Finite Element Analysis of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 551

The unit of these loads is force per unit area. Distributed edge tractions
(general, shear, normal, or transverse) and edge moments can be applied
to shell elements as element-based or surface-based distributed loads. The
unit of edge tractions is force per unit length. The unit of edge moments
is torque per unit length. Distributed line loads can be applied to beam ele-
ments as element-based distributed loads. The unit of line loads is force per
unit length. It should be noted that in some cases, distributed surface loads
can be transferred to equivalent concentrated nodal loads and can provide
reasonable accuracy provided that a fine mesh has been used.
The application of boundary conditions is very important in finite ele-
ment modeling. The application must be identical to the actual situation in
the investigated steel or steel-concrete composite bridges. Otherwise, the
finite element model will never produce accurate results. Boundary condi-
tions are used in finite element models to specify the values of all basic solu-
tion variables such as displacements and rotations at nodes. Boundary
conditions can be given as model input data to define zero-valued boundary
conditions and can be given as history input data to add, modify, or remove
zero-valued or nonzero boundary conditions. Boundary conditions can be
specified using either direct format or type format . The type format is a way of
conveniently specifying common types of boundary conditions in stress/dis-
placement analyses. Direct format must be used in all other analysis types.
For both direct format and type format, the region of the model to which
the boundary conditions apply and the degrees of freedom to be restrained
must be specified. Boundary conditions prescribed as model data can be
modified or removed during analysis steps. In the direct format, the degrees
of freedom can be constrained directly in the finite element model by spec-
ifying the node number or node set and the degree of freedom to be con-
strained. As an example in ABAQUS [1.29], when modelers specify that
(CORNER, 1), this means that the node set named (CORNER) is con-
strained to displace in direction 1 ( u x ). While specifying that (CORNER,
1, 4), this means that the node set CORNER is constrained to displace
in directions 1-4 ( u x , u y , u z , and y x ). The type of boundary condition can
be specified instead of degrees of freedom. As examples in ABAQUS
[1.29], specifying “XSYMM” means symmetry about a plane X¼ constant,
which implies that the degrees of freedom 1, 5, and 6 equal to 0. Similarly,
specifying “YSYMM” means symmetry about a plane Y¼ constant, which
implies that the degrees of freedom 2, 4, and 6 equal to 0, and specifying
“ZSYMM” means symmetry about a plane Z¼ constant, which implies
that the degrees of

freedom 3, 4, and 5 equal to 0. Also, specifying