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Calculating collision force

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bridgebuster

bridgebuster

Active member
Jun 27, 1999
3,965
My office has been asked to determine if a formwork system for a bridge deck would be able to sustain a collision force. Here's the problem: The contractor is placing a concrete slab above a low clearance roadway. The slab will span about 30 feet. The formwork is being supported by W10 beams; the ends of the beams are supported on columns.

The client is afraid that a bus could hit the formwork. We couldn't find written guidance or examples. One of our engineers came up with a procedure, as follows:

The bus weighs 20 Tons and we assume it's traveling at 30 mph.

First he calculated the kinetic energy, .5mv^2,

Then he equated potential energy to the strain energy, where the strain energy equals .5* P^2 * L^3/(48 * E * I)

and P = collision force

After finding P, he assumed that 10% was transferred to the formwork, which worked out to be 125 K or 85 ksi.

Does anyone have any thoughts or guides we can refer to?

I also posted this in the structural engineering forum
 
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Wow. Must be a REAL low clearance roadway.

My recollection of Caltrans falsework design is that vehicular collision forces are not considered in design. There are minimum horizontal forces to be applied in different areas, but nothing near 125 kips! This seems to be avoided by strict adherance to the required clearance envelope.

It sounds like the answer to the question is "No, it would not be able to sustain a bus collision."


 
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Hi bridgebuster,

In Canada, vehicle collision load is equated to a static force of 1400kN at 1.2m above ground. Just to give you and idea, in kips that is 315 kips. We typically require guiderails or ensure clearances to avoid such a force. Simply put structurally designing for such an unnecessary high lateral force is not the best way.

If your client is adamant about this scenario, have bus and transport truck traffic detoured to remove this possibility. Way cheaper than designing for that type of load during construction.

Regards

VOD
 
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I'd put up temporary height indicator beams (similar to those found at the entrance to multi-storey carparks) perhaps 10-20 metres from the bridge. Also used when working adjacent to power lines. The clanging noise alerts the driver to stop and is cheap, quick and easy.
 
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Thanks for eveyone's input.

The owner intends to have traffic officers on-site as well as an audible sensor warning system for overheight vehicles but they're still nervous.

We've kicked around some ideas for the formwork, which will probably end up costing more than the slab.

 
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I've seen jobs with overhead access platforms damaged by overheight loads. I strongly recommend height indicator beams as DaveMinter suggested.

Make the indicator beams strong enough to damage the overheight vehicle. Overheight drivers are not likely to stop without some strong encouragement.
 
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coloradobridges:

The owner is against the indicator beam and telltales due to potential liability.

We've developed a way for them to brace the formwork. Will it work? Depends on whether the bus crushes as we assumed. ;-)

Anyway, the owner will post traffic agents around-the-clock; hopefully that will do the trick.
 
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Our owner was also against a heavy indicator beam due to liability. This allowed an overheight vehicle to pass by the posted height limit, pass the sleeping highway patrol, and impact the platform at full speed. The platform sustained significant damage but did not collapse. Presumably the truck suffered less damage than the platform since it sped away never to be found.
 
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I agree with whoever said, "The formwork will not be able to sustain a collision." In my opinion, your guess that 10% of the energy will be transferred to the formwork in a collision is inadequate. I think there are sometimes where the client has to be willing to detour traffic. I don't know what your clearance is but I don't see how you can make any guarentee here. Recently, a digital photo was sent to me showing what happened to a five girder P/S concrete bridge when an overheight truck hit it at (I assume) highway speeds (the road was a four lane divided highway). All five girders were compromised. Two or three were completely removed up to a point about twelve inches below the deck for a length of twenty or thirty feet near midspan. All the P/S strands were broken, and the stirrups were also broken. I don't think any formwork could take that kind of load. Good Luck. - Ed
 
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Dinosaur,

We refined the analysis and did away with the arbitrary 10%. We modeled the bus and calculated how much energy we think it will absorb. It's possible we could make it work work but only a crash test will tell. However, that's not in the cards. We didn't have much time to analyze this, and of course, they didn't want to spend a lot of money (no surprise there.)

Anyway, we can't give the client any guarantee; just an opinion based on assumptions. These are the same people who, a few years ago, asked us to prove that a DOT standard guiderail would function as designed. Maybe it's time to get out of this business.

Also, would like to see those photos you mentioned. A friend of mine in Florida dealt with a similar problem.
 
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Not as impressive as Dinosaur but on a road reconstruction project that I was involved with, an excavator (unconnected with the site) was pulled off its low loader at highway speed by concrete bridge beams placed a week earlier. The excavator arm punched through the prestressed box girder.
 
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