Estimating Seismic Shear in Columns of RC Special Moment Frames

Abstract eng:
In the United States, state-of-practice methods for approximating seismic shear in special moment frames (SMRFs) may underestimate the magnitude forces that SMRF columns need to sustain during strong ground shaking. Overloading of reinforced concrete columns in shear is likely to reduce the flexural deformation capacities and, in extreme cases, may cause shear failure. This, in turn, may lead to localized damage and failure of the structural system which is why a conservative estimate of seismic shear is imperative to an adequate overall design. In this paper, the results of a series of numerical analyses of four archetype buildings conforming to ASCE 7-10 and ACI 318-14 are presented in order to: a) illustrate the varying estimates of seismic shear in SMRF columns based on several common ACI 318 interpretations and b) develop an approach that better estimates these forces and that can be used as the more appropriate lower bound design seismic shear than the code-based elastic forces from procedures such as the modal response spectrum analysis. It is shown that consideration of system overstrength, dynamic amplifications, and statistical distribution of peak responses leads to a method that produces the most consistent estimate of column shear demand with satisfactory level of conservatism for the buildings studied.

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