Numerical analysis of potential failure modes in shear walls of the tunnel form concrete system: Performance-based approach

Similar to other structural systems, the nonlinear modeling process of the tunnel-form concrete system necessitates an understanding of the potential failure modes in its primary lateral load-bearing members. With the identification of failure modes in the elements, deformation-control actions, force-control actions and deformation parameters are determined, making the analysis/design process straightforward. Given the unique characteristics of the tunnel-form concrete system, the applicability of design requirements and assumptions proposed for other similar systems for this structural system remains uncertain. In this study, to assess the potential failure modes of the walls of the system, two different modeling scenarios were utilized, employing pushover and time-history analyses. Within the scope of the studied 5- and 10-story buildings and the adopted assumptions, the results revealed that in the walls, shear is controlled by deformation and bending is force-control. In the incremental analysis, the bending failure mode of the walls (reaching yielding in vertical rebars) was estimated to be almost 10% higher than that of the shear failure mode (achieving an immediate occupancy limit). In the time-history analysis, under the design hazard level (475-year return period), the shear strain in the walls exceeded 13 times the strain at the onset of nonlinear shear behavior. This is because the wall sections still retained sufficient flexural-axial capacity and, contrary to code-based predictions, remained far from the flexural failure mode. The investigations further demonstrated that shear failure in the system was sufficiently ductile and capable of adequately absorbing and dissipating seismic input energy.