Nonlinear procedures for seismic evaluation of buildings

New static and dynamic displacement-based procedures have been developed by the Federal Emergency Management Agency (FEMA) for the seismic evaluation and design of buildings. The static procedures calculate displacements in yielding buildings as the product of an elastic spectral displacement and coefficients C_i. Mean elastic and mean inelastic displacements are assumed to be equal for elastic periods, T_o, greater than the characteristic site period, T_g. The data presented in this paper and elsewhere support this assumption for values of the strength ratio greater than or equal to 0·20. For T_o ≤ T_g, mean inelastic displacements can substantially exceed mean elastic displacements for all values of the strength ratio; FEMA 273 accounts for this observation by the coefficient C₁, However, the FEMA 273 cap on C₁ (= 1·5) is not sufficiently conservative and should be increased to 3·0 for the analysis and design of modern construction. The effects of stiffness degradation, strength deterioration and pinching are represented in FEMA 273 by the coefficient C₂. The values assigned to C₂ appear to be most reasonable. Nonlinear dynamic analysis is being widely used to estimate maximum deformations and displacements in buildings. The values calculated for the maximum deformations and displacements will be dependent upon the means used to characterize structural damping in the building frame. Although structural damping is routinely implemented through a damping constant (or matrix), such an implementation will overestimate the effects of structural damping in a yielding building and underestimate maximum deformations and displacements. Structural damping should be characterized by the target damping ratio (typically 5% of critical) at the point of maximum displacement.