While the nonlinear finite element analysis methods have been commonly used for the performance assessment of existing structures, their use for the retrofit design of concrete foundations has remained limited. One reason for this is the sophisticated modeling process which requires knowledge, experience, and caution. This study demonstrates the applicability and benefit of the nonlinear finite element modeling for the performance-based structural retrofit design of caisson foundations. The foundation system investigated supports a self-supporting telecommunication tower located in Canada. The addition of new antennas and the change in the design standards requires the caisson foundations of this tower to be retrofitted with new cap beams and helical piles to resist significant additional tensile forces. A two-stage analysis and design process is conducted with the help of a continuum-type finite element analysis method, treating reinforced concrete as an orthotropic material and employing the constitutive relations of the Disturbed Stress Field Model. General modeling guidelines and the points for caution are discussed for the retrofit design of caisson foundations using nonlinear analysis methods.
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