Design and Development of Steel Breakaway Posts

Abstract

The current generation of NCHRP Report 350-compliant guardrail terminals all use wooden breakaway posts in the terminal section. The first two end posts are typically breakaway cable terminal posts inserted into steel foundation tubes that are joined with a ground strut to provide the anchorage capacity. Wood is readily available and inexpensive, but it also has many drawbacks. The quality of the wood and the associated breaking forces vary widely. The strength of a wooden post is affected by many factors, including post size, ring density, knot location and size, cracks and checks, species, and moisture content. Results are presented of an effort to design and develop steel breakaway posts for guardrail terminals. The breakaway steel post system described has been successfully tested for use in a tangent terminal. The post exhibited consistent strength for redirection impacts and failed at very low loads during head-on impacts. The breakaway steel post actually improved the redirective performance of the SKT-350 during a 2020-kg (4,450-lb) pickup truck impact at the beginning of the length of need. These breakaway steel posts have been shown to meet NCHRP Report 350 performance criteria and should provide highway agencies with an alternative where wood posts are unacceptable.