Energy absorbing roadside posts

Abstract

This paper provides an overview of the energy and crush space required to provide survivable impacts between motorcars and energy-absorbing poles. In addition this paper considers some of the constraints that would apply if those poles or posts would be required to be remaining upstanding and serviceable after the crash. A simplified acceleration/displacement pulse for the combined car and pole is used in all the modeling described here. This pulse is arrived at using results from full-scale tests and FEA simulations for impacts from 60 km/h and for a maximum acceleration of 20 g. These limitations and the shape of the pulse give us the total crush depth for any given collision energy. The depth into the car, at the point when 20 g is reached is arrived at using published analysis of full-scale car-to-pole tests and it is a function of the car’s mass, width and length, and the pole diameter. The division of crush depth between the pole and car is then estimated using the assumption that the pole can be designed to crush from the time that the car reaches 20 g acceleration. The results show that the poles that bring about 20g acceleration in the car can be provided only for a narrow range of car masses. A pole that suits medium sized cars will cause reduced acceleration to heavier cars, to the point that it may be overrun and demolishing by much heavier cars. The same pole will cause higher accelerations for cars that are lighter than the chosen mass, to the point that it may be equivalent to a rigid pole for very light cars.