Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 27

F i g u r e 2.4 Shear wall boundary elements.

2.3 Load Distribution into a Diaphragm

Wind pressure, seismic forces, and soil pressures are the primary lateral forces that are
typically applied to roof and floor diaphragms and their supporting shear walls. IBC
Section 1609.1.1 allows the determination of wind loads to be in accordance with
ASCE 7-05 Chap. 6 or by the “Alternate all-heights method” of IBC Section 1609.6. Any
method can be used as long as the method is applicable to irregular-shaped structures
and separates the wind loads onto the windward, leeward, and side walls of the build-
ing and properly accesses the internal forces in the main wind force-resisting system
(MWFRS). Simplified procedures intended for rectangular structures should not be
used for irregular-shaped structures. The wind pressure on the sheathing is transferred
uniformly into the supporting studs, as shown in Fig. 2.5. The combined internal and
external pressures times one-half of the stud height ( qh /2) plus the wind pressures act-
ing on the roof surface apply a uniform strip load (plf) to the roof and floor diaphragm.
Seismic forces are applied to the roof diaphragm when the ground moves away
from the structure, causing inertia forces within the mass of the roof structure, as shown