ETL 1110-2-355
31 Dec 93
stem has on its flange. The stiffened width of the
half as many elements by taking advantage of
thinner slab depends on the specific geometry of the
symmetry. Boundary conditions can be used so that
base slab and the designer's assumptions.
only a portion of the structure, such as a single wall
or a portion of a wall, needs to be modeled as
opposed to the entire structure, once again reducing
g. Location of design moments and shears.
the input and output required (see Figure A-8). This
Joint flexural design moments at the face of support-
aspect becomes very important when performing two-
ing concrete or toe of a fillet should be used. This is
dimensional analyses on portions of a three-
also where reinforcement development length starts.
dimensional monolith. The designer should use care
Moments at these points can be found easily by
in the selection of the applied boundary conditions so
modeling a joint at the desired location. Maximum
that behavior of the model represents similar behavior
midspan moments are available from computer runs
of the real structure.
or can be found using free-body diagrams. Shear
should be checked, initially, at the face of the sup-
d. Foundation modeling. Modeling of the foun-
porting concrete. Alternative locations for checking
shear, in compliance with American Concrete Insti-
dation can typically be accomplished through the use
tute (ACI) 318, can be used if required.
of elastic springs which may be computed from the
pile stiffness coefficients. Some finite element pro-
7-4. Finite element analysis.
grams contain pile elements which may be used or it
may be possible to employ a soil-structure interaction
(SSI) model. Close coordination with the geotechni-
a. General. Finite element analysis is a numeri-
cal engineer is required when selecting the spring
cal method which can be used to determine the
constants for the foundation, whether it be a pile-
stresses, strains, and displacements. Finite element
founded or a soil-founded structure. A pile-founded
analysis can be performed in two or three dimensions.
structure should be analyzed so that the piles carry
While the finite element method is a powerful and
the entire load. If an SSI analysis is being per-
useful tool, it must be used with care. Results from
formed, then provisions must be made so that the
finite element analysis often appear accurate even
piles carry the load. Finite element modeling of the
when the input is incorrect. The apparent accuracy of
foundation is an option. In many cases it is not used
finite element results stems from the fact that results
because the increased accuracy of the results is not
are often given to the fourth or fifth decimal place.
improved enough to justify the increased cost of the
To ensure results that are accurate, it is imperative
analysis. If the foundation is modeled, guidelines for
that finite element input data be thoroughly reviewed
developing the foundation mesh can be found in
prior to proceeding with a design. Numerous texts
Jones and Foster (in preparation).
are available on the subject and guidance for model-
ing with finite elements is provided in ETL 1110-2-
e. Variable thickness slabs. Typically, variable
332. ETL 1110-2-332 should be reviewed by any
designer who will be performing finite element analy-
thickness slabs occur only in gate monoliths, and the
sis. In addition, for the designer not familiar with
shapes that result in these monoliths can be very
finite elements, review of Will et al. (1987) is an
diverse as seen by the cross sections shown in
excellent example of how a novice should approach
Figure A-9. Because of the shape of the base and
finite element modeling. Finally, ETL 1110-2-254
because the loading on a miter gate monolith is three
should be referenced for the purpose of documenting
dimensional, a three-dimensional finite element analy-
finite element results.
sis will likely provide the best available solution for a
variable thickness slab. Due to the fact that a three-
dimensional analysis is a difficult procedure, even for
b. Strip selection. Information on strip selection
experienced finite element users, simplified two-
for two-dimensional monoliths can be found in
dimensional finite element analyses may be used to
paragraph 7-3b on strip selection for frames.
design a base slab. Two-dimensional models which
may be used to model a base slab using shell ele-
c. Boundary conditions. Boundary conditions
ments are the sloped plate model, the stepped plate
become very important when using finite elements.
model, or the offset beams model. These models are
In the case of symmetrical structures, boundary con-
described in the paragraphs below. Any of these
ditions can be used to reduce the amount of input and
models may be used by the designer. Selection of
output produced by an analysis. This is shown in
the best method may depend on the specific geometry
Figure A-7 where the structure can be modeled with
A-22
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