Aggie Ingeniero, June 2013

NMSU Bridge Inspection Program helps keep drivers, commercial traffic safe

The recent bridge collapse on Interstate 5 in Washington serves as an unfortunate reminder that much of our nation’s infrastructure is aging and requires inspection on a regular basis to keep both commercial and personal travelers safe. In New Mexico, the New Mexico State University Bridge Program, housed in the Civil Engineering Department of the College of Engineering, does much of that work for the New Mexico Department of Transportation.

David Jauregui, the Wells Hatch Professor of Civil Engineering at NMSU and head of the NMSU Bridge Inspection Program, says that the incident in Washington is an example of what can happen when non-redundant bridges – those without multiple fail-safe systems in place – are damaged.

The program Jauregui leads is about to begin a new research project on bridges in New Mexico – those that are older and might have engineering plans that are too faded to read, or that have been lost during the long service lives of the bridges – to protect New Mexico travelers from similar catastrophe.

“NMSU and NMDOT have a great working relationship,” said NMDOT Bridge Management Engineer Jeff Vigil. “NMSU has provided structural experts to aid our department for many years. Their work helps to ensure the safety of our structures.”

Bridges without existing design plans is not just a New Mexico problem.

“Between 10-15 percent of bridges in the U.S. don’t have design plans [on file],” Jauregui said. “The owner of the bridge may be a local government agency, a city or a county, and these particular agencies may not have a lot of bridges on their inventory. Over the years, this documentation may be misplaced or lost. Or, ownership of the bridge was transferred from one agency to another, and as a result, they no longer have the documentation.”

The typical lifespan of a bridge when the interstate system was first being designed was about 50 years; many of those bridges from the 1950s are either at or near their service life end. Newly constructed bridges have expected service lives of 75-100 years. Material durability, the quality of the concrete and the quality of the steel used to build the structures are important factors in the longevity of the structures.

“We’re also concerned with limiting defects like cracking in concrete, corrosion of steel – those are factors that can shorten the life of a bridge. By ensuring the quality of the material is high, we’re moving toward making the bridge durable for many years.”

One of the major bridge issues in the country is the ever-increasing load levels going over bridges. With heavy truck traffic, those loads tend to at times exceed the designed load limit of bridges. Because of that, bridge components can become overstressed and cracking, buckling and crushing can occur.

When evaluating a bridge without plans for the first time, NMSU engineers will start with an overall visual inspection of the structure. They evaluate the deck, superstructure, substructure, channel and channel protection, and approach roadway alignment for each structure.

“The primary method we use is visual inspection, which requires us to make a field visit to a bridge to get within arms length of an element,” Jauregui said. “You need to be able to see it up close and that’s important because you’re looking at things like the width and length of concrete cracks and how much section loss you have on a steel component that has corroded. A professional engineer will lead a team to document these type of effects based on visual observations.”

Other techniques employed in the field include sounding, surveying the top surface of the bridge deck to determine if and/or how much settlement has occurred and general documentation of the clearances under the structure, both lateral and vertical.

Jauregui’s team, comprised of four graduate students from around the world and two NMSU colleagues, Craig Newtson and Brad Weldon, will meticulously examine bridge structures in NMDOT Districts 1 (Dona Ana County area), 2 (Roswell area) and 3 (Albuquerque area).

“Our first step is to visit the bridge site and collect information related to not only the condition, but also the geometry that exists. Dimensions like the width of the beams, the spacing between the beams and the thickness of the deck are going to be crucial so that we can then move forward to create a sophisticated computer model of the structure,” Jauregui said. “We’re also going to be doing non-destructive testing with equipment like a Ferroscan, which is basically a rebar scanner, that will allow us to scan the side of a beam and determine the spacing of the steel reinforcement, the concrete cover to the reinforcement, and using this information, we will then start planning some field testing in addition to the computer analysis.”

The group will use the measurements and estimates of the reinforcement on concrete bridges to perform diagnostic load testing and proof testing after creating the computer model. These are experimental tests conducted using a typical loaded dump truck provided by the NMDOT.

“As we load the bridge with a dump truck, we will monitor deformations such as strains, vertical deflections, and based on those measurements, we’ll come back to the office and calibrate the computer so that it fits the field measurements,” Jauregui said. “By doing so, we have confidence of how the structure is responding to the load.”

The NMSU team uses equipment to very carefully monitor how the bridge deforms under the heavy loads.

“The last thing we want to do is damage the structure,” Jauregui said. “We can plan the test and also stop the test if we find that perhaps damage is occurring. We don’t want to damage the bridge during the test.”

Data collection will begin as soon as notice to proceed is given. The project will last for about two years. At the conclusion of the project, load capacities for the bridges will be filed with the NMDOT and other appropriate agencies, which will allow for more accurate inspections in the future.

“It’s important for us to maintain safe structures for the traveling public in New Mexico,” Vigil said. “It’s especially important for industries like mining, agriculture, oil and gas that we have the proven capacity to carry the loads they need to transport their products across the state.”

And Don’t Miss These Stories