First-ever two-hour fire testing on a mass-timber exposed structure shows that engineered wood can meet stringent U.S. fire codes for tall buildings. The test results, announced recently, move the 12-story Framework Tower, sited in quake-prone Portland, Ore., a step closer to construction. The 140-ft-tall high-rise is on course to be the tallest mass-timber building in the U.S.
“Until this test, no one had shown that the beam-to-column connection can meet the two-hour fire rating” for high-rises, says Thomas Robinson, founding principal of LEVER Architecture, which is designing Framework with structural consultant KPFF Consulting Engineers. “The tests are a confirmation of what we already modeled and designed,” he adds.
The test specimen consisted of an exposed glue-laminated-timber (GLT) beam-to-column assembly—with embedded steel connectors—and an exposed cross-laminated-timber (CLT) floor with a top layer of gypsum concrete. After two hours of exposure in the fire chamber, the specimen still carried the full load—2,500 lb of pressure imposed by an actuator.
“No one had done that [test] before,” says David Barber, a principal of Arup, Framework’s fire-protection engineer. In Europe, one-hour tests are typical.
Since 2010, there have been more than 17 seven-story or taller mass-timber buildings constructed outside the U.S. The tallest is a 14-story residential building in Bergen, Norway.
There are no tall-timber U.S. buildings, primarily because of regulatory resistance. The mixed-use Framework aims to change that. Construction is expected to begin in March and take about a year.
Framework’s tests are funded by a $1.5-million grant from the U.S. Dept. of Agriculture, in partnership with the Softwood Lumber Board and the Binational Softwood Lumber Council (ENR 9/28/15-10/5/15 p. 16). The funders are trying to spur a forest-to-frame supply chain for mass-timber high-rises, which are considered sustainable because timber is a renewable building material.
The Framework team tested GLT from different suppliers, to avoid getting locked into a single source. In a fire, a GLT beam chars on the outside, creating natural insulation for the embedded steel connector. Without protection, the connector would lose strength in a fire and fail. Computer-numerically controlled (CNC) machining allows the connector’s steel to be embedded economically in the beam, says Robinson.
In the tests, the specimen matched Framework’s perimeter column-to-beam detail. All embedded steel was 4 in. from the wood’s surface on all sides. “We are adding more wood [to the member] to create a fire rating, but the extra wood is not a big cost driver,” says Robinson.
The CLT panels—perpendicular layers of solid-sawn lumber bonded with structural adhesives—were supplied by D.R. Johnson Wood Innovations, the first certified CLT supplier in the U.S. Johnson recently conducted successful CLT fire tests (ENR 9/19 p. 18).
Results of structural tests—including for Framework’s vertically post-tensioned “rocking” shear-wall core, which is designed to minimize structural damage in a quake—are expected next month. If built, Framework would have the tallest rocking wall and the only one built using CLT.