Construction crews finished work in late May on the South Dearborn Street off-ramp bridge in Seattle, part of the Alaskan Way Viaduct Replacement Program.

The bridge is one of 32 projects in the larger program, and will handle traffic heading to sports stadiums or downtown Seattle.

What makes this span special is that it is testing a method for making bridges earthquake resistant. It was built using memory-retaining metal rods and bendable concrete composite. These elements are expected to let the bridge to flex when the ground shakes, then snap back to its original shape once the shaking ends.

Current Seismic Design Philosophy

The Washington State Department of Transportation says that the current seismic design philosophy for most bridges is to maintain "life safety" by providing a low probability of collapse. It is uneconomical to construct a bridge that can withstand expected earthquakes without some damage. So, significant damage to some elements of a bridge is considered necessary and acceptable.

For concrete columns, damage could include concrete cracking and/or spalling and rebar stretching, the agency says. These could lead to permanent deflection or considerable leaning of the bridge.

The damaged conditions may require the bridge to be restricted to limited access, or the bridge may be closed. Potential damage may even restrict emergency vehicle access.

The location where this damage occurs typically is in the hinge region of columns. Generally, these are in the vicinity of the top and/or bottom of columns, where they connect either to the foundation or crossbeams.

Repairing columns may require patching broken concrete, shoring for the entire column replacement, or even complete bridge replacement.

Thanks for the Memory

Shape memory alloys (SMAs) are a special type of metal manufactured from either a combination of nickel and titanium, or are copper based.

These alloys have been used in robotic, automotive, biomedical, and aerospace applications. For a bridge application like the one being tested in Seattle, the metal is shaped into round bars to replace steel rebar in certain locations on bridge columns.

Seattle's waterfront and downtown. Credit: Washington DOTSeattle's waterfront and downtown. Credit: Washington DOTWashington DOT says that steel rebar used in bridges is similar to paper clips. If bent a little, it will spring back to its original shape. If bent too much, of course, the paper clip won’t spring back and has exceeded the yield point. Earthquake energy is dissipated using this seismic design philosophy, by stretching steel rebar beyond the point that it won’t spring back.

With SMA, the alloy deforms like steel beyond the yield point, but will spring back. This means the alloy is superelastic. Energy can still be dissipated by stretching the SMA.

The SMA isn’t needed for the entire height of the column in part due to its expense compared to steel rebar. So, the SMA is only used in certain regions of the column. It is connected to steel rebar using couplers. The couplers require the SMA and steel to be headed at the ends. The coupler slips over the head and screws together, to attach the SMA to the steel rebar.

In addition, engineered cementitious composite is used in the column wherever the SMA is located. ECC is similar to conventional concrete, except that is has an added ingredient. Small polymer fibers are mixed in to stop cracking when the columns are bent during an earthquake.

A research team at the University of Nevada – Reno, led by Dr. M. Saiid Saiidi, have been testing SMA and ECC with various types of equipment. Materials have been tested in tension, compression, bending, and shear. Researchers also have used shake-tables to simulate a real earthquake with a one-quarter-scale bridge.

Three scaled-down bridge columns were tested to evaluate the performance of SMA and ECC. Two columns were constructed with SMA and ECC. The other column was similar to the current column construction using conventional concrete and steel rebar, used to compare and contrast the results of the SMA and ECC.

Each column was subjected to displacements similar to an actual earthquake. The columns were pushed and pulled at increasing stages. Tests incremented with higher and higher force until the ultimate capacity of the SMA and ECC were reached. The test results showed a significant reduction in permanent displacement and limited damage as compared to the conventional construction.

The Alaskan Way Viaduct, an elevated section of State Route 99 in Seattle, was built in the 1950s, and decades of daily wear and tear have taken their toll on the structure. Because of the viaduct’s age and vulnerability to earthquakes, replacing it is critical to public safety.

Project Elements

The Alaskan Way Viaduct Replacement Program includes projects led by the Washington State Department of Transportation, King County, the City of Seattle and the Port of Seattle. The Federal Highway Administration is a partner in this effort.

Major elements include:

  • A two-mile-long tunnel beneath downtown Seattle.
  • A mile-long stretch of new highway that connects to the south entrance of the tunnel, near Seattle’s stadiums.
  • A new overpass at the south end of downtown that allows traffic to bypass train blockages near Seattle’s busiest port terminal.
  • Demolition of the viaduct’s downtown waterfront section.
  • A new Alaskan Way surface street along the waterfront that connects SR 99 to downtown.