The Mineta National Transit Research Consortium’s latest peer-reviewed study, Fatigue Evaluation of the Increased Weight Limit on Transit Railway Bridges, proposes a method to determine remaining “fatigue life” of steel railway bridges for transit lines based on analysis and field testing. For example, heavy freight cars and their bridge-crossing frequency were found to have a significant effect on the critical locations near the supports and short span bridges. This data can help transit agencies and operators to prioritize and schedule repairs and rehabilitation. Principal investigator was Hani Nassif, PhD, PE, working with Kaan Ozbay, PhD; Peng Lou, PhD candidate; and Dan Su, postdoctoral associate.
The report can be found at http://transweb.sjsu.edu/project/1143.html
Nassif said, “Transit agencies own and operate thousands of bridge structures that undergo repetitive train crossings, leading to what is known as load-induced structural fatigue. The majority of these bridges were built a century ago, and many have exceeded their theoretical fatigue lives, which could cause partial or complete bridge failure.”
After the failure of the Minnesota I-35 Bridge on August 1, 2007, left 13 people dead and 145 injured, several transportation and transit agencies have recently begun self-evaluating their bridge stock. This research report offers the advanced tools necessary to catalog thousands of bridge structures with periodic train schedules and weight data. The proposed approach can be applied to existing steel bridges.
Increased weight limits can compound the issue
The closure time of track lines and obstruction for passengers can be disruptive and costly, especially for lines with only one active track. Furthermore, the recent increase of freight railcar weight limits from 263,000 lbs. to 286,000 lbs. raises additional concerns for transit passenger rail systems bridges if those systems were not designed to withstand the increased weight. It is necessary to assess the impact of the increase in railcar weight on those bridges prior to allowing passenger lines for freight train use.
“The simplest analysis involved a review of the regional passenger and the freight train load data regarding weight, volume, and the number of crossings,” said Nassif. “The next analysis level involved simulation, where stresses on bridge girders and/or components were determined using finite element analysis methods. Results from field tests helped to verify and validate these computer models, and the proposed approaches were applied to existing steel railroad bridges.”
Researchers developed a probabilistic model for the fatigue evaluation of railway bridges. They considered various random loading variables, including annual train frequencies, dynamic impact, passenger volume, and freight car loading. The probabilistic fatigue load spectra were derived using the Monte Carlo simulation and the Rainflow Counting method. In terms of resistance, the relevant S-N curves were randomized with constant variance in fatigue strength.
Infrastructure components must be protected and managed
Infrastructure systems constitute a major part of the national investment and are critical for society’s mobility as well as its economic growth and prosperity. The US has invested an estimated $25 trillion in civil infrastructure systems, including all installations that house, transport, transmit, and distribute people, goods, energy, resources, services, and information. Infrastructure system components, such as bridges, tunnels, traffic systems, road pavement, airports, seaports, dams, and other systems, are considered assets that should be protected and properly managed.
Nassif said, “Unfortunately, the current degree of deterioration and damage from natural and malicious events leaves the system dangerously vulnerable. Major decisions must be made to allocate limited funds for maintaining and safeguarding the nation’s bridges. To maximize long-term cost efficiency, solutions should be based on an integration of various computational models and simulations, as well as on infrastructure deterioration models.”
The methodology recommended in the report is a practical tool to perform fatigue analysis for an owner or agency managing hundreds of structures with different regional trainload characteristics. Recommendations for appropriate bridge maintenance are provided, as well, to help operate the bridges safely and cost-effectively.
