The relationship between earthquakes and man-made structures has been one-sided for as long as humans have been capable of building. That relationship is now being tested at the University of California San Diego thanks to the Natural Hazards Engineering Research Infrastructure Tallwood project.
The project aims to investigate the resilience of tall buildings by simulating a series of large earthquakes on a full-scale, 10-story mass timber building. The Tallwood structure is the tallest full-scale building ever to be constructed and tested on an earthquake simulator. The seismic shake table tests not only the structure’s resiliency but also a multitude of elements, including the integrity of glass and the resiliency of window seals.
“Resilient design must also account for the building’s nonstructural systems, which are not part of the structural load-resisting system but play an important role in the building’s function and its ability to recover after the earthquake,” says Keri Ryan, a project co-investigator and engineering professor at the University of Nevada, Reno.
Among the many professionals who traveled to La Jolla, Calif., is Lothar Erkens, an engineer at Winco Window Company. The Saint Louis-based custom aluminum window manufacturer provided windows for the project. Erkens says the tests afford the fenestration industry a perfect opportunity to validate testing on smaller shake tables and test whether window installation clearances are designed to handle actual earthquakes.
“We’re basically trying to validate the engineering assumptions made,” he adds.
Tallest Full-Scale Earthquake Simulator
The Tallwood shake table boasts the world’s largest payload capacity, carrying and shaking structures weighing up to 2,000 metric tons (4.5 million pounds). Recently, the table underwent an upgrade courtesy of funding from the National Science Foundation. It now has the capability to accurately replicate the complete three-dimensional ground motions experienced during earthquakes, encompassing all six degrees of freedom: longitudinal, lateral, vertical, roll, pitch and yaw.
“The rocking wall system consists of a solid wood wall panel anchored to the ground using steel cables or rods with large tension forces,” says Shiling Pei, principal investigator and associate professor of civil and environmental engineering at Colorado School of Mines. “When exposed to lateral forces, the wood wall panels will rock back and forth–which reduces earthquake impacts–and then the steel rods will pull the building back to plumb once the earthquake passes.”
Tests simulate earthquake motions recorded during prior earthquakes covering a range of earthquake magnitudes on the Richter scale, from magnitude four to magnitude eight. This is done by accelerating the table to at least 1g, which could accelerate the top of the building to as much as 3gs.
The testing began officially in early May, says Erkens, and will last around four weeks. The process involves shaking one level before walking the structure to check for damage. The process is then rinsed and repeated at differing levels.
The first three levels of the building feature non-structural elements, such as a curtainwall provided by Technical Glass Products and windows provided by Innotech and Winco.
Integrity of Fenestration
Erkens says the test came about after the Kobe, Japan, earthquake in 1995 that killed more than 6,000 people and the Auckland, New Zealand, earthquake several years ago. Those earthquakes showed that windows, doors and other non-structural components might not initially exhibit any failures. However, “two to three weeks later, the first heavy rainstorm comes, and suddenly there are major water intrusions into the building,” says Erkens.
Glass failures are evident to everybody, he adds. Glass cracks and breaks. If only the seal fails, it’s not noticeable unless you know what to look for. Those failures don’t appear until you have excess air filtration during winter or excess water filtration during the first heavy rainfall.
“For window installers, they use various silicone products to seal the frames and the glazing or dry glaze,” says Erkens. “The materials used and the clearances designed are based on tribal knowledge and theoretical engineering calculations, but the actual seismic loads are not completely understood to date.”
Erkens adds that the Tallwood shake test is one of the means of gaining a more fundamental understanding so that the industry does not over-engineer. As of the second week of testing, no windows have failed yet, says Erkens. However, he explains that his team is currently focused more on data acquisition. The evaluation will begin after the shake testing is completed.