9.1 Introduction

The lateral forces that act on conventional wood-frame buildings (bearing wall system) were described in Chap. 2, and the distribution of these forces was covered in Chap. 3. In the typical case, the lateral forces were seen to be carried by the wall framing to the horizontal diaphragms at the top and bottom of the wall sections. A horizontal diaphragm acts as a beam in the plane of a roof or floor that spans between shearwalls. See Fig. 9.1.

Figure 9.1: Typical horizontal roof diaphragm.

The examples in Chap. 3 were basically force calculation and force distribution problems. In addition, the calculation of the unit shear in a diaphragm was illustrated. Although the unit shear is a major factor in a diaphragm design, there are a number of additional items that must be addressed.

The basic design considerations for a horizontal diaphragm are

1. Sheathing thickness

2. Diaphragm nailing

3. Chord design

4. Collector (strut) design

5. Diaphragm deflection

6. Tie and anchorage requirements

The first item is often governed by loads normal to the surface of the sheathing (i.e., by sheathing loads). This subject is covered in Sec. 6.19 for lumber diaphragms and in Chap. 8 for wood structural panel diaphragms. The nailing requirements, on the other hand, are a function of the unit shear. The sheathing thickness and nailing requirements may, however, both be governed by the unit shears when the shears are large.

In this chapter the general behavior of a horizontal diaphragm is...