By Nathan J. Hoesly, PE, SE
The use of engineered wood products is an essential component of nearly all wood-framed buildings. This article will focus on two specific types of engineered wood products, structural composite lumber (SCL) and glue laminated (Glulam) timber framing. Understanding the intended uses and differences between various SCL products and glulam framing is essential for design professionals.
Structural composite lumber (SCL) is a term used to describe a family of engineered wood products created by layering wood veneers or strands and bonding them with moisture-
Laminated Strand Lumber (LSL) is manufactured from flaked wood strands and resembles oriented strand board (OSB) in appearance, though the strands are arranged parallel to the longitudinal axis of the member. Members are commonly fabricated in 1 ¼”, 1 ½”, 1 ¾” and 3 ½” widths, and in 9 ¼”-16” depths to match common i-joists. Stud options are available in equivalent 2×4, 2×6, and 2×8 sizes that are stronger, straighter, and (as needed) longer than sawn lumber. LSL is typically less expensive than other engineered wood beams.
Due its high allowable shear strength, LSL beams have capacity for larger penetrations than other engineered wood beam options. While not as strong as LVL or PSL beams, LSL is generally cheaper and are ideal for short spans. LSL is also ideal for use in rim conditions due to minimal shrinkage, cupping, and high fastener holding strength when used in highly loaded diaphragms or for shear transfer at plywood shear walls.
Parallel Strand Lumber (PSL) is manufactured from veneers laid into long, parallel strands and bonded together. PSL beams are primarily used in beam and header applications where high strength is required. Common PSL beam sizes are available in widths of 3 ½, 5 ¼” and 7”, and depths matching I-joists from 9 1/2” – 24” deep. PSL columns are also available in sizes comparable with sawn wood members from 4×4 to 8×8 in size.
PSL beams are generally more expensive than glulam, LSL, or LVL beams. PSL beams can be stained or finished where an aesthetically pleasing exposed application is desired.
Laminated Veneer Lumber (LVL) is a commonly available engineered product that is manufactured similarly to PSL. Available sizes, strengths, and stiffnesses are similar to PSL but are generally cheaper, making it a commonly specified beam type. A benefit to LVL is that it can be fabricated in narrower beam widths (1 ½, 1 ¾”), and multiple plys can be nail-laminated together to form a larger beam. This is especially beneficial in retrofit options where lifting a wide, heavy beam into place is cumbersome or infeasible. LVL stud and columns are available as well from some manufacturers.
Glued Laminated Timber (Glulam) is manufactured by face-bonding layers of kiln-dried timber members, typically 2×4 or 2×6 in size, together with waterproof adhesives to form timber section. Glulams are popular due to their engineered strength, versatility, availability, and cost. Typical stock beams widths are available in 3 1/8”, 3 ½”, 5 1/8”, 5 ½”, 6 ¾” widths and depths exceeding SCL beams. However, custom glulams can be fabricated in almost limitless widths, depths and profiles, giving glulam beams a distinct advantage over SCL beams in their versatility and architectural appeal. Glulams have a long history of being used beautifully in exposed, large open areas such as vaulted ceilings, churches, theatres and a vast array of other public spaces. Manufacturing processes for glulams allow for members to be cambered, curved, and fabricated in unique shapes, such as arches or as bridge members. Different appearance grades for exposed conditions may also be specified to increase architectural appeal.
For exterior or weather-exposed conditions, glulam beams are generally preferred over SCL beams. Weyerhaeuser, one of the few manufacturers of PSL in the U.S., has a Wolmanized PSL product that is approved for weather-exposed framing beam applications, but it is relatively expensive. Few other SCL treatment options exist. Alternatively, pressure treated or preservative treated options exist for glulam members. Additionally, several naturally durable species of glulam beams are produced in the U.S., including Alaskan Yellow Cedar and Port Orford Cedar, which provide green alternatives to chemical treatments.
Both SCL and glulam beams may be used where a fire-rated exposed member is required, subject to meeting the provisions of Chapter 16 of AWC’s National Design Specification® (NDS®) for Wood Construction. Typically, only wider beam sections will meet the required fire rating due to the depth of charring of any exposed face. This often eliminates the use of LSL, and glulams are usually preferred over LVL and PSL due to cost, appearance, or available beam sizes.
Design professionals should be knowledgeable about specific product availability and costs in their areas during design as this can help drive which types of engineered wood beams are specified. Although SCL and glulam beams can be used interchangeably at times, they also have unique advantages and limitations to be aware of.
Nathan Hoesly, PE, SE is an Associate of Nishkian Dean, a structural engineering consulting firm in Portland, Oregon
Wood is often seen in multi-family mid-rise buildings as the most economical construction material. While the purpose of joists, beams, and wood stud walls are easily understood in their role carrying the gravity loads of a building, the lateral load resisting elements and how they work can be more confusing.
Lateral Load Resisting Shear Walls
When lateral loads due to wind forces or a seismic event hit a building, the loads travel through the floor, collecting into and being resisted by wood shear walls. The top and bottom plates of the shear walls act as continuous collectors, moving the lateral loads from the diaphragm into the shear wall. Plywood sheathing, the nailing of the plywood to wood studs, and anchor bolts at the sill plate resist the shear forces. As the lateral loads move from a horizontal plane (i.e. the floor) to a vertical plane (i.e. the wall), the lateral loads also create vertical tension and compression forces, which are resisted by end posts and a hold-down system located at each end of the shear wall.
Figure 1: Components of a Shear Wall
At the beginning of a project, as unit layouts and floor plans are being set, it is important to ensure that the building will have an appropriate shear wall layout. In multi-family buildings, corridors usually afford a sufficient length of interior shear walls, but exterior shear walls must be carefully coordinated between the client, architect, and structural engineer. Façade features, window sizes, and window locations can all critically affect exterior shear wall designs.
Stacking Shear Walls
Structurally, it is always more efficient to stack shear walls from the top of the building to the foundation (or podium). This allows the components that resist the compressive and uplift forces to be continuous. When shear walls do not stack, the building code requires that components be designed for an increased load, and extra framing members and connections are required to transfer the loads.
Figure 2: Stacked Shear Walls
The shear wall design is not just determined by the number of shear walls, but also the length of each shear wall. As shear walls get shorter, the hold-down system gets loaded more heavily. The minimum length of shear wall permitted on a project depends on the floor-to-floor height – the taller a floor, the longer a shear wall must be. This aspect ratio is determined by taking the height (h) and dividing it by the shear wall length (bs). Shear walls cannot have an aspect ratio greater than 3.5 but as a rule of thumb, aspect ratios should be less than 2. Where the aspect ratio exceeds 2, the wall’s shear capacity is penalized. In other words, shear wall lengths should always aim to be greater than half the floor height.
Nishkian Chamberlain has extensive experience with multi-family construction and would be happy to help you find cost-effective solutions to your construction and development needs. Please do not hesitate to contact us at NCInfo@Nishkian.com, or give us a call at (310) 853-7180. You can also go to our Contact page to connect with any one of our offices in your region.
Figure 3: Aspect Ratio
The demand for affordable housing in urban areas is increasing as rent prices skyrocket. Many developers are now required to include a percentage of affordable housing units to obtain approvals for their new multi-family construction projects. Additionally, affordable housing projects that wish to access alternate funding sources are often required to meet various green building requirements.
Recently, Nishkian Chamberlain began work on a new affordable housing project in Ventura, California. The project is unique in that it is entirely comprised of affordable housing apartment units. In order to receive some state and federal funding, the project is required to meet several green building targets. One such target is the use of “Advanced Framing”.
In short, Advanced Framing is energy and material-efficient wood framing. Conventional wood framing, typically used for many years, includes many structural redundancies (double top plates, three-stud corners, multiple jack studs, double or triple headers, etc.). The goal of Advanced Framing is to eliminate unnecessary redundancies and achieve savings in material usage while also taking advantage of opportunities for increased energy efficiency.
There are various techniques commonly used in Advanced Framing. Which technique(s) are used is determined on a project-to-project basis. One technique applied to a recent project was the use of engineered wood floor joists. Using engineered wood floor joists allowed larger joists spans and wider joist spacing, leading to savings in materials, increased construction efficiency, and reduced construction costs. It is critical for the structural engineer on the project to coordinate joist depths, which may be larger due to the increased spans and spacing, with the architect and MEP early on in the project to minimize conflicts and rework later on.
Additionally, for this project, the increased floor joist spacing affected the design of the floor sheathing. A larger span rating was required, and serviceability characteristics such as floor vibration were investigated. It was recommended to use a thicker floor sheathing than what we use for Conventional Framing to achieve the required span rating, reduce vibrations and increase the perceived stiffness of the floor.
In addition to using engineered wood floor joists, other Advanced Framing techniques include:
These techniques can be mixed and matched to accommodate the unique aspects of each project while still achieving the targeted green building requirements.
In conclusion, the demand for affordable housing is increasing, especially in high density urban areas where housing is in short supply and rents are high. Affordable housing projects often target green building requirements such as the use of Advanced Framing techniques to obtain State and Federal funding. Advanced Framing techniques can be used individually or in combination to achieve the desired green building criteria.
If you are interested in finding out how using Advanced Framing techniques can benefit your next project, Nishkian Chamberlain has significant experience in this technique and would be happy to discuss your construction and development needs. You may contact Craig Chamberlain at CChamberlain@nishkian.com or (310) 853-7180.
Edwin T. Dean, PE, SE
Wood frame is an economical construction type and if properly detailed durable and fire-safe. The level of fire resistance required of a building is established by the building code and is a function of the size, use and occupancy of the building. The fire rating is driven by the need to provide ample time for occupants to exit the facility, retain structural stability long enough for fire-fighting personnel to combat the fire and for the protection of the contents of the building and adjacent structures.
There is a new wave of “baby boomers” who are looking for housing solutions that allow them to age in place while maintaining an active lifestyle in Montana. According to the current data from the Population Reference Bureau report and the U.S. Census Bureau, the youngest of the 76 million boomers have begun turning 50 in 2014 and 10,000 boomers per day will turn 65 from now through 2030. These demographics alone are driving the increased demand for 55-plus communities. The concept has gotten an even bigger boost in recent years as more boomers find themselves sitting on an empty nest in an active housing market.
Greenhouse Village is a new 55-plus community bringing 10 single-family condominiums that are approximately 2,300 square feet in size to the Southside neighborhood in Bozeman. Prugh & Lenon Architects took on the role as lead designers and project managers for this redevelopment project. Nishkian Monks served as the structural design consultant providing development and design of the structural system, construction administration, and special inspection of the building design collaborating with general contractor Tim Dean Construction.
The Pierce is a new landmark structure bringing 232 luxury apartment and 8,730 square feet of retail space to San Jose’s South of First Area (SoFA) district. Project owner, Sares Regis, together with Steinberg Architects envision that this project will help to revitalize the area and become the gateway to downtown San Jose. In 2014, The Silicon Valley Business Journal awarded The Pierce “Best Mixed-Use Project.”
Standing seven stories tall – five stories of wood framing over a two-story concrete podium – The Pierce is an exceptional project from an engineering standpoint, as well. In order to maximize exterior window openings and allow for offset exterior walls, the wood frame portion of the building was designed using rigid diaphragm analysis. Traditional wood frame design uses flexible diaphragm analysis where lateral forces are distributed evenly between the short exterior shear walls and the long interior corridor walls. Using a rigid diaphragm approach, however, Nishkian Menninger was able to eliminate the need for exterior shear walls, instead distributing the lateral forces to the longer and more effective corridor and unit separation walls. Having been approved by the San Jose building department, the Nishkian offices are now applying this technique to other multi-unit residential buildings throughout the Bay Area.
Located at the corner of Cottonwood and Fallon in Bozeman, the West Edge Condominiums are a development of the Human Resource Development Council of District IX, Inc. or HRDC, a non-profit community action agency, as part of the Neighborhood Stabilization Program funded by the Montana Department of Commerce. Funds were awarded to the HRDC in partnership with Gallatin County to purchase foreclosed properties, make necessary improvements, and construct additional units. The units are then sold to households living and working in the community who were formerly priced out of the market.
Cross-laminated timber (CLT) continues to receive more attention nationally and locally as an innovative and economical solution for utilizing wood construction in taller buildings. While multi-story CLT buildings have been constructed in Europe, Canada, and Australia over the past 20 years, its use as a primary building material in the United States is still in the early stages of development.
CLT may also be classified as mass timber construction, which is building construction that uses large prefabricated wood panel members such as CLT and engineered wood for wall, floor, and roof construction. Glulam material may also be used in beam and column applications.
Construction was recently completed on Fire Station 76 for Multnomah County Rural Fire Protection District No. 10, located in Gresham, OR. Nishkian Dean participated in the project as the structural engineer of record, working directly with Hennebery Eddy Architects and Bremik Construction. The station is operated by The City of Gresham fire department.
The new 11,600 square foot facility replaces a smaller facility located across the street near 302nd Avenue and SE Dodge Park Blvd. A new station was required to serve the increasing demands caused by growth of the surrounding areas.
Basics of Lateral Load Resisting Systems in Wood Frame Buildings
Buildings resist wind and seismic forces through a combination of horizontal and vertical lateral force resisting systems. The lateral forces are first transferred through the horizontal elements at each floor which then act as deep beams to distribute the loads to the vertical elements. In wood frame construction, the horizontal elements are typically floor and roof diaphragms consisting of plywood sheathing nailed to wood framing members, such as joists, beams, and blocking. The vertical elements are typically shear walls consisting of plywood sheathing nailed to studs and blocking. Shear walls are anchored to the building foundation or an elevated concrete podium slab, both of which are designed to resist lateral loads and uplift forces.