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?

 

[JAE]

It can't resist that in my opinion. The bollard would just lay over.

Vehicle impact resistance is defined for guardrails in AASHTO. They don't use calculations but rather full scale vehicle impact tests to verify load compliance.

Bollards are more of a psychological barrier vs. a true mechanical barrier.

 

[Bribyk]

It would depend highly on the vehicle. Newer vehicles absorb a lot of the energy and reduce your peak forces. A forklift or something similar wouldn't crumple as much (or at all) and would have higher forces. If you can assume a newish vehicle you can probable pull up some acceptable g-loads (peak and average accelerations) from the NHTSA for head-on collisions at 30 mph. I don't know if they test a point load on vehicles though, it's usually a wall-ish type structure. Then your impacting force can be calculated as F = ma, assuming your bollard does hold up and is rigid. You could get more complex from there, if required...

 

[CarlB]

You might check out the documents at

http://www.wbdg.org/design/provide_security.php

and other "force protection" design guides. Bollards to resist a speeding vehicle will likely need large concrete foundations.

 

[GregLocock]

You can get an upper bound by equating the fully plastic moment of the bollard, *pi/2, to the KE of the vehicle.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dik]

I did a bollard for a mining project a couple of weeks back and used a 12" sched 80 stainless steel pipe, conc filled, embedded 6'of 30" dia reinf conc that came to just above grade and then did a sloped surface for runoff. It's a corrosive environment and use of SS is common. When the client rep asked how strong it was, I told him that I couldn't find any literature on bollard design and I made it to resist some impact, but that I had essentially made it fairly resistant. I couldn't find any literature on bollard design other than prescription. I felt that once you deviated from a simple bollard, the added cost of making it stronger was relatively little. If some equipment actually strikes the bollard, it is possible to shear it off or possibly pull it out.

 

[kslee1000]

After studing a few articles on the web, I found the following parameters are useful in develop a simple method to estimate the impact force, with the condition that the driver is aware of the barrier, and has attempted to stop the vehicle, but failed due to inadequate stopping distance. Please comment on the method present below.

Parameters:

Vehicle weight = 4000# (ave. 2300#-3500# for passenger car to SUV)
Deceleration, D = -15 ft/s2 (a ball park number from research, measured from touching the brake to a complete stop. See linked site for more info)
Speed, S = 30 mph = 51 ft/s
Required braking distance for complete stop, B = 59 ft (again, see linked site, also for dist. required for other speed)

Assumptions on Site Condition:

1. Assume barrier is located 15' (can be any) off the drive way. (d = 15')
2. The drive way is level, and frictionless.

Solution:

Time required for a "reactive" stop:
t(R) = (Vf-Vo)/a = (0-51)/15 = 1.7 s
Time for a "forced" stop:
t(F) = t(R)*(d/B) = 1.7*(15/59) = 0.432 s
Effective deceleration:
a(E) = (Vf-Vo)/t(F) = (0-51)/0.432 = -118 ft/s2
F = m*a(E) = 4000*118/32.2 = 14659# = 15k (rounded)
The load shall be applied at the level of impact.
From here, we can design the barrier and its foundation.


For un-conscientious case (no effort to make stop), since a = 0 at constant speed (Vf just before collision equals the initial speed, Vo), Kinematics couldn't solve the problem. It has resorted to more rigorous energy method.

 

[kslee1000]

Note:

The derivation of effective deceleration is based on the assumption that the vehicle has crashed onto a barrier with infinite rigidity (no damping), which results in an instant stop (Vf = 0).

 

[JedClampett]

What's the bollard protecting? If it's some bushes or flowers, I'm not sure making hitting it a death sentence for the driver is appropriate.
Trying to design these type items for a high rate of speed (and 30 mph is high) is nearly impossible. You make it stronger and the force on it goes up. And 30 mph head on almost has to be an intentional act.
One of my first designs in my career was to design bollards to protect the frame of a roll up door. When my boss asked what it was designed for, I responded, "Who are they going to be madder at, us for under-designing the bollard or the guy who couldn't steer his truck?" Or something like that. It was a long time ago.

 

[kslee1000]

Gorgot the link mentioned above.

http://www.csgnetwork.com/stopdistinfo.html

 

[GregLocock]

klee100

I'm afraid I can't follow your logic, and I KNOW that you can't easily estimate a peak (or even average) force without making some assumptions about the impact process. The link you gave is a bit disturbing in its lack of rigor.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kslee1000]

Greg:

Thanks for commenting. Based on your background (Automotive), I would guess you can provide better sources on similar studies, which may provide easily undersandable clues to solve the long standing puzzle that has bothered many of us in the engineering field.

The method presented is just a quick thinking. It yet to be validated, or discredited, by collective thoughts and sound reasoning. I am eagerly await all constructive critics.

 

[MaddEngineer]

I had a client last year who thought they might need me to design some bollards, I did some limited research, but the project never materialized.

Basically the Department of State has some standards for Anti-Ram Vehicle Barriers and they specify different levels of protection based on a 15,000 lb vehicle traveling at 30, 40 or 50 MPH. With different stopping distances 0-3ft, 3-20ft, 20-50ft. (The actual barriers are crash tested per an ASTM Standard) The understanding is that the barrier will deform into the plastic range (or move)and will absorb the impact energy. The NAVY also had a method based on 10,000 lb vehicle traveling at 15mph or 50mph (Low Security or High Security). The documents note the Kinetic Energy for each. For the 10,000 lb vehicle at 15mph the energy will be 75,000 ft-lb (arrived at by using 1/2m V^2 = 75,155 ft-lb). A true energy analysis could get tricky (you have to look at crumpling of the bumper, bollard, soil, concrete, etc.) but assuming the vehicle will stop in 3ft would give a force of 25,000 lb. This can easily be taken by an 8in/sch80 pipe. The bollards will be also most likely 3ft apart?? so at least two (2) would be engaged I would think. Stopping in 1ft would be 75,000 lb/2 Bollards = 37,500 per bollard. Might have to do iterations, checking deflection vs. load, however, the stiffness of the bumper/vehicle is an unknown.

 

[kslee1000]

That's the problem with energy method, which I tended to avoid simple for the reason - too many variables and unknowns.

For engineering needs, there should be a practical method to approximate the effect. Or more testing should be performed to backup the theory. Try crash GM cars, as they are much cheaper now

Note: I am not quite convinced that energy method is correct way to determine impact force. It predicts the collapse mechanism (deformation) by including effect of dampping, but the question remains - the force at the split second when two rigid body colide, tome, damping is part of thereafter, which does not have relevance in determine the force at collision. Again, I could be deadly wrong on this.

 

[GregLocock]

OK, the KE=plastic energy in bollard approach gives an upper limit, that is, if the KE of the vehicle is less than that required to bend the bollard out of the way then by definition the bollard must be still in the way.

But, for real vehicles that is almost absolutely useless, since vastly more of the plastic deformation will occur in the vehicle, not the bollard.

Passenger cars/SUVs aren't really designed to cope well with poles, frontally, which will tend to cheesewire through the structure. That's why Armco fencing is designed the way it is - the contact stiffness is low and there are several weak mechanisms for absorbing energy.

Your best bet would be to study some pictures of poles cheesewiring though cars, to get an estimate of the crush distance involved for a given speed. You can then turn that into an average force, and double it to get a sort of peak force, and double it again, for luck.

At 30 mph 13.4 m/s I guess that you'd see 0.5-1m of intrusion, so for a 2000 kg vehicle the 'average' force by this method is 1/2*m*v^2/s=180-360 kN

So, that is 9 to 18g, which is in the ballpark for a crash pulse.

So if you double it and double it again I'd design the bollard for 1440 kN, exerted 0.6m off the ground (the height of the engine).

Note that this will damage the car and its occupants far more than a deformable barrier. This is a pretty severe event.

Also you need to make sure the car doesn't jump over the bollard, that's why an elastic solution is better. If the bollard lays over then all bets are off.






Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kslee1000]

My intent wasn't limited to bollard, it's for general applications.

Using energy method, assume linear motion with known structural rigidty of system (vehicle and barrier, replaced by an equivalent spring), here is the foundamental relationship between parameters m (mass), K (equivalent spring constant), x (displacement), V (final velocity), a (acceleration/deceleration):

U (work/energy) = K*x^2/2 = m*V^2/2 = m*a*x, (F*x)
From the relationship, we can setup two equations to solve 2 unknowns - "x" & "a".
The result is, a = V*(K/m)^2/2
K is either from test, or could be calculated from known data on barrier support and the vehicle.
V (at time of impact) can be safely assumed as equal to the given speed (ie. 30 mph for the case under discussion)

The the impact force is simple, F = m*a.

 

[GregLocock]

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.

Hey, here's an article we need to read

http://www.springerlink.com/content/p6843r6780p2t536/

but its 34 bucks, so I guess the OP gets to buy it!

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

A quick look at the search results for pole impact at

http://www.sae.org

indicates that there isa fair amount of literature on this.

I would come back to a previous point, cheesewiring through a vehicle could inititiate multi million dollar lawsuits, is your structure really that valuable?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[kslee1000]

Greg:

Bollard has never intended to completely stop a vehicle. It generally serves as a warning by the vehicle entrance, and as a back-in guide. The 30 mph request was not reasonable, no one would try to achieve that without given the owner the warning as you mentioned. However, I was tring to take upon this opportunity to find a practical method to calculate the impact force, which has bothered many of us, and has broad, though rare, applications. Thanks for keep coming back with valuable informations.

 

[bridgebuster]

Before you do anything, look at the attachment. It has some very important imformation on bollard design.

You can try a down and dirty approach by assuming the bollards are cantilevered beams or piles with lateral loads. The point of fixity would be somewhere within the concrete foundation. Check the foundation to see if it has enough breakout strength and check the depth to see if it has enough passive resistance against overturning.

Where I work the building is surrounded by bollards. They were put in five years ago; just before the Republican convention as a security measure against a truck bomb. They are 8"-dia pipes filled with concrete; the bollard spacing is 5'-0; they extend 2'-6" above grade. The footing is 18-inches wide. I don't recall how deep the pipes are embedded. The top of the foundation is about 6" below grade; I think it's 3- deep.

On a bridge project we put in bollards as a pedestrian safety measure against a bus mounting the sidewalk. The bollards are 6"-dia Schedule 80 pipe; 3'above grade. They're mounted to a 3' wide x 2' deep footing, 10" below grade using 8 - 1" dia anchor bolts with 10" embeddment.