Alki Elementary School, mass timber

Alki Elementary School is a three-story structure nestled in the heart of a West Seattle neighborhood, adjacent to an existing community center and within walking distance to Alki Beach. Designed by Mahlum Architects, the facility utilizes cross-laminated timber (CLT) decking and a glulam beam and column structure that is accompanied with buckling restrained braces. The CLT is showcased via exposed ceiling conditions throughout the building’s corridors and commons areas. CLT was attractive to Seattle Public Schools (SPS) as a design and structural element that would not only achieve their sustainability goals, but also their desired architectural vision.

Early in design, Cornerstone collaborated with Mahlum and their design partners to validate the district’s available construction budget by analyzing any cost differences between the proposed timber structure versus a conventional steel structure. Cost considerations explored by the team laid out the differences in foundation design, bracing strategies, and brick anchorage as well as acoustic performance, finishes, MEP routing and moisture controls during construction. 

Armed with proper cost information from the Cornerstone preconstruction team, SPS was able to determine a path for building Alki Elementary School with the desired CLT design and structural elements. 

Cornerstone continued to collaborate with Mahlum to optimize the structural design for maximum efficiency. Prior to schematic design, Cornerstone conducted a second cost study comparing a 5-ply floor + 3-ply roof system with a 15-foot x 28-foot grid versus an alternative approach that utilized 3-ply CLT decking for both floors and roof decking with an 11-foot x 28-foot grid. Although the 3-ply floor system required closer spans, it proved to be more efficient overall utilizing 0.50 cubic feet of fiber per square foot of floor area versus 0.75. 

Mass Timber Stats:

3-ply CLT roof and floors | CLT panel area: 76,850 SF | Total wood fiber: 38,110 CF

Project Highlights:

• Conducted early cost and constructability studies comparing multiple mass timber grids to determine the most efficient span layout

• Developed erection sequencing strategies for a tight urban site with no lay-down space

• Integrated pre-applied moisture protection systems to safeguard CLT through Seattle’s wet season

• Balanced budget, performance, and design intent through early pricing studies