A new design for the framework of columns and beams that support bridges could speed construction and reduce earthquake damage.
The new design, devised by engineers at the University of Washington in Seattle, allows the columns and beams that make up the framework, known as a ‘bent’, to be pre-fabricated off-site before being shipped to where they are needed and connected quickly, rather than the current time-consuming process of casting concrete in situ.
And unlike previous methods, the new design allows the high-strength steel cables inside the columns that help create resistance to earthquake damage to be pre-tensioned and embedded in the concrete at the plant where they are pre-fabricated.
“Pre-fabricating means the pieces need to be connected on-site, and therein lies a major difficulty," said John Stanton, a professor in the university’s Department of Civil and Environmental Engineering.
"It is hard enough to design connections that can survive earthquake shaking, or to design them so that they can be easily assembled, but to do both at once is a real challenge.”
The high-tensioned steel cables act to re-centre the columns after an earthquake so they are vertical and not leaning over at an angle, meaning the bridge can be used by emergency vehicles in the critical moments immediately following the event.
“A good analogy is to think of a series of a child's wooden building blocks, each with a hole through it. Stack them on top of one another, put a rubber band through the central hole, stretch it tight and anchor it at each end,” said Stanton.
“The rubber band keeps the blocks squeezed together. Now stand the assembly of blocks up on its end and you have a pre-tensioned column. If the bottom of the column is attached to a foundation block, you can push the top sideways, as would an earthquake, but the rubber band just snaps the column back upright when you let go."
The team’s work, which is due to be presented in a paper during Quake Summit 2014, adapts technology that was pioneered in the building industry in the 1990s. According to Stanton, the design of reinforced concrete bridges in seismic regions has barely changed since the mid-1970s.
When the columns rock during an earthquake, they experience high local stresses at the points of contact, and without special measures the concrete there would crush.
To counteract this possibility, the researchers protected the ends of the columns with short steel tubes, or "jackets," that confine the concrete, not unlike the hoops of a barrel, or the steel cap that ranchers use to protect the top of a fence-post while driving it into the ground.
"Cyclic tests of the critical connections have demonstrated that the system can deform during strong earthquakes and then bounce back to vertical with minimal damage," Stanton said.
Those tests were conducted on individual connections and in July, the team will test a complete bridge built with the system at 25 percent of full-scale on the earthquake-shaking tables at a facility at the University of Nevada, Reno.
The columns, which also contain some conventional rebar for extra support, have been made using only common construction materials, which Stanton says should smooth the way for owners and contractors to adopt the new approach.