Elastomeric Bearings for Steel Trapezoidal Box Girder Bridges Page: 4
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design consultant for the US 71 / Ben White / IH 35 Interchange (now under construction in
south Austin). These guidelines and recommendations have been considered but not yet adopted
as TxDOT policy or procedures. Thus, these guidelines reflect the views and opinions of the
authors and do not necessarily reflect the official view or policy of the Texas Department of
The following is list of design guidelines are not presented strictly in order of execution because
some iteration of the design process is required:
" The TxDOT Bridge Division procedures for the design of the standard reinforced elastomeric
bearings used for conventional type bridges" should be understood before the engineer tries
to design non-standard reinforced elastomeric bearings for a steel trapezoidal box girder
" For load effects, use service loads (i.e. unfactored loads) and concomitant movements. Add
0.005 radians to the calculated trapezoidal box girder transverse and longitudinal twist angles
for rotational capacity to accommodate fabrication and placement errors.
" Except as modified for slip as discussed below, use a 50 F rise and a 70 F fall in temperature
(unless a larger range is warranted) to determine overall thermal movements to be
accommodated, assuming at installation the mean ambient temperature in the region of the
" Assume thermal movements are along a chord from the free bent to the assumed fixed bent
(the traditional idea of a fixed bearing is not really achievable with elastomeric bearings). The
relative stiffnesses of the bents may be considered when calculating movements only if the
stiffnesses of the bearings are appropriately included in the analytical model. Because the
actual direction of movement is uncertain, guided bearings are discouraged.
" Use a constant thickness bearing pad resting on a level surface (a variable thickness sole plate
is required) to avoid developing gravity load induced horizontal forces in the bearing. In the
recent TxDOT research study12, researchers found that a flat bearing pad resting on a level
surface supporting a sloping superstructure is not subject to significant horizontal forces from
gravity loads while a tapered bearing or a bearing have a sloping face is subjected to such
" Rectangular pads are preferred over round pads. The rotational capacity required is dominated
by rotation about the transverse axis of bridge. Thus, minimizing the length of the pad parallel
to the longitudinal axis of the bridge will provide the most rotational capacity. If required
rotational capacity is low (near 0.005) round pads may be used, however, round pads are not
" TxDOT. "Bridge Design Manual", http://manuals.dot.state.tx.us:80/docs/colbridg/forms/des.pdf, Texas
Department of Transportation (TxDOT), Austin, Texas, December 2001., pp. 9-83 to 9-86.
12 Research Study 3-5-92/4-1304, "Elastomeric Bearings".
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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/m1/4/: accessed September 24, 2018), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting UNT Libraries Government Documents Department.