Perimeter protection against vehicles 2

[ajk1]

Is there any reference material on how to calculate the resistance of granite benches against vehicles of given weight travelling at specific speed?

 

[jayrod12]

I've always considered all of that type of stuff as sacrificial. Is someone wanting to use it as a barrier?

It's like bollard design, I always ask the client if they're expecting it to stop a travelling vehicle, or a deterrent. They always say stop until I show them the difference in the design, then magically it's a deterrent.

 

[JAE]

ajk1

1. Build the granite bench.
2. Decide on the size and speed of the vehicle to be stopped.
3. Take such vehicle and drive into the bench at that speed.
4. See what happens.
5. If it stops the vehicle - there you go.
6. If it doesn't stop the vehicle, design a larger, heavier, stronger bench and repeat steps 1-5.



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[BridgeSmith]

There should be force numbers for different size vehicles at different speeds and angles of impact in both AASHTO’s Manual for Assessing Safety Hardware (MASH, appropriately enough) and NCHRP Report 350, “Recommended Procedures for the Safety Performance Evaluation of Highway Features.” So, if you have access to either of those, that should help.

 

[JAE]

HotRod10 - I've never seen "force numbers" on vehicles for barriers like that. Not familiar with recent AASHTO's though.

I thought all impact-type barriers needed to be field tested to verify this (based on all the military force protection barrier guides I've seen). Thus my snarky post above.

Does AASHTO really give you forces for various vehicles?

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[BUGGAR]

m1v1 = m2v2
How fast do you want the vehicle and bench to be going after impact?
They make sand filled dummies to place on your bench to increase the mass until you get the speed down to where you want. Better the vehicle hits the dummies, too.

 

[ajk1]

I asked the question in the context of stopping an act of terrorism, such as driving a truck into a the front of a building. So it does not matter if the bench barrier is badly damaged in the process. Does that change any of the answers?

 

[jayrod12]

I would say that a dedicated individual would be able to drive a truck through anything smaller than a small vehicle sized chunk of granite. If you're expecting to make it out of granite slabs, I feel like they're just going to get crushed into rubble as they drive through it.

 

[ajk1]

Thank you all for this information. I will try to follow up.

I thought all impact-type barriers needed to be field tested to verify this (based on all the military force protection barrier guides I've seen).

Yes JAE I would expect that any design would have to be field tested if it were to have any official validity, but I am not sure if that would be a practical thing to do for a one-off for a particular location.

 

[helfreco]

Why are we focusing on park benches here? Are you trying to protect an existing area that already has granite benches around, or is this a new build and there is some architect requirement that calls out 'defensive benches'?

If it is the latter, can you install concrete bollards and dress them up like benches? Two bollards about four feet apart with wood slats joining them, and some sort of granite or faux granite covering to make the bollard look like a fancy bench.

 

[JStephen]

I assumed "benches" here meant a step in elevation with vertical wall, not park benches.
I find some interesting videos on Youtube of impact testing with different bollards, etc. They mention various ASTM specs that the testing is done to.
Note that in all cases, they'll define some sort of "maximum vehicle" limited by weight, speed, etc., and don't try to design so that nothing can ever penetrate.
With the vertical wall idea, a big question is how high it needs to be, and that's where the "maximum vehicle" idea comes in.

 

[JAE]

We did a force protection entrance for a Veterans Administration Hospital recently and used concrete bollards that had previously been built and tested. There were all sorts of proprietary bollard systems out there as well - all built and tested (in other words - no analytical design basis but rather build and destroy to verify integrity).

ajk1 - Most all bollards and other traffic barrier systems are not meant to fully survive the impact but simply stop the impact by absorbing all the energy.

AASHTO may have developed a way to analytically design traffic barriers (guide rails, etc.) so I was hoping HotRod10 could respond.

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[JLNJ]

If you don't want the bench to budge, then all the energy goes into the crumpling of the vehicle.

The force all depends on the guess as to the deformation of the vehicle. There are some resources out there with examples. I think you'll find that the forces will be sky high (like 100+ kips) and the bench will just be moved out of the way.

 

[BridgeSmith]

Yes JAE, AASHTO does give impact forces for vehicle impacts to railings. They are needed in order to design the railing anchorages and bridge decks for railings, and to design columns for vehicle impacts. For instance, one of the values that made it into the spec is a 600kip impact load applied to unprotected columns, corresponding to an 80kip truck traveling 50mph. I haven't looked at the MASH document, but I'm fairly sure the impact forces for the different railing classes (test levels) are in there. In the spec, all it shows is the vehicle weights, speeds, and impact angles for the 6 test levels (TL1 - TL6).

 

[GC_Hopi]

I believe the standard in the U.S. at the federal level is ASTM F2656-07, Standard Test Method for Vehicle Crash Testing of Perimeter Barriers. See Link and Link for a few reference documents. Usually the barrier has two criteria which they are judged on: 1) barrier rating - what kind of vehicle it can withstand. 2) level of penetration - expecting distance the vehicle will travel after hitting the barrier.

 

[ajk1]

Thank you all for the time you spent on this and the information. Much appreciated.

Why are we focusing on park benches here? Are you trying to protect an existing area that already has granite benches around, or is this a new build and there is some architect requirement that calls out 'defensive benches'?

There are existing benches. They are planning on adding some more.