Design Considerations for Earthquake Forces - Simplified Design of Reinforced Concrete Buildings - page 276

Chapter 11
Design Considerations for
Earthquake Forces
11.1 INTRODUCTION
The objective of this chapter is to introduce the basic seismic design provisions of the International Building
Code (2009 IBC) that apply to the structures intended to be within the scope of this publication where seismic
forces are resisted entirely by moment frame or shearwalls. The 2009 IBC refers or adopts with modification
provisions from ASCE 7-05 2 . Reference to both documents will be made throughout this chapter as applicable.
The material in this chapter does not cover structures with horizontal or vertical irregularities (ASCE 12.3.2)
and assumes rigid diaphragm typical for cast-in-place concrete floor systems. Examples of structural systems
with horizontal or vertical irregularities are shown in Figures 11-1 and 11-2 respectively. For comprehensive
background on seismic design and detailing requirements for all cases, refer to References 11.1, 11.2 and 11.3.
The ASCE-7 contains contour maps for the maximum considered earthquake (MCE) spectral response
accelerations (5 percent of critical damping) at periods of 0.2 second (S s ) and 1.0 second (S 1 ). The mapped
values of S s and S 1 are based on Site Class B (see below for site class definition). The earthquake effects that
buildings and structures are proportioned to resist are based on what is called Design Basis Earthquake (DBE).
The design seismic forces prescribed in the ASCE-7 are generally less than the elastic inertia forces induced
by the DBE (Reference 11.3). Figure 11-3 shows the relation between the force generated by the DBE if the
structure were to be designed for elastic response for this force (V e ) and the actual Code prescribed force used
in design (V). The Figure also shows the inelastic response under the Code prescribed force (V). Structures
subjected to seismic forces must resist collapse when subjected to several cycles of loading in the inelastic
range. Therefore, critical regions of certain members must be designed and detailed to safely undergo sufficient
inelastic deformability. The building code contains structural detailing requirements to enable the structure and
members to dissipate seismic energy by inelastic deformation in order to prevent collapse.
11.2 SEISMIC DESIGN CATEGORY (SDC)
The ASCE 7-05 requires that a Seismic Design Category (SDC) (A, B, C, D, E, or F) be assigned to each
structure (11.6). A SDC is used to determine:
(1) Permissible structural systems
(2) Level of detailing
(3) Limitations on height and irregularity
(4) The components of the structure that must be designed and detailed for seismic resistance