Need to confirm bollard design for vehicle impact 3

[BrianBoen]

I have specified a schedule 80 steel bollard with an 8" diameter. The bollard is 3' above ground and 3' below ground with a 42" deep concrete encasement with minimum 4" cover on all sides of the steel pipe.

The client would like me to confirm that the bollard will withstand the impact of a 4000 pound vehicle at 30 mph. How do I convert the 4k pound @ 30 mph impact into a point load or factored load of some sort?

I have the allowable bearing pressure and steel information. It seems like I have everything that I need except for a load or force that I can use. I know
KE = 1/2 x m x v^2 and
F = m x a
but I don't have an acceleration and kinetic energy doesn't really help does it. Can anyone help?

 

[kslee1000]

Greg:

When you say,

"Yes, if you know K then you can derive a peak force quite easily.
But the effective k in a highly non linear event is not obvious, and in my opinion is almost meaningless."

I would like to remind you that, while I agree with you that the whole event (car crash) is likely ends up in the state that renders K meaningless, it (total energy involved) is not really in our interest. For most of engineering problems, we don't need to get everything "exact" as scientist does, but to obtain a "reasonable accurate" solution through a manageable method that is in line with the theories behind it. That (manageable method) is what we are looking for.

How to get K? For structural support system, it is tedious to cal, but solvable. For vehicle, it is much murky, since I don't think there is any study been done for that. Therefore, I would suggest to treat the car as a rigid body, and the structural support system as the spring. By doing so, though it is not the "exact" condition, but rather conservative (to assume no damping effect from the deformation of the vehicle).

 

[BrianBoen]

I must say thank you to all that posted. Your information is valuable.

There were a few questions that I saw: First, the bollards are going to be in front of a Federal Courthouse. So the strength of these bollards must meet criteria as stated above.

MaddEngineer,
I was wondering how you came up with 75,155 ft-lb of energy for a 10000# vehicle @ 15mph using the 1/2 * m * v^2 formula.

MaddEngineer was correct in mentioning the stopping distance requirement. The ASTM standards vary from one of three options: 0-3ft, 3-20ft and 20-50ft. I forgot to mention that in my original post. The requirement that needs to be met is 3-20 seeing as the building is about 20 ft set back from the bollard.

I don't think the cheesewiring would be an issue with that being said. We are not designing to completely demolish the car and stop it on a dime rather we want to slow it down and not allow it to travel any further than 20 ft beyond the bollard.

Does that new requirement shed any light on your theories? It seems that kslee1000 is on the right track and the 20ft stopping distance requirement should be usable in the deceleration equations you provided.

 

[JedClampett]

This is the kind of issue that comes up in Nuclear Generating Station design. There might be some literature available there. However, they aren't worried at all about aesthetics, so there's a lot of wire with multiple layers of barriers.
I'd use a group of staggered bollards with a 4'-0" spacing and another row about 4'-0" behind them such that there's no way you can enter without hitting one bollard square and then two behind it or vice versa. If anyone gets through the first set, the second set will certainly stop them. And it shouldn't impede wheelchairs or pedestrians.

 

[Larryhd2]

Here is an article you might want to review...

http://www.structuremag.org/article.aspx?articleID=784

 

[kslee1000]

JED's suggestion is worth to think about. Or how about concrete block barriers?

By throw in the allowrance on stopping distance made this problem even trickier, because now it involves inelastic behaviors of both subjects (car & barrier), and failure mechanism of the support system. Not an easy task for computation, hope something readily available is out there somewhere.

 

[BrianBoen]

It's not a calculated solution that I found but it still works. Looks like I'll have to increase the depth of the steel pipe and the size of the footing a little bit.

See the attachment for the detail. It has been tested to withstand the impact of 4500# @ 30 mph. It was found in FM 5-114 Appendix A and in the US Department of Transportation document titled "Transit Security Design Considerations" (FTA-TRI-MA-26 7085-05) in Appendix E.

 

[Larryhd2]

The article I posted will pretty much answer your questions regarding the load imparted to a bollard.

Assuming a zero deflection at the bollard I came up with about 61kips to the bollard.

You would need to do a trial and error with bollard deflections figured in to dial in a more accurate loading.. making sure the bollard stays within the elastic range...

 

[BrianBoen]

Do you mind explaining how you got 61kips?

 

[kslee1000]

Brian:

Very interesting site.
Below is the link for persons interested in Transit system. I guess there are quite a few free reports for dowload.

http://transit-safety.volpe.dot.gov/publications/order/singledoc.asp?docid=356

 

[Larryhd2]

Honestly, I looked at the chart in Figure 5 in the article and scaled down for a 4000# vehicle...

Using equation 5 given, I come up with a vehicle crush of (30^.5)/3 = 1.83'

Using equation 2:
F=mv^2/(2*(1.83+0)) = (4000/32.2)((30*1.4667)^2)/(2*1.83)
=65,700#

Assuming the bollard moves (deflects) 6", the load goes down to mv^2/(2*(1.83+.5)) = 51,600#.....

It seems you would then need to decide on some bumper height to apply the load, determine bollard deflection under the applied force, and iterate...

 

[Bribyk]

How about putting some stairs in instead?

 

[BrianBoen]

wow, I feel stupid. I wasn't dividing the weight by 32.2 to get mass. I kept coming up with this astronomically big number. haha. Thanks.

I believe about 2 to 2.5 feet would be an appropriate location to apply the load.

 

[BrianBoen]

Stairs are a good idea for a new site. This is an existing site and unfortunately is not a possibility.

 

[Larryhd2]

Good ole slugs will do it every time...

 

[JStephen]

This won't help with the design problem, but is a cool video. An Anti-Ram wall at work- takes a few minutes, but about halfway through, they show a loaded dump truck smacking into it. Impressive.

http://www.jmworks.com/Adler/index.htm

 

[kslee1000]

Brian:

I have to praise Larry for the paper he provided. It seems reasonable yet simple to use. How is the project going?

 

[patprimmer]

If you want to stop an aggressor, look to WW11 tank trap designs or designs used in ancient times for foot soldiers to ward off calvery with strong sharp steel spikes firmly anchored and pointing at the approaching vehicle.

To prevent entry and give maximum protection to the driver, look at drag race catch nets where a woven wire mesh is used and mounted in a way to yield quite a distance it absorbs the energy and distributes it over the entire front f the vehicle.

Really solid bollards tend to cut cars in two up to at least the engine, or if less solid, knock down and then the car gets airborne as it goes over the top of the sloping post. If the concrete the post is embedded into has insufficient bearing area in the soil it will just knock the post over. I think there is a real need to think the real objectives and possible results and consequences through. The bollard in a relatively small amount of concrete in variable ground with various vehicles at various angle has a huge number of variables.



Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[kslee1000]

Unfortunately, the request for barriers set-up will keep rising, as long as we live under constant fear. As Brian pointed out, he was working on behalf of the Fed, I don't think the barrier is for you and me, but... So the criteria for speed, 30 mph, is way high than normal used for warehouse, or shipping-receiving terminal.

 

[MaddEngineer]

Sorry Brian, for not responding sooner, but looks like you figured it out (m = Weight/32.2)... Also I didn't realize you noted the design criteria (4000lb @ 30mph) - (Must have had a brain cramp, was cranking calcs out on a Saturday night and I have two twin baby boys at home)... Larry great article that Equation 5 is the motherload. Looks like Brian has enough info to rationalize his design if he distributes the load to two bollards he will remain in the elastic range. However, I guess the problem still remains for mass-velocity relationships that put you into the plastic range for the bollard(s). I was thinking of modeling the bollard as a rotational spring that would rotate at the plastic hinge. I found a paper online discussing plastic deformation of a cantilever under tip impact but its a little heavy and I haven't really took a hard look at it for relevance (if anyone wants a stab check it out). Also does anyone think some (or alot of)energy will be initially absorbed by the foundation since theoretically the foundation should slide until it is stopped by something, either the soil or the roadway and then will it engage the bollard in bending (albeit this may be a very small distance). I saw evidence of this at the drive thru a McDonald's, bent bollards and cracked/crushed pavement. You could use the EA of the footing (a large number) plus the k of the soil... Any thoughts

 

[kslee1000]

Madd:

Glad to see others to join in.

I thought about rotational spring analogy as well. There are two scenarios on how the system behaves:

1. Barrier on massive concrete (infinite rigid) block below grade: The barrier acts more or less like cantilever beam.
2. Barrier on foundation with intermediate rigidity, like in the shaft filled with concrete: the barrier may work with its foundation as an integral unit, the failure mode is linear in nature (the system rotates like the needle arm in clock). At the very moment, the impact force has caused the soil wedge in front of the footing to reach passive state, thus the soil resistance is lost.

If a sliding distance is allowed after initial impact, one can manipulate to design a more flexible system, which is allowed to fail, but effects in slow down the vehicle. The vehicle would then come to a complete stop by friction within the pre-set distance.

The concept is simple, which can be expressed as:

Ff = Fi - R - N = 0

Fi can be tetermined by displacement obtained using spring analogy; R is the resistance force could be estimated from the subgrade condition; N is the friction force.

How the above matches your thinking?