Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 416

416 Earthquake Engineering for Structural Design

Influence of
sitesoilconditions

Figure 9.21 Elastic displacement response spectrum (after Sabetta and Bommer, 2002)

In this approach, the elastic acceleration spectrum is replaced by the elastic
displacement spectrum. Since damage of structures subjected to earthquakes is
certainly expressed in terms deformations (strains at fibers, curvatures at sections,
rotations at members and drift at story levels), the displacement-based approach is
conceptually more appealing (Bommer and Elnashai, 1999).
For design purposes, Sabetta and Bommer (2002) proposed an elastic displacement
spectrum format, presented in Figure 9.21. The proposed values of T b, T c and T d are
presented in Figure 9.17, while for T e and T f are in Figure 9.21, in function of the site
soil classes. The influence of these conditions is to enlarge the field of maximum
amplification of spectrum.
The displacement response design method is considered to offer the following
advantages with respect to the spectral acceleration response design method
(Medhekar and Kennedy, 2000):
- Displacements play a major role at the preliminary design stage itself, resulting in

a good control on displacements over the entire design process. Target
displacement criteria are selected for the serviceability and ultimate limit states
and thus the damage control is achieved directly.

- Strength and stiffness of the lateral load resisting system are chosen to satisfy the

desired deformation criteria.

- The estimation of the fundamental period of the structure is not required for the

preliminary design.

- The displaced shape may be explicitly linked to the member ductility demands.