Dynamic Shear Amplification in High-Rise Concrete Walls: Effect of Multiple Flexural Hinges and Shear Cracking

Abstract eng:
Concrete walls are becoming a very popular seismic force resisting system for high-rise buildings up to 600 ft (180 m) high along the west coast of North America. One critical design issue is what shear strength is required to ensure a brittle shear failure will not occur. Linear dynamic response spectrum analysis (RSA) is usually used in Canada to determine the relationship between maximum bending moment and maximum shear force. Nonlinear time history analysis (NLTHA) has shown that flexural yielding of the wall does not limit the shear force in the wall, and that scaling the maximum bending moment and maximum shear force from a response spectrum analysis by the same reduction factor may result in unsafe design. The shear force tends to increase as the magnitude of ground motion is increased, and this is referred to as dynamic shear amplification. The influence of flexural yielding at multiple locations over the wall height and influence of shear deformations due to diagonal cracking of the wall were investigated. The results indicate that both significantly reduce the maximum shear force in the wall. When multiple hinges occurred and the ratio of maximum bending moment at base determined from RSA to flexural strength at base was equal to 5, the maximum shear force near the top of the wall reduced 50%, while the maximum shear force at the wall base reduced 20%. Reduced shear stiffness due to diagonal cracking further reduced the maximum shear force at the base of the wall. When the shear rigidity of a cracked concrete wall is equal to 10% of the uncracked section shear rigidity, which is a typical value, the maximum shear force at the base reduced a further 27%. The combined influence of multiple hinging and diagonal cracking reduced the maximum seismic shear force at the base of the wall by 40%.

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