Guardrails are important structures that protect vehicle occupants during roadside accidents. The expected function of a roadside safety barrier includes redirecting a vehicle from running off the road and shielding a vehicle from hazardous objects such as a concrete bridge pier at the roadside. The desired safety behaviour is ensured not only by the guardrail structure itself, but also by the interaction between the supporting soil and the guardrail post. One of the most important factors influencing the performance of guardrail systems is the interaction between the soil and the post holding these guardrails. In the present work, we study the soil-post interaction; a numerical simulation of a rigid impactor hitting a roadside post is presented. The interaction of cohesionless soil with W152 × 13.4 post is analysed and compared to impact test results. Traditionally, two approaches have been used to model soil-post interaction: (i) the continuum method where the soil is modeled as a solid element with nonlinear constitutive laws and (ii) the subgrade method where the soil reaction is simulated by series of lumped nonlinear springs. The last method is widely used in the field of safety roadside because of its simplicity and computational efficiency, even if it presents many deficiencies. The numerical simulation using subgrade model showed that, in general, the initial force spike developed in the early stage of the crash event is missed. This phenomenon is due to the absence of the soil inertia and results in inaccurate prediction of the guardrail reaction during vehicle impact. As a remedy, we propose an enhancement the subgrade approach where the soil is modeled as a system of lumped springs, masses and dampers attached to the post. A simple procedure to calculate the lumped soil masses and the damping coefficients in cohesionless soils is also developed. The results of the new method and the continuum method are in comparison to published results of crash tests. The presented enhancement shows that the numerical simulation results are in better agreements with crash tests than the conventional subgrade method.
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