safety barriers

1. Road barriers

Safety barriers are devices that are longitudinally placed on the road in places where safety conditions were not provided for one reason or another. It should be noted that in this situation, a collision with a safety barrier constitutes a full-scale accident. Thus, it is about mitigating the severity of such a collision compared to the severity of a collision in a hypothetical and similar scenario in which such a collision does not exist.

Thus, various types of safety barriers appear. On the one hand, metal and non-fixed concrete barriers are classified as deformable safety barriers because they are capable of absorbing large amounts of energy through deformation. On the other hand, fixed concrete barriers do not support this deformation, but try to direct the dissipation of kinetic energy through friction.

1.a) metal safety barriers

Traditionally, metal safety barriers used in our country consist of a two-wave profile, a number of equidistant posts, and dividing parts. Thus, after assembly, each element that composes it contributes to the correct operation.

When a car loses control and hits a metal safety barriers, it does so against a double wave. The energy that is then released in a collision is partly absorbed by the vehicle itself and partly by the safety barriers. The forces that then reach the barrier cause it to deform laterally until it reaches the separator. When the deformation reaches the spacers, they completely transfer this force to the deformable pins with low stiffness.

This system always prevents the lower part of the legs from sticking out under the fence,  ensuring that the wheels of the car will not collide with them and cause the car to roll over. To facilitate this behavior of the barrier, the posts are not concreted but hammered. This is achieved in such a way that after the deformation potential is exhausted, the pole can easily come out of its position.

1.b) concrete safety barriers

Rigid concrete barriers are composed of prismatic parts with specific cross-sections that guide vehicles colliding with them,  dissipating some of the kinetic energy through friction.

The forces generated at the points of contact between the barrier and the vehicle should be directed parallel to the barrier without returning it abruptly to the roadway or causing excessive damage or deceleration. This ”  guide  ” must be done through the wheels, avoiding excessive contact between the barrier and the vehicle body. In addition, the surface of the barrier must be smooth so that the vehicle does not “climb” too high and does not stop, but does not tip over.

In contrast, the  effectiveness of  these barriers is limited by  exposures that occur at small angles around 15º  . For larger angles, there will be some contact between the barrier and the body, causing the latter to absorb energy and deform, and also increase the collision gravity.

And it is at this last moment that we are faced with a big dilemma:  increase or decrease the rigid barriers on the rails? Moving them closer together makes an impact more likely because the driver has less room to maneuver, but the angle of impact is very small and the gravity of the collision is reduced. In contrast, if we remove them, the driver will have more room to maneuver, but the collision will be much more severe if it occurs at a higher angle. It is the engineer’s decision to assess these possibilities and establish a barrier depending on the situation.

2. Pretiles

These containment devices have  two main purposes:

  1. Contain light and heavy vehicles
  2. Limit bids to the handrail-supporting board with plastic anchors.

The ductile  anchor is  reduced to a steel bar, which connects the handrail, at its base, to the board, allowing the barrier to deform within specified limits. After they are overcome, the anchor stops working, transferring efforts to the nearest anchors.

Thus, when exposed to a light vehicle, the forces generated are completely absorbed by the dashboard, without deforming the armature. Conversely, in the event of a  strong impact, the handrail does not transfer all the forces to the board, but part is absorbed by the deformation of the anchor. If the impact were even stronger, the anchor would deform to its limit position, in which it would be torn off the board, and the attached anchors would work. Thus, the maximum force transferred to the board is always limited.

In stiffer sections, where there is not enough space for such deformation,  concrete  handrails are used, the deformation of which is much less, since in no case can the car wheel be suspended in the air.

3. Shock absorbers.

In conclusion, it should be noted that shock absorbers are used to  protect hazardous areas or obstacles , which are usually part of the road infrastructure itself,  from frontal collisions for which safety barriers are not suitable.

Its purpose is to mitigate the impact of the vehicle by absorbing its kinetic energy through deformation in the form of an “accordion” depending on the length of the shock absorber of the device. On the other hand, shock absorbers should act as a protective barrier against side impacts.