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Builders nationwide are becoming increasingly familiar with shear walls, especially along the earthquake-prone West Coast and in the wake of recent hurricanes in the East. They're aware of the kinds of metal connectors involved, and they're accustomed to having the inspector check plywood nailing schedules and framing layouts.

But shear walls are just one element of earthquake- and wind-resistant construction. Less obvious, and somewhat more difficult to build, are the collectors. Also known as drag-struts or drag-ties, collectors gather the lateral earthquake loads from a large area of a building — a roof or floor diaphragm, for example — and deliver them to a structural element, such as a shear wall, that can resist the force. But unless the collectors are properly built, the shear walls will be ineffective.

The lateral forces from earthquakes or high winds are spread out over the entire area of the roof or floor diaphragm. And yet many contemporary custom homes have only small areas of shear walls compared with roof and floor areas. A collector gathers the force spread through the diaphragm and transfers, or "drags," it to the shear wall.

Tension and Compression Forces

For a collector to work, it must meet the following conditions:

• Forces must actually get to the collector.

• The collector must be continuous (or be composed of elements joined together to act as one continuous member).

• It must have tensile capacity.

• It must be able to resist compression.

Seismic forces cycle back and forth, which is why the collector is stressed alternately in compression and then in tension. To design a collector that will resist both tension and compression, engineers must consider that high winds may come from any direction.

The most common collector in a typical wood-frame house is a wall top plate. But with more complex house configurations, there's not always a wall plate available to act as a collector. In such a case, you may have to use another member for the collector — a truss or beam, for example — or you may need to assemble one from blocking and metal strapping.

Top Plates as Collectors

Look at the example in Figure 1, a 40-foot-long house with a long window wall along the eaves. Lateral forces transfer from the diaphragm sheathing in the roof to the eaves blocking, and from there into the top plate. To ensure a complete load path, you must nail the sheathing into the eaves (frieze) blocks, which in turn must be securely fastened to the top plate.

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Figure 1.The top plate above the windows in the front wall of this house must be able to collect the cumulative lateral force from the roof diaphragm — 120 pounds per foot in this example — and deliver it to the shear wall at the left end of the building. An ordinary double top plate, as commonly built, would not be up to the task.

In a typical house, these forces might range from 80 to 120 pounds per foot along the eaves. In this house, the designer left only 8 feet at the end of the window wall for the engineer to use as a shear wall. We have 32 feet of windows that can resist no shear, so the top plate must collect all the force in the 32 feet of roof diaphragm above the windows and drag it to the shear wall.

The diaphragm force in this example — 120 pounds per lineal foot of roof, acting parallel to the eaves wall — builds up in the collector as we get closer to the shear wall. At the right end of the house, the force is zero. Ten feet closer to the shear wall, the force is 1,200 pounds (10 feet x 120 pounds/foot = 1,200 pounds); another 10 feet closer, the force is 2,400 pounds. Finally, when we have collected all 32 feet worth of diaphragm force, we have a total of 3,840 pounds of force in the top plate. Note that we're showing tension force in the illustration, but if the wind was blowing in the opposite direction, or the earthquake forces were reversed, the collector would act in compression along the top plate.

Top plate collectors can typically carry the compression force, but they often need additional tension capacity. A single 2x4 top plate in a standard grade would not be able to carry 3,840 pounds of tension force. A double top plate might work if we reinforced any splices in the plates. As a minimum, the IBC (International Building Code), the IRC (International Residential Code), and the UBC (Uniform Building Code) all require eight 16-penny nails at lap splices. This requirement was new as of 1994 and essentially doubled the required nailing at splices. Even so, using allowable code values, this minimum nailing requirement would resist only about 1,000 pounds, assuming you used sinkers. Because this is far short of the 3,840 pounds we need in the example, we need to splice across the joints in the double plate with sheet metal straps to carry the tension force.

Other members as collectors. Trusses, joists, rafters, and other such continuous elements can easily serve as collectors. Figure 2 shows two instances where a shear wall is connected by a strap to a collector element in line with it. The strap provides tensile capacity.

To resist compression, the truss must bear against eaves blocking, which in turn must connect to the top of the shear wall.

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Figure 2.The strap in the bottom photo carries tension forces from the collector truss to the shear wall beyond (lower left in the photo). In the photo at top, straps running in both directions connect the wood I-joist second-floor framing to the shear walls below.

Building Collectors From Scratch

When you don't have a continuous member to act as a collector, you have to build one. While it's easy to reinforce the splices in a top plate, assembling a collector that runs perpendicular to framing members takes greater effort.

Figure 3 shows a collector assembled between second-floor framing by installing blocks tied together with a steel strap. In this case, the strap and blocks are collecting the force from the floor diaphragm and carrying it to a shear wall beyond.

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Figure 3.Blocking is a critical component of a collector when the framing runs perpendicular to the shear wall. The solid blocks in the photo handle compression force, while the strap collects tension force and carries it to a nearby shear wall, as shown here.