Rethinking Wood as a Material of Choice

Costs less, delivers more
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Sponsored by reThink Wood
Layne Evans

In addition to material costs, an aggressive construction schedule was one of the main drivers for the choice of wood in Emory Point, a mixed-use project near Emory University in Atlanta, Georgia. Designed by Cooper Carry and The Preston Partnership, the 442-unit project includes one five-story wood-frame building over slab-on-grade and three four-story wood-frame buildings over one-story concrete podiums. According to Brad Ellinwood, PE, of Ellinwood + Machado Consulting Structural Engineers, a number of systems were considered but wood was by far the most economical. For the structural frame portion only, the wood design cost approximately $14/square foot compared to $22/square foot for a 7-inch post-tensioned concrete slab and frame. Despite the need for significant site preparation, wood's ease of use allowed the entire project to be completed in just over a year.

Environmental recognition with local green building programs was a plus for the Marselle Condominiums in Seattle, WA, designed by PB Architects, but cost was the driving factor in the decision to use wood construction.

Photo by Matt Todd, courtesy of WoodWorks

 

COST CALCULATOR

A cost calculator is now available to demonstrate wood’s cost advantage. Users simply select a building type and U.S. average or state/city, and the calculator draws on current construction cost data to provide a comparison of wood and non-wood materials (aggregate of steel and concrete) for the shell or whole building. Each calculation is accompanied by graphs showing cost index and price variation of materials to demonstrate performance over time.

Source: www.woodworks.org

Often, even when wood is chosen to meet other goals, cost is still the deciding factor. For the Marselle Condominium project in Seattle, Washington (see the case study1 at the end of this article), wood construction helped the building meet requirements of the local Master Builders Association Built Green program. But while the environmental recognition was an added benefit, the developer considered the decision to use wood framing purely financial. “If the project had been built using all concrete, for instance, it would have cost about 30 percent more,” according to Kory Knudson, vice president of Norcon, NW, Inc. “If we had built the entire project out of steel, it would have taken much longer and we would have had to make many energy modifications.”

Innovative Uses for a Traditional Building Material

Building codes recognize wood's structural performance capabilities in a broad range of applications—from the light-duty repetitive framing common in small structures to the larger and heavier framing systems used to build arenas, schools and other large buildings. However, around the world, architects and structural engineers are extending the boundaries of wood design, while innovative technologies and building systems continue to expand opportunities for wood use in construction. It's a symbiotic relationship that has also influenced the evolution of building codes and standards.

For example, the Cathedral of Christ The Light in Oakland, California, is an extraordinary timber cathedral designed to last 300 years using a unique structural system. Designed by Skidmore, Owings and Merrill LLP (SOM), the soaring 36,000-square-foot, 1,500-seat structure replaces another cathedral destroyed during a 1989 earthquake. Architecturally stunning, the new building features a space-frame structure comprised of a glulam and steel-rod skeleton veiled with a glass skin. Given the close proximity of fault lines and non-conformance of the design to a standard California Building Code lateral system, the City of Oakland hired a peer review committee to review SOM's design for toughness and ductility. Through the use of advanced seismic engineering, including base isolation, the structure has been designed to withstand a 1,000-year earthquake. Engineers were able to achieve the appropriate structural strength and toughness by carefully defining ductility requirements for the structure, using three-dimensional computer models that simulate the entire structure's nonlinear behavior, testing of critical components relied on for seismic base isolation and superstructure ductility, and verifying their installation.

Speed record: Taking speed of construction to an entirely new level, the two-story Long Hall in Whitefish, Montana, designed by Datum Design Drafting and engineered by CLT Solutions, took just five days to erect and gave the owner a sustainable, energy-efficient building. It was the first commercial building in the U.S. made from CLT.

Photo by gravityshots.com

 

THE TREND TOWARD
TALLER WOOD BUILDINGS

Multi-family housing was one of the first market segments to rebound from the recession, because it’s more affordable than single-family housing while offering advantages such as less upkeep and closer proximity to amenities. Wood construction is attractive for multi-family projects because it offers high density at a relatively low cost, as well as adaptability on site, faster construction, and reduced carbon footprint. The IBC allows wood-frame construction for five stories and more (e.g., with the use of mezzanines and terraces) in building occupancies that range from business and mercantile to multi-family, military, senior, student and affordable housing. However, there are indications that this may increase as new products continue to enhance wood’s ability to add value in multi-story applications. For example, cross laminated timber (CLT) is widely used in Europe and is gaining ground in North America. In the UK, there are eight- and nine-story examples of CLT buildings and a ten-story CLT project is near completion in Australia.

An example with farther-reaching implications is the Long Hall in Whitefish, Montana, the first commercial building in the U.S. to be built from cross laminated timber (CLT). Although the Type VB structure was built to 2009 International Building Code (IBC) requirements, CLT was completely new to code officials. Darryl Byle, PE, of CLT Solutions worked with the local building department more than six months in advance to address concerns and keep the project on schedule. Among the challenges, the team needed approval of the CLT system as a stand-alone, one-hour rated assembly in order to feature exposed CLT on the interior. Byle used data on fire design from sources such as the National Design Specification® (NDS®) for Wood Construction and experimental CLT fire test data from manufacturers and independent sources to demonstrate that CLT panels could be expected to perform well in a fire event.

Galleria Italia at the Art Gallery of Ontario. (See case study on page 5.)

Photo by Sean Weaver

In addition to CLT, parallel strand lumber (PSL), glued laminated timber (glulam) and prefabricated paneling systems are among the products contributing to a wider range of wood buildings. They have made wood a viable choice for applications such as arenas, gymnasiums and lobbies, which require tall walls and large open spaces with minimal, intermediate supports. For example, glulam can be manufactured to achieve spans as long as 100 feet and walls up to 20 feet. (See the case study at the end of this article of the Art Gallery of Ontario renovation designed by Frank Gehry.)

Wood and the Environment

Wood grows naturally and is renewable. Life cycle assessment (LCA) studies also show that wood yields clear environmental advantages over other common building materials in terms of embodied energy, air and water pollution, and greenhouse gas emissions.

In the past, the green building movement has taken a prescriptive approach to choosing building materials. This approach assumes that certain prescribed practices—such as using local materials or specifying products with recycled content—are better for the environment regardless of the product's manufacturing process or disposal. Today, however, it is being replaced by the scientific evaluation of actual impacts through LCA.

LCA is an internationally recognized method for measuring the environmental impacts of materials, assemblies or whole buildings over their entire lives—from extraction or harvest of raw materials through manufacturing, transportation, installation, use, maintenance and disposal or recycling. When integrated into green building codes, standards and rating systems, LCA encourages design professionals to compare different building designs based on their environmental impacts and make informed choices about the materials they use.

 

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Originally published in GreenSource
Originally published in September 2012

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