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Nabholz Explores the Cutting Edge of Construction: CLT Timber

Timber

This content is sponsored by Nabholz.

Nabholz, along with client University of Arkansas and architects Modus Studio, Mackey Mitchell Architects, and Leers Weinzapfel Associates, is currently constructing the nation’s first large-scale mass timber residence hall project. The five-story Stadium Residence Halls will comprise of more than 200,000 square feet and 368 rooms and are expected to be completed before the start of the fall 2019 semester.

Nabholz’ research into cross-laminated timber (CLT) began long before construction started. This building material is new to the United States, so the project team traveled to visit some of the only CLT job sites the country. These sites included one in North Carolina on the Duke Campus, the other at the First Tech Corporate Headquarters in Portland, Oregon.

CLT panels consist of several layers of kiln-dried lumber boards stacked in alternating directions, bonded with structural adhesives, and pressed to form a solid, straight, rectangular panel. Think plywood on steroids.

CLT offers several safety advantages. While at the processing mill, CLT panels are cut to size, including openings for doors, windows openings, utility piping, and ductwork, with state-of-the art CNC (Computer Numerical Controlled) machines. With this many major cuts done offsite, this reduces the chance of accidents on site and speeds up the installation of systems. CLT panels are also exceptionally stiff, strong, and stable, handling load transfer on all sides.

Building Rendering

For many, both inside and outside of the construction industry, timber construction equates to a higher fire risk. This is not necessarily true, however. Solid wood and cross-laminated timber performs much better than their stick frame cousins. According to Greenspec®, to understand how efficient CLT is in a fire, it is important to start with the understanding that fire resistance is the ability of a material to confine a fire or to continue to provide a structural function or both. The measure of fire resistance is the time elapsed from the start of the fire up until the point the material fails to function. Typically, resistance is expressed in minutes (e.g. FR 30, 45, 60 or 120). CLT’s fire resistance actually comes from charring.

For CLT, if the face of the timber panel is exposed to temperatures in excess of 752° Fahrenheit from fire, the surface of the timber ignites and burns at a steady rate. As the timber burns, it loses its strength and becomes a black layer of char. The char becomes an insulating layer preventing an excessive rise in temperature within the unburnt core of the panel. This unaffected core continues to function for the period of the fire resistance. To achieve the designed fire resistance period there must be sufficient virgin solid timber remaining behind the char layer to sustain the loads applied. Therefore, each CLT panel within the building must be designed for the fire resistance period and the specific loadings applied to that panel.

Though relatively new to the United States, this building material is frequently used in Europe and Canada. Builders and designers cite several advantages to using cross-laminated timber. First and foremost, the environmental impact of CLT is far less than traditional building materials such as concrete and steel. With responsible forestry management, timber is a sustainable and renewable resource. Cross-laminated timber’s carbon footprint pales in comparison to that of concrete and steel. Also, it is made to order from computer generated models, eliminating waste.

If CLT continues to grow in popularity, it could translate to big economic gains for the state. In Arkansas, timber is a large industry, meaning more CLT buildings could directly profit Arkansans. The University of Arkansas is laying the path for this building material to become more mainstream in the U.S. at potential gain for Arkansans. Just think of this – the 142,000 cubic feet of timber that will be utilized on a massive project like the UA Stadium Drive Residence Hall is grown in Arkansas forests in a matter of hours.

Work in Progress

This product offers other benefits specifically to builders. First, it is a familiar material for many craftsmen. It is a “friendly” material, requiring less specialized tools. Second, it makes for a cleaner job. Less concrete means less overages, drips, and over pours. It also requires no welding and there is none of the rusting associated with steel. Third, it makes building faster. Nabholz recently constructed a similarly sized project using traditional materials and it took 18 to 20 weeks to erect the structure. With CLT, Nabholz erected the Stadium Drive Residence Hall’s structure in 12 to 15 weeks. Cross-laminated timber offers some relief for noise pollution, as well. It is quieter to install, and eliminates a lot of the clanging that comes with working with steel.

With obvious benefits to workers, the environment, project owners, and the state’s largest industries, Nabholz has made several investments to ready our team to handle the material. First, Rob Dodd, Nabholz Project Executive traveled to Vancouver, BC with the members of design team, UA housing, and UA School of Architecture to visit a CLT manufacturing facility to better understand how this product is made. This group also visited several projects constructed of CLT. The Nabholz project team traveled to Oregon to observe the largest CLT project in the U.S. during the installation phase (the Stadium Drive Residence Hall project will surpass this as the largest in the U.S. However, there are already other projects in the works that will surpass Stadium Drive).

Nabholz will self-perform the installation of the timber package on the UA Stadium Drive Residence Hall project, and with the experience gained on this project, will look to install timber packages on other projects in the future. Already, Nabholz is constructing another CLT project in Northwest Arkansas along with the Stadium Drive Residence Hall project, the 5th Street Office Renovation in Bentonville.

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