WRITER: Kristen Sullivan
One of the highest bridges in the United States, the Rio Grande Bridge has stood proudly for 50 years. As the bridge enters its 50th year, a group of New Mexico State University engineers and students through the Bridge Inspection Program conducted an inspection to determine the status of the welded and bolted connections as well as examine the steel for signs of corrosion for the New Mexico Department of Transportation.
Located 10 miles northwest of Taos, New Mexico, the bridge has trusses approximately 100 feet deep from bridge deck to the top of the piers which narrow down to 20 feet at the top of the center span. The first NMSU inspection took place in 1973, just eight years after the bridge was dedicated in 1965.
NMSU’s Bridge Inspection Program, in the Civil Engineering Department, handles a large number of inspections for the NMDOT. David Jáuregui, department head of civil engineering, directs the program. NMSU has a long history of cooperative partnership with NMDOT, beginning in the 1950s with the joint direction of the Annual Transportation Engineering Conference—an event that continues today with the 2015 conference held in Las Cruces this past April.
Co-op undergraduate students in the NMSU College of Engineering perform on-site bridge inspections and are supervised by professional engineers. Undergraduate students travel throughout the state and inspect approximately 150 bridges as part of a six-month co-op assignment. Graduate students, focusing in structural engineering, conduct load capacity ratings.
Both the bridge inspections and ratings are conducted in accordance with professional standards distributed by the NMDOT, the Federal Highway Administration and the American Association of State Highway and Transportation Officials.
“The major goal of the NMSU Bridge Inspection Program is to ensure that highway bridges continue to provide safety and serviceability to the traveling public,” Jáuregui said. “The documentation that we provide to the NMDOT is used to make decisions related to future maintenance, rehabilitation, retrofit or replacement of structures. The basic approach in which an engineer would inspect any type of structure that experiences some type of damage imposed by an event, such as an overload, earthquake, flood or wind, can be related to bridge inspection. NMSU students participating in the program gain technical expertise and valuable experience they can apply in the evaluation of various structural systems such as bridges, buildings, culverts, and even historical monuments.”
Jáuregui directs a team that includes eight undergraduate students, 10 graduate students, two professional engineers, and three NMSU colleagues, Professor Emeritus Ken White, Professor Craig Newtson and Assistant Professor Brad Weldon. Jáuregui said the engineering skills students are learning are not limited to bridge inspection; they are really learning about the field of forensic engineering.
“An added benefit of the program is that once the students finish this six-month appointment they are given other opportunities through the NMSU Department of Civil Engineering to pursue applied research related to capacity evaluation of structures, which directly ties into the inspection,” Jáuregui said. “We are investigating and developing analytical and experimental techniques to determine the load capacity of New Mexico bridges, in particular those without design plans, and other states have expressed interest in adopting our procedures.”
Research is an important part of the relationship between NMSU’s Bridge Inspection Program and NMDOT. The first “smart bridge” incorporating fiber-optic technology was installed in 2004. Developed by NMSU faculty, this technology has since been installed in two additional NM bridges. Other ongoing research projects involve non-destructive evaluation techniques such as acoustic emission and ultrasonic testing, and innovative construction materials and design.
One such project is the testing of ultra-high performance concrete bridge girders on a large scale. Civil engineering graduate students, under the direction of Weldon, are conducting this research to aid the development of bridge design procedures for the state of New Mexico that could lead to a variety of improvements to the state’s infrastructure.
The concrete possesses dramatically increased compressive strengths, a very dense microstructure, and steel fibers that greatly improve post-cracking strength. These properties allow for the design of bridges that can have much longer design lives compared to those constructed with normal strength concrete. A modified mixture using primarily local products, developed by Professor Craig Newtson and his graduate students, drives down the ultimate cost of the final product.
Normal strength concrete bridges are designed to last approximately 50 years, UHPC bridges are estimated to have design lives of up to 150 years. Additionally, UHPC is significantly stronger, exceeding 22,000 pounds per square inch, as compared to the strength of average concrete between 4,000 – 6,000 psi.
Currently, no bridge design specifications for concrete of this strength exist in the United States. The unique material properties are not accounted for in the current design standards. The next phase of Weldon’s research includes the instrumentation of a new bridge being constructed in New Mexico using UHPC. This will be the first bridge in the US constructed with non-proprietary UHPC.
“Students come out of the program with a very special and unique set of skills that is not totally available at other universities in the U.S. I’m very proud to be the director of this program and appreciative of the collaboration we have with the NMDOT in bridge-related projects that contribute to the safe travel of the public here in New Mexico,” Jáuregui said.