In a recent study of inspection reports, design documents and other data to evaluate the safety performance of bridge barriers, investigators found that the most commonly used barrier designs meet newer safety requirements and keep Minnesota drivers safe.
In the 1980s, safety testing and standards were developed for bridge barriers and tested against vehicles like the Geo Metro (a small, 1,600-pound economy sedan) and standard two-door, 4,000-pound pickups. Since then, safety standards have been modified in part to accommodate larger automobiles and trucks.
Vehicles today are heavier and taller than vehicles in the 1980s, driven by a market flush with sport utility vehicles (SUVs). They also have higher centers of gravity. Vehicles tested against the latest safety standards are 2,400-pound Nissan sedans and 5,000-pound, four-door Dodge Ram pickups.
“This research will lead to published tables that will allow engineers to determine which existing barriers meet MASH Test Level 3 and to identify which barriers can remain in place and which must be repaired or retrofitted,” said Paul Rowekamp, bridge standards and research engineer, MnDOT Bridge Office.
National bridge barrier safety standards were revised in 2016 with the issuance of the Manual for Assessing Safety Hardware (MASH). Barriers meeting this new standard are typically made from concrete and steel, and are expected to last for decades. With responsibility for over 2,000 bridges and over 1.5 million lineal feet of barriers, MnDOT manages some barriers that have been in place since about 1950.
New MASH standards for Test Level 3 (TL-3) require that barriers should be at least 29 inches tall; designed without features that could catch on bumpers, hoods or other vehicle design elements; and strong enough to handle impacts from modern vehicles.
What Was Our Goal?
MnDOT sought to evaluate the safety of older bridge barriers in its bridge inventory to see if they meet MASH TL-3. The researchers needed to examine the condition of barriers, determine how well they fit newer safety criteria and develop designs for retrofitting existing barriers, if necessary, to meet standards.
What Did We Do?
The evaluation focused on the four most common barrier types used on MnDOT bridges: one-line rails and type G, J and F barriers. During on-site visits, the research team examined barrier types and identified critical elements of each design to consider in their analysis. Investigators studied drawings, plans and bridge inspection records of Minnesota bridges to determine how much of the inventory includes each of the barrier design types and the inspected condition of these barrier types throughout the MnDOT system.
The research team then evaluated critical barriers of each type—those with representative or significant features—in terms of the most recent national safety standards to determine how well these barrier types satisfy requirements. Finally, the research team determined retrofit approaches that could bring unsatisfactory designs into compliance with current safety standards.
What Did We Learn?
Results of the evaluation are summarized below:
One-Line Barriers. Built between 1949 and 1970, one-line barriers typically entail concrete posts standing above curbs and concrete beams spanning between the posts as rails. About 13% of MnDOT’s bridge barriers are one-line; based on MnDOT inspections, researchers determined that 75% were in good condition, and 6% were in poor condition, with severe delamination or cracking and some exposure of reinforcement steel.
All of the 14 critical one-line barriers evaluated were rated “not satisfactory” in terms of strength and performance. Thus, MnDOT’s one-line bridge railings require retrofitting or replacing to meet MASH TL-3.
G Barriers. Built between 1970 and 1977, these concrete barriers have a wide sloped base designed to redirect or raise a colliding vehicle’s tires to diminish the impact of vehicle bumpers, hood and other features on the barrier. Researchers determined that 10% of MnDOT’s bridge barriers were G designs, of which 80% were in good condition and 2% were in poor condition. Three specifically evaluated G barriers were all found to be unsatisfactory in terms of performance and require retrofitting to meet MASH TL-3.
“Some of the older barriers were a problem, but we developed an end-post retrofit with a drilled shaft to make them satisfactory,” said William Williams, associate research engineer, Texas Transportation Institute.
J and F Barriers. The differences between J and F barriers are subtle; F barriers have a somewhat steeper sloping face to better accommodate taller vehicles and have been erected more recently than J barriers, which MnDOT began building in 1976. These two barriers comprise about 55% of the MnDOT bridge barrier inventory. Researchers determined that about 72% were in good condition, and about 1% were in poor condition. Of 25 critical F and J barriers, 15 earned satisfactory MASH TL-3 performance ratings.
Researchers developed retrofit designs for bringing unsatisfactory designs, mostly one-line and G barriers, up to safety requirements.
Most of MnDOT barriers appear to meet safety standards. Retrofitting or replacing bridge barriers can be conducted as bridges are rehabilitated or replaced, and the report allows designers to identify the retrofit needs a particular bridge would require. MnDOT’s Bridge Office will develop recommendations and add them as tables to the MnDOT Bridge Preservation and Improvement Guidelines to identify which barriers meet MASH TL-3 requirements.