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In many cases, the effect of
temperature on 3D structures can yield
surprisingly different results than
those obtained from a simplified 2D
model. Having the ability to model a
structure with 3D shell finite elements
as well as simple 2D frames, as is the
case in Sam, can prove extremely
beneficial.
Consider the case of a simple,
orthogonal, two span portal slab and
wall structure. It is required to
obtain the in-plane slab forces, in the
direction of the span, due to a
temperature rise in the slab. This will
intuitively result in longitudinal slab
compression due to the vertical bending
resistance of the walls. This can be
verified using a simple plane frame
analysis.
Now consider the same structure
modelled with 3D shell finite elements
and discover that the compressive force
derived above will be severely
underestimated along the centre line of
the structure and will in fact become
significantly tensile at the edges.
This may seem surprising but can quite
readily be explained by considering
both longitudinal and transverse
expansion of the slab and walls acting
together. The transverse expansion is
unrestrained, except along the lines of
the walls. This results in lateral
bowing at the edges of the slab which
induces lateral bending along the lines
of the walls. The resultant
distribution of longitudinal axial
force is shown in the following contour
diagram.
By using the composite member facility
in Sam we can show the axial forces
along a number of 1m wide strips, and
the tensile forces at the edge of the
slab can be clearly seen.
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