March 12, 2009

UNIVERSITY OF ARIZONA

To simulate earthquake conditions on an actual building, the research team erected a three-storey, 450-tonne, half-scale pre-cast concrete parking garage on a giant shake table at UC San Diego’s Englekirk Structural Engineering Center.

Concrete garage shaken to bits in quake test

Three-storey half-scale model yields ‘gobs’ of seismic information

PETER KENTER

correspondent

What’s the best way to test the effects of a major earthquake on a concrete parking garage? Build one and shake it for all it’s worth.

Professor Robert Fleischman, a Professor in Civil Engineering and Engineering Mechanics at the University of Arizona, heads a project that did just that — a five-year, US$2.5-million earthquake engineering study that researchers hope will lead to improved building codes, which will protect buildings and occupants from earthquakes. The research project is being carried out jointly by The University of Arizona, the University of California at San Diego and Lehigh University.

Fleischman notes that many concrete structures, such as overpasses and parking garages should have been able to withstand the 1994 Los Angeles earthquake, which killed 57 people and caused an estimated US$20 billion in damage. Lehigh University had already tested many individual building components, such as floors and critical connections at full-scale and developed sophisticated computer analytical models from the data collected.

A critical floor, for example, was fabricated in Pennsylvania and assembled in a lab at Lehigh. Actuators attached to the components could be made to shear and open and close the joint, allowing researchers to simulate how the joint would behave in a real quake.

To simulate earthquake conditions on an actual building, the research team erected a three-story, 450-tonne, half-scale pre-cast concrete parking garage on a giant shake table at UC San Diego’s Englekirk Structural Engineering Center.

But building a half-scale parking garage is more complicated than merely reducing all of the dimensions by half.

“If you’re faithful to similitude and you scale everything by the same number some things work out, but others don’t,” says Fleischman.

“If the building is scaled by half, then something that has the strength to withstand 20,000 pounds of force now must be designed to withstand 5,000 lbs. of force instead. If we’re testing a V-shaped vector connector, we had to go to a fabricating shop and painstakingly make a half-scale replica that’s four times as weak with a deformation capacity of one-half. We had to find plants that would retool just for one custom job to make a series of double-T pre-cast floor units to our specifications. We had to go to South America to find Number 3 and Number 2 rebar rods that look like you could break them with your hands.”

Each floor of the structure used a different type of construction, with pre-cast double-Ts on the bottom, hollow core with ductile mesh and topping on the middle floor and straight pre-cast on top.

The earthquake simulations also had to be speeded up and amplified to provide accurate data from the half-scale building.

The million-dollar parking building took two years to plan, one month to build and about five minutes of quake time to destroy. More than 600 instruments, including transducers, accelerometers and GPS locators located throughout the structure measured its responses to fifteen 20-second quakes simulated by the shake table.

“As one of my advisors used to say, a successful structural test ends in failure,” says Fleischman. “But we didn’t want the floor to drop out prematurely. We wanted to squeeze every last drop out of the tests.”

Fleischman says the tests have netted “gobs and gobs of data at a bargain price.” Computer models can now predict to a high degree of accuracy how any given structure, or component of a structure, will behave when an earthquake strikes, he says.

“Products can now be tested and qualify as low, medium or high deformability based on our design parameters. If a connector is a low performer it may be better suited to a squat building, rather than a tall one.”

The work has already been incorporated into the National Earthquake Hazards Reduction Program, which was created by the U.S. Congress to reduce earthquake-related losses through improved design and construction methods and practices.

“Think of this program as the guidelines that inform the code,” says Fleischman. “The next step is to put it into the code.”

Phase two of the project involves using computer models to build and test many different types of virtual structures.

“We need to make sure that we have our design factors right and that our models are making sense,” says Fleischman.

The research project is being funded by the Precast/Prestressed Concrete Institute, the National Science Foundation’s Network for Earthquake Engineering Simulation and Grant Opportunities for Academic Liaison with Industry, and the Charles Pankow Foundation.

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