Building a better shear wall

By Scott Soule, S.E., P.E.

Environmental issues, declining timber quality, and unpredictable lumber prices have caused builders and designers to explore alternative construction materials. Trus Joist's FrameWorks Building System of engineered lumber products effectively satisfies the demanding criteria placed on today's building products.

In addition to quality control in the manufacturing process and labor savings on job sites, this system is environmentally responsible as well. Of all the alternatives marketed in the United States to replace traditional wood framing materials, only engineered lumber is derived from a renewable resource.

Trus Joist relies primarily on secondgrowth and alternative timber species rather than controversial old-growth timber resources. These fast-growing trees usually have limited value in structural applications. However, the company's manufacturing technique converts as much as 75 percent of each log into high-value, engineered lumber, creating a structural product that is strong, straight, and consistent.

One of the newest products in the Boise, Idaho-based company's engineered wood arsenal is the TJ-Shear Panel, a prefabricated wood shear panel that offers a solution for structures needing a high-aspect-ratio shear wall.

Designed for applications in wind and seismic regions, the TJ-Shear Panel delivers better performance than highly variable, site-built alternatives.

All panel sizes use only two, 7/8-inch anchor bolts for the foundation connection, reducing installation time.Pre-drilled access holes and a concentric design provide ample room for electrical wiring and fixtures without field modification.

The panel was developed as an alternative for builders and engineers to comply with the changes in the 1997 Uniform Building Code, which placed restrictions on aspect ratio for site-built shear walls. These code changes were precipitated by observations of the performance of narrow, site-built walls during the 1994 Northridge earthquake in California. Specifically, high-aspect ratio walls were overly flexible and unreliable, resulting in increased structural damage. Prefabricated shear panels were developed to address these concerns.

Some of the specific, critical factors to be aware of in narrower, site-built walls include the following: Construction variables - How well is the wall put together? Are nails missing? Are nails hitting the studs correctly? Are nails overdriven? Are there gaps in the sheathing? Are construction tolerances adequate, such as plumb-cut studs and sheathing seams centered on framing members? A wall with these issues may fall far short of a designer's expectations of lateral performance. The TJShear Panel is prefabricated under strict quality control requirements, thus eliminating site-built performance variability.

Material variables - What is the moisture content of the lumber and will it shrink with time? Is there wane and twist in the lumber? Is it the correct species for the design? Are the sheathing products certified? Are you using the correct fasteners, nails, staples, or screws? The TJ-Shear Panel is manufactured using engineered lumber, which by its nature is straight, consistent, dimensionally stable, and dry. The sheathing material (or web) uses TJ-Performance Plus oriented strand board, the same highperformance material used as the web of the Silent Floor joist.

Hold down forces - As the wall gets narrower, the uplift forces increase; this makes conditions at the hold down very critical. Is the hold down attached tightly to the foundation? Is the connection to the stud (or rail) solid with little “slop”? Will there be crushing of the bottom plate, allowing undesirable movement? Does the hold down induce bending forces on the stud or rail? A small vertical movement at the base of a high-aspect ratio wall translates to a big horizontal movement at the top, resulting in a wall with greater flexibility than anticipated.

Because Trus Joist's TimberStrand LSL, the engineered wood used in the rails of the panel, is dimensionally stable, it is able to hold tight tolerances in the attachment of the specially designed hold down. This eliminates slop. The hold down is concentric so it does not induce bending moments in the rail, and the fit is very tight, resulting in little movement under high loading.

Bending versus shear - As a wall gets narrower, a larger percentage of the movement comes from bending forces rather than shear forces. A long wall will rack in the shape of a parallelogram, a short wall will bend more like a column.

Plastic racking absorbs a great deal more energy than elastic bending and, therefore, it is desirable in a shear wall to have more racking and less bending.

The TJ-Shear Panel uses a pinconnected frame to force the deflected shape into a parallelogram. This shape causes most load resistance and energy dissipation to be resisted by the web/rail interface, where it can be dealt with predictably.The result is a better performing shear wall, even at high-aspect ratios.

Trus Joist introduced the TJ-Shear Panel in 2002 to the California,Nevada, and Arizona markets.Today, the panel is available throughout the country, and it is gaining widespread acceptance as an effective alternative to code-prescribed braced panels.

In addition to meeting the needs of today's home designs, some of the benefits experienced by current users include the following:
• quick and simple installation;
• engineered wood provides consistent, predictable performance that resists bowing, twisting, and shrinking;
• compatibility with traditional wood frame construction;
• pre-drilled holes allow placement of utilities without field modification;
• the panel was specifically designed and optimized for high-aspect-ratio shear wall applications;
• an installation-ready, raised floor kit that includes preassembled components, simple instructions, and all the hardware needed for installation, including for a raised wood floor application for homes built with crawl spaces;
• a new anchor installation system that acts as a template for properly placing the hold-down anchors, and that also works as supplemental reinforcement to transfer the shear forces to the foundation without the need for site-fabricated alternatives; and
• ready acceptance by building and code officials.

Designers and builders throughout the country are finding the TJ-Shear Panel to be the right solution for their lateral design needs.

Scott Soule, S.E., P.E., is a corporate engineer for Trus Joist, A Weyerhaeuser Business in Boise, Idaho. He can be reached at soules@trusjoist.com. The firm is recognized as a leading manufacturer and marketer of engineered lumber products. Trus Joist operates 19 manufacturing plants in North America and Australia, and has sales offices worldwide.