Diaphragms With Vertical Offsets - The Analysis of Irregular Shaped Structures - page 237

F i g u r e 7.5 Shear transfer options at vertical offset.
system and reduce the rotation of the shear panels. Since the shear panels are discon-
tinuous at the supporting beams, the supporting beams and columns must be designed
in accordance with ASCE 7-05 4 Section 12.3.3.3, which requires the application of an
over-strength factor for seismic design categories C through F. The shear wall shown at
the first-floor level provides lateral support for the mezzanine or second floor. In con-
figuration 2, wall panels are not allowed due to architectural constraints. In that event,
a steel rigid frame or braced frame can be used. The installation of interior columns and
a lateral-force-resisting element at the first floor accomplishes the same effects as that
described for configuration 1. The deflection of the shear panels or frame must comply
with allowable code drift limits to avoid damage to the framing and connections along
the clerestory wall line.
When an open front, simple span diaphragm configuration exists, the longer span
diaphragm that is supported at grid line 3 is assumed to have simple supports. As such,
theoretically there is no moment or chord force at grid line 3. The diaphragm chords of
each section are assumed to act independently. As a minimum, ASCE 7-05 Section 12.1.3
requires that both sections of the diaphragm be interconnected to prevent separation or
tearing at the offset. This can be accomplished by installing full-height vertical transfer
walls between grid lines 2 and 3, at both grid lines A and B, that extend from the foun-
dation to the upper roof diaphragm of section 1, as shown in Figs. 7.6 and 7.7. The low
roof diaphragm chord must be embedded into transfer walls SW2 and SW3 as shown
in Fig. 7.7. The displaced shape of the diaphragm sections and the warping of the clere-
story wall shown in Fig. 7.9 suggest that forces transferred from section 2 into section 1
through the transfer walls act more as chord forces than simple interconnection tie
forces.
The method of distributing the forces into the transfer wall and across the vertical
offset can be seen in Figs. 7.7 and 7.8. The forces at the top of the wall of section 1 must
be determined by a rotational analysis of the open front diaphragm section as presented
in Chap. 6. Figure 7.6 shows that there is only one wall along grid line A. The force at
the top of that transfer wall is equal to the accumulated shears from grid lines 1 to 3. The
direction of the diaphragm shears being transferred into the boundary element shows
that the member is in compression. The strut force along grid line B is shared by two