BRIDGE TO THE PAST
One often hears the phrase, “They just don’t build ’em like they used to.” It’s true of many things, including bridges. In Fort Worth, Texas, local historic preservation groups expressed concerns that historically significant bridge structures in the area could be endangered due to infrastructure upgrades demanded by modern traffic.
In 2011, the West 7th Street Bridge — built in 1913 to carry traffic over the West Fork of the Trinity River — was identified for replacement due to severe deterioration. Historic interest groups and local citizens expressed their concerns about the loss of the old structure.
To help address these concerns, the Texas Department of Transportation’s Fort Worth District turned to Texas A&M Transportation Institute (TTI) Research Engineer Mary Beth Hueste and her colleagues to assist Tarrant County, the City of Fort Worth and Historic Fort Worth, Inc., a nonprofit dedicated to historic preservation, in developing a system to rank historic bridges in the area.
One of the first issues was determining how many bridges in the area were historic due to age, type of construction, type of bridge rail and overall design, among others considerations. From this, a quantitative rating system was used to assess a bridge’s historic qualifications. After TTI assembled a list of 2,860 bridges in Tarrant County, the top 37 historic bridges were identified and ranked.
“The purpose of the research was to identify older bridge structures in Tarrant County that should be retained and kept in operational condition due to their historic value, and to provide a framework for developing preservation plans,” Hueste says. In addition to her position with TTI, Hueste is an associate professor and holder of the E.B. Snead ’25 Career Development Professorship II in the Zachry Department of Civil Engineering at Texas A&M University.
A significant challenge in maintaining historic bridges is to preserve the historic integrity, aesthetic value, commercial value and functionality of bridges while limiting expenditures. In the case of historic bridges, nearly every element of every bridge would benefit from some level of service beyond routine maintenance. Reality and reduced maintenance budgets may limit some work that bridge owners may want to perform.
“Preventive maintenance is the key to preserving these historic structures. Waiting until problems occur can require more maintenance dollars over time and reduce the chances of keeping a bridge in service,” Hueste notes. This new framework for prioritizing significant bridge structures will allow for more of them to be preserved and eventually qualify for inclusion on the National Register of Historic Places maintained by the U.S. National Park Service.
BRIDGE TO THE FUTURE
The Texas Department of Transportation (TxDOT) is always looking for ways to stretch road-building dollars. So, the department tasked Texas A&M Transportation Institute (TTI) Research Engineers Mary Beth Hueste and John Mander, both professors in Texas A&M University’s Zachry Department of Civil Engineering, with designing and testing a new bridge design for spans of 50 feet or shorter.
For two years, Hueste and her team designed and constructed a bridge at TTI’s Riverside Campus Proving Grounds Research Facility. The bridge uses a new concrete spread-slab beam-bridge design instead of the more traditional Texas design that places beams adjacent to each other. Using fewer beams is expected to reduce construction costs.
The test bridge was instrumented with multiple load cells, strain gages and string potentiometers. “We have load cells placed at each beam end so we can determine how the load is being distributed,” Hueste explains. “These data give us more insight into the performance and the best practices for designing this type of bridge.”
After construction and instrumentation, dynamic load tests were conducted with heavy vehicles, including dump trucks and water tankers, both loaded. Researchers measured the vertical deflections of the bridge under heavy loading.
“As we look at the difference in vertical movement from one beam to another, we get a better idea of how that load is being distributed,” Hueste notes. Data obtained from these tests will be used for computer modeling and development of design guidelines.
The results will allow TxDOT to use this new method instead of the conventional side-by-side slab beam technique, currently the norm. Hueste says the use of the new system is expected to be more cost effective, providing safe, long-lasting bridges that save scarce highway construction dollars.