Lateral-Force Collectors for Seismic and Wind-Resistant
Gaps in rows of blocks can lead to
failure. In contrast to the collector in Figure 9,
Figure 11 shows a collector that will fail in compression. The
gap in the row of blocks will need to close before the
collector can deliver much force to the shear wall to the right
(outside the photo). You can bet this gap would not close
gently during an earthquake. While the floor sheathing would
handle some level of lateral loads, the repeated back-and-forth
movement of the house would cause this gap to close and open
many times, slamming the separate building segments against
each other with thousands of pounds of force.
Figure 11.The missing block in this collector
between the girder and the I-joist to its left could prove
costly in an earthquake: The back-and-forth cycling of the
quake will smash the two members together, folding and
stretching the strap below until it fails.
Furthermore, the strap spanning the gap will buckle and then
straighten out each time the gap closes and opens. After
several cycles, the strap will fatigue and break, which could
allow the gap to open up. Then the two building sections might
separate completely or just bash each other to bits.
Many commercial buildings collapsed or suffered major damage
in the 1994 Northridge earthquake due to collector failures.
Those failures prompted code changes that reflect the
importance of collectors. When sizing collectors in such
buildings, designers must now use an additional safety factor
of about 175% of what the codes previously required. These
changes do not yet affect buildings with wood-framed shear
walls, but they do indicate an awareness of the importance of
collectors to structural safety.
Just because you can bend straps
doesn't mean you should. For a steel strap tie to work
effectively, it must be installed without kinks, bends, or
twists. The strap shown in Figure 12 has lost some of its
strength because of the twist pounded into it.
Figure 12.Straps should not be bent: The kink in
this strap weakens it.
It will also tend to straighten out when it's put in tension,
which in this case could displace the top plate or the joist
that it connects to, as well as rip nails out of those
Watch out for plan changes.
Figure 13 shows what can happen when a stock plan gets changed
without accounting for the lateral load path. These two houses
are located in the same tract and have identical floor plans.
Presumably to make the houses look different, the front-facing
gable of House A was changed to a hip in House B.
Figure 13.These photos were taken of
two nearly identical houses in the same tract and
illustrate how a change in the engineered plans can
render a collector useless. In House A, which was
built according to the stamped plans, the section
of roof in the small front-facing gable is tied by
a strap running from a shear wall at the corner of
the house to a collector truss running in the same
direction (see plan).
In House B, the builder
chose to modify the small gable by making it a hip
roof. That resulted in hip trusses running
perpendicular to the shear wall. The carpenters
made an effort to connect the strap to something
solid by adding some blocking between the first two
hip trusses. However, since the blocking doesn't
extend to the roof, where it could connect to the
sheathing, it will do a poor job of transferring
diaphragm forces from the roof to the shear
The effect was that the same strap running from the shear
wall to the collector truss in House A runs to nowhere in House
B. The carpenters had nothing to connect the strap to in House
B, so they added some blocks perpendicular to the bottom chords
of the first two trusses. But because the trusses have almost
no strength in that direction, there's no way for the forces
from the roof diaphragm to even get to the strap and the shear
wall beyond. Correcting this problem would involve installing
sheathed frames between the trusses from the bottom chord to
the roof sheathing, then nailing the strap along the entire row
of frames at the bottom, and nailing the roof sheathing to the
frames at the top. As installed, the blocks shown in House B
will not collect any force except from the ceiling; during an
earthquake or windstorm, they would do no more than rip out a
small area of ceiling drywall.
This is a common problem in tract home construction. Too often
the plans get changed by the architect, owner, or truss
manufacturer after they have left the engineer's office. That
is one reason hiring the design engineer to observe
construction progress is recommended almost universally by the
experts who examine destruction after hurricanes or
earthquakes.Thor Matteson, S.E.,is a structural engineer in Mariposa,
Calif. This article was adapted with permission fromThe Wood Framed Shear-Wall
Construction Guide, available at
www.shearwalls.com or through the
International Code Council (888/699-0541).