Elastomeric Bearings for Steel Trapezoidal Box Girder Bridges Page: 3
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applied loads and allowable pad stresses indicate that very large pads, impractical to fabricate,
would be required, or that the pad dimensions limit the rotational capacity below that required. If
reinforced elastomeric bearings are not suitable, an alternate system such as pot or disk type
bearings is indicated. However, pot or disk type bearings are not suitable for light loads or girder
up-lift conditions. Such conditions may be accommodated by elastomeric bearings using the
design guidelines and recommendations presented in this paper.
TxDOT has sponsored research to study elastomeric bearing behavior and design. Research
results were published in October 1995. The study findings are important to TxDOT's design
method for elastomeric bearings of steel trapezoidal box girders. Of note, researchers found that
elastomeric bearings can tolerate far greater rotations than is allowed under the AASHTO
"Based on the AASHTO Specifications, bearings of both hardness ratings with 6 shims
would be allowed rotations of from 0.004 raidians (0.235 degrees) at 3.85 MPa (550 psi)
to 0.010 radians (0.573 degrees) at 7.7 MPa (1100 pis). During rotational stiffness tests
in this study [the 9"x14"] bearings were routinely subjected to rotations of 0.033 radians
(1.9 degrees). In no case was there any observable sign of damage to the specimen. Lift-
off to a 20-30mm (0.79-1.2") depth of separation between the bearing surface and the
lever arm was noted at a rotation of 0.0176 radians (1.01 degrees) under 3.85 MPa (550
psi) compressive stress for the 69 durometer bearings and 0.0293 radians (1.68 degrees)
for the 54 durometer bearings at 7.7 MPa (1100 psi). In none of the tests was any uplift
of the bearing from the bearing seat noted although this is cited as one of the key reasons
for the restrictive specifications concerning rotations8"9
In 1998 TxDOT completed the plans for the US 290 & IH 35 Interchange in north Austin, which
is now in service. The bridge layouts called for each of the four direct connectors to contain one
or two twin steel trapezoidal box girder superstructure units. The required curvatures and the
underlying roadways and other obstacles, including the main lanes of IH 35, dictated the span
arrangements of these trapezoidal box girders. The resulting span and unit lengths presented
opportunity to use elastomeric bearings in lieu of the more complex pot and/or disk type bearings
normally associated with trapezoidal box girders.
The first draft of these design guidelines and recommendations10 was developed after the
completion of the plans for the US 290 trapezoidal box girders, in response to a request from the
Research Study 3-5-92/4-1304, "Elastomeric Bearings".
8 Standard Specifications for Highway Bridges, 15th Edition, AASHTO, Washington, D.C. (1992) (As Amended
by 1993 and 1994 Interim Specifications-Bridges).
9 Muscarella, J. V. and Yura, J. A. "An Experimental Study of Elastomeric Bridge Bearings with Design
Recommendations", Research Report 1304-3, Center for Transportation Research, Bureau of Engineering
Research, The University of Texas at Austin (October 1995), p. 121.
10 Bradberry, Timothy E. "Design Guidelines for Non-Standard Reinforced Elastomeric Bearings for Steel
Trapezoidal Box Girder Bridge Units", draft document, Texas Department of Transportation (1999).
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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/m1/3/: accessed October 19, 2018), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting UNT Libraries Government Documents Department.