Need footing loads for post struck by vehicle
Need footing loads for post struck by vehicle
(OP)
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?





RE: Need footing loads for post struck by vehicle
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).
RE: Need footing loads for post struck by vehicle
RE: Need footing loads for post struck by vehicle
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.
RE: Need footing loads for post struck by vehicle
RE: Need footing loads for post struck by vehicle
RE: Need footing loads for post struck by vehicle
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
RE: Need footing loads for post struck by 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
RE: Need footing loads for post struck by vehicle
RE: Need footing loads for post struck by vehicle
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
RE: Need footing loads for post struck by vehicle
RE: Need footing loads for post struck by vehicle
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