March 21st, 2012

STRONG MOTION

Preserve Infrastructure, by Catherine Higgins.

Placing instruments that measure strong motion can help departments of transportation design and build bridges that can withstand an earthquake.

A researcher places downhole instrumentation for monitoring seismic activity. Placing and maintaining monitoring equipment is expensive, so researchers working with UDOT have identified the best and most cost effective sites for other potential locations.

It’s important that bridges on interstates withstand a potential earth quake – experts agree that Utah “is a seismically active region”  with the potential of experiencing a major earth quake some day. The primary threat from earthquakes is the intense shaking that can cause structures, including bridges, to collapse. UDOT has taken that eventual future event into account in and has built structurally sound bridges and retrofitted existing bridges. Standards for bridge design and construction are based on past research using data collected from actual earthquakes.

Research can help provide more information about how to design and build bridges that can withstand earthquakes. UDOT has one seismic station in the I-15 spaghetti bowl. The protected equipment is monitored to make sure it continues to be in working order. Placing and maintaining equipment is expensive, so finding the best and most cost effective sites for other potential locations is important.

UDOT has identified a process for determining the appropriate location of other stations, if funding is identified,  in a report just issued by the Research Division. Research Project Manager David Stevens explains that having a method to select the best sites is important “so that the information is useful, not redundant.”

Researchers place instruments on bridges– including accelerometers and other equipment – designed to collect data on how quake motion can affect bridges. A new UDOT Research Division report identifies criteria to consider when placing equipment:

  • Proximity – Bridges to be instrumented should be chosen based on close proximity to an identified fault line. However, placing the equipment near another instrumented bridge near the same fault line could mean the two sites collect essentially the same information, so the data could be redundant and not useful.
  • Importance – Three factors help identify how critical the bridge is to the transportation system. ADT measures are easy to come by and a good indicator of public dependence on the bridge. The number of viable alternative routes and the current value the bridge also point to the relative importance of the bridge.
  • Structural form – For initial data collection, a simple structure is best “to lay a foundation of strong motion behavior knowledge” and then progress to collecting data from more complex bridges. Researchers describe a simple bridge as one with no skew, one span, two girders and no curvature.
  • Local soil – Seismic events affect soil types differently, so bridges chosen for instrumentation should be distributed among different soil types.
  • Age Older bridges that are due to be replaced are obviously not good candidates for instrumentation.

Continuing research in Utah and other states can help contribute to the body of knowledge about how bridges react during a major seismic event. For more about UDOT’s Strong Motion Instrumentation Plan, including details about a decision-making process and also types of equipment that could be used, read the report.

The UDOT Research Division currently oversees and maintains downhole instrumentation and equipment near the I-80 to SR-201 flyover bridge.  UDOT Structures Division and UDOT Research Division “work together to keep it ready for recording strong motion, as well as to explore near-term research uses for the instrumentation,” according to Stevens.

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