Nonlinear Static and Dynamic Analysis of Steel Structures with ...

Chapter 2 Finite element modeling with ABAQUS/Standard
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Fig. 2.6: Local axis definition for beam-type elements
For beams in a plane the n1-direction is always (0.0, 0.0, –1.0); that is, normal to the plane
in which the motion occurs. Therefore, planar beams can bend only about the first beamsection
axis. For beams in space the approximate direction of n1 must be defined directly
as part of the beam section definition or by specifying an additional node off the beam axis
as part of the element definition. This additional node is included in the element's
connectivity list.
If an additional node is specified, the approximate direction of n1 is defined by the
vector extending from the first node of the element to the additional node.
If n1 is defined directly for the section and an additional node is specified, the direction
calculated by using the additional node will take precedence.
If the approximate direction is not defined by either of the above methods, the default
value is (0.0, 0.0, –1.0).
2.2.2 Shell elements
Shell elements are used to model structures in which one dimension, the thickness, is
significantly smaller than the other dimensions. Conventional shell elements use this
condition to discretize a body by defining the geometry at a reference surface. In this case
the thickness is defined through the section property definition. Conventional shell
elements have displacement and rotational degrees of freedom. In contrast, continuum
shell elements discretize an entire three-dimensional body. The thickness is determined
from the element nodal geometry. Continuum shell elements have only displacement
degrees of freedom. From a modeling point of view continuum shell elements look like
three-dimensional continuum solids, but their kinematic and constitutive behavior is
similar to conventional shell elements. In this thesis, only conventional shells are used.