources. (March 2012)|
A pin and hanger assembly is used to connect two plate girders of bridges. These assemblies are used when the space between two bridge piers is too wide to be spanned by a single set of girders. To overcome this, steel beams are set with one end resting on a bridge pier, while the other end is connected to adjacent steel beams that are cantilevered from the next pier. The two steel beams are connected using a pair of connecting plates (one on each side of the bridge joint). A pair of bolts are inserted through the plates and girder webbing: one through the cantilevered beam and one through the suspended beam. Exceptionally long spans may have two sets of girders cantilevered from opposite bridge piers with a third set of girders suspended by pin and hanger assemblies from both cantilevers.
Pin and hanger assemblies are considered fracture critical bridge components, meaning that the assemblies are non-redundant and failure of these systems could cause part or all of the bridge to collapse. The collapse of the Mianus River Bridge in Connecticut exposed potential flaws with pin and hanger bridges that could lead to catastrophic failures, if left unchecked. Because of this, state departments of transportation incur costly expenses on bridges with pin and hanger assemblies, as they require constant inspection and maintenance. As a result of these safety concerns, and advances in bridge design to allow longer spans, pin and hanger assemblies are no longer used on new bridges in the United States.
Attempts have been made to increase the safety of bridges with pin and hanger assemblies by adding some form of redundancy to the assembly. Retrofits that add redundancy to pin and hanger assemblies include adding a "catcher's mitt"--a short steel beam attached to the bottom of the cantilevered girder that extends out beneath the suspended girder to "catch" the suspended girder should the pin and hanger assembly fail. Another redundancy is connecting the cantilevered and suspended girders at the pin and hanger assembly with welded blocks and tie rods. Finally, a bridge may be retrofitted by replacing the pin and hanger assembly with a pair of gusset plates that are much larger and thicker than the pre-existing hanger plates.