Need footing loads for post struck by vehicle

[jcas51]

The problem is a hollow rectangular post embedded into a small cylindrical concrete footing. When the post is struck by a vehicle, it will bend due to the low section modulus. I am using LPILE to determine the foundation size and need confirmation on the footing design loads. I am currently assuming that a small car with bumper at 20" above grade will strike the post and the applied shear force is calculated based on the bending moment capacity of the post. I am applying this shear force and the bending capacity of the post as the applied moment to the foundation. This method is resulting in pretty large foundations. I assume that once the post bends, there is no more load on the foundation, that the load is applied only up until the point the post fails in bending. Can anyone comment on whether this approach is reasonable, or provide any reference on what loads are actually applied to the foundation in this situation?

 

[JedClampett]

This is a difficult problem with a lot of "it depends" answers. If you're designing a government facility security barrier, you want the post to stop the vehicle cold. If you're designing a post on a highway, you want to absorb energy so the vehicle can be brought under control safely without killing the occupants.
In other words, if you're trying to protect something from intentional harm, you need to design an extremely stiff system. If you're trying to define a roadway or property boundary, something energy absorbing is more appropriate. And if there's something worse beyond the post, like a steep cliff, you need to use judgement.
The forces are based on the speed of the vehicle, the energy absorbing capacity of the vehicle and the energy absorbing capacity of the post. At best you can guess at one of the three (speed), and design one other (the post energy capacity).

 

[jcas51]

The intent is not to stop the vehicle. The post is not being design here, only the foundation, and the post is intended to fail upon impact. The mindset is that no matter how fast the vehicle is going, the post will be loaded to the point of failure. Basically, the vehicle can drive into the post, flatten it, and deep going. So does it really matter how fast the vehicle is going? If the post will bend over, doesn't it feel the same force no matter how large or fast the vehicle is?

 

[weab]

Jed,
My first impression is the same as yours, that this is an energy problem. But since the issue is just the foundation, jcas51 might be on to something. Assuming that the post fails plastically after initial yield, the load will increase just slightly based on steel stress-strain curve.

However, a sudden impact may cause the steel to strain differently, maybe even a brittle fracture. I don't believe that you will get a plastic failure. I think that the force will be more or less, but I can't determine which.

jcas51, if you do use this approach, keep in mind the highest possible range for yield. For example, a 36 ksi steel pipe can actually be over 50 ksi yield.

 

[BigH]

So, if I may, what is the intent of the post? I thought, perhaps wrong, that many posts and signs near highways are designed to give way so as not to "kill" the occupants by non-yielding. if the intent is not to stop the car and can be flattened, then isn't the only thing you should worry about for the foundation is the loads imposed by wind loading, etc. BTW, I had a high school friend killed when the car in which he was riding met a large sign post pole - back in 1970.

 

[jcas51]

Big, that is where the problem lies. It is basically a short, rectangular fence post. It is holding a wire fence, but the DOT says the foundation must be designed to not crack because they don't want concrete projectiles if there is an impact. So, in order to design adequate reinforcing, I need to know what loads to apply.

 

[msquared48]

Has the DOT performed any tests to see whether the post will fail in bending or in shear? Or how they want it to fail?

I ask this because, to me, and if I understand the intent of the cable restraint system, if the post fails in bending, then the cable will rotate down to the ground, and so will the cable, not stopping the car and absorbing little energy.

However, if the post fails in sudden shear, the post should remain in an upright position long enough for the cable to catch the car and transfer the load in the cable to other posts to either side.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ACtrafficengr]

Cable guide rail posts are designed to fail in bending. So are the fasteners holding the cables to the posts. The post bends out of the way, leaving the cable do grab the vehicle sheet metal to decelerate and redirect the vehicle.

Here is a crash test one one of the new high-tension cable barriers:

http://www.youtube.com/watch?v=PTrbUokgohk

and a conventional low-tension cable barrier:

http://www.youtube.com/watch?v=6Wdbcwlu1gU

Jcas, is this a barrier, or just a fence? What is the point of having a foundation rather than just direct-embedding the post?


"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[jcas51]

The DOT will not allow direct embedment. Apparently after impact it is easier just to remove the bent post from the sleeve in the concrete and replace it with a new post than to install new posts. I have seen numerous videos and am aware of what happens to the post. But that doesn't explain how much load is transferred to the foundation.

 

[ACtrafficengr]

Sorry, the vids were for msquared.

I must admit that I'm a bit out of my area of expertise. I design guide rail installations, not the guide rail components. Consider this as input from a possible user, not another designer.

It seams to me you want the post to fail in bending, not shear, so that the upper section doesn't become a projectile. What if you calculate the force needed to form a plastic hinge at ground level, and apply that to the foundation?

Large foundations aren't necessary bad. The foundation should be heavy enough so it doesn't shift much when hit, so the replacement post won't be out of proper alignment. This is probably more important for a high-tension system.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[wannabeSE]

If the L-pile result seem unreasonable, you might see how the results compare to results using the pole footing formulas in the IBC.

 

[ACtrafficengr]

If you are using a standard light guiderail post, I think the Turner Fairbanks Highway Research Lab has a finite element model of it. Perhaps their results would be useful to you, if not the model itself.

They are part of FHWA, so their work should be public domain.

I also recall something about posts in weak soils being the worst case scenario. In strong soils, the posts yielded at ground level. In weak soils, they were dragged through the soil, increasing the g forces on the vehicle and its occupants. I suspect a concrete foundation would act more like a strong soil than a weak soil.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust