Researchers at the University of California, San Diego’s (UCSD) Jacobs School of Engineering have applied post-tensioning, a technique commonly used in the construction of concrete buildings and bridges, to create a new class of weld-free steel-framed structures.
An initial test conducted in 2001 on a large-scale assembly indicated that the post-tensioned steel frames could absorb strong earthquake motions with little or no damage. A further large-scale test is scheduled for May-June, 2002.
‘By adding tremendous tension to rods that run horizontally through the beams and columns, squeezing them together like an elastic band, a steel frame can be constructed without welded joints,’ said Andre Filiatrault, professor of structural engineering.
After the Northridge, California earthquake in 1994, it appeared that steel-framed structures had escaped unharmed. However, upon careful inspection it was discovered that the joints connecting the beams to the columns had lost structural integrity. These welded joints were fractured and had large cracks which propagated all the way through the columns. ‘The structures had very little resistance left and probably would have collapsed during another large quake,’ explained Filiatrault.
Since 1994, much research has been underway to remedy the problem and the UCSD design offers one alternative solution.
‘Strong post-tensioned rods clamp the beams to the columns, holding them together,’ said doctorate student Constantin Christopoulos. ‘These high-strength rods provide the spring force, which allows the system to return to its initial position after it is deformed during an earthquake. But there is still a need to dissipate energy, so we introduced some unique sacrificial bars.’
On the top and bottom of the beams where they meet the columns, a weaker sacrificial bar, contained in a cylinder to avoid buckling, slides back and forth during an earthquake. Another unique feature is that the behaviours of the structure can be modified as its function changes over time, which cannot be done with welded joints.
The initial real-time test in 2001 was conducted on a three-column, two-beam assembly. The results validated the behaviour of the proposed weld-free connection in a frame system under realistic earthquake loads. The loads were said to be representative of the displacement expected between two adjacent floors of a building during a strong earthquake.
According to Filiatrault, various forms of post-tensioning have been used on concrete structures for many years, but never on steel-framed structures. ‘This concept should really benefit the steel industry. One or two workers can add tension to the rods and squeeze the frame together very quickly. This may equate to less ‘dead time’ and cost savings,’ said Filiatrault.