Structural trauma caused by earthquakes can wreak havoc on roads and bridges, particularly when they are poorly engineered and constructed. Even in cases when bridges and overpasses avoid complete catastrophic collapse, they are often rendered unusable after large earthquakes, hampering the flow of traffic as well as rescue and rubble clearing efforts.
The two earthquake-resistant columns will support a new exit bridge ramp from downtown Seattle’s busy state route 99. (Image source: University of Nevada, Reno).
At the University of Nevada, Reno, civil engineering professor and researcher Saiid Saiidi has pioneered technology that will help keep critical infrastructure open and usable following large earthquakes, and downtown Seattle is about to gain the first benefits. The technology – the result of 15 years of research – is currently being used in the construction of a new exit bridge ramp on a section of the busy State Route 99 corridor through downtown Seattle. The ramp, which is supported by special columns designed to shake and flex rather than collapse, even during a major earthquake, is scheduled to be completed in spring of 2017.
The materials are the most critical element of the design. Bridge columns made using memory-retaining nickel-titanium rods, or “Nitinol” – as opposed to steel rebar – and a flexible concrete composite were able to return to their original shape even after an earthquake as strong as a magnitude 7.5.
“The material used in the most critical part of the columns is unique because it’s not actually steel, unlike traditional rebar,” Saiidi said. “It’s made of an alloy of titanium (44 percent) and nickel (56 percent). The material is ‘smart’ because it remembers, as a shape memory alloy, its original shape. If you stretch it or compress it, it goes back to its original length once you let it go. teel cannot recover its elongation once it gets past certain limit.”
The concrete used in the project is known as “engineered cementitious concrete”, or ECC. While it’s not accurate to call it “smart concrete,” it’s still special in its composition.
“The concrete is still superior to ordinary concrete because of its high tensile strain capacity that prevents it from falling apart under back and forth action of earthquakes,” Saiidi said. “There are polyvinyl fibers in the concrete with special coating that control cracking and maintain the integrity of the material.”
Since he began his research, Professor Saiidi has experimented with a variety of combinations of materials to replace traditional concrete and steel rebar materials as well as traditional designs. In his “earthquake simulation lab” at the University of Nevada, Reno, he has built and destroyed 200-ton bridges, bridge columns, and concrete abutments. So how do you simulate an earthquake in a lab? Computer-controlled hydraulics allow the lab to simulate earthquakes of various intensities to test new materials.
“We use shake tables that are able to simulate earthquakes,” Professor Saiidi told Design News . “We typically use the recorded motions from previous earthquakes and duplicate them in the lab.”
An experiment conducted last year in the lab moved a test bridge more than