Timber!

An Olivieri Brothers White Paper, Spring 2016

 

By:  Kevin Walsh, NCARB, LEED GA

 

 

Wood Construction: Emerging Use and Application

 

If you’ve driven by a construction site for a large building in the past twenty years, chances are you did not see much wood construction on site. In fact, you probably only saw wood being used to help construction crews erect a different material such as concrete or brick. The reason for this is that wood has fallen victim to many false perceptions as a structural material in buildings outside of the residential industry. Sure, you’ve seen the occasional 3-4 story hotel or apartment building stick-built (framed with dimensional lumber), but probably not much else.

 

That is, however, until now. Recent trends have wood construction emerging as a desirable material in many applications, including some low-to-mid-rise buildings. Concrete and steel as the primary structural elements will always have validity in many, if not most, commercial buildings, and will always be highly recommended (if not required) in specific building typologies. This new wave of interest in wood construction stems from costs savings, versatility, sustainability, evolving lateral (seismic and wind) load resistance research and testing, speed of construction, and new design guidelines and practices to meet fire code requirements.

 

Construction Costs

 

When designing a building, the architect and owner typically have a few choices when it comes to structural materials. The end material choice is selected either due to building code requirements, structural performance, or construction costs. When permitted by building code, wood construction can offer significant costs savings over structural steel and concrete to an extent; at some point, the structural requirements, particularly those in large buildings, may require too much material to perform under high load conditions for wood to be considered the most economical choice.

Wood is typically less expensive due to the availability of locally sourced material and ease of construction. Carpenters are incredibly efficient in erecting wood construction structures, and modern techniques allow for quick construction, therefore, reducing labor costs.

 

Versatility

 

When most people think of wood in construction, they think of the 2x6 stud walls in their house, or maybe the maple wood floor in their local recreation center gymnasium. Both examples are great uses for wood, but wood can be utilized for far more. Wood members can be laminated together to form what is known as glulam (glued laminated timbers) and these wood members can achieve strength and spans comparable to steel, while portraying the warmth and aesthetics of wood. Glulam members are typically straight beams or arches. For example, look at the Jackson Hole Airport or the Richmond Olympic Oval from the 2010 Olympic Games in Vancouver. Another innovative wood product is the Structural Insulated Panel, or SIPs. A typical SIP consists of closed cell insulation sandwiched by two layers of standard OSB plywood using high strength adhesives. These are panels are prefabricated on a project-by-project basis. Advantages of using SIPs include significant increases in R-value due to the continuous closed cell insulation, reduced thermal bridging, and quick erection time. SIPs as a structural, load bearing system have the potential to be four stories in some applications.

 

Considering glulam members, conventional framing, trusses, SIPs, cross laminated timbers, timber framing, and others, wood construction can respond to a large variety of needs and challenges.

 

Sustainability

 

Both steel and concrete are non-renewable materials that require a great deal of energy to produce. Steel is recyclable; in fact, most steel today is not virgin steel but a recycled product. Regardless, the amount of embodied energy in the recycling or production of steel and concrete is far greater than the energy used to harvest, mill, kiln dry, and transport wood. Advancements in sustainable forestry regulations and practices have secured the availability of the material. Many green accreditation organizations focus on reducing a building carbon footprint and wood construction offers that opportunity.

 

Lateral Stability

 

In some areas such as California, Alaska, and believe it or not, southeastern Missouri, seismic load is a critical factor in building design. Research institutes and university around the world have studied wood construction and its performance under earthquake forces. The result is that wood performed well under such loads. Wood is lightweight, and coincidentally, the earthquake forces are proportional to a building’s mass. The lighter material and high-strength-to-weight ratio of wood allows it to perform well in a seismic situation. Also, wood construction typically has many connections which allows for some ductility (ability to move and flex), which helps dissipate the forces. This load transfer, along with redundant members, is also the reason wood construction responds well to wind forces.

 

Fire Codes

 

One of the biggest concerns with wood construction is its natural ability to burn. However, due to modern sprinkler systems and fire rated assemblies, fire resistance is achievable in many applications. Exposed timbers, meaning wood members at least 6-8 inches in each direction, can achieve a 1-hr fire resistive rating in many applications. This is because the exterior face of a heavy timber beam or column will char, which reduces the rate at which the member burns and lengthens the amount of time the timber member can perform structurally. Fire resistance can also be achieved with Type X or C gypsum board applied to wood structure.

Conclusion

 

Modern techniques, regulation, and strategies have allowed wood construction to emerge as a viable construction material for many applications, not just the residential sector.  Wood construction can be restricted by occupancy use of the building and building size (height, stories, and/or area) in many building codes, therefore, be sure to discuss any challenges and opportunities associated with wood construction with your area professional.

 

 

References:

 

Softwood Lumber Board

http://www.rethinkwood.com/

www.softwoodlumberboard.org

 

APA – The Engineered Wood Association

www.apawood.org

 

Woodworks – Wood Products council

www.woodworks.org

 

 

 

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