Elastic Connections May Aid Bridge Design
Shape shifting materials
sound like the stuff of science fiction, but they’ve been known for decades and
are used in a wide range of applications, from bio and mechanical engineering
to dentistry. Seismic engineering researchers now have latched on to the material
and promise to expand its use even more by designing “elastic” bridge
connections that would deform during an earthquake but then spring back to its
original shape. If successful, the design would prevent serious structural
damage or collapse and allow a bridge to remain open after a quake, a time when
roads and bridges are critical for emergency response.
Nickel titanium is the primary Shape Memory Alloy civil
engineering professor M. Saiid Saiidi is
working with at the University of Nevada, Reno, because of its
“superelasticity,” he says. Other SMAs are generally only temperature
sensitive, requiring a heat source to return to its original shape. Nickel
titaniumis a bit different and has 10 to 30 times the elasticity of steel or
other standard metals. In many ways it performs like steel, but it differs in
its ability “to undergo a large deformation and come back to its original
shape,” Saiidi adds.
Building robust bridges and
other significant infrastructure in highly seismic areas such as California and
the western U.S. could significantly lessen damages after a large quake. The
1994 Northridge earthquake in southern California caused an estimated $20
billion in damage to roads and buildings.
Shake Tests
With funding from the National Science Foundation using the
foundation’s George E. Brown, Jr. Network for Earthquake Engineering
Simulation, Saiidi and colleagues tested the material at Nevada-Reno’s shake
table. They used bridge models of 120 feet to 130 feet long, with three types
of columns: standard reinforced steel and concrete, nickel titanium and
concrete, and nickel titanium and engineered cementitious composites, with
cement, fiber, water, and chemicals. The columns were first tested using
Open SEES, and earthquake simulation
program developed at the University of California, Berkeley, and then built and
tested on the shake table. The columns with nickel titanium outperformed the
standard design under forces equaling or exceeding those of a magnitude 6 quake
on the Richter scale, Saiidi says.
“In
bridges, typically the column is most susceptible [to seismic shaking],
specifically at the end zone at the top and at the bottom, by the footing,” he
says. Because nickel titanium is much more costly than standard steel
reinforcing bar, the researchers specified its use only in those zones, a
little more than one-tenth the length of the overall column. Nickel titanium
bars were assembled into cages just as a standard rebar cage, and connected to
the column’s main cage in the plastic hinge zones, he says.
Saiidi’s test caught the attention of the Federal Highway Administration and Washington State’s Department of Transportation, which funded similar research. The agencies are using the design for a highway ramp that is expected to begin construction in 2015.
History
Nickel titanium—the
generic name for the family of alloys is nitino—has been around since the early
1960s from work at the Naval Ordnance Laboratory. It has many applications in a
number of disciplines. Among other applications, the U.S. military has used nitinol
couplers in F-14 aircraft to join hydraulic lines. In medicine, the material is
used as a guide for catheters in blood vessels and anchors to attach tendons to
bone in orthopaedic surgery. It also is used in robotic actuators and
micromanipulators to simulate muscular movement.
“The first time I
learned about it was from a mechanical engineer building an actuator,” says
Saiidi. “Later, the question I asked myself was, ‘What if we used it inside a
concrete column in a bridge in a high seismic zone?’”
That
was about twelve years ago, and Saiidi’s research has progressed, also using
other copper-based shape memory alloys. He says copper holds promise but civil
seismic work is hampered by its scarcity. Only one manufacturer—in
Japan—currently fabricates the material.
Cost-Benefit
Nitinol also costs more than standard steel and
suffers from limited sourcing. Saiidi says a bridge built with columns
incorporating nickel titanium will cost about 3% more than a structure with
standard materials. The savings will come in reduced maintenance costs—nickel
titanium is highly resistant to corrosion—and its ability to withstand seismic
forces, he says.
“Several
manufacturers make SMA wires,” he says, noting nitinol is commonly used for
orthodontic braces and eyeglass frames that “bounce” back to their original
shape if bent. “Only a few make bars.” In Washington, the bridge design using
the material calls for sizes up to 30 millimeters, he says.
“But
if you look at lifecycle costs, it will become economical after an earthquake.
You won’t have to shut the bridge down,” he says. “After an earthquake is when
you need bridges the most.”
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