Diaphragms With Vertical Offsets - The Analysis of Irregular Shaped Structures - page 233

CHAPTER 7
Diaphragms With
Vertical Offsets

7.1 Introduction

Vertical offsets frequently occur in floor and roof diaphragms. Whenever a full-height
interior shear wall or lateral-force-resisting element cannot be installed at the location
of the offset, it will cause a disruption in the diaphragm web and chords. Very little has
been written on this type of discontinuity. ATC 7 1 briefly addressed the effects of dia-
phragms with vertical offsets but did not provide any details on how to design for this
type of irregularity. A complete discussion and brief example for the analysis of vertical
offset diaphragms can be found in a publication written by Edward F. Diekmann. 2
These types of disruptions or discontinuities create very complicated load paths that
require innovative solutions. Steps in roof and floor diaphragms are usually caused by
the desire to create a clerestory wall with windows or other architectural feature, as
shown in Figs. 7.1 and 7.2. The cross section in Fig. 7.2 shows the typical condition
where an interior mezzanine occurs between grid lines 1 and 3. In this case, the roof
shears at grid line 3 are not transferred down into the mezzanine below or down to the
foundation. The figure shows that the magnitude of the uniform loads that are applied
to the roof diaphragm sections will vary, partly because of the presence of the mezza-
nine and partly because of the difference in the wall heights. Assuming that the place-
ment of a transverse interior shear wall is not possible at grid line 3, the challenges
become these:

• How to support the edge of the upper and lower diaphragms along grid line 3
• How to transfer the diaphragm shears across the vertical offset for transverse
loads
• How to effectively splice or transfer the disrupted diaphragm chord or strut
forces across the vertical offset for transverse and longitudinal loading
• How to limit the diaphragm deflection to acceptable limits and prevent P -delta
problems
• How to calculate the deflection for this type of structure

If any of these design challenges cannot be resolved, the diaphragm will be subject
to failure. Figure 7.3 shows a diaphragm with unequal sections on each side of the ver-
tical offset. Section 1 complies with the requirements for an open front diaphragm

233