Mayor Ed Murray’s Housing Affordability and Livability Agenda committee presented a comprehensive set of recommendations for affordable housing Monday, which included a proposal to raise the height limit of wood structures built on a concrete base.
The recommendation reads: “consider building and fire code modifications to allow six stories of wood frame construction.”
While high-rise buildings must be made of concrete, this particular code refers to mid-rise structures currently maxed out at five wood stories built on top of two concrete stories at the base. The type of structure is called V-A (5-A) and is often used for apartment buildings with retail establishments on the ground floor.
But adding a sixth story of wood construction requires a major review to make sure fire and earthquake safety standards are still met.
A principal engineer with the Seattle Department of Planning and Development told KIRO 7 that some engineers doubt the addition of a sixth wood story would work, given the additional risk it might pose to the occupants inside.
While the national standard is to allow four wood stories on top of two concrete stories, Seattle and some other jurisdictions added a fifth wood story decades ago.
“This five-on-two style has become pretty attractive. It’s economical, it makes a lot of sense for developers,” said Jeff Berman, an associate professor of civil engineering at the University of Washington.
He said adding a sixth wood story is unusual in other places, due to concerns about fire and earthquakes.
“We have lots of information and shake table tests on six
-story timber buildings, but not on two stories of concrete,” Berman said.
He also said more wood on the structure means more material that could burn easily. The structure would also have to be tested to make sure its weight would not cause the building to collapse quickly in a fire.
The mayor’s committee also recommended looking at expediting the approval process for new building technology, like cross-laminate timber or CLT.
The material, described by the DPD engineer as “plywood on steroids,” is supposedly stronger and would burn more slowly in a fire.
While CLT could potentially be used to build higher, as it has been in other countries, it is not currently allowed in building V-A structures.
“The idea is that Seattle would be a test bed,” Berman said.
Jeff Loy, the owner of North Seattle Home Services, said the benefit of CLT is that it is made from smaller trees without the need to sacrifice large trees for large beams.
“They’re stable. And they’re stronger. They’re not cheaper though,” Loy said.
He said a piece of CLT is about twice as expensive as conventional wood of the same size. So while it could someday be used to build higher, it is not a more cost-effective option.