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Block 9.2 – Analyze Structural Strength Limit State The complete structural design of a deep foundation must consider combined axial load, shear, and bending.
The nominal axial, shear, and flexural resistance of a foundation element cross section must exceed the factored axial load, shear, and bending moments. Based on the results of the Geotechnical
Strength Limit State analysis, the trial foundation size and depth should be analyzed to verify that the structural section is adequate for the factored load cases. The structural design includes the following considerations (Brown et al. 2010):
1. Factored loads are used to determine axial load, shear, and bending moment.
2. The nominal structural strength of the foundation element is determined and must exceed the combined factored axial forces and bending moments for each load combination or for the critical load combination.
3. The nominal shear resistance of the foundation element is determined and compared to the factored shear forces.
If the factored axial load, factored maximum bending moments, and/or factored shear forces exceed the nominal structural resistance
of the foundation elements, then the foundation design must be modified. This may include adding additional reinforcement in the case of concrete elements, using a heavier steel section
in the case of steel piles, and/or increasing the size (diameter or exterior dimensions) of the foundation elements. If the size of the foundation is increased, the Geotechnical Strength Limit State should
be reviewed and reanalyzed, if necessary, to see if the length can be reduced based on the increased size. This may therefore bean iterative process. The structural design of foundations for lateral loading is described in detail in Chapter 11.
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