Sizing of boulders for security barrier (vehicle impact)
Sizing of boulders for security barrier (vehicle impact)
(OP)
I am trying to find guidance into sizing of boulders for use in a security barrier perimeter in order to resist vehicle impact.
Vehicle Mass: 1800 kg
Vehicle Speed: 50 kph
Any advice would be greatly appreciated.
Vehicle Mass: 1800 kg
Vehicle Speed: 50 kph
Any advice would be greatly appreciated.






RE: Sizing of boulders for security barrier (vehicle impact)
B.E.
The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor
RE: Sizing of boulders for security barrier (vehicle impact)
I've never sized something like this before (probably better called a bollard) but I'm assuming FEMA should have some guidelines as it relates to anti-terrorism. Although consider you are using Si units I'll assume you are out of the US.
In the end it will boil down to F= ma. You know the mass, but the tricky part is finding the acceleration as that depends on the crushing of the car and the deflection of the bollard. I'd say check out the FEMA website or hopefully someone else has some better guidance.
RE: Sizing of boulders for security barrier (vehicle impact)
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
If you can do concrete, thats about 150pcf, and you can shape it as needed.
RE: Sizing of boulders for security barrier (vehicle impact)
don't know that crumple zones would help much. they're designed to prolong the contact, disapate the energy. i'm sure any terrorist worth their salt would figure out that reducing the impact time, stiffening up their vehicle, would improve their chances.
surely (yes, i know, don't call me shirley) there's a Mil Spec.
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
As to F = ma I have read a couple of papers which recommend a design force based on an average vehicle deceleration of a = 28g. Also that the load calculated should be considered factored as it is already conservative. This yields a design force of 495 kN.
I have ufc 4 022-02 but I could not locate anything pertaining to sizing of boulders. I have also looked through FEMA 427 and it does suggest barrier types though it does not detail any design specifics.
As to conservation of momentum I had thought about this too however I think it is more likely an energy problem. A lot of energy will be absorbed through the vehicle crumpling. In addition there will be energy transferred through friction and through the lateral stiffness of the soil.
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
maybe disgruntled taxpayer ??
RE: Sizing of boulders for security barrier (vehicle impact)
I have worked on projects in the past where we did full scale testing of boulders that were outside the standard detail to prove that they met the requirements.
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
RE: Sizing of boulders for security barrier (vehicle impact)
For a basic, first-principles approach, I've successfully used the following procedure:
1. Neglect vehicle crush, boulder spalling, etc.
2. Assume a fully plastic collision.
3. Use Conservation of Momentum to determine post-collision velocity of the vehicle-boulder system.
4. Determine post-impact kinetic energy of vehicle-boulder system based on velocity obtained in 3, above.
5. Estimate friction coefficient between boulder and ground. This can be tricky... be conservative.
6. Determine stopping distance using Work-Energy Principle.
The maximum allowable stopping distance may be governed by the AHJ (authority having jurisdiction), depending on which agency it is. Otherwise you (or the security consultant, if there is one) will need to determine if the penetration distance is acceptable based on your site-specific requirements (e.g. required standoff distance). As you mentioned, you can increase the resistance of the boulder by partially embedding it; this will add soil mass/passive resistance to the Momentum and Energy considerations.
Hope this helps.
Brandon
RE: Sizing of boulders for security barrier (vehicle impact)
This this for stopping terrorists, or is it for stopping accidents?
This affects your attitude towards the people in the vehicles. I have no objection to decelerating a suicide bomber at 100Gs. I would be concerned about some poor guy who hit an ice patch and went through a T intersection towards your building.
RE: Sizing of boulders for security barrier (vehicle impact)
I imagine a stopping distance of around 1 meter is acceptable as I had seen mention of it being the average distance to the cargo in a truck even though for this case it has not been clearly stated.
Brandon, this was the approach I was considering. Substantial soil deflection will engage passive soil resistance to deflection. I will take a closer look at it from that perspective. I have to assume the boulder will not shear upon impact.
RE: Sizing of boulders for security barrier (vehicle impact)