Diaphragm Basics - The Analysis of Irregular Shaped Structures - page 23

CHAPTER 2
Diaphragm Basics

2.1 Introduction

The methods for analyzing and designing simple rectangular box systems have been
common knowledge for decades. A number of publications and textbooks that have
been written to date provide a fairly complete coverage of the topic. However, a natural
progression of architectural creativity has taken structures from simple rectangular
floor plans to ones that contain horizontal and vertical offsets and other irregularities
that complicate load paths. Because of time constraints, the length of undergraduate
classes on wood design often limits the coverage of diaphragms to simple rectangular
sections. The extent of mentoring after graduation varies greatly, and because there are
very few topics or examples that explain how to analyze complex diaphragms and
structures, some individuals may not have an in-depth understanding about how com-
plex diaphragms and their components really work. This may cause some engineers to
approach the analysis and design of these irregular-shaped structures as though they
were still rectangular diaphragms. A cursory review of simple diaphragms and their
components will be provided here as a base from which to extend the discussion into
irregular-shaped diaphragms.

2.2 The Basic Lateral-Force-Resisting System

The structure shown in Fig. 2.1 represents a typical bearing wall system, also known as
the box system. Lateral forces are resisted by a flexible wood roof diaphragm and light-
framed shear walls at the upper level and by a flexible wood floor diaphragm and
concrete or masonry shear walls at the lower level. Wind or seismic loads are typically
applied at, or transmitted to, the roof level as a uniform load into the roof diaphragm.
These loads are resisted by the roof diaphragm which acts as a horizontal beam, or deep
girder. Girder analogy, as described in ATC-7, 1 assumes that the flanges of the dia-
phragm take tension and compression forces only due to bending and the web takes
the entire shear. Plywood diaphragm testing has indicated that the girder analogy can
be used to predict the performance of wood-sheathed diaphragms and that the use of
the analogy for the determination of the flange forces is conservative. Figure 2.2 shows
the resulting shear and moment diagrams for a uniformly loaded rectangular dia-
phragm. Plywood diaphragms behave differently from shallow beams due to the depth
of the diaphragm and the jointed construction. The APA noted that it has been shown
that shear stresses have been proved to be essentially uniform across the depth of the
diaphragm, rather than showing significant parabolic distribution as in the web of a

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