The objective of this project was to develop a long-span guardrail design that meets NCHRP Report 350 evaluation criteria and is less expensive to construct than the existing design.
The researchers reviewed design details of the long-span guardrails previously developed, especially the design presented by Mak*, to evaluate expected behavior of the guardrail when subjected to NCHRP Report 350 tests. Design features that have been found to be important in terms of capacity of the guardrail to contain and redirect a vehicle are the structural strength and geometrics of the guardrail. Lateral stiffness of the guardrail system is the primary design feature that determines maximum deflection of the guardrail during a collision and changes in lateral stiffness of the guardrail system along its length are the key features influencing pocketing of the vehicle.
The BARRIER VII computer program was used to evaluate expected performance of the proposed design in NCHRP Report 350 tests. This program models a collision in two dimensions with a simple characterization of a vehicle and a detailed characterization of the guardrail. It was used to evaluate the expected deflections of the barrier and to predict any potential of the barrier to allow any unacceptable pocketing of the vehicle.
The report linked below presents the details and results of the first full-scale crash test performed on the long-span guardrail. In this test, the impact point was 3 ft-1 inch downstream of the beginning of an 18 ft-9 inch long span. As the collision progressed, the rail element was deflected toward the field side. Posts on either end of the long span were also deflected toward the field side. Maximum deflection of the rail element, prior to its rupture, was 57 inches. As the front of the vehicle approached posts on the downstream end of the long span, the rail element was being bent into sharper curvature. After the front of the vehicle passed the splice where the rail section changed from nested to a single layer, the single layer of the rail element was ruptured.
* K.K. Mak, R.P. Bligh, D.J. Gripne and C.F. McDevitt, “Long-Span Nested W-Beam Guardrails over Low-Fill Culverts,” Transportation Research Record No. 1367, Transportation Research Board, National Research Council, Washington, D.C., 1992.