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Unconstrained Bollard Design Check

Blackfairy

Blackfairy

Civil/Environmental
Jul 22, 2024
11
Can someone check and see if I am doing it right.

I am designing a steel bollard that will take 6K impact at around 2.25ft from the ground. I am using the pole/embedded post design unconstrained. I am using 150 psf of lateral pressure and doubling it as per IBC 1806.3.4. Since it is unconstrained, my allowable lateral soil-bearing pressure will be 1/3 x depth x 150 psf. Using 1.5" diameter casing, I found that my depth is around 11 ft or so which is too deep not to mention that the allowable vertical bearing pressure will exceed the 2000 psf due to little area and high weight of the casing.

So I changed it to 2.5' diameter and got 9ft of embedment and my vertical bearing pressure capacity ratio is now less than 1. But looking at it, is it normal to have 2.5' of casing on bollards 9ft deep for a 6K impact? If that is the case I might have to reinforced it and not just fill it with grout since it's a huge diameter. For comparison, the other bollard that is constrained by a slab on grade, I calced it 1.5ft diameter 6ft deep. That 1/3 depth for allowable lateral pressure for unconstrained is what is really is the big difference between constrained and unconstrained.
 
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Is there a very special need for this bollard to remain in place after such an impact? If this is for a typical parking lot or warehouse bollard, it is normal to assume foundation failure in the event of a collision. Bollards tend to function primarily as visual warnings, and secondarily as a structural barrier. There will always be a big impact which could take one out, you just want to make the thing being protected is unharmed.
 
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hemiv said:
Is there a very special need for this bollard to remain in place after such an impact? If this is for a typical parking lot or warehouse bollard, it is normal to assume foundation failure in the event of a collision. Bollards tend to function primarily as visual warnings, and secondarily as a structural barrier. There will always be a big impact which could take one out, you just want to make the thing being protected is unharmed.
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It is just protecting electrical panels on the that controls a fish release system. The place is not public, the only thing that can hit the bollards are the trucks that carries the fish into the fish release pipes while backing up or maybe a electrical maintenance truck while they are checking the electrical panels.

If that is the case how should I design it though Should I just design it the same and just make non conservative assumptions?
 
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Another idea to consider: Isolated pole factor to increase lateral pressure.
 
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I believe you're using the IBC equation properly.
I get 8.5' embedment for the 2.5' diameter unconstrained.
Maybe 2.0' dia, 9.0' deep would be better.

And for constrained case- 1.5' diameter, 6.33' deep.
 
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My experience is like others here, bollards are a warning and slight deterrent when hit, but not intended to stop a vehicle in its tracks (not speaking for AASHTO rated stuff).

Impact load magnitudes can be huge depending on what assumptions you make about the "collision".
 
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Just get some big rocks, keep the surroundings natural. If a truck is backing up to this area add a wheel stop a distance away, no damage to the truck if hit but let’s the driver know they should stop.
 
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Thanks, there is actually a wheestop to protect the actual fish release pipe when the trucks back off, plus bollards at the back of the wheelstop. From what I am understanding on the replies, is it safe to just design the unconstrained bollards as constrained or just detail them the same as what I get from the constrained version. Most of the bollards there are constrained since they are on the slab on grade, there are only couple that is on the soil.
 
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You can design the bollards as either unconstrained or constrained. But for both cases, if you design the bollards to withstand that impact load with minimal deflections, you will end up with unrealistically large foundations. Not to mention injure the driver of the vehicle! It would be like driving into a solid concrete building. In all likelihood, the owner or the contractor may just not follow your design and put in something more typical.

The bollard is expected to fail and release some of that impact energy. I do not design the bollards on my projects with structural calculations at all. I follow a standard detail of my firm, or create a new detail if needed for some reason. 6" dia. Sch. 40 steel pipe filled with concrete, painted yellow, 48" above grade, embed in a 2' dia x 4' deep concrete pier is, to my mind, a pretty typically arrangement. This kind of bollard has been vetted by virtue of extensive use - "empirical design," let's say.
 
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There are about 100 posts on E-T about bollards and similar protection devices.

Most all the ones you see are just empirical and as everyone is saying if something that big drives into it the post / bollard needs to flex or allow the foundation to move or you're into unrealistic sized elements and foundations.

do a quick search and you will see many posts and designs.

RoT - height of post above ground should be twice that buried then it won't move much. Anything less and it will but still protect the item.

Many highway specs and designs exists also.

A 6,000 (lbf?) design load means nothing without saying what deflection you can accept or what speed the vehicle is going at. Where did this figure come from in the first place?
 
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