Dynamic Amplification of Bending Moments and Shear Forces in Cantilever Walls

Student: Alejandro Amaris
Supervisor: Prof. M.J.N. Priestley

Abstract

In recent years, Displacement based design procedure has been used to achieve a specified acceptable level of damage under the design earthquake. The inappropriateness of the Force based design assumptions of initial stiffness and ductility capacity suggests that results of base moments and shear reached in a structure when inelastic response had occurred are not valid. For that reason, it is proposed in this analysis determine appropriate dynamic amplification factor for flexure and shear for a wide range of cantilever wall buildings of 2, 4, 8, 12, 16 and 20 stories which were design using the fundamental of displacement based design principles and compare the results with Force Based Design analysis and time history dynamic analyses.

In addition, the relationship between ductility demand and the dynamic amplification factor in each wall system was investigated. This was carried out using time history analyses for five different earthquakes intensities for each wall, and analysing the bending moment and shear force envelopes.

The effects of some of the issues discussed above were analysed through the use of the inelastic dynamic analysis program, Ruaumoko and the results were compared with existing code requirements.

It was found that dynamic amplification of both shear and moment envelopes became more severe as the initial elastic period of the structure increased, and also as the ductility increased (effected by increasing the seismic intensity). Since all the walls were designed to the same drift limit of 0.02, the level of ductility corresponding to the design seismic intensity decreased as the number of stories increased.

It was further found that most of the dynamic amplification resulted from second mode response, and that existing design equations for dynamic amplification for walls were grossly non-conservative.

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