Bollard Impact Simulation: Hollow vs Concrete-Filled Steel Bollard Under Vehicle Strike

A steel fabricator that makes safety bollards for a US distributor of facility protection products came to us with a commercial problem. Plastic bollard brands publish impact data for their products. Steel bollards usually come with none, and the distributor's customers had started asking for it. Over the years the bollard's pipe wall had also been thinned to control cost, so the distributor wanted to know what the current design could take and whether filling the tube with concrete would be a sensible upgrade to offer.

Shirsh Technosolutions carried out a bollard impact simulation using explicit dynamics FEA. Two versions of the same bollard were struck by the same vehicle in the model: the standard hollow steel tube, and the same tube filled with a concrete core. The report gave the distributor deformation, stress, plastic strain and energy results for both, in a form that could go straight into customer technical submissions.

Defining the impact. The vehicle mass including payload, the speed and the strike height were confirmed with the end user through the client before the load cases were frozen. Impact energy was worked out from mass, speed and impact angle with the kinetic energy formula used in PAS 13 style impact ratings, so the result could be read against how other barrier makers quote their products. The strike was taken head-on at the middle height of the bollard.

Model. The bollard was built in 3D from the client's drawings: the steel pipe, the base plate with its four anchor holes and the pipe-to-plate weld. The concrete-filled version added a solid concrete core inside the tube. The vehicle was represented as a rigid impactor block with the full vehicle mass. Pipe and base plate steels were given nonlinear stress-strain curves taken from the mill test certificates, so yielding and plastic flow follow the actual material rather than a textbook value.

Explicit dynamics set-up.

• Nonlinear explicit dynamic analysis, chosen for the short contact event and large plastic deformation.

• Hexahedral solid elements with at least two elements through the pipe wall and refinement around the impact zone.

• Automatic surface-to-surface contact between impactor and bollard, plus self-contact so the tube cannot pass through itself if it folds.

• Pipe-to-plate weld tied, base plate held on a rigid foundation so the result reflects the bollard alone.

• The impactor set moving at the impact speed and the run continued until the vehicle had come to rest and the bollard had settled.

What we reported. Stress and displacement plots at regular intervals through the impact for both versions, time histories at monitor points on the pipe and on the base plate, effective plastic strain in the steel, and the energy balance showing the vehicle's kinetic energy being taken up as internal energy of the bollard. A side-by-side comparison of the hollow and concrete-filled designs closed the report.

Common questions

Can FEA replace a physical bollard crash test? Not for certification. A crash test to a named standard is still the formal route. What explicit dynamics FEA gives is reliable comparative data on the actual product geometry and material, quickly and for several design options, which is often what a supplier needs to support a quote or decide which version to take to test.

Does filling a steel bollard with concrete make it stronger? In this study the concrete core made the bollard stiffer. It bent less, resisted local buckling of the tube at the strike point and spread the impact more evenly down the post. The base weld and plate still carry the load in both versions, so the anchorage detail matters whichever option is chosen.