Limits of Acceptable Rail-and-Post Deflection in Crash-Damaged Strong-Post W-Beam Guardrail

The guidelines for the testing of strong-post W-beam guardrail, intended to ensure the safety of errant vehicles, are specified in NCHRP Report 350. A limitation of these tests is that they are always performed on new, undamaged guardrail, whereas guardrail along highways is in a continuous cycle of damage and repair. No tests have ever evaluated the performance of deflection-damaged guardrail. A full-scale, two-part crash test was conducted to evaluate the performance of a guardrail with 14.5 in. (368 mm) of prior crash damage. When this guardrail was struck by a 4,409-lb (2000-kg) pickup truck traveling at 62 mph (100 km/h), the vehicle vaulted and came to rest upright behind the barrier. A critical factor was the failure of a post to separate from the rails. Finite element models were used to evaluate the guardrail performance at lower levels of deflection. The crash tests and finite element models demonstrated that rail-and-post deflection of 11 in. (279 mm) or higher resulted in vaulting or rollover. Repair was recommended for strong-post W-beam guardrail with combined rail-and-post deflection exceeding 6 in. (152 mm). This limit allows a margin of safety for variations in soil strength and vehicle height. The existence of rail-only deflection had a minimal effect on the crash performance up to the maximum tested value of 6 in.