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Lateral-Force Collectors for Seismic and Wind-Resistant Framing, continued

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.

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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.

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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 members.

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.

House A

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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).

House B

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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 wall.

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).