October 22, 2019
By Yanling Leng, PhD, PE | Senior Project Engineer | firstname.lastname@example.org
Posting bridges for load limit is a serious matter. Extra engineering effort is justified when evaluating for posting. This article presented three alternatives that may be conducted by professional engineers to assist bridge owners in making optimal posting decisions based on load ratings. - Author's Note
Gephyrophobia is a common phobia that affects many people. People with this phobia are afraid of crossing bridges, driving over bridges, or even seeing one from afar. There is not a scientific name for the fear of posting a bridge; however, it can bring on the same anxiety for the people involved with the posting. They may think, “Who am I going to upset by changing the load limits on this particular bridge today?” As a matter of public safety and as a way of safeguarding vital transportation infrastructure, bridge owners must restrict truck weights, often through load posting. A professional engineer licensed in your state can assist with making that determination based on the load ratings.
Bridges may need to be posted for restrictive loads when the capacity of the bridge decreases and/or when the demand on the bridge increases. The capacity of the bridge may decrease due to deterioration, damage, etc. The demand on the bridge may increase due to changes in the dead load (bridge deck, wearing surface, etc.) or the live load (legal trucks, permit trucks, or special loadings). Subjecting bridges to vehicles that are heavier than the bridges were designed to carry shortens the service life, and can cause both visible and hidden damage. The cumulative effect of the damage caused by these heavy loads will eventually force the roadway jurisdiction owning the bridge to restrict the weight of vehicles using the bridge or, in extreme cases, to close the bridge to all traffic.
All states are required to load rate and post bridges in order to comply with federal standards. Load ratings are performed in order to determine the safe live load capacity of a bridge, considering the existing conditions of the bridge. Based on the load ratings, the bridge is evaluated for load posting or strengthening. Currently, approximately one out of every five bridges on the public roads in the state of South Dakota is posted.
Posting bridges for load limit is a serious matter, additional engineering effort is justified when evaluating for posting. Choosing not to post a bridge may create safety issues. On the other hand, posting a bridge can create hardship on the motoring public and industry in the vicinity of the bridge. To ensure that posting is justified, alternatives should be considered when evaluating for posting.
First, re-rate the bridge using alternative methods to determine whether the bridge can accommodate higher loads based on currently accepted code criteria. The current specification for load rating and posting bridges is The Manual for Bridge Evaluation, Second Edition (MBE, 3nd Edition) (AASHTO, 2017), developed by the American Association of State Highway and Transportation Officials (AASHTO). Bridges may be evaluated using any of three methods: allowable stress rating (ASR), load factor rating (LFR), and load and resistance factor rating (LRFR). It is mandatory for all new bridges to be load rated using LRFR. If an existing bridge was designed using Load and Resistance Factor Design (LRFD) methodology, it must be rated using LRFR. If an existing bridge was designed using LFD methodology, it shall be rated using either LFR or LRFR, yet there may be considerable discrepancies in the results. Thus, an old bridge rated low using LRFR may be re-rated using LFR to determine whether it can accommodate higher loads or vice versa. In addition, there are two rating levels, inventory rating, and operating rating. Operating rating normally yields a higher load, however, posting at operating rating will reduce the service life of the bridge. Thus, the bridge owner’s maintenance strategies should be taken into consideration when deciding on a rating level.
Second, conduct an inspection to confirm the condition, measurements, and other properties of the bridge, and carry out a more in-depth, refined analysis. Traditionally, most bridges are rated with substantial margins of safety built in to compensate for the unknowns. Through field inspection and refined analysis, the uncertainties associated with the evaluation could be greatly reduced. For the purpose of evaluation, the reduction in distribution factors will raise the bridge ratings somewhat for the same checking procedures. The new analysis is more accurate and reliable. From a reliability point of view, higher ratings are justified.
Finite Element Analysis, as a typical refined analysis method, promotes a fundamental change in the practice of bridge engineering and attempts to move our industry past the use of simplistic design specifications to achieve more optimized solutions. In evaluating existing structures, there is also a growing interest in having engineers perform a direct risk assessment to determine the future course of rehabilitation investments and the balance of replacement costs with continued operation.
Lastly, a one-lane alternative may be considered when evaluating for posting. Normally, a bridge will be rated for the number of traffic lanes as designed; however, if the capacity is not adequate, the bridge may be checked for a reduced number of lanes. Reducing the number and locations of the loaded lanes and restricting lanes with barrels or stop signs, can keep a bridge from being posted with a weight restriction.
A good bridge posting strategy should seek a balanced approach to extending the service life and maximizing the usage of your bridges. A licensed engineer can analyze your needs and assists you in making optimal posting decisions with confidence. Please contact us for any clarifications or questions you may have.
Yankton County has four major county bridges crossing the James River. With each being well-beyond its expected life span, we are very concerned with an affordable replacement timeline for all. Clark Engineering presented each of these bridges, asked our priorities, asked our timeline and helped us identify the most at risk bridge for first replacement. Clark helped us to apply proper load limits for both short-term replacement bridges and long-term preservation bridges in order to meet our goal for complete turnover in a 20-year period. - Cheri Loest, PE | Vice Chairman, Yankton County