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Flexible bollard

Flexible bollard

Flexible bollard

(OP)
I am investigating bolting "flexible" bollards at about 5 foot centres to the top of an existing 4" high (above the adjacent asphalt) continuous concrete curb that is only 8" wide and 8" deep (if necessary I can have the existing curb broken out and replaced by a new wider and deeper curb). The bollard will be around the perimeer of a parking lot and are intended to keep non-paying patrons out of the parking lot. I need to check that the bolts fastening the bollard to the concrete curb will not break out of the side of the curb and that the curb will not overturn when the bollard is subject to about a 3,000 pound force applied 20" above the asphalt paving (to simulate a 6000 pound vehicle backing into the bollard at about 7 km/h; correlation of velocity and impact force based on a paper by Iqbal). I expect that the "flexible" bollard would reduce the moment applied to the curb as compared to a rigid bollard, but I don't know by how much? Anyone got any idea?



RE: Flexible bollard

Make the bollards out of 1 1/2" light gage tubing (EMT conduit). It will likely bend before it can impart enough moment to overturn the curb or blow the bolts out.

RE: Flexible bollard

Bollards imply barriers.....high loads.

If, in the future, there is a condition whereby a vehicle plows through your warning but should have been stopped as with a true bollard, then you will be criticized for an inadequate bollard design. In short, if you put up a bollard, make it sufficient to withstand the appropriate loads. Otherwise, don't put one up.

RE: Flexible bollard

(OP)
Ron - If the bollard is to protect something from damage (like a piece of electrical equpment, etc.), then I agree that it must be strong enough to stop the vehicle. But in my case, that is not the purpose of the bollard. It is only to indicate to drivers not to come into the parking lot (except at the toll gate). True that someone could elect to drive over a flexible bollard, but I doubt that many would, particularly if it is of the 4" diameter type, as they don't give the impression of being able to be driven over even though it can be done.

Anyway, this is wandering away from my question...namely, how do I determine the moment and force that would be imposed on the supporting concrete curb by a flexible bollard? Would it be 50% less than a non-flexible bollard, or some other percentage? I don't need a precise number, as this is all only an approximation at best. Any suggestions? So far the bollard manufacturer can't give me an answer, but he is checking with his European source.

RE: Flexible bollard

you need to run the numbers using the appropriate section modulus to see what force it requires to bend the bollard. there is no magic number

RE: Flexible bollard

Why not just raise the curb?

RE: Flexible bollard

(OP)
cvd - The section modulus of the bollard won't give me the answer, because this is a proprietary "flexible" bollard, meaning that it has some sort of mechanism (perhaps a stiff spring) that lets it rotate up to 90 degrees!

ExcelEngineering (Structural) - Yes, raising the curb is one of the options that I had been considering, but then pedestrians could trip over it, and also it would restrict pedestrian access to the building by pedestrians who come from the street across the parking lot to the building, which is not something that we would want to do. One of my options is to drill concrete piers several feet into the ground, embed round steel post down to the bottom and fill the dilled hole with concrete, and design it for say a 3000 pound impact force from cars inadvertently hitting the post when they back up (as now happens), but that is likely relatively expensive because I find the pier has to be fairly deep into the ground. If you have any other ideas, I would be eager to hear them. Thanks very much.

RE: Flexible bollard

Either way you want to stop a vehicle from entering the parking lot, so why not just some regualar bollards. Drivers problem if they cant follow the road and not hit a bollard.

RE: Flexible bollard

isn't this yet another "impact" problem ? sure you can design for a load, x000 lbs, which makes your job easy ... much easier than a multitude of impact scenarios. now it might be sensible to design the foundation of the bollard to a higher load than the body can absorb. you might use UHMW plastic for the bollard body, with a separate base, so that the bollard can collapse, and rip out of the base, without affecting the base.

Quando Omni Flunkus Moritati

RE: Flexible bollard

ok, then if this is some proprietary spring, than you need to get the answer from the manufacturer of the spring

RE: Flexible bollard

When I was working for Chevron, we came up with a concept of providing small diameter pipe with thick walls that failed into the plastic range with very large deflection, rather than a large diameter, thin wall pipe section that was more rigid yet failed suddenly and totally by buckling.

RE: Flexible bollard

Also provide a larger diameter section near the base that the upper section is telescoped into and welded. Let the upper section do all the bending upon impact.

RE: Flexible bollard

Ron - thanks. I will study that. I have searched the internet but did not come across that particular site. What I am after is something that looks good, will stand vertical and can take small impact like when a car backing out of a parking space hits it at low speed.

RE: Flexible bollard

Ajk1:
It seems to me that what you want is something that looks a little tougher than it might actually be, to discourage the non-paying people, without ruining the curb. Why not something which looks like it would do some damage to the underside of a car if driven over, but will bend at a lateral force somewhat greater than a couple frat boys could apply after a drunken evening. If it looks like it will remove their muffler or catalytic converter, or oil pan, it doesn’t have to be real strong to have the intended affect. Maybe a heavy pipe, 1.5 or 2" in dia., 4-5' tall, with a 4"x8" base pl. and two bolts into threaded couplings, or some such, permanently set in the curb top 5' o/c, all parallel to the curb. It shouldn’t have to resist 3000lbs. at 20 or 24" before it bends or rips the base pl. off. This will damage the curb or require an outlandish foundation condition every 5'. And, this system will be less expensive to replace once the couplers are installed in the curb, than your fancy flexible bollards. You are not trying to stop a car on impact, you are trying to make it look too costly to drive over one of these warning pipes. You could go back to your old thread where the bollards just hook the front fender on the car if the driver isn’t careful.

RE: Flexible bollard

Ron's recommendation is similar to something I used, called qwickkurb. We used it on a bridge replacement project to separate opposing traffic.

http://www.qwickkurb.com/

RE: Flexible bollard

(OP)
bridgebuster (Civil) - thanks for this information. I think there are 2 choices:

a) Use some form of flexible bollard/post, or

b) put is a rigid post designed to resist a significant impact force; this would require putting in a drilled pier foundation. Bolting to a puny curb without a deep foundation will result in unending and frequent repairs to the curb (i.e. the situation that we have now)or repairs to breakaway bolts.

I don't know if the base plate on a 2" diameter flexible bollard is small enough to properly bolt to our existing 8"x8" curb.

One problem I have with fastening to our existing curb is that there is nothing there except the curb weight, to prevent the curb from rotating over, although perhaps as the curb starts to rotate the applied force would greatly diminish....don't know. If it were not for this problem, perhaps I can fix the existing problem of curb breakout at the embedment of the post, by welding a base plate on the bottom of the post and using 4 Hilti 3/4" dia adhesive anchors on the centreline of the continuous curb to bolt the plate to the curb. This would not get the allowable load up as high as I would like, but it would significantly increase the allowable load from what it is now with the 3.5" dia. post embedded 4.5" into the 8" deep x 8" wide continuous unreinforced curb.

I have to go out now, but when I get back I will try to attach some sketches of what I have in mind.

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