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NMSU's civil engineers take bridge evaluation to new level

State-of-the-art procedures in bridge evaluation have shown that several New Mexico bridges can handle load capacities that are 10-20 percent higher than originally thought.

Dr. David Jauregui, left, and graduate student Brice Carpenter discuss a computer model of a bridge created by Carpenter. (NMSU photo by Darren Phillips)

Greater bridge load capacities could lead to savings for the trucking industry, eliminating the need for detours around bridges with low load capacities. The new evaluation methods could lead to reduced costs in bridge refurbishment and new construction.

David JŠuregui, associate professor of civil engineering at NMSU, recently concluded a research project for the New Mexico Department of Transportation and is currently conducting research for the Federal Highway Administration utilizing new methods to evaluate load capacities of bridges.

Load ratings determine the level of load a bridge can routinely handle over its service life and also the maximum occasional overload (those exceeding the legal limit, usually 80,000 pounds) that the bridge can safely accommodate.

JŠuregui evaluated the Tortugas Arroyo Bridge in Las Cruces and another concrete slab bridge that was retrofit with a carbon-fiber polymer system on Interstate 10 near Lordsburg, N.M. The new procedures have also been used to evaluate a Las Cruces bridge on U.S. Highway 70 over Sonoma Ranch Blvd. and a bridge near Hatch, N.M., on N.M. Highway 187. These two steel bridges were designed and constructed using an innovative technique, only a few of which actually exist in the U.S. The two graduate students involved in these projects are Brice Carpenter and Andrea Solis.

The NMSU bridge research team uses field testing and computer analysis methods that capture the three-dimensional behavior of bridges. First, sensors attached to various points collect strain measurements as a loaded vehicle travels over the bridge. The data is compared to a finite-element computer model of the bridge (representing the slabs and beams) to determine if the actual behavior of the bridge recorded by the sensors is similar to that of the model.

The new evaluation method could be applied to more than 80 percent of the bridges in New Mexico: those constructed of steel girders, pre-stressed concrete girders and reinforced concrete slabs. The NMSU civil engineering department is contracted by NMDOT to inspect and rate many of the state's bridges.

"The findings will provide NMDOT with an alternative approach for assessing bridge capacity. Having more accurate information will aid transportation officials in making decisions related to important issues such as future rehabilitation work and permit load requests," JŠuregui said.

Normally, bridge load capacities are based on the design assumptions of a bridge and typically these estimates are conservative.

JŠuregui said the new evaluation techniques could be used starting with the construction of a new bridge to create a baseline and track the bridge behavior over its service life to help engineers inspect, evaluate and develop better construction methods. "It's more quantitative - most bridge inspection is visual."

In the case of the retrofit Lordsburg bridge, the analysis was used to determine the effectiveness of the repair. It also showed that the concrete traffic barriers on the sides of the bridge relieved some of the stress to the structure; something that design-based analysis would not have revealed.

"This is a cost-effective application of technology to evaluate bridge performance and supplements visual data gathered during inspections," said JŠuregui. "It's a good investment to ensure the safety and serviceability of our bridges in the long-term."