Code Sections and Analysis - The Analysis of Irregular Shaped Structures - page 8

provides no room for workmanship. This puts a high demand on diaphragms,
shear walls, and connections.
• Splitting, using gun-nails when common nails are specified, using different
species of wood than specified, overdriving nails, and slack in light-gage metal
straps lead to failures.
Edward F. Diekmann provided an interesting note in his engineering module
“Design of Wood Diaphragms” 5 regarding a misconception for the requirement of
wood diaphragms and shear walls. He noted that it was unfortunate that interest in
diaphragms and shear walls was developed primarily on the West Coast, which gave
rise to an impression that they were required only because of the earthquakes that occur
in that region. Experience has shown that most wood-framed structures on the West
Coast are governed by wind forces which are larger than seismic forces. Because of this
misconception, it appears that a large number of wood-framed structures in many other
regions of the country are apparently erected without thought as to how they are to be
braced against wind forces. Another problem noted in ATC 7-1 6 was the lack of provi-
sion of complete load paths and detailing. Engineering has become a highly competi-
tive business. ATC 7-1 noted, “Nothing is more discouraging to the conscientious
engineer endeavoring to deal with lateral forces with all the detailing requirements on
diaphragms and shear walls than to contemplate the absence of attention paid by some
of his fellow engineers to the most basic shear transfer problems. It is a sobering experi-
ence to see structural plans for a wood-framed apartment complex without a single
wood-framing detail and to realize that you were not given the job because your pro-
posed fee was too high.” It is hoped that the information provided in this chapter and
the remainder of this topic will provide clarity to the importance of complete load paths
and designs.
The diaphragm and shear wall layout shown in Fig. 1.1 is a good example of
structures currently being designed and built. The code and standards definitions
and sections just presented should be carefully reviewed for applicability to each
irregularity discussed for this structure. In the transverse direction, two diaphragms
exist. Diaphragm 1 is supported by the first-floor shear walls along grid line 1 and at
grid line 6. Diaphragm 2 is supported by the shear walls located at grid line 6 and at
grid line 7. Diaphragm 1 has multiple discontinuities and irregularities within the span
that must be resolved. Starting at grid line 1A, it can be seen that a two-story entry con-
dition exists, which typically occurs in many office or shopping center complexes. The
upper level is usually an architectural feature commonly referred to as a pop-up . The shear
walls at grid line 1A are two stories in height and support the pop-up roof. The walls at
grid line 2 and grid line B also support the pop-up roof but are discontinuous shear
walls because they are supported by the main roof and do not continue to the founda-
tion. The pop-up section should be designed as a second story that transfers its forces
as a concentrated load into the main diaphragm. The diaphragm sheathing and framing
is often omitted below the pop-up section at the main roof level. Diaphragm boundary
members are not allowed at the main diaphragm level at grid line A from 1 to 2 or at
line 1 from A to B, due to architectural constraints. This condition creates a horizontal
offset in the roof diaphragm in the transverse and longitudinal directions. The offset
disrupts the diaphragm chords, creating a notched diaphragm effect. Because of the
offset, the question arises as to how to provide continuity in the chord members
and transfer its disrupted force across the offset. It also raises the question of how to