27 February 2015

Technical Talk: Road Safety Barrier Rigid to Semi-Rigid Transitions

Road safety barrier transitions provide progressive stiffening between barrier systems of different rigidity. The difference in barrier rigidity will be evident in different deflections for the same crash impact energy. The greater the difference in stiffness, the greater the difference in crash deflections. A vehicle impacting near the connection between barrier systems of different rigidity will “pocket” without the appropriate transition. Pocketing limits the redirective ability of the barrier systems and can result in outright capture of the vehicle. To prevent this, a transition in the leading (i.e. approach) direction is designed to progressively stiffen the barrier when transitioning from a weaker to stiffer system (e.g. W-section to concrete). In a trailing or departure direction, the barrier stiffness progressively decreases between the stiffer to weaker barrier systems.

The New Zealand Transport Agency has developed a standard detail for transitions between rigid and semi-rigid systems. The detail Test Level 4: W-Section to Concrete Transition Detail, Road Safety Barrier Series RSB-5 is available at the New Zealand Transport Agency. This transition is between a semi-rigid TL-3 W-Beam strong post timber barrier system and a rigid TL-4 (or higher) concrete barrier system. The W-Beam is mounted at 550mm to the rail centre and the Thrie-beam at 610mm to the rail centre.

In this detail for a leading transition, the system progressively stiffens through a 10 metre length. It consists of a W to Thrie transition section of guardrail which increases the material cross-section from W-section to Thrie-beam. It is then followed by a length of Thrie-beam with the last half of that having an additional post at 1000mm post spacing. This is followed by a nested length of Thrie-beam with an additional post in the first half of the rail length at 1000mm post spacing, and two additional posts at 500mm post spacing in the last half of the rail length. A Thrie-beam structural connector then connects the transition onto the rigid concrete barrier using five bolts to either a cast in bracket in the concrete or through the concrete using a backing plate. The steel posts with modified blockouts used in the transition are in accordance with NZTA M23 Appendix A. In situations where standard length posts cannot be installed to the correct embedment depth, a ground beam with bolt-down posts is an option.

In a trailing installation where the road is divided by a median barrier the entire nested Thrie-beam section including posts one through six may be omitted. This is because the risk of pocketing is lower in transitioning from the stiffer rigid system towards the semi-rigid barrier as it cannot be impacted by a vehicle travelling in the opposite direction.

Any design variation to the Transport Agency standard detail has to be signed off by the Transport Agency National Office before installation

The Transport Agency has accepted the CSP detail for transitioning the semi-rigid TL-4 Nu-Guard® 31 system to rigid concrete barrier.

CSP is available to provide technical assistance in using this detail.