|
Home > Resources > Featured Editorial
Earthquake-Proof Buildings Designed September 3, 2009
An earthquake-resistant structural system for buildings designed by a team from Stanford Univ. and Univ. of Illinois will not only help a multi-story building hold itself together during a violent earthquake but also return it to standing up straight on its foundation afterward.
The design, which was tested successfully in Japan, survived simulated earthquakes in excess of magnitude 7, bigger than either the 1994 Northridge or 1989 Loma Prieta earthquakes in California.
After a shaking test, a (formerly flat) steel fuse shows the deformation caused by the energy it dissipated during shaking. The fuses are designed to absorb damage and are easily replaced after an earthquake. | "This new structural system has the potential to make buildings far more damage resistant and easier to repair, so people could reoccupy buildings a lot faster after a major earthquake than they can now," says Greg Deierlein.
The system dissipates energy through the movement of steel frames that are situated around the building's core or along exterior walls. The frames can be part of a building's initial design or could be incorporated into an existing building undergoing seismic retrofitting. They are economically feasible to build, as all the materials employed are commonly used in construction today and all the parts can be made using existing fabrication methods.
"What's unique about these frames is that, unlike conventional systems, they actually rock off their foundation under large earthquakes," says Deierlein.
The rocking frames are steel braced-frames, the columns of which are free to rock up and down within steel "shoes" secured at their base. To control the rocking and return the frame to vertical when the shaking stops, steel tendons run down the center of the frame from top to bottom. These tendons are made of high-strength steel cable strands twisted together and designed to remain elastic during shaking. When shaking is over, they rebound to their normal length, pulling the building back into proper alignment.
At the bottom of the frame sit steel "fuses" designed keep the rest of the building from sustaining damage.
"The idea of this structural system is that we concentrate the damage in replaceable fuses," says Deierlein. The fuses are built to flex and dissipate the shaking energy induced by the earthquake, thereby confining the damage. Like electrical fuses, the steel fuses are easily replaced when they "blow out."
Deierlein and his colleagues conducted shake testing of the new system in the last few weeks at the Hyogo Earthquake Engineering Research Center in Miki City, Japan. Using different types of fuses and various shaking parameters, they conducted four major tests. They had previously developed and tested the individual components of the system and performed computational analyses to simulate the system's performance at laboratories at Stanford and the Univ. of Illinois.
The tests of the system were conducted using a three-quarters size model of a standard modern three-story office building with a footprint 120 by 180 ft. The 26-ft tall model sat on a massive vibrating shake table-–the largest in the world, measuring over 3,000 ft2–-that's designed to reproduce the shaking from different earthquakes.
For testing, Deierlein's group constructed a complete three-story steel-braced frame that is sandwiched between two concrete and steel structures in which they concentrated all the mass that would normally be in a building that size. Each of the three stories weighed 100 metric tons.
The researchers subjected their model to ground motions recorded during the 1995 Kobe, Japan, earthquake, magnitude 6.9, and the 1994 Northridge earthquake, magnitude 6.7. The U.S. Geological Survey characterizes the Northridge temblor as the most costly in U.S. history, with losses estimated at more than $40 billion. The Kobe earthquake caused over 6,000 fatalities and economic losses are estimated to have been three to five times greater than Northridge.
For some of the shake tests, Deierlein said his group amplified the ground motion shaking from the actual earthquake records to simulate the shaking that would happen during the largest earthquake that each fault is considered likely to generate.
For the fourth and final test, the group used a motion from the Northridge earthquake and scaled it up 1.75 times greater than the recorded motion, well in excess of the Maximum Considered Earthquake. "The only damage that occurred to the test frame was in the replaceable fuses," says Deierlein. "This final test demonstrated that the rocking frame is a reliable and effective system."
"Most buildings that we design today for large earthquakes are designed such that when there is a large earthquake, the building, in a sense, sacrifices itself to save the occupants," Deierlein said. Buildings that survive earthquakes often have to be torn down because they are too deformed or damaged during the shaking for it to be economical, or even physically possible, to repair them.
Source: Stanford Univ.
|
|
|
|
|
|
