Thriebeam guardrail is a stronger version of the w-beam guardrail systems. The additional corrugation in the rail stiffens the system, making it less prone to damage during low speed and moderate speed impacts. It also allows higher mounting of the rail, which increases its ability to contain larger vehicles under some impact conditions.
Safe vehicle containment and re-direction is developed through a combination of the flexural resistance of the thriebeam guardrail and the bending resistance of the supporting posts.
Upon impact, the posts absorb some of the crash energy through post rotation in the surrounding soil prior to separation from the thriebeam as they fully yield. The blocking pieces, positioned between the thriebeam guardrail and posts, minimise vehicle snagging and reduce the potential for the vehicle to vault over the barrier by maintaining rail height during the initial stages of impact.
Once the thriebeam guardrail separates from the posts, the rail forms a re-directive ribbon, guiding the vehicle away from the nearby hazard.
Thriebeam guardrail may be installed using standard or modified blocking pieces.
To improve the performance for heavy vehicle impacts, a steel blocking piece, known as the modified or notched block was developed.
The modified block has a triangular notch cut from its web and allows the lower portion of the thriebeam guardrail and the flange of the block to bend inward during impact. This bending feature keeps the face of the thriebeam guardrail nearly vertical throughout the impact zone as the posts begin to yield. As the posts yield, rail elevation is maintained and the likelihood for a vehicle to roll over the barrier is reduced.
The modified block design also raises the rail height when compared to the use of standard blocks. This further reduces the potential for high-centre-of-gravity vehicles to vault the barrier.
Posts for the installations of thriebeam guardrail are spaced at 2m centres.
Straight thriebeam guardrail panels can be used when the radius is greater than 45m. When the radius is less than 45m, factory curved thriebeam guardrail is required.
Posts, blocking pieces and thriebeam guardrails are hot dip galvanised.