Approximate and refined analysis methods - Computational Analysis and Design of Bridge Structures - page 21

constant or varying depth are joined together contiguously by the
cast-in-place reinforced concrete and are transversely prestressed to
make them act effectively as a void or solid slab with orthotropic
properties.
4. Shear-Key Slab (Figure 2.5f). A shear-key slab is constructed of pre-
stressed beams or reinforced concrete beams connected along their
length by cast-in-place concrete to form joints but which are not pre-
stressed transversely. The name shear key is used because the joints
are not fully continuous for transverse moments. The distribution of
loads between beams is by the torsional stiffness of individual beams
and vertical shears between key joints.
The slab bridge can be simplified as a strip beam 1D model, 2D grillage
model, or 3D plate/shell FEM model.
2.2.3 Beam-slab deck type
A beam-slab bridge consists of a number of longitudinal beams connected
either compositely or noncompositely across the tops by a continuous slab.
It is the most popular type for the small- to medium-span bridges.
Spaced beam-slab bridges, as shown in Figure 2.6, are usually made of
beams spaced between 2 m (7 ft) and 4 m (13 ft) apart. The bridges can
be designed with precast, prestressed concrete beams or steel beams acting
compositely with the concrete deck. The deflection behavior is different
from that of orthotropic plate. Beams along the longitudinal direction are
taking most of the loads. Diaphragms are usually placed in the middle/end
or other places to help distribute live loads laterally. When dead loads are
the only concern, a 1D beam model can be used in analysis. When analyz-
ing live loads, a 1D beam model (with two degree-of-freedom beam ele-
ment) employed by live load distribution factors on influence lines can be
used. However, with advanced computer technologies and widely available
comprehensive analysis tools, a completed 2D grillage model with influ-
ence surface loading is preferred, which will be more accurate in live load
analysis and also simplifies the procedures of live load distribution factor
calculations.
2.2.4 Cellular deck type
Box (also called cellular ) deck-type bridges consist of a box or boxes
enclosed by slabs and webs. They contain one or a few large cells, attached
or detached. Small- and medium-span concrete bridges are usually cast in
situ or precast in segments. Long-span concrete or steel bridges are fre-
quently constructed as segmented cantilevers. In addition to the less mate-
rial used, light weighed, and high longitudinal bending stiffness, box girder