Finite Element Analysis of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 550

5.9 APPLICATION OF LOADS AND BOUNDARY CONDITIONS
ON THE BRIDGES
Chapter 3 has previously detailed different loads applied on steel and steel-
concrete composite bridges as well as highlighted different supports of the
bridges. Different loads applied on the bridges must be simulated accurately
in finite element models. Any assumptions or simplifications in actual loads
could affect the accuracy of results. The loads detailed in Chapter 3 com-
prised dead, live, wind, static, dynamic, thermal loads, etc., in addition to
any loads that designers would like to check the safety of the bridges against
them. However, most of these loads applied to steel and steel-concrete com-
posite bridges are either concentrated loads or distributed loads . Concentrated
forces and moments can be applied to any node in the finite element model.
Concentrated forces and moments are incorporated in the finite element
model by specifying nodes, associated degrees of freedom, magnitude,
and direction of applied concentrated forces and moments. The concen-
trated forces and moments could be fixed in direction or alternatively can
rotate as the node rotates. On the other hand, distributed loads can be pre-
scribed on element faces to simulate surface distributed loads. The applica-
tion of distributed loads must be incorporated in the finite element model
very carefully using appropriate distributed load type that is suitable to each
element type. Most computer programs specify different distributed load
types associated with the different element types included in the software
element library. For example, solid brick elements C3D8 can accept distrib-
uted loads on eight surfaces, while shell elements are commonly loaded in
planes perpendicular to the shell element midsurface. Distributed loads can
be defined as element-based or surface-based . Element-based distributed loads
can be prescribed on element bodies, element surfaces, or element edges.
The surface-based distributed loads can be prescribed directly on geometric
surfaces or geometric edges.
Three types of distributed loads can be defined in ABAQUS [1.29],
which are body, surface, and edge loads. Distributed body loads are always
element-based. Distributed surface loads and distributed edge loads can be
element-based or surface-based. Body loads, such as gravity, are applied
as element-based loads. The unit of body forces is force per unit volume.
Body forces can be specified on any elements in the global X -, Y -, or Z -
direction. Also, body forces can be specified on axisymmetric elements in
the radial or axial direction. General or shear surface tractions and pressure
loads can be applied as element-based or surface-based distributed loads.