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TTI researchers develop cost-effective guardrail design

As the automobiles on our roadways have changed from predominately passenger cars to trucks and sports utility vehicles (SUV), so must the roadway design standards used by the Texas Department of Transportation (TxDOT) change to accommodate these vehicles. Guardrail bridge transitions represent one of these areas. The increased stiffness needed in the transition to safely contain and redirect heavier trucks and SUVs made guardrail bridge transitions costly and complex to construct.

Researchers at the Texas A&M Transportation Institute (TTI) recently completed a project for TxDOT that modified the standards for low-speed guardrail-to-bridge rail transitions. A transition is a section of guardrail that ties a roadside guardrail and bridge rail together at a bridge approach.

“If a flexible roadside guardrail is connected to a rigid bridge rail, there is a basic incompatibility between the two in terms of their stiffness,” says Roger Bligh, associate research engineer with TTI and project supervisor. “If a vehicle impacts the guardrail just in advance of the bridge rail end, the guardrail will deflect, but the bridge rail will not. So the vehicle ‘pockets’ into the end of the bridge rail, resulting in a severe crash. In this project, we designed a transition section that solves this problem by smoothly transitioning the stiffness from the flexible guardrail to the more rigid bridge rail.”

Using computer modeling, researchers analyzed several guardrail transition designs for low-speed roadways. The selected system achieved smooth transition stiffness by reducing the spacing of posts and using two guardrail beam elements.

“Initially, we looked at design alternatives using computer analysis to provide an indicator of successful impact performance before full scale crash testing,” says Bligh. “We wanted to make sure that the design being crash tested had a good probability of passing, and yet was the most economical and cost effective solution for the state.”

The project concluded with a successful first crash test that was performed on the design selected in conjunction with TxDOT engineers. TxDOT has implemented a new standard detail sheet of the design that is now available statewide for use on roadways. It has also gained some national attention.

“Safety is the top priority for TxDOT. We are always looking for ways to improve the safety of our roadway system for the traveling public,” says Rory Meza, TxDOT director of roadway design. “Research projects such as this one provide us with the opportunity to be innovative and to implement the improvements in a short time frame. The low-speed transitional section provides the added safety at a reasonable cost, using standard materials readily available in the industry.”