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Study of Soft-Story Woodframe Buildings Begins Full-Scale Tests

 


Exterior of NEES-Soft test structure

The NEES-Soft Test Building as of July 12, representing 1920's to 1930's Bay Area construction methods.

Interior of NEES-Soft test structure

One of the rocking walls located throughout the NEES-Soft test structure

Interior of NEES-Soft test structure

Plywood on the underside of the second floor diaphragm in the garage of the test structure

NEES-sponsored research is set to test new retrofit methods for making soft-story woodframe buildings much safer during an earthquake.

Dr. John van de Lindt of Colorado State University is leading a team of researchers from five universities to study the behavior of soft-story woodframe buildings, which are multi-story buildings designed with the first floor used either for parking or commercial space with large openings and few partition walls. These buildings, built before the 1970s, are therefore susceptible to collapse at the first story during an earthquake. In fact, a nine-year study conducted by the city of San Francisco found that 43 to 80 percent of multi-story woodframe buildings would be deemed unsafe after a magnitude 7.2 earthquake, and 25 percent of these buildings would be expected to collapse.

The NEES-Soft Project Team has begun testing of a full-scale 4-story soft-story woodframe building at the NEES@UCSD site that will take place over the next four weeks. The structure has been built to San Francisco mandatory retrofit ordinance passed only a few months ago, and the tests are meant to validate the new retrofit guidelines and provide vital data on the structure's behavior.

The first test took place on Monday, July 15, with two small shakes. Two significantly larger shakes were run early Tuesday morning. Further testing will take place throughout the next four weeks to test three other types of retrofit design standards.

The first retrofit method being tested utilizes cross-laminated timber with eight two-foot-wide rocking walls and plywood on the underside of the second floor diaphragm. Complying with San Francisco city ordinance, the structure is designed to 50% maximum considered earthquake (MCE), which is an earthquake that is expected to occur once in approximately 2,500 years for a specific area.

The tests run on Tuesday included shakes at the 50% MCE level. The structure behaved just as expected, with the garage-level plywood ceiling transferring shear to the cross-laminated timber as designed. The upper three stories also behaved as expected, with some very moderate hairline cracks starting on the second floor. A video of the test can be found here.

The second retrofit method that will be tested adheres to the FEMA P807 guidelines, followed by a third technique using Performance Based Seismic Retrofit (PBSR) design requirements. A fourth and final technique will utilize a toggle brace viscous fluid damper PBSR provided by Taylor Devices.

The NEES-Soft project began in 2010 as part of the National Earthquake Hazards Reduction Program (NEHRP) with a grant from the National Science Foundation. The first part of the project involved developing and testing numerical models and constructing the test structure.

Dr. van de Lindt and his team will use the data obtained from their experiments to provide experimental verification of the ATC 71.1 retrofit procedure and draft recommendations for future building code updates. The data will also be publicly available on the NEEShub to provide a better understanding of the collapse mechanisms and processes for soft-story woodframe buildings.

A full schedule chart of the NEES-Soft testing stages may be found here.