Retrospective Structural Mitigation of Buildings Damaged By the 2011 Tohoku Japan Tsunami Using the Asce 7 Tsunami Design Provisions

Abstract eng:
A number of building tsunami failures were analyzed by an ASCE tsunami reconnaissance team after the devastating Great East Japan Earthquake and Tohoku Tsunami of March 11, 2011. In previous studies by the authors, structural failures of buildings were analytically replicated from their structural response, as calculated for field-estimated maximum flow depths and velocities. The structures were selected for a range of effects, including buoyancy, lateral hydrostatic, hydrodynamic and debris damming loading conditions. Overall, the analyses demonstrated that the ASCE 7 tsunami design provisions are sufficiently reliable engineering design tools for structural load characterization and analysis of suitably defined tsunami flow conditions. In this paper, possible modifications to the examples of damaged structures are developed based on the ASCE 7 tsunami design provisions, together with a discussion of the mitigation options to upgrade various types of structures for tsunami resistance. These example buildings are retrospectively used to demonstrate design strategies to achieve tsunami resiliency of buildings based on tsunami flow characteristics observed at different sites during the Tohoku Tsunami, which represent relatively extreme tsunami design criteria. Resistance to hydrostatic buoyancy forces may be achieved by several approaches, selected depending on performance objective. The methods include installation of deep foundations with sufficient tensile capacity to resist a buoyant condition of a structure and/or to preclude pressurization of the underlying soil. Alternatively, the structure may be designed to relieve hydrostatic uplift pressures through breakaway slabs or sacrificial exterior wall or cladding elements, allowing internal flooding of the building. Components of a structural steel frame building can be feasibly designed for additional lateral resistance to resist the hydrostatic and hydrodynamic lateral loads, including accumulated debris damming and impact effects when the inundation height is at or around the building height for the low rise building studied. There are similar mitigation techniques for concrete frame structures. Concrete structural walls can be designed to resist in-plane tsunami forces and out-of-plane forces, including bore and debris impacts, with an increase in the out-of-plane strength generally being required. The analysis demonstrates that when buildings are significantly overtopped it is difficult to design wall elements for tsunami loads. When tsunami inundation depth is not more than the height of concrete or steel framed structure, it can be more feasibly designed for tsunami forces. Special debris impact loads are not considered.

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