A simple method for determining seismic demands on gravity load frames

Abstract

This paper presents a simple method based on modal response spectrum analysis to compute internal forces in structural elements belonging to gravity framing not part of the seismic force resisting system (SFRS). It is required that demands on these gravity load resisting system (GLRS) be determined according to the design displacement profile of the SFRS. The proposed new method uses the fact that if the linear stiffness properties of the GLRS not part of the SFRS have negligible values compared to those of the SFRS, only the latter will provide lateral resistance. Displacements of the GLRS then correspond to those of the SFRS alone. The new method is illustrated by computing the seismic responses of a symmetric and an asymmetric multi-storey reinforced concrete building. These results are compared to those obtained from the application of the simplified analysis method proposed in the Canadian standard for the design of concrete structures. Nonlinear time history analyses are also performed to provide a benchmark for comparison. Results show that the new method can predict shear and bending moment in all members at once with ease. Therefore, this new simplified method can effectively be used to predict seismic forces in elements not considered part of the SFRS.

Influence of soil-structure interaction on seismic demands in shear wall building gravity load frames

“…Beauchamp et al [10] proposed a more general method to determine seismic demands in the GLRS. The method requires a single linear model of the building that can be used to facilitate the seismic design of both the GLRS and the SFRS based on modal response spectrum analysis (RSA).…”

“…To consider the inelastic displacement profile of the SFRS, Beauchamp et al [10] suggested, following comprehensive parametric analyses, to further reduce the stiffness of walls in the plastic hinge zone, giving the linear plastic hinge (LPH) model. They used an effective elastic modulus of E e f f = 0.35E c , which is half the value assigned to the remainder of the walls.…”

“…As will be discussed in Section 3, the flexibility of the foundation generally increases displacements in the structure, which increases forces in the GLRS. Thus, it is important to evaluate the influence of the foundation movements on the GLRS and improve the proposed simplified method accordingly [10].…”

“…They are labelled SS and RO and are described in the next paragraphs. The demands in the elements part of the GLRS are then determined according to the procedure proposed by Beauchamp et al [10] on a model that incorporates one or the other of the studied methods.…”

“…Seismic demands in the GLRS are presented only for columns, although the proposed method is also efficient for other elements. The effects of torsion on the determination of seismic demands in the GLRS have been studied by Beauchamp et al [10]; thus, torsion is not included in the present paper to simplify calculations and to focus on the effects of flexible soil conditions.…”