Design Load on Overhead Clearance Bar

[ReverenceEng]

Hi all,

I have a project to design an overhead clearance bar / headache bar. It does not have a swing-away capability. I know ASCE 7 chapter 4 prescribes a 6,000 lb force for barrier systems applied at I believe 18" above grade, but certainly this isn't a barrier and doesn't need to function as one. So, the 6,000 lb seems a bit large to me. However, the force will be applied at the top of the bar, inducing quite a large moment at the base, so I need to select a manageable force, but also design appropriately.

Does anyone have any guidance on what magnitude of a point load might be appropriate here? I'd like it to withstand an occasional bump at height, or at least not completely get knocked over by one.

Thank you for your advice,

 

[EngineeringEric]

IS the purpose to stop something? or scrape? or bend when impacted?

 

[ReverenceEng]

It's really just a clearance indicator that you would have at a drive-thru for a bank or fast-food restaurant. The purpose is for a tall vehicle to hit it before it can hit the building. It doesn't need to stop anything, and it would probably need replaced after any sort of significant impact, but I'd like it to possibly survive a love-tap here and there...

Their design is to have the vertical HSS welded to a base plate...I am also thinking of over designing the footing to try and ensure that whatever force I go with for the clearance bar, the plate will yield long before we have footing issues, thus making replacement easier.

I was thinking 1000 lb at the top for the steel components, and then doubling that for the foundation design...thoughts?

 

[XR250]

Two columns and a cross bar?

 

[TehMightyEngineer]

I'd either design it to be a light-weight, easily replaceable item with break-away bolts when hit. Or design it with a lot of ability to deform inelastically so that you don't have heavy pieces of steel breaking off and falling onto something or someone.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[EngineeringEric]

Yeah, i would design everything permanent for say 2000# and then design a fuse element or connection for maybe 25% load. I would not do a barrier type load, those are meant to stop/really slow down a vehicle traveling fast. You are trying to make an audible sound for a truck moving 5 mph...

love taps depend on how fast the driver hits that brake!

 

[XR250]

If it is not meant to physically stop the vehicle, might as well make it pretty light weight as Ted said. Could use some 1 1/2" tube steel for beam and columns and let it bend.

 

[JAE]

Many of the headache bars I've seen were designed with two columns - a solid cross bar - and then the actual headache bar was hung below it with short chains such that any impact wouldn't be transmitted into your frame and cause the owner to have to continually replace the assembly every week.

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[XR250]

Funny story - In college, my roommate had a pretty jacked up truck. We pulled into a parking garage but the headache bar was set too high. As soon as we started going up the ramp, the truck got wedged against the concrete beam. The parking deck owner's insurance paid for the damage.