Elastomeric Bearings for Steel Trapezoidal Box Girder Bridges Page: 2
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procedures, their background and basis, and presented in the Appendix to this paper is a brief
history of the use of bridge bearings in Texas.
Bearings placed between the bridge girders and their supports have two main functions: support
the gravity loads (dead load and live loads) and accommodate the changes in the length of the
bridge resulting from temperature variations and rotations caused by bending4. Thus, the primary
function of a bridge bearing is to transfer forces from the superstructure to the substructure while
maintaining stability. To ensure this functionality, all probable forces, environmental effects, and
associated movements should be considered, including but not limited to thermal effects, creep
and shrinkage effects, wind on superstructure, wind on live load, dead load, live load, and braking
forces. The bearings can then be designed to accommodate these phenomena in a stable manner.
Although elastomeric bearings have been developed for precast prestessed concrete girder and
steel I-girder bridges in Texas, there are no standard reinforced elastomeric bearings for steel
trapezoidal box girders. Bridge engineers must custom design such bearings. Each steel
trapezoidal box girder unit should be evaluated for the suitability of reinforced elastomeric
bearings, pot bearings, and/or disk bearings, and the suitable bearings designed specifically for the
given box girder unit.
According to NCHRP Report 469, "[e]lastomeric bridge bearings, which have been used since
1950, have had remarkably good performance records. A recent survey (Chen and Yura, 1995) of
all state DOTs established only a few instances of poor performance. Some pad deterioration,
resulting from large shear strains on plain pads, was generally dismissed as poor initial design and
not the result of problems with the elastomer material or fabrication of the bearing. There were no
reported problems related to fatigue or low temperature behavior. The most common
performance problem was slip when the pad was not directly connected to the pier using sole
plates or other mechanical devices. Repeated slip that has resulted in the "walking out" of the
bearing has been traced to excessive paraffin wax in the rubber that has been added for ozone
protection (Muscarella and Yura, 1995; McDonald, 1999)."5 This long term performance record
is a significant consideration in the choice of bridge bearing systems for steel tubs.
In the design of these bearings for steel trapezoidal box girders the support forces and movements
(translations, rotations, and twists) should be calculated by a rational analysis of the
superstructure, including all relevant applied loads and thermal effects, to determine if reinforced
elastomeric bearings are suitable. Table 18.104.22.168-"Bearing Suitability" of the AASHTO Standard
Specifications for Highway Bridges6 (AASHTO Specifications) may prove helpful in determining
the suitability of these bearings. In general, reinforced elastomeric bearings are not suitable if the
4 Yura, J., Kumar, A., Yakut, A., Topkaya, C., Becker, E., and Collingwood , J. "Elastomeric Bridge Bearings:
Recommended Test Methods", NCHRP Report 449 , Transportation Research Board, National Research Couincil,
Washington, D.C. (2001), p. 9.
s Ibid, p. 3.
6 Standard Specifications for Highway Bridges, 16th Edition, AASHTO, Washington, D.C. (1996) (As Amended
by 1997, 1998, 1999, 2000, 2001, and 2002 Interim Specifications-Bridges), p. 347.
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Bradberry, Timothy E.; Cotham, Jeffery C. & Medlock, Ronald D. Elastomeric Bearings for Steel Trapezoidal Box Girder Bridges, text, Date Unknown; Austin, Texas. (texashistory.unt.edu/ark:/67531/metapth637370/m1/2/: accessed November 20, 2018), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting UNT Libraries Government Documents Department.