Modifications to Seismic Provisions

There are two substantial changes that have been made to Chapter 21 in ACI 318-08. First, the sections of Chapter 21 have been reorganized so that the requirements for "ordinary", "intermediate", and "special" systems are presented in this respective order throughout the chapter. As a result, the numerical section corresponding to a given provision has changed for the majority of the sections. Second, the concept of a seismic design category has been introduced to the provisions. The individual provisions of Chapter 21 that a building must satisfy are now dependent on both the structural category (ordinary, intermediate, or special) and the seismic design category.

The following table provides a summary of the code sections a building must satisfy based on frame type.

Table 1. Required Chapter 21 Code Sections by Frame Type
Frame Type	Applicable Code Sections
OMRF	21.2
IMRF	21.3
SMRF (cast-in-place)	1.3 through 21.1.7 21.5 through 21.7
SMRF (precast)	1.3 through 21.1.7 21.5 through 21.8
OSW	None
ISW (precast)	21.4
SSW (cast-in-place)	1.3 through 21.1.7 21.9
SSW (precast)	1.3 through 21.1.7 21.10

The following table provides a summary of the code sections a building must satisfy based on seismic design category.

Table 2. Required Chapter 21 Code Sections by Seismic Design Category
Seismic Design Category	Applicable Code Sections
A	None
B	21.1.2
C	21.1.2, 21.1.8
D, E, and F	21.11 through 21.13