Straight and curved steel box girder bridges - Computational Analysis and Design of Bridge Structures - page 247

8.2.2 consideration of modeling
steel box girder bridges
8.2.2.1 Design considerations
Steel box girders are at their critical stage during construction because the
noncomposite steel section must support both the fresh concrete and the
entire construction loads. A comparison study using different modeling tech-
niques was made recently (Begum 2010). Curved and straight box girders
in the study are two span bridges that have the same general construction,
consisting of a bottom flange, two sloped webs, and top flanges attached to
the concrete deck with shear connectors. The negative bending region, where
the bottom flange is in compression, is stiffened by longitudinal stiffeners. There
are internal diaphragms or cross frames at regular intervals along the span
and lateral bracing at top flange. The cross frames maintain the shape of the
cross section and are spaced at regular intervals to keep the transverse dis-
tortional stresses and lateral bending stresses in flanges at acceptable levels.
8.2.2.2 Construction
From a designer's point of view, the most critical stage is during construction
when the box is quasiclosed and the casting sequence of the concrete may
affect girder stresses and deflections. Most steel box girder bridges are using
disk, pot, or spherical bearing (Figure 8.16), although elastomeric bearing
pads have been successfully employed in some applications. Collectively,
these bearings are known as high-load multirotational bearings and suited
for curved steel box girder bridges. Of the three bearing systems, spherical
bearings have the greatest rotation capacity and most trouble-free mainte-
nance record. Pot bearings have been troublesome; disk bearings, on the
other hand, have fewer documented failures than pot bearings. The main
purpose of these bearings is to allow the girders to expand and contract to
accommodate daily and annual thermal changes that the bridge undergoes
as well as accommodating construction and live load rotations.
Free or fix of bearings should be correctly simulated in the superstructure
analysis model to accurately analyze the response of the structure to various
loading conditions. The bearing orientations must be reproduced and mod-
eled correctly, especially for curved bridges, not only for thermal load analy-
sis but also for dead load (DL), live load, and centrifugal force analyses.
Depending on the specific configuration of a structure, improper modeling
of bearing conditions (boundary conditions) could have a significant impact
on the correctness of the analysis results. Boundary conditions should be
carefully modeled, and, in cases where the support stiffness is not known
with certainty (e.g., with integral abutments), it may be advisable to run
more than one analysis with different assumptions to assess the sensitivity
of the structural response to the different boundary condition assumptions,