Laminated strand lumber helps meet special demands of tall wood buildings

TTtuE MULTTFAMtLv industry is I expected to remain strong through 2013 and into 2014, according to the National Association of Home Builders. The organization has forecast 299,000 multifamily housing starts for 2Ol3, a 22Vo improvement over the previous year.

As builders and developers seek cost-effective ways to meet the demand, many are turning to wood framing for four- and five-story buildings-and even up to six stories in Canada. However, wood shrinkage can be an issue. Laminated strand lumber (LSL) provides an effective means of dealing with shrinkage in wall plates.

Wood framing in taller buildings offers a number of benefits, from product availability to strength and durability. Plus, the reduced material and labor cost of building with wood, when compared to steel or concrete, means a cost-competitive option.

But wood framing isn't without its challenges, especially when it comes to shrinkage. Shrinkage occurs in solid-sawn lumber as the wood's moisture content dries from manufacturing conditions to equilibrium. The percent of shrinkage across the grain of lumber is roughly 40 times greater than along the length. Because of this, the shrinkage in a building caused by wall plates will be significantly greater than that caused by the studs.

With moisture content of up to I9Vo, traditional surfaced-dry lumber will shrink as it reaches its moisture equilibrium in the finished buildingtypically somewhere between 8Vo and l2%o. As the lumber dries, it's not uncommon to see a quarter of an inch of shrinkage per story in a typical multifamily building. For a four- or fivestory building, that's more than an inch of shrinkage, with the wall plates alone accounting for nearly a third of the total, according to a technical report from Western Wood Products Association.

Wall shrinkage in multifamily developments can affect both the shear wall performance and structural capability of the building. Additionally, shrinkage can lead to moisture infiltration. Window seals can break as framing misaligns with brick and stucco veneer. Plumbing components running through the building's framing members can also be damaged if those members shrink.

Precise compensation for building movement with flashing and detailing is difficult and expensive because of lumber's natural variability in moisture content and other properties. Little can be done to fix building shrinkage retroactively, so builders and developers planning to construct tall wood buildings need a solution for mitigating shrinkage on the front end. Using LSL for horizontal framing members in the wall plate system can help.

LSL is created from a mixture of hardwoods. The raw logs are debarked, cut into strands, blended with waterproof adhesives that contain no formaldehyde, and formed into dense mats. A massive steam injection press then uses steam and pressure to convert the mats into billets.

The final product typically has moisture content around J7o to 87o, less than the equalized framing in most enclosed structures. Wall plate shrinkage is then minimal or nonexistent because the moisture content of LSL is so close to the moisture equilibrium of the building.

Further mitigation of wall shrinkage along the length of the grain can be obtained with LSL in stud and rim board applications. Plus, LSL studs are strong, straight and true, helping reduce product waste and labor time when used instead of traditional studs. Due to its strength and consistency, LSL rim board is ideal for supporting both high vertical and lateral loads in multi-story construction and is more dimensional stable than lumber.

The benefits of LSL don't end there. The manufacturing process for LSL provides a uniform cure and even density gradient across the thickness of the product, adding to its strength and making it more resistant to warping, shrinking and twisting.

The strongest LSL on the market is rated up to 1.758, making it an ideal alternative to traditional lumber fbr beams, headers and tall wall stud applications. LSL products can be a direct replacement for 2x4,2x6,2x8, and 2xl0 lumber products. LSL can also serve as a cost-competitive alternative to LVL and glulam beams.

LSL has a number of additional application benefits, including:

Roof Framing: LSL is ideal for complex and contemporary roofs or vaulted ceilings because it stays straight.

Door & Window Headers: Due to twisting and warping, LSL can reduce

Building-Products.com resistance to likelihood of drywall cracking around windows or framing, especially in sliding glass door and specialty window applications, and shallow headers allow for transoms and arched windows.

Tall Watl Framing: LSL is excellent for framing in walls over l0 ft. high, as it reduces movement that could lead to stress cracks and leaking seals.

Columns: LSL provides a solid, continuous load path for hold-downs in high-wind areas.

Truss Chords: Well suited for attic or girder applications, LSL has excellent plate and nail-holding capabilities.

Stair Stringers: Because LSL resists shrinking and twisting, it reduces the likelihood of squeaks.

To learn more about LSL and to view an infographic on the effects of wall shrinkage, visit www.whatislsl.com.

Thermo-treated wood

The Most Stable & Beautiful Hardwood Decking Available

Environmental ly friendly manufacturing process

Made from American hardwoods o Jf [!lru (Joint End Matched) Machined End Joints

Thermal modification stabilizes the wood, making it 7 times less likely to warp, check, bow, cup, twist, expand or contract

Class I durability rating

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By Carla Waldemar