An earthquake-resistant structural system, successfully tested in Japan, will not only help a multi-storey building hold together during a violent earthquake, but return it to an upright position afterwards, with damage confined to a few easily replaceable parts.
The team that designed the system was led by researchers at Stanford University and Illinois University. During testing on a massive shake table, the system survived simulated earthquakes in excess of magnitude 7, bigger than the 1994 Northridge earthquake and the 1989 Loma Prieta earthquake in California.
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 and all the parts can be made using existing fabrication methods.
‘What is unique about these frames is that, unlike conventional systems, they actually rock off their foundation under large earthquakes,’ said Greg Deierlein, professor of civil and environmental engineering at Stanford, who led the team.
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 centre 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 correct alignment.
At the bottom of the frame sit steel ‘fuses’, designed to protect the rest of the building from damage.
‘The idea of this structural system is that we concentrate the damage in replaceable fuses,’ Deierlein added.
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 system at the Hyogo Earthquake Engineering Research Center in Miki City, Japan, using different types of fuse and various shaking parameters.
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 Illinois.
Deierlein, who is the principal investigator on the project and oversaw the testing in Japan, said that while various researchers have been working for 10 or 15 years on some of the ideas and techniques encompassed in the system, this is the first time anyone has put them all together to demonstrate their performance.
Deierlein said the system is applicable to steel-framed buildings up to about 15 storeys tall, but that the general approach could be modified for other types of buildings.
Schematic diagram of the rocking frame set up for shake-table testing. The steel-braced frame is shown in red. The white structure behind the frame simulates the weight of a three-storey building. The inset shows the replaceable steel fuse, in yellow, at the base of the rocking frame. Behind and in front of the fuse are the vertical steel cables that pull the building back into plumb after an earthquake. During testing, the frame was sandwiched between two of the white structures