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Vertically post-tensioned mass timber rocking walls resist quake loads induced by shake table.
On May 9, a 10-storey mass timber frame was put through its paces at the largest high-performance outdoor shake table, located at the University of California San Diego. (main pic: Dr Shiling Pei, 10-storey mass timber building shown here at four stories in August 2022. image credit: David Baillot/University of California San Diego.)
As first reported in Engineering News-Record, the frame, designed to be resilient enough to withstand powerful earthquakes with little or no structural damage, performed exceptionally well during seismic simulations of two major quakes; the 1994 Northridge quake, a magnitude-6.7 temblor in Los Angeles, and the magnitude-7.7 Chichi quake that struck Taiwan in 1999.
According to Shiling Pei, principal investigator for the National Science Foundation’s Natural Hazards Engineering Research Infrastructure TallWood Project, “The building remained damage free after two major design-level earthquakes back-to-back.” Pei is also associate professor at Colorado School of Mines, which serves as the contractor for the TallWood Project.
“We found a way to build tall wood [structures] that are earthquake-proof for design-level shakes. There was very little damage to non-structural systems, on first inspection. After the entire project is done, we will try to put the rocking-wall system into the building codes.”
The footprint of the test structure, designed by LEVER Architecture, is 9.8 metres x 10.4 metres. The columns sit on the 12-metre x 7.6-metre shake table, but the floor deck cantilevers beyond the table on one side, according to LEVER.
The 34-metre-tall test structure has four partial façade assemblies, a number of interior walls and a 10-storey stair tower. It’s built from mass timber floor panels, consisting of cross-laminated timber (CLT), glue-laminated timber, laminated-veneer lumber (LVL), or nail/dowel-laminated timber panels, depending on the floor level.
The mass timber floor panels work with LVL beams and columns primarily to resist gravity loads. For resistance to quake loads induced by the shake table, vertically post-tensioned mass-timber rocking walls, 3-metre wide and the full height of the frame, exist on all four sides of the test building, though one wall is set inside against the stairwell.
Two walls are CLT panels and two are mass plywood panels. MPPs are certified as a type of CLT but instead of using boards stacked in alternating layers, MPPs use layers of thick plywood, according to Freres Engineered Wood, which supplied the MPPs.
Watch a video render of the project
Reid Zimmerman, structural technical director of the Portland, Ore., office of KPFF Consulting Engineers, and a member of the TallWood design and construction team, said, “For the post-tensioning, the rocking walls each have two high-strength threaded bars running full height, which pre-compress the CLT panels or mass plywood panels.”
During shaking, the mass timber panels rock, lifting at the heel end and compressing at the toe end of the walls, while always being controlled and brought “back to center” by the post-tensioned rods.
The test building is based on the unbuilt 12-storey Framework project, designed by LEVER with KPFF. The building, Framework, had all its relevant construction permits when the developer pulled the plug. But, the research done for the tall-timber building is serving as the basis for code changes and more.
Watch a video of the completed structure
“Framework’s impact on this test was huge,” says Zimmerman. “The majority of the lateral connections were inspired by or similar to Framework.” These include the steel plates at the corners of the base of the rocking walls, which react against steel shear keys to resist horizontal base shear, preventing sliding.
Jonathan Heppner, LEVER’s principal for the test frame, calls the research “exciting,” adding the design guidelines “will help with the permit approval process,” even before the rocking frame is incorporated into the building code.
