Shape Memory Alloys Aid Design of Earthquake-Resistant Bridges
Posted on: 09/05/2013
University of Nevada, Reno (UNR) researchers have identified several “smart materials” that could be used as alternatives to steel and concrete in the development of more earthquake-resistant bridges. Specifically, shape memory alloys (SMAs) have demonstrated an ability to re-center bridge columns, which minimizes the permanent tilt columns can experience after an earthquake.
Nickel titanium, or nitinol, the shape memory alloy tested in the UNR project, has shown particularly unique abilities. While the majority of SMAs are only temperature-sensitive, nitinol is also superelastic. This means that it can absorb the stress imposed by an earthquake and return to its original shape, which makes nitinol a particularly advantageous alternative to steel. In fact, the superelasticity of nickel titanium is between ten to 30 times the elasticity of normal metals like steel.
To assess the performance of nickel-titanium reinforced concrete bridges, the researchers analyzed three types of bridge columns: traditional steel and concrete, nickel titanium and concrete, and nickel titanium and engineered cementitious composites (ECC), which include cement, sand, water, fiber and chemicals. First, they modeled and tested the columns in OpenSEES, an earthquake simulation program developed at the University of California, Berkeley. Finally, they assembled and tested the columns on a shake table.
To strengthen the concrete and prevent immediate failure in an earthquake, the researchers used the shake tables to test glass and carbon fiber-reinforced polymer composites. Both composites substantially enhanced the reinforcing properties of concrete and the columns resisted strong earthquake forces with minor damage.
The results of both the modeling and shake table tests were extremely promising. The nickel titanium/ECC bridge columns outperformed the traditional steel and concrete bridge columns on all levels, limiting the amount of damage that the bridge would sustain under strong earthquakes.
While the initial cost of a typical bridge made of nickel titanium and ECC would be about 3 percent higher than the cost of a conventional bridge, the bridge's lifetime cost would decrease. Not only would the bridge require less repair, it would also be serviceable in the event of moderate and strong earthquakes.