6.1.4 Effectiveness of Anti-Ram Barriers

The effectiveness of an anti-ram barrier is based on the amount of energy it can absorb versus the amount of kinetic energy imparted by vehicle impact. The angle of approach reduces this energy in non-headon situations, and the energy absorbed by the crushing of the vehicle also reduces the energy imparted to the barriers. The kinetic energy imparted to the wall is one-half the product of the vehicle mass and its impact velocity squared. Because the velocity term is squared, a change in velocity affects the energy level more than a change in vehicle weight. For this reason, it is important to review lines of approach to define areas where a vehicle has a long, straight road to pick up speed before impact.

The vehicle weight used for the design of barriers typically ranges from 4000 lb for cars up to 15,000 lb for trucks. Impact velocities typically range from 30 mph for oblique impact areas (i.e., where the oncoming street is parallel to the curb) up to 50 mph where there is straight-on access (i.e., where the oncoming street is perpendicular to the curb).

The kinetic energy of the vehicle at impact is absorbed by the barrier system. For fixed systems (like a concrete bollard), the energy is absorbed through the deformational strain energy absorbed by the barrier, soil, and the vehicle. For movable systems (like a surface-mounted planter) energy is absorbed through shear friction against the pavement and vehicle deformation.

Barrier effectiveness is ranked in terms of the amount of displacement of the system due to impact. Standard ratings defined by the federal government define the distance the vehicle travels before it is brought to rest. The most effective systems stop the vehicles within three feet, moderately effective barriers stop the vehicle within 20 feet, and the least effective systems require up to 50 feet.