Design Considerations for Earthquake Forces - Simplified Design of Reinforced Concrete Buildings - page 281

c) The diaphragms are rigid as defined in 2009 IBC Section 1602.

Applicable to buildings of moderate height, this exception can make a substantial difference in the SDC of the
building which impacts the type of detailing required and ultimately plays a key role in the building economy.
The utilization of this exception is illustrated in the examples given in this chapter.

S D1

I

II B C

A D

III

IV

C

S DS

Structures located where S 1 ≥ 0.75 are assigned SDC E if Occupancy Catagory I, II, III, and SDC F if
Occupancy Category IV.
The SDC is the more severe in accordance to S D1 or S DS
Where S 1 < 0.75, the Seismic Design Category is permitted to be determined base on S DS alone,
if T < 0.8T s where T s = S D1 /S DS (see ASCE 11.6 for other limitations)

Figure 11-6 Seismic Design Category Based on S DS and S D1

11.3 REINFORCED CONCRETE EARTHQUAKE-RESISTING STRUCTURAL SYSTEMS

The basic reinforced concrete seismic force resisting systems are shown in Figure 11-7. The permitted
structural system, height limitations, and reinforcement detailing depend on the determined SDC. A brief
description of each system follows:

(1) Bearing Wall System: Load bearing walls provide support for most or all of the gravity loads. Resistance
to lateral forces is provided by the same walls acting as shearwalls.

(2) Building Frame System: A structural system, with essentially a complete space, frame provides support
for the gravity loads. Resistance to lateral forces is provided by shearwalls.

(3) Moment-Resisting Frame System: An essentially complete space frame provides support for the gravity
loads and resistance to lateral loads at the same time (simultaneously).