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

Blocks. The nails from the diaphragm sheathing into the blocks transfer a small amount of lateral force from the diaphragm into each block. How the force gets from each block to the shear wall depends on the direction in which the force acts. If the diaphragm force is pushing toward the shear wall from the row of blocks (from right to left), the collector is acting in compression and the steel strap is not engaged at all. Starting with the block farthest from the shear wall, each block pushes against the next in the direction of the shear wall. The second farthest block from the shear wall has one other block pushing against it; the next block has two blocks pushing against it, and so on. By the time we reach the shear wall, the last block in line pushes against the shear wall with the force collected from the whole row of blocks.

Strap. When the force in Figure 3 acts in the opposite direction, each block transfers a small amount of force to the strap. The strap collects the force from all the blocks and delivers it to the shear wall.

You can nail the strap either directly to the blocks or through the sheathing into the blocks. Figure 4 shows a strap nailed to the top of the blocks before the floor sheathing was installed.

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Figure 4.This strap was installed from the top before the floor sheathing was nailed down. The carpenter routed out the tops of the framing members to avoid a bulge in the carpeted floor above.

The carpenter routed a 1/8-inch slot for the strap so it would not create a lump in the sheathing. Note that this requires nailing the strap to the blocking, then nailing the sheathing to the blocking. If a strap is installed on top of the sheathing, as in Figure 5, the same nails can connect both the strap and the sheathing to the blocks. Remember that the blocks are important not only because they provide backing for the strap but also because they carry the compression forces when the collector acts in compression.

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Figure 5.This strap was nailed to blocking below through the floor sheathing.

Figure 6 shows a collector assembled from wood I-joist blocks. The strap nailed along the bottom of the blocks collects the force from each block and carries it to a shear wall just outside the photo.

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Figure 6.This collector was assembled from wood I-joist blocks and strapping. Ideally, the ends of the I-joist block webs would bear solidly against web fillers in the floor joists.

Note that because there are no web stiffeners in the I-joists, the blocks bear solidly only at the joist flanges and have gaps at the webs. When the sheathing exerts a force in one direction at the top of the block and the strap exerts a force in the opposite direction at the bottom, the block will want to roll. Connecting the ends of the blocks to the joists will prevent this. For a relatively small collector such as this one, I would usually consider that the blocks are held tightly enough on three sides so that they will not tend to roll. But if this collector was designed to carry a large load, the blocks would need to be toe-nailed to web stiffeners installed on both sides of every I-joist. Obviously, compared with using solid lumber framing, this becomes very labor intensive.

Use only as many nails as needed. Believe it or not and contrary to common advice, you don't usually have to fill every nail hole in a strap used for a collector. Remember that what you're doing is transferring forces from the floor sheathing into the strap and then to the shear wall. Look at Figure 6 again. Notice that the strap has about ten nails connecting it to each block — obviously installed by a carpenter intent on filling the holes. Yet how many nails do you think connect the sheathing to each block? If only three or four nails connect the sheathing to the block, we certainly don't need more than three or four nails from the block to the strap. Too many nails may actually split the wood member, which would defeat the purpose of the strap. Some inspectors may insist that you nail all the holes in a collector strap. If you want to push your luck, you could try to explain to the inspector why straps do not always need complete nailing. Otherwise, do what the inspector tells you, trying your best not to split the wood.

The strap in Figure 5 was designed and inspected by a structural engineer. It has just the right number of nails to collect the forces along 45 feet of diaphragm and deliver them to a shear wall.

Follow the Load Path

The collector provides the load path to the shear wall. But the forces we want to collect are in the floor or roof diaphragm. So it is critical to nail the sheathing to the collector truss or beam, or to the row of blocks acting as the collector. If the collector member does not fall on the regular framing layout, it's possible that nailing the sheathing to the collector will get overlooked and there will be a gap in the load path.

Figure 7 shows a strap that will connect to a roof truss. Because none of the roof trusses in the regular 24-inch on-center layout fell in line with the shear wall, the builder had to add an extra truss to pick up the strap. In a case like this, the carpenter nailing off the sheathing must be careful to nail the roof sheathing to that truss to complete the load path. The plans should clearly note this additional nailing requirement. Similarly, when collectors run perpendicular to the framing, it is easy to overlook nailing the sheathing to the row of blocks unless the strap happens to land on top of the sheathing to remind you. Again, the plans should show these connections.

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Figure 7.This strap, which runs from the top of a shear wall, will connect to the bottom of a roof truss above. Note that none of the trusses on the regular layout fell in line with the shear wall, so the builder had to add an extra off-layout truss to catch the strap. The roof sheathing will also have to be solidly nailed to this truss so that it picks up lateral forces in the roof diaphragm.

The photos in Figure 8 show a logical framing sequence. First, the straps get nailed to the top plates of adjoining support walls. Then, after the roof trusses are installed, the straps get nailed to the bottom of the girder truss and to the blocking between the common trusses. The girder truss, blocks, and straps all act as the collector. Nailing the main roof sheathing to the blocks completes the load path from the roof diaphragm to the shear wall.

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Figure 8.In this framing sequence, straps attached to the tops of shear walls are left hanging (top) until the installation of a blocked girder truss (bottom left), which will act as the collector for a section of roof. Finally, the straps are nailed to the bottom of the girder truss (bottom right).

Common Collector Pitfalls

Most problems with collectors arise when the framing members run perpendicular to the force we need to collect.

Many short straps do not equal one long strap. The line of blocks shown in Figure 9 will serve as a collector in compression, but will fail miserably in tension.

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Figure 9.These photos illustrate a common mistake in building collectors — using many short straps instead of one long one. Each short strap must carry the cumulative load to that point in the collector, meaning that the strap near the end of the collector is likely to fail.

At some point along the line, the collected forces will overwhelm the individual strap connections. Think of it in terms of a game of tug-of-war. If the members of one team are each holding a separate length of rope, they'll have a hard time beating a team whose members are combining their grips on a single length of rope (Figure 10). The person at the very front of the line on the separate-rope team would be the only person holding the rope that the other team is pulling on, and his grip would have to hold the combined force of his entire team.

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Figure 10.Think of a collector strap in terms of the game of tug-of-war. The team holding many short ropes will have a tough time beating the team whose force is combined in one continuous rope.

It's important for building designers not only to account for the tension force that a collector must carry, but also to detail how to build the collector. Problems like the grossly overloaded collector in Figure 9 can arise when the plans use vague notes like "drag-tie" or "block and strap in line with wall." Engineers shouldn't expect carpenters to build something that is not shown adequately on the plans.