Shake Table Tests Prove Structures Can Be Seismically Sound With Less Steel Than Called for by Current California Code

January, 2006, San Diego – University of California, San Diego (UCSD) structural engineers have announced that a violent simulated reproduction of the Northridge earthquake shook a seven-story structure at the UCSD Jacobs School of Engineering’s Englekirk Structural Engineering Center, and resulted in only minor cosmetic damage to the building.  The experiment, along with earlier shakes, was conducted to test a revolutionary new theory that mid-rise concrete apartment, condominium and hotel structures can be built to survive powerful earthquakes using less steel reinforcement than currently required by California building codes.

“Many people don’t realize that excessive building strength can actually promote poor structural performance and non-structural damage during an earthquake,” said Robert Englekirk, founder of the Englekirk Companies and a co-principal investigator of the project. “The structural engineering community wants to develop regional design procedures that allow for the development of more suitable buildings in Southern California.”  UCSD structural engineers said the building held up as well as the theory. 

The test was conducted under tight safety precautions with the powerful mechanical jolts delivered by a 25 ft. by 40 ft. shake table. The experiment duplicated ground motions from the Jan. 17, 1994, Northridge earthquake that were recorded at the Olive View Hospital in Sylmar, CA.  The 275-ton, 65-foot-tall building was also tested on Nov. 22, 2005, with ground motions that were recorded during the Northridge earthquake farther from the epicenter, at a seven-story hotel in Van Nuys, CA. The November, 2005 test produced horizontal accelerations that were 30 percent of the force of gravity, and the Jan. 14 test simulated the earthquake closer to its epicenter, with horizontal accelerations of 82 percent of gravity.

“What we found is fairly simple; if we use an intelligent design strategy that reduces the demands required by the current California building standards, and use about half the reinforcing steel that’s required, mid-rise buildings will survive powerful earthquakes with only minor damage,” said Jose Restrepo, professor of structural engineering at the UCSD Jacobs School of Engineering Department of Structural Engineering and co-principal investigator of the project.  Prof. Restrepo likened the design to that of a car, where upon impact, a large amount of the force is dissipated at the bumper, prior to spreading through the car to the passenger area.  So to in this design, the intent was to have the brunt of the impact absorbed in an energy absorbing  “plastic hinge” located at the ground floor, causing a dissipation of the earthquake forces prior to their spreading throughout the building.  Restrepo said the performance of the seven-story building was even better than the sponsors of the project had expected. “The professional engineers we have talked to were thrilled,” said Restrepo.

“I can’t tell you how satisfying it is to see the BauGrid Welded Reinforcement Grids (WRG) withstand such tremendous large tension and compression forces,” said Hanns U. Baumann, S.E., president of BauTech, Inc., which donated the WRG that were used as the confinement reinforcement in the walls of the 7-story test structure.  “This design is one that Bob Englekirk has espoused for some time, and I’m glad to see that Prof. Restrepo’s test program has done such an excellent job of showing that he was absolutely right.” said Baumann.   Baumann added that “Because of public safety concerns, it is quite important that design engineers understand that the WRG used in this structure is not the same as the much weaker Welded Wire Reinforcement (WWR).  WRG is manufactured with high strength welds at every location where the steel rods intersect, which provides the unique and valuable inter-cell confinement that was exhibited during the test.”

Restrepo said the seven-story building may be put through additional tests to provide further scientific confirmation that less reinforcing steel than currently required could improve the performance of mid-rise concrete buildings in densely populated and seismically active regions in California.

Sixty people were killed and 7,000 injured during the Northridge earthquake which left 20,000 homeless, and damaged more than 40,000 buildings in Los Angeles, Ventura, Orange, and San Bernardino Counties. The death toll and roughly $40 billion in property damage prompted professional structural engineers to call for more scientific testing of mid-rise residential buildings.

When asked how soon new design procedures such as this could be incorporated into Southern California buildings, in that implementation of new ideas into building codes is a process that is notoriously slow moving, Englekirk stated that the current code already allows use of this “performance-based” design.  “The code allows engineers two ways to design structures.  There are prescriptive designs, as spelled out in the codes, and there are performance-based designs, where through the use of testing and proven scientific methods, it is shown that a design is safe.”

Full-scale tests of such large buildings have previously not been possible because of weight, space, and technical limitations of smaller indoor shake tables. UCSD’s shake table can actually support a building roughly 10 times heavier than the seven-story structure it currently holds.

The $9 million shake table at the Jacobs School’s Englekirk Center is one of 15 earthquake testing facilities in the National Science Foundation’s Network for Earthquake Engineering Simulation. The UCSD-NEES shake table, the largest in the U.S. and the only outdoor shake table in the world, is ideally suited for testing tall, full-scale buildings.  Click here for more information on the facility.

Construction of the seven-story test building was led by Highrise Concrete Systems, Inc. of Dallas, TX, a company which specializes in the construction of multi-story concrete buildings using the latest formwork technology.  Additional financial support donated equipment and labor was provided by Tech, Inc.; Dywidag Systems International, USA;, Inc. (DSI); HILTI; Associated Ready Mix; California Field Ironworkers; Cemex: Concrete Steel Reinforcing Institute (CRSI); Douglas E. Barnhart, Inc.; Englekirk & Sabol, Inc.; Fontana, Grace, Hanson Aggregates; Morley Builders; Pacific Southwest Structures; Schuff Steel-Pacific Inc.; Southern California Ready Mix Concrete Association; the Portland Cement Association(PCA); and Carpenters/Contractors Cooperation Committee, a nonprofit labor and management group. 

The Englekirk Structural Engineering Center, part of UCSD’s renowned Powell Structural Research Laboratories, is named in recognition of Robert and Natalie Englekirk’s support of structural engineering research and education at UCSD Jacobs School of Engineering Department of Structural Engineering. Construction of the Englekirk Center and the earthquake research program there are supported by the Englekirk Center Industry Advisory Board, a group of 43 structural engineering firms and associations in Southern California. Patron members include Carpenters/Contractors Cooperation Committee, Englekirk Systems Development, Inc., and Highrise Concrete Systems, Inc. Other partners include: American Segmental Bridge Institute; Anderson Drilling; Baumann Engineering; Brandow & Johnston Associates; Burkett and Wong Engineers; Charles Pankow Builders, Ltd.; Clark Pacific; Douglas E. Barnhart, Inc.; Dywidag Systems International, USA, Inc. (DSI); Englekirk and Sabol Consulting Structural Engineers, Inc.; EsGil Corporation; GEOCON; Gordon Forward; HILTI; Hope Engineering, Inc.; John A. Martin and Associates; Josephson Werdowatz & Associates Incorporated; JVI, Inc.; KPFF Consulting Engineers; Matt Construction Corporation; Morley Builders; Nabih Youssef and Associates; Oak Creek Energy Systems; Occidental Petroleum Corporation; Pacific Southwest Structures; PCL Construction Services, Inc.; Portland Cement Association; Precast/Prestressed Concrete Manufacturers Association of California (PCMAC); Saiful/Bouquet Consulting Structural Engineers, Inc.; Schuff Steel-Pacific, Inc.; Structural Engineering Association of Southern California (SEAOSC); Simon Wong Engineering, Simpson Manufacturing Co., Inc.; Smith-Emery Company; Stedman & Dyson Structural Engineers; The Eli & Edythe L. Broad Foundation; Twining Laboratories; UC San Diego Design and Construction; Verco Manufacturing Co.; Weidlinger Associates, Inc.; and the Structural Engineering Association of San Diego (SEAOSD).