Appendix A15-tonne gross vehicle mass rationale#

15-tonne gross vehicle mass (GVM) requirement for truck mounted attenuator host vehicle#

Critical to the development of a TMA that affords protection to the public, the road workers and the TMA Operator, is the selection of the host vehicle. The vehicle must be appropriate for the use intended and also comply with all legislative requirements.

There are a number of requirements that affect the selection of the host vehicle. A discussion of critical requirements follows.

Minimum tare mass#

For acceptable impact performance, minimum tare mass requirements for host vehicles are set by the manufacturers of Impact Attenuator Units. The two currently available Impact Attenuator Units have minimum host vehicle tare mass requirements of 7.3 tonnes (Safe Stop) and approximately 9.07 tonnes (20 000 lbs) (Scorpion).

The host vehicle tare mass is the mass of the truck with all the components necessary for operation as a TMA.

Weight distribution#

To enhance the effectiveness of the automatic impact braking (AIB) system, the rear axle/s should carry a significant proportion of the total TMA mass.

Use of ballasting#

The use of ballasting is discouraged. The mounting points of all attachments to a TMA host vehicle are required to withstand a force of 20 times the mass of the attachment. While the attachment of the ballasting to the truck body may meet this requirement, the attachment of the body with ballast to the chassis is unlikely to meet the 20 times mass requirement without significant modification to the mounting points on both the body and to the truck chassis.

Chassis size and strength for impact attenuator unit mounting#

Under impact, the loads imposed on an impact attenuator unit are transferred through the mounting assembly into the chassis of the truck. The truck chassis shall be of a size that allows mounting of the impact attenuator unit in accordance with the manufacturer’s specifications. The truck chassis shall also be of sufficient strength to absorb applied loads without significant failure or distortion.

Conclusion#

It is recommended that TMA host vehicles with a manufacturer’s GVM rating of at least 15 tonnes will meet these critical requirements.

Vehicles with a lesser GVM rating are unlikely to meet all or possibly any of these critical requirements.