Short-Radius Thrie Beam Treatment for Intersecting Streets and Drives (1442-1F)

At sites where a driveway or secondary roadway intersects a primary roadway in close proximity to a bridge end, the available space will not accommodate a standard length of approach guardrail, and alternate treatments are required. In Study 1263, a short-radius nested W-beam treatment was developed for use at these locations. Although the design offered improved impact performance over existing systems, it failed to pass one of the design test conditions. This study was undertaken to develop and test a new short-radius thrie-beam guardrail treatment suitable for use by TxDOT that meets nationally recognized safety standards.|The new treatment consists of a single 10-ga. thrie-beam rail mounted at a height of 787 mm (31 in.) and supported on weakened, round wood posts. The system extends approximately 9.75 m (32 ft) from the bridge end along the primary roadway at which point it is curved in a 4.87-m (16-ft) radius and extended down the secondary roadway. A series of five crash tests was used to evaluate the impact performance of this short-radius thrie-beam system. Although it failed to contain a 3/4-ton pickup truck as acquired by NCHRP Report 350, subsequent testing showed that it successfully meets the guidelines and evaluation criteria set forth in NCHRP Report 230, and is suitable for implementation where site conditions warrant such a treatment. In addition to offering significantly improved impact performance over existing designs, the thrie-beam design should be much easier to install and maintain than the interim nested W-beam design developed under Study 1263.