Finite Element Analysis of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 470

section to help readers to choose the best finite elements to simulate the
different components of steel and steel-concrete composite bridges.
5.2.1 Main Continuum, Structural, and Special-Purpose
Finite Elements
To choose the best finite element for the structural steel members, we have
to look into the classification of the cross section, which is normally specified
in all current codes of practice. There are three commonly known cross-
section classifications that are compact, noncompact, and slender sections.
Compact sections have a thick plate thickness and can develop their plastic
moment resistance without the occurrence of local buckling. Noncompact
sections are sections in which the stress in the extreme fibers can reach the
yield stress, but local buckling is liable to prevent the development of the
plastic moment resistance. Finally, slender sections are those sections in
which local buckling will occur in one or more parts of the cross section
before reaching the yield strength. Compact sections in 3D can be modeled
using either solid elements or shell elements that are able to model thick sec-
tions. However, noncompact and slender sections are only modeled using
shell elements that are able to model thin sections. It should be noted that
many general-purpose programs have shell elements that are used to simulate
thin and thick sections.
Let us now look in more detail and classify shell elements commonly
used in modeling noncompact and slender structural members. There are
two main shell element categories known as conventional and continuum shell
elements , examples shown in Figure 5.1 . Conventional shell elements cover ele-
ments used for 3D shell geometries, elements used for axisymmetric geom-
etries, and elements used for stress/displacement analysis. The conventional
shell elements can be classified as thick shell elements, thin shell elements,
and general-purpose shell elements that can be used for the analysis of thick
or thin shells. Conventional shell elements have 6 degrees of freedom per
node; however, it is possible to have shells with 5 degrees of freedom per
node. Numerical integration is normally used to predict the behavior within
the shell element. Conventional shell elements can use full or reduced numerical
integration , as shown in Figure 5.2 . Reduced-integration shell elements use
lower-order integration to form the element stiffness. However, the mass
matrix and distributed loadings are still integrated exactly. Reduced integra-
tion usually provides accurate results provided that the elements are not dis-
torted or loaded in in-plane bending. Reduced integration significantly
reduces running time, especially in three dimensions. Shell elements are