NCHRP Research Report 892 provides proposed load and resistance factor design (LRFD) bridge design pier protection specifications and proposed occupant protection guidelines to update the AASHTO LRFD Bridge Design Specifications and AASHTO Roadside Design Guide, respectively. The proposed specifications and guidelines are based on a comprehensive analytical program that used a risk-based approach for investigating the effects of a heavy-truck hitting one or more bridge columns or piers. The report also includes four examples that illustrate the use of the proposed specifications and guidelines for shielding bridge piers. The material in this report will be of immediate interest to bridge and safety engineers.
Bridge piers are generally close to the travelway to minimize bridge lengths. As a consequence, barriers are normally placed around piers to reduce the potential of vehicle crashes damaging the piers. However, the design and placement of the barriers may not have considered the possibility that vehicles, particularly large trucks, might still impact the pier. The AASHTO LRFD Bridge Design Specifications require piers that were not designed to withstand large impact load to be protected by a 54-in. high structurally independent barrier if the barrier is within 10 ft of the pier. If the barrier is more than 10 ft from the pier, a 42-in. high barrier is specified. There is no consideration of the risk of a high-speed impact, traffic volume, truck usage, operating speeds, facility type, and other factors in the bridge specifications. In addition, while the bridge specifications specify a height of barrier, they don’t specify the length of barrier in advance of the pier. The specifications also don’t specify the transition that might be appropriate.
The requirement for protecting bridge piers from truck impacts may have a significant effect on passenger car safety. Rigid barriers are generally believed to cause more injuries and fatalities than semi-rigid and flexible barrier systems. In addition, having barriers close to the travelway may significantly increase the number of passenger car crashes. Crashes involving heavy trucks hitting bridge piers are rare, and a barrier designed to protect a bridge pier from impact may create a new hazard especially for passenger vehicles. In the aggregate, the personal cost in lives and property to drivers who hit barriers is more than the cost of repairing bridge piers damaged by heavy trucks.
Further, there are operational concerns associated with the use of tall concrete barriers near the travelway. Concrete barriers are much more likely to produce deep snow drifts than are other more open barriers. Drifting is also a problem during severe sand storms. The increase in snow and sand drifting will increase operational costs as highway agencies are forced to make more frequent passes with snow plowing equipment to keep highways open. Also, tall concrete barriers placed near interchanges can adversely affect sight distances.
In fact, most of the issues mentioned above result from following the approach “one size fits all” in protecting bridge piers without quantifying when and how the bridge pier protection should be applied. States are spending significant funding on projects that may not provide the expected benefits, particularly on lower traffic volume or functional class bridges.
Under NCHRP Project 12-90, Roadsafe LLC was asked to develop (1) risk-based guidelines that quantify when bridge piers should be investigated for vehicular collision forces per AASHTO LRFD Bridge Design Specifications or be shielded with a longitudinal barrier considering as a minimum: site condition, traffic, bridge design configurations, geometry of the roadway section passing beneath a bridge, operation characteristics, and benefit/cost and (2) guidelines for barrier selection, length-of-need, and placement for shielding bridge piers and protecting the traveling public.
A number of deliverables, provided as appendices, are not published but are available at http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP12-90_Appendices.pdf. These appendices are titled as follows:
- Appendix A – Proposed LRFD Bridge Design Pier Protection Specifications
- Appendix B – Proposed RDG Occupant Protection Guidelines
- Appendix C – Survey of Practice
- Appendix D – Lateral Impact Loads on Pier Columns
- Appendix E – Nominal Resistance to Lateral Impact Loads on Pier Columns
- Appendix F – Heavy Vehicle Traffic Mix and Properties