Now that the long-anticipated upgrade is complete, the NHERI at UC San Diego LHPOST6 shake table is up and running – and ready to simulate earthquake ground motions with 6 degrees of freedom (6DOF). Earthquake engineers from around the world can again test their designs at the UC San Diego facility, the worlds’s largest outdoor shake table.

In April and May, 2022, the first project to run on LHPOST6 will shake and characterize a brand-new NHERI community resource: a full-scale, modular, steel-framed test structure. Coined the Modular TestBed Building (MTB2), this work is being led by principal investigators Tara Hutchinson from UC San Diego and Chris Pantelides from the University of Utah. They and their team, including co-PI Gilberto Mosqueda of UCSD, worked closely with industry partners, including CoreBrace LLC, who built the structure’s buckling restrained braces, and DuraFuse Frames, who built the special moment resisting frames.

Like a super-sized erector set, this three-story test-building is assembled and taken down using a professional steel erection team, cranes, and the UC San Diego site team.

“We wanted an adaptable structure that could easily be erected and de-erected,” says Tara Hutchinson, who helped design the testbed building. “As you might imagine, we had to make many decisions, including defining a suite of performance attributes for this adaptable testbed building — decisions that are generally governed by geometry, detailing, and load distributions. What we will have in the end, for community use, is a building that can be configured to model a variety of building types that the earthquake designer uses in practice today. That was our aim.”

For instance, the building has modular floor plates and decks, and it can be configured with three different lateral force resisting systems. Full testbed specifications are available at the NHERI UC San Diego website.
 

Shakedown Tests

The spring 2022 tests will be a “shakedown” of the structure — tests to characterize the building’s behavior. The team will gather performance data and document that for future users, especially in regard to accelerations and interstory drift profiles under different seismic intensities.

“We are going to test the structure in three different configurations – with motions scaled to capture the testbed building’s elastic through intended design behavior. In some configurations, due to the ease in replacing yielding elements, we will be able to scale motions to achieve above-design behavior,” Hutchinson says.

They will start by erecting the structure in a special moment frame configuration. They will instrument it and perform a series of shakedown tests using uni-directional (longintudinal) motion input, and systematically will add bi-directional and then tri-directional input motions – benefiting from the shake table’s upgraded multi-DOF capabilities.

Next, they’ll adapt the base plate anchorage into something called a “compliant base” with yielding replaceable anchors. They’ll repeat the earlier series of tests with this more flexible system – and also increase the shaking intensity.

“We want to induce the stretch of those anchors, which are quite inexpensive to replace. That’s the aim of exercising this configuration,” Hutchinson explains. Next, they’ll call the steel erector team back, remove the moment frame connections and their yielding shear plates, and install buckling restrained braces on all levels in the longitudinal direction – and repeat the series again.

“We have three weeks to perform all of this, so it will be very intense,” Hutchinson says. At the end of it all, the team will update the numerical models used during the testbed building’s design phase, validating them against the shakedown test results. They plan to post these numerical models for community use within their public data report on the DesignSafe DataDepot, so that future researchers may use them in planning a test program using the testbed building. “And of course, we’ll have the response data from the three extreme configurations we’re testing in this shakedown phase,” she added.

Fortunately, thanks to its supporting steel erector company Asbury Steel, the testbed team was able to practice erecting the testbed building in October 2021 on the NHERI@UC San Diego staging slab, while the shake table upgrade was underway (see photographs). Impulse tests were also conducted during this phase to help verify the testbed building’s dynamic characteristics while constructed in one of its configurations.

Future possibilities

The entire team at the NHERI UC San Diego facility is looking forward to researchers submitting proposals that utilize the testbed.

Hutchinson describes the versatile testbed structure as a vehicle for delivering seismic demands to structural and nonstructural systems. It is specifically designed for researchers to modify and tune, depending on what they’d like to achieve.

The testbed can be constructed in a three-story configuration; however, it also could be erected with a base isolation or other foundation, or with lower or upper structural floors. It can easily support additional structural elements. It accommodates nonstructural elements. It can be erected on isolators. Researchers can explore alternative lateral force resisting systems, for instance, and different walls, such as rocking walls.

“There are so many cool things you can do,” Hutchinson says. For instance, one configuration Hutchinson would be interested in seeing is a combination of buckling restrained braces in the very stiff upper stories and soft lower stories.

Researchers interested in using the testbed are invited to contact the team at NHERI UC San Diego to discuss ideas and research proposals.
 

By Marti LaChance, NHERI Communications

Related Links

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LHPOST Live Cameras

NHERI @ UC San Diego

NHERI Experimental Facilities