×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Sizing of boulders for security barrier (vehicle impact)

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.  

RE: Sizing of boulders for security barrier (vehicle impact)

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

RE: Sizing of boulders for security barrier (vehicle impact)

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.  

RE: Sizing of boulders for security barrier (vehicle impact)

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.

RE: Sizing of boulders for security barrier (vehicle impact)

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

RE: Sizing of boulders for security barrier (vehicle impact)

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.

RE: Sizing of boulders for security barrier (vehicle impact)

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

RE: Sizing of boulders for security barrier (vehicle impact)

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.

RE: Sizing of boulders for security barrier (vehicle impact)

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.  

RE: Sizing of boulders for security barrier (vehicle impact)

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.

RE: Sizing of boulders for security barrier (vehicle impact)

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

RE: Sizing of boulders for security barrier (vehicle impact)

(OP)
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.  

RE: Sizing of boulders for security barrier (vehicle impact)

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

RE: Sizing of boulders for security barrier (vehicle impact)

there's a difference ?

maybe disgruntled taxpayer ??

RE: Sizing of boulders for security barrier (vehicle impact)

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.   

RE: Sizing of boulders for security barrier (vehicle impact)

(OP)
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.  

RE: Sizing of boulders for security barrier (vehicle impact)

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.

RE: Sizing of boulders for security barrier (vehicle impact)

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

RE: Sizing of boulders for security barrier (vehicle impact)

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

RE: Sizing of boulders for security barrier (vehicle impact)

(OP)
drawoh, It is for maintaining a perimeter for defense against terrorists. Blast resistance of structural members are determined based on the standoff distance which is maintained by the bollards, boulders, walls ect.

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)

Do a Test Run!!!!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources