Frangibility and deflection zone recommendations for Bollards secured using the Impact Recovery System:
Bollards secured on sustainable foundations using the Smart Impact Recovery System are designed for use in car parks and roadside applications, specifically designed for areas where bollards may be subject to impact from vehicles travelling at low speed.
With high speed impact from a heavy vehicle (such as large truck) the internal securing post has the ability to bend to ground level (although the 165mm diameter of the bollard disables the post from completely being flattened)
The post will bend to approximately 80 degrees. A distance the length of the bollard (IE: 800 mm for an 800 mm high bollard) is recommended from any glass windows or building perimeters.
STOPPING YOUR BOLLARD DEFLECTING
If you ned to STOP the deflection we insert a solid steel Rod inside the bollard. This prevents the bollard from deflecting beyond the 20 degrees. If badly impacted by a vehicle this bollard will not deflect but may need replacing after a bad impact (this is still relatively low cost compared with replacing the foundations every time a bollard is badly impacted)
FRANGIBILITYThe measure of post frangibility is based the post’s bending strength. The posts are vertical cantilevers so limiting the post bending strength in turn limits the force that can develop between post and vehicle.
> Posts with design bending strength up to 3.34 kN.m are regarded as frangible for all speeds.
> 60 OD CHS 3.6 Posts having a bending strength below 3.34 kN.m are considered frangible, for all speeds.
The bollard is secured using a 60 OD internal securing post. It is the Impact Recovery Rings that take the initial impact and if the vehicle continues to propel forward the force is transferred to the internal 60 OD CHS post.
The bending factor of a CHS post is determined by the wall thickness of the CHS post. (We specify 3.6 mm wall thickness, for car-parks, although this can be altered to suit the client’s purposes).
LOW SPEED IMPACT
When impacted at slow speed (as in carpark > 15 km/hour) the Impact Recovery Rings absorb the impact force and allow the bollard to deflect (up to 18 degrees, which represents a maximum of 250 mm for a bollard of 800 mm height) and self-recover.
RELATIVELY LOW SPEED > 60 KM/HR
When a vehicle impacts a post at relatively low speed, the response of the post is similar to that shown at the right. i.e. the post will initially deflect elastically and if there is sufficient momentum and energy in the errant vehicle, the post’s bending strength will be exceeded, leading to plastic deformation concentrated near the top of footing as the post bends over.
The post has the ability to bend to ground level (although the 165 mm diameter of the bollard disables the post from completely being flattened). The post will bend to approximately 80 degrees. A distance the length of the bollard is recommended from any glass windows or building perimeters.
MAXIMUM IMPACT FORCE
The maximum impact force can be estimated from the following:
F1 x Height of Impact = Ms
(Ms = Nominal section moment capacity of post = 3.34/0.9 = 3.71kN.m for 75x50x2.5RHS Gr350. Note the ‘design
strength’ has been divided by the capacity reduction factor0.9 to obtain the ‘nominal strength’). Assume Height of Impact = 0.5m. F1 = 3.34/0.9/0.5 = 7.42 kN, approx = 750kg force.
> For this low speed impact, the force involved in accelerating the post has been ignored, as it is expected to be of minor significance. An estimate of this fraction of the total force is included for the higher speed impact, as the post speed attained will be higher, and the time available to accelerate the post will be smaller.)
HIGH SPEED IMPACT
When impacted at high speed the Impact Recovery Rings absorb the initial impact, slowing the vehicle. The vehicle will continue to propel forward and the force is then transferred to the internal 60 OD CHS post, which will bend at ground level.
When a vehicle impacts a post at a high speed, the response of the post is somewhat different to the low speed impact. For example, the inertia of the post above the point of impact is likely to cause an extra zone of plastic bending (or hinge) to form close to the point of impact, as show on the right. (This extra hinge in fact relieves the impact forces compared to a post remaining relatively straight but quickly gaining rotational or angular velocity centered at the footing top.)
> For the same post section size and height of impact, the corresponding force to cause this bending will be twice the previous value, ie F2 = 2 F1 = 1,500kg force (as an inflection point will be at a height of 0.25m). IE: The post will bend more easily, reducing risk of injury to drivers.