Sizing of boulders for security barrier (vehicle impact) 1

[munnin]

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.

 

[berkshire]

Granite weighs 165 lbs per cu foot
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

 

[jdgengineer]

Berkshire --I'm guessing that's not all that relevant to the discussion

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.

 

[paddingtongreen]

If you are supplied the mass and velocity, they should be able to give you the "crumple" force and distance (work done) for the truck, that would be a good start. You could calculate the energy used to crumple the truck front, and the residue would go to moving the boulder.



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[mijowe]

http://www.wbdg.org/ccb/DOD/UFC/ufc_4_022_02.pdf

 

[mijowe]

btw berkshire, that was "any" advice and it made me lolz

 

[hokie66]

That's only the size of a car, and not travelling very fast. Boulders of about the same mass should do the job. As to bollards, that is a different thing.

 

[chicopee]

Impulse and momentum problem with full plastic collision and assumingly immovable rock.

 

[CANPRO]

Do you have limits on the displacement after the collision? I think you will need this to solve the problem. This first thing that came to my mind to solve this was to use conservation of momentum to get the velocity rock+car after the impact, from there you can look at how long and far it will take for that lumped mass to come to a stop.

 

[ztengguy]

Granite is 165, but it would also depend on the surface area of contact. I would hate for a car to hit it, and the boulder roll into your building.

If you can do concrete, thats about 150pcf, and you can shape it as needed.

 

[rb1957]

it's not the size of the vehicle that matters, it's the size of the bomb inside the vehicle !

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.

 

[mijowe]

how do we know munnin is not a terrorist trying to see how we think, looking for holes?

 

[munnin]

Current specification is for boulders which are 1800 kg minimum. Material is limestone. Partially embedding the boulder in the soil is also an option provided it extends 600 mm above grade. A stopping distance was not specified however I have seen 3 ft referenced in a couple of places which relates to the distance from the front of the vehicle to the cargo.

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.

 

[chicopee]

It may not be a terrorist,just a suicidal maniac.

 

[rb1957]

there's a difference ?

maybe disgruntled taxpayer ??

 

[Gumpmaster]

Who is the client? Certain federal clients have their own standard details for boulder sizing.

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.

 

[munnin]

There is no requirement for testing just a compliance statement requiring it stop a certain vehicle weight going a certain speed. I am curious as to how you sized your boulders prior to testing. I had thought this would be a standard somewhere in the US based on rounds of testing or something by the department of defense maybe.

 

[ACtrafficengr]

The Texas Transportation Institute has some experience with testing barriers, both for preventing vehicle intrusion, and highway safety barriers. You could give them a call.

 

[bmrossetti]

Munnin,

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

 

[drawoh]

munnin,

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.

JHG