Doggerel Ι August 23, 2017
After a long time lost in the woods, architects and engineers are rediscovering timber.
Wood has been a default building material for millennia. Historically, one of the easiest and most effective ways to keep buildings standing upright was to fell large trees and shape them into load-bearing beams and columns. This changed in the 20th century, when the pliable possibilities of concrete and the seemingly immortal strength of steel lured builders away.
Thanks to a technology known as mass (or engineered) timber, wood construction is making a comeback for mid- and high-rise buildings. Mass timber was invented in the late 1800s, but attracted little interest until recently, when growing concern about buildings’ environmental impact led designers to search for alternative structural solutions.
Offering benefits ranging from carbon sequestration to job creation, mass timber (which includes variants such as glulam and CLT) has gained powerful allies among designers and policymakers alike. But in the United States, it has remained confined largely to high-profile showcase projects.
Research conducted this spring could soon change this.
Over the years, advocates have studied mass timber’s material properties in order to demonstrate that it provides the level of safety that building codes require. As a result, the approval process for some mass timber construction projects can be very easy, said David Barber, a fire engineer in Arup’s Washington DC office.
But there’s still work to be done. Until recently, relatively little was known about the fire-safety properties of structural connections — the industry term for the hardware used to link beams and columns. (Connections are often the weakest part of a structural system.) Building codes offer guidance for connection design in cases where timber structures are covered by drywall, but provide little help for exposed wood beams and columns. Because of this, architects who want to maximize wood’s aesthetic potential by leaving timber structures in view have been forced to hire specialists to conduct extensive, expensive research into connection fire safety.
Seeing a gap in the marketplace, Barber (with Arup’s support) approached the Softwood Lumber Board, an industry organization dedicated to increasing wood construction. His idea: identify a mass-market-friendly, code-compliant timber connection solution.
The ultimate goal of this plan is to remove barriers for architects and developers, enabling the construction of a wider range of wooden buildings — think low-and medium-rise offices, apartment buildings, schools, and libraries.
Seeing value in the concept, the Software Lumber Board agreed to fund several fire tests of structural connections. Arup stepped in to provide engineering advice from its global team through the firm’s internal research program. Wood products company D.R. Johnson supplied glulam, and connections manufacturer MyTiCon donated hardware.
The team selected three different steel-screw connection systems to test, prioritizing wide availability, ease of installation, and modest price.
Convening in a San Antonio test facility in March, the team used these connections to link mass timber beams and columns designed to support four- to eight-story buildings. The structural systems were then loaded with weight simulating a real building and placed in a furnace.
The team repeated the tests three times over four days. During each test, the temperature rapidly ramped up to approximately 1,250 degrees Fahrenheit before rising to 1,750 degrees. (In the future, Barber plans to test glulam connections for two hours.)
The result: success. All three connections complied with the professional code that sets minimum standards for the length of time structural systems must withstand fire.
Design for the public good
The test reports are complete and have been published on reThink Wood, a website produced by the Softwood Lumber Board and other industry partners. With proof of fire-safety compliance to show regulators, designers wanting to build exposed timber structural systems can simply incorporate the fire-rated connectors into their buildings.
For Barber, this represents a huge advance. Although more complicated mass timber buildings will still likely require custom fire testing, wood’s benefits will reach a much wider swath of the public than has been possible in the past.
Designers and developers working on standard typologies “won’t have to reinvent the wheel on fire safety every time,” he said.