Reinforced concrete bridges - Computational Analysis and Design of Bridge Structures - page 113

For materially orthotropic finite elements, five elastic constants, E x , E y , G xy ,
ν x , and ν y , need to be specified.
In the third illustrated example with slab bridge decks, in-plane orthotropy
was disregarded as the analysis tool used only permits bending orthotropy.
However, in-plane (axial) and out-of-plane (bending) effects are uncoupled,
and therefore this approximation does not affect comparisons for live load
effects (bending) obtained from the model tests.
As described in Chapter 3, the finite element response to applied
loading is based on an assumed displacement function. This function
may be applicable only to the elements of certain shape; quite often the
program will allow the user to define the elements that do not conform
to this shape. Recommendations for FEA (O'Brien and Keugh 1999) are
listed as follows:
1. Regular-shaped finite elements should be used wherever possible.
These should trend toward squares in the case of quadrilateral ele-
ments and toward equilateral triangles in the case of triangles. In the
case of quadrilateral elements the perpendicular lengths of the sides
should not exceed 2:1 and no two sides should have an internal angle
greater than 135°.
2. Mesh discontinuities should be avoided.
3. The spacing of elements in the longitudinal and transverse directions
should be similar.
4. Elements should be located so that nodes coincide with the bearing
locations.
5. Supports to the finite element model should be chosen to closely
resemble those of the bridge slab.
6. Shear forces near points of support tend to be unrealistically large
and should be treated with skepticism. However, results at more than
a deck depth away from the support have been found in many cases
to be reasonably accurate.
A beam-and-slab or cellular bridge deck may require a 3D FEA. It is possi-
ble to approximate the behavior of slabs and webs to thin flat shells, which
can be arranged in 3D assemblage. At every intersection of shells lying in
different planes, there is an interaction between the in-plane forces of one
shell and the out-of-plane forces of the other, and vice versa. For this reason
it is essential to use finite elements, which can distort under plane stress
as well as plate bending. Because it is assumed that for flat shells, in-plane
and out-of-plane forces do not interact within the plate, the elements are
in effect the same as a plane stress element in parallel with a plate (or flat
shell) bending element.
There is no logical limit to the cellular complexity, structural shape, or
support system of a bridge that can be analyzed with a 3D flat shell model.