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The types of loading include ship weight, static pressures, and seismic
force. The soilstructure interaction, such as wall friction and subgrade
response, are also included in the loading conditions.
To analyze the effects of loading, we use some loading combination as shown in
Fig. 1. They may include ship load, different earthquake magnitudes, and
lateral restraint provided by the crane rail tie beams which connect to the
top of the drydock wall. In most cases, we use six (6) combination cases in
the analysis.
Solution: An advanced and complete structural analysis using the finite
element method was therefore implemented by the Navy. The analysis began with
the establishment of the structural and geotechnical parameters. Once the
structural configuration is set up by the structural engineer, the
geotechnical engineer develops the static loads, earthquake loads,
soilstructure interaction, and the possible lateral restraint provided by the
adjacent structures.
In most of the cases, the concrete tie beams which connect to the drydock wall
must be included in the analysis because of the lateral restraint to the wall
displacement and the added rigidity to the drydock wall. See Fig. 2.
The numerical analysis is performed by the NASTRAN computer programs, which
are widely used in the Naval Ship Research and Development Center, in
Washington, D. C. The first step of the analysis is to formulate the finite
element mesh which is most suitable to the analysis. About six finite element
models, meaning six (6) loading cases, are generally selected for the
analysis. Some of the results are shown by Figs. 3 through 6. Fig. 3 shows
the formulation of a finite element mesh, Fig. 4 shows a displacement pattern
of the drydock subjected to a loading case, Figs. 5 and 6 show stress profiles
of the floor slab.
From the experience of drydock structural analyses, we have learned that: (1)
we cannot use the elementary strength of material concepts (i.e. f=My/I
equation); (2) we cannot use the equilibrium of free body concepts; (3) we can
use two dimensional finite element method, considering linear or nonlinear
material properties; and (4) we may use Navy's earthquake loading criteria and
computation. The method of computations are described in the Navy's design
manuals DM7.2 and DM7.3. The Navy's DM7.3 discusses the factors affecting
liquefaction and the empirical method of liquefaction potential evaluation.
The soil liquefaction problem during earthquake is still a great uncertain
field. Many difficulties arise when assessing the liquefaction potential.
Some methods have been used, but the methods are still incomplete or
questionable. To evaluate liquefaction potential quickly, a simple flow chart
is developed to check the problem, as shown on Fig. 7. The flow chart gives
an idea whether a more sophisticated evaluation is warranted.
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