Tested on the world's largest shake table measuring 64x49 feet, in Miki City Japan, the E-Defense system, which can recreate the conditions of large earthquakes, tests to 7.5 on the Richter scale. The tests were conducted at the Hyogo Earthquake Engineering Research Center, to ascertain if a six-story, timber-framed condominium block, atop a one story of concrete and steel representing the ground level retail, could survive intact in the face of a strong earthquake.
The structure of the seven-story building, measuring at 14,000 square feet with 23 residential one and two-bedroom units for living space, was designed specifically not to exceed certain inter-story drift limits, which correlate strongly with damage and loss, and was tested three times with simulated earthquakes, ranging from 6.7 to 7.5 on the Richter scale. Carefully examining the building for signs of damage, including cracks and ruined stairs, the researchers found only cosmetic damage to the building.
The shake test is part of a larger experiment in which engineers are trying to use wood – a cheap, abundant material - for earthquake-proof construction projects. With earthquake research typically focusing on testing high-tech materials, such as flexible concrete or metal alloys, a group of researchers from five universities are trying to earthquake-proof wood mid-rise buildings, as wood is more prevalent in the construction business.
The first design changed the pattern of nails in a building to better distribute stiffness among different floors. John van de Lindt, an engineer at Colorado State University, US, explains that tall wood buildings in an earthquake are vulnerable to what can be called ‘soft story’, a phenomenon in which one story does not remain as stiff as the floors above it.
Though engineers previously took into consideration only a building's initial stiffness, this new model is based on measurements of how stiffness changes in a building during an earthquake. After looking at the pressure points around the building, engineers changed the nail patterns to make points of strength that coincided with the structural pressures experienced during a quake.
To add stability, the researchers also used 63 anchor tie-down systems from Simpson Strong-Tie. Running from the building's steel-frame foundation to the roof, these steel-rod systems work to prevent the building from rocking. Steel straps and plates were attached at adjacent levels to resist shearing - the tendency for different levels to slide sideways relative to each other, and come apart.
The quake is simulated by forcing large amounts of fluid in and out of hydraulic pistons. Those pistons, placed under the table, move it up and down, while others cause left and right, as well as front and back motions.
E-Defense, with the capacity to support models weighing up to 2.5 million pounds, is the only shake table in the world able to accommodate the seven-story condominium, weighing almost 1 million pounds.
Meanwhile, the University at Buffalo, New York, will test two 72-feet bridges, under earthquake-like vibrations, for another earthquake project. These bridges use seismic isolation technology, which isolates a structure from its foundation to prevent it from feeling ground vibrations. Tests on these bridges are scheduled to begin in July 2010.