In wood-framed building design and construction, many of the old rules of thumb no longer apply. Residential and light commercial buildings are becoming larger and more complex, yet framing practices often have not advanced accordingly. As a result, structural engineers increasingly must help architects and contractors so that their designs meet code, and provide additional details on framing member placement and connections for crews to use in the field.

Wall systems, in particular, provide a number of framing challenges in modern building designs. For stick-built construction, engineered wood products can help solve these challenges in a cost-effective and easy-to-construct manner. Key points to keep in mind when designing with two common engineered wood types are discussed below, along with wood framing design tools available to engineers.

Designers can use engineered wood in many residential and light commercial construction projects. Photo: iLevel by Weyerhaeuser

Greater demands on walls
A primary change in load-bearing walls is that they frequently are no longer monolithic. Gone are the days of my grandfather, who after laying the block and framing the rectangular ranch house floors himself, used the floor deck to sketch the house “designs” in a matter of minutes — each of which simply included a couple of doors and a handful of windows. Continuous, long walls without varied facades and few windows allowed for greater freedom in sizing and placing structural elements, often providing all of the necessary load paths without any additional considerations.

Today’s buildings, however, tend to have more open floor plans, higher ceilings, walls with one or more step-backs, and extensive window and door openings. The result is inviting and aesthetically pleasing buildings, but the architectural plans do not allow as much space for structural members.

Higher-end multi-family structures, condominiums, lodging facilities, and even residential homes provide a clear example of this trend. Many incorporate walls taller than 10 feet — as found in two-story great rooms and common spaces — along with multiple tall windows to bring in natural light.

Commodity lumber often is not available in sufficient lengths, load capacities, or quality to frame such walls. Adding steel columns to the wood framing can support the gravity and lateral loads, but increases the design complexity since wood and steel respond differently under varying environmental conditions. Steel also has different attachment details, which may not be familiar to the residential or light commercial contractor.

An alternative in some wall configurations is to increase the wall thickness using 2x6 or 2x8 framing. Because interior dimensions, window jam sizes, and other architectural details must be changed with deeper walls, contractors may resist this.

Engineered wood products provide a way around these challenges in many wall designs. They allow for wood to be used throughout the wall without mixing classes of materials and provide high strength in conventional 2x4 walls, or in thicker walls, as needed. With few exceptions, they can accommodate the necessary vertical and lateral loads placed on residential and light commercial structures.

Engineered wood product attributes
Two common types of engineered wood products used in wall framing are laminated strand lumber (LSL) and parallel strand lumber (PSL). Classified as structural composite lumber (SCL), these materials resist bowing, twisting, and shrinking; are consistently straight; and are available in lengths up to 64 feet, depending on the product. Members include studs, columns, headers, and beams. Both LSL and PSL provide predictable performance, and each has notably greater shear and moment capacities than commodity lumber. Other engineered wood products are available for wall framing, including laminated veneer lumber (LVL) headers and glulam columns. Consult a wood products manufacturer or building material dealer for information on these and other materials’ performance characteristics.

LSL and PSL are manufactured from wood strands bonded together with structural adhesives. The strand size and alignment and overall composition of PSL make it stiffer than LSL. For example, the modulus of elasticity (E x 106 psi) of available members includes:

• LSL studs: 1.3E, 1.5E, 1.55E, and 1.6E;
• LSL headers and beams: 1.3E and 1.55E;
• PSL columns and posts: 1.8E; and
• PSL headers and beams: 1.8E and 2.0E.

Values may vary by manufacturer, so consult product literature for specific details and sizes.

LSL provides high shear capacities in shallow headers — as much as 400 psi — allowing a 3.5-inch-wide header to support high design loads and fit into a 2x4 wall. This can facilitate placement of girder trusses over the header, where shear forces are the controlling design factor.

PSL works well for larger headers and taller wall columns, especially where high lateral stiffness is required. Its high strength also enables engineers to rotate PSL columns 90 degrees to a plank orientation within walls, providing high load capacities while keeping wall thicknesses to a minimum.

Designing with LSL and PSL
When using LSL and PSL, the design process remains unchanged: Determine specific design considerations for the project and then select the material and size best suited for the application. Two significant design considerations — site geography and finish materials — lend themselves to using SCL in a variety of applications, including tall wall framing and as lateral bracing. An added benefit of using SCL in these applications is the familiarity contractors have with wood products and connections.

Prefabricated LSL shear panels accommodate high lateral loads in narrow wall segments. Combining engineered wood and specialty lumber can help balance performance and cost.

Site geography — As with any framing material, a key consideration when designing engineered wood walls is the environmental exposure. The design wind pressure and seismic design category will help determine the appropriate type and size of the various framing members. Manufacturers’ literature typically includes tables for maximum allowable loads on studs, columns, and headers, along with engineered design assumptions and design examples. In some cases, these details also may be built into framing design software — either as general values for broad classes of materials or as specific values for given product brands.

Finish material — As with marble or stone on floors, the interior and exterior wall cladding plays a key role in determining the code-specified deflection criteria to avoid cracking in the finish and prevent moisture intrusion. Exterior walls covered with plaster or stucco typically have a maximum allowed deflection of L/360, while walls with interior gypsum board only require an L/180 maximum deflection criteria (per 2009 IRC). Members supporting windows (mullions) typically require L/175.

These values may vary based on specifications from the finish-material provider and/or building code authority. For example, large windows with single panes of glass may require stiffer walls.

Tall walls — LSL and PSL members are engineered to meet code requirements for walls as tall as 30 feet. The use of long, continuous studs and columns helps avoid hinge points from stacked walls. Their straightness helps create smooth walls over great heights for clean interior and exterior finishes. The materials also have high bending strengths, an important factor for designing walls to resist out-of-plane wind loads.

In tall walls, LSL typically is used for studs and smaller columns and window mullions, while PSL is used for larger columns and window mullions. Either material can be used for headers, depending on the stiffness and load capacity requirements. In headers, the materials allow for shallower depths than conventional lumber, which can provide more room for tall windows with less framing material.

LSL and PSL members can help create straight and stiff walls as tall as 30 feet. Photo: iLevel by Weyerhaeuser

Lateral bracing — Numerous bracing methods for wood-framed structures are permitted in the IBC and IRC, including site-built braced wall panels and prefabricated shear panels (see the alternative materials section of the code). Prefabricated lateral-resisting elements are not limited to steel moment frames. Prefabricated shear panels, some made from LSL, can provide lateral resistance where other site-built methods are limited.

Given the typical modern aesthetic of multiple large openings within a wall, the required braced wall lengths or height-to-width ratios specified in code often are too restrictive to accommodate braced wall panels or site-built shear wall segments. As an alternative, prefabricated shear panels can be used in narrower wall sections while resisting high in-plane lateral loads. For example, an 18-inch-wide by 9-foot-tall prefabricated LSL shear brace can be used to resist 2,090 pounds of shear in wind design or 1,905 pounds of shear in seismic design.

LSL shear braces are available in heights up to 20 feet, are trimmable to the proper height in the field, and coordinate well with the remainder of the wood structure. These features combine to help meet specification in a variety of projects and applications.

Wood and fasteners — One other consideration when specifying LSL and PSL in any application is to remember that the products are wood and have similar characteristics to other wood-framing members. For example, during construction the materials should be protected from excess moisture to avoid swelling, and fasteners should not be placed too close together or near the edges to reduce the risk of splitting. Fortunately these concepts are the same as conventional lumber, so field crews do not need to learn new methods.

Framing design software
To help avoid tedious hand calculations, a range of software is available for wall designs. This includes programs that can address the entire structure and track loads from the ridge to the sill plate, as well as tools for sizing individual members.

Engineers can use single-member sizing software to streamline the design of vertical and horizontal wall-framing members, including freestanding posts, studs and columns embedded in wall systems, and headers. Some programs have capabilities to account for seismic and wind loads and include design details for engineered wood products and dimension lumber. Single-member sizing software is available from a number of vendors, and some framing product manufacturers offer programs available for free download.

Conclusion
As project teams operate under increasingly limited budgets, wood framing is receiving additional attention. In many cases, engineered wood products provide the structural capabilities necessary for both residential and light commercial construction, and can integrate with dimension lumber to balance performance and cost. An understanding of engineered wood’s capacities can help engineers provide architects and contractors with the assurance that their designs can meet both functional and aesthetic requirements for the overall building.

Jason O. Shumaker, P.E., is a structural frame engineer for iLevel by Weyerhaeuser. He provides technical support to design professionals, contractors, and code officials on engineered wood products and structural frame design software for residential, multi-family, and light commercial buildings. For more information, visit www.ilevel.com.