Straight and curved steel I-girder bridges - Computational Analysis and Design of Bridge Structures - page 193

Straight and curved
steel I - girder bridges
7.1 BehavIor of Steel I - gIrder BrIdgeS
7.1.1 Composite bridge sections
under different load levels
A composite steel I-girder bridge can be considered as a series of
I-girders with their concrete deck acting compositely with the steel
girders (Figure 7.1). Figure 7.1c shows three different noncomposite or
composite steel sections and their respective stress diagrams. For steel
girder bridge analysis, the respective section properties are used at dif-
ferent load stages. For steel multigirder bridges with cast-in-place con-
crete decks, there are four general loading stages in the construction
sequence:
• Stage 1 —Erection of structural steel framing (girders and cross
frames)
• Stage 2 —Placement of the structural deck slab (wet concrete)
• Stage 3 —Placement of appurtenances (e.g., barriers, railings, over-
lays) representing the long-term (LT) loading
• Stage 4 —Bridge in-service condition (e.g., carrying live loads; vehicu-
lar, rail, pedestrian) representing the short-term (ST) loading
The normal stress distribution σ( x ) in the concrete slab of a composite
beam does not have a constant value but varies where the maximum
flexural normal stress occurs at the junction point of the slab and steel
girder web, as illustrated in Figure 7.2. This phenomenon is caused by
the lag of shear strain at the top of the concrete slab and is referred to
as shear lag effect. Effective width of a cross section at a given loca-
tion depends on the structural layout and loads. For design purposes,
it is convenient to define the effective width for the concrete slab. The
effective ( b e ) and transformed widths ( b tr ) are illustrated in Figure 7.2.
Results of a recent study as shown in NCHRP Report (Chen et al. 2005),
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