ASD/LRFD Manual - American Wood Council

98 M14: SHEAR WALLS AND DIAPHRAGMS
M14.4 Diaphragms
Subdiaphragms
The subdiaphragm (also known as the mini-diaphragm)
concept has been recognized and extensively
used to provide a method of meeting wall attachment and
continuous cross-tie code requirements while minimizing
the number and length of ties required to achieve continuity
between chords. A formal definition of a subdiaphragm
can be found in SDPWS, “SUBDIAPHRAGM portion of
a larger wood diaphragm designed to anchor and transfer
local forces to primary diaphragm struts and the main
diaphragm.”
In practice, the subdiaphragm approach is used to
concentrate and transfer local lateral forces to the main
structural members that support vertical loads. The subdiaphragm
approach is often an economical solution to code
required cross-ties for the following reasons:
• Main structural members are already present
• Main structural members generally span the
full length and width of the buildings with few
connectors.
• Main structural members are large enough to
easily accommodate loads.
• Main structural members are large enough to
allow “room” for requisite connections.
Each subdiaphragm must meet all applicable diaphragm
requirements provided in the applicable building
code. As such, each subdiaphragm must have chords,
continuous tension ties, and sufficient sheathing thickness
and attachment to transfer shear stresses generated within
the diaphragm sheathing by the subdiaphragm. In addition,
building codes may contain aspect ratios that are specific
to subdiaphragms.
The subdiaphragm is actually the same structure as
the main roof diaphragm, thus the subdiaphragm utilizes
the same roof sheathing to transfer shear stresses as the
main diaphragm. As such, sheathing nailing and thickness
requirements of the roof diaphragm may not be sufficient
for the subdiaphragm requirements. In this case, the subdiaphragm
requirements would control and dictate roof
sheathing and fastening requirements in subdiaphragm
locations. Fortunately, the portion of the main diaphragm
that is utilized as a subdiaphragm is a choice left to the
designer; thus dimensions of the subdiaphragm can be
chosen to minimize potential discontinuities in sheathing
thicknesses or nail schedules. Similarly, the roof diaphragm
requirements may be more stringent than those
for the subdiaphragm.
American Wood Council