Loads on Overhead crash gantry

[rteja]

I am designing an overhead crash gantry of 30m span (see attached image). The client's requirement is "structure to resist momentum of 8000 kg vehicle".
I do not have experience in this type of structures. The purpose now is to give steel estimate for my costing team.
My question is, can i apply equivalent static loads, instead of time history loading? i do not have enough time vs force data. If yes, what will be the equivalent static load?
In the attached image i followed design forces recommended in AASHTO section 13, table A13.2-1 (Design forces for traffic railings - TL-4: 240kN applied over 3m span). But i'm not sure if this is the right design guidelines i should be referring to.
Any suggestions or guidance in designing this structure will be very helpful. Thanks.

 

[Tomfh]

I would be inclined to do it properly and calculate it via energy method, rather than kludge method of static equivalent load.

You need to know how fast the 8 tonne vehicle is moving. That gives you an energy to resist.

The structure must then be capable of absorbing that much energy prior to failure. Hence you also need to decide what constitutes “failure”...:

 

[rteja]

Thanks for the response Tomfh.
Posted speed limit for this road is 120kmph. This would mean nearly 4.4 MJ of energy to be absorbed. This seems a bit extreme for this steel structure. And i have seen very similar structures already existing on roads.
May be as you pointed out, "failure" is the deciding factor. Failure criteria can't be just exceeding yield.

 

[Tomfh]

An 8 tonne truck doing 120 kmh. That’s a fair whack. I’ll be impressed if you keep it from yielding!

 

[Settingsun]

Is it 5.5m above road level? What vehicle is it? Trucks are lower than that and even normal overheight trucks would just have the roof ripped off. The rest would go right through a la Terminator 2.

Is this some terrorism requirement? That's a different field than normal crash barriers.

TL4 is 8000kg at 80km/hr but also 15 degree angle, a glancing blow. A TL4 barrier is parallel to the road. That doesn't seem to be what you have.

 

[BUGGAR]

Pretty big loading. You could use a small ship fender to absorb the energy. Fender catalogs will give you the reaction load for a given energy absorption amount, and you just design the structure for that reaction. Same as designing a wharf for ship impact. You will have to put a fender wherever the vehicle can hit the structure.

 

[Settingsun]

The truck energy is roughly equivalent to berthing of the largest ships in service. It's not going to be a small fender.

The rubber is probably not far from being a solid block at these speeds anyway. I'm sure fenders have never been tested for it.

 

[HTURKAK]

I will suggest you to look Eurocode 1 — Actions on structures —Part 1-7: General actions — Accidental actions ,

SECTION 4 IMPACT and
Appedix C DYNAMIC DESIGN FOR IMPACT .

 

[ldeem]

The energy method is the way to go with this but your client needs to specify a speed in addition to mass to get momentum.

I have done several impact energy type projects and getting client to decide on speed of impact is hard. It doesn't take much speed to have a very large impact energy. A true system would need some kind of energy absorbing mechanism instead of just steel frame work.

I am not sure where you are in the world but here in the US I would be very concerned about liability exposure. I would insist on the client specifying both mass and speed (or some other design criteria).

 

[r13]

Is the crash gantry meant to slow down the runaway truck to protect another structure?

 

[LittleInch]

The clients demands and scope is nuts.

From what you've said this is a motorway.

At 5.5 m you're only ever going to take the top off something. If you want good stuff look at 11foot eigth .com or whatever it is for loads of crashes at relatively low speed.

http://11foot8.com/.

Very few actually stop and it is much lower than your 5.5m barrier.

We've also discussed this sort of thing for this bridge

https://www.eng-tips.com/viewthread.cfm?qid=459466

with lots of alternatives.

Do a search for "vehicle barrier" on this site - too many to list.
Here's something similar - read down a bit as its a railway bridge protection but starts off as something else

https://www.eng-tips.com/viewthread.cfm?qid=342382

But the key point is that any vehicle barriers you do see are all about 5 to 6m wide, not 30m wide and on low speed roads. The only thing you can get on a motorway is a set of poles or chains or lights etc etc. A vehicle barrier at 5.5 m is unheard of and won't work. IMHO

If a truck - and where the hell does 8 tonnes come from?? Nothing weighs 8 tonnes. It's either a small vehicle or it's a 40 tonne semi / artic or a tipper lorry - hits this thing it will either fall over, rip the top off, rip the raised tipper section off or if it does work as described stop so fast the driver will exit via the windscreen or the following truck /car / vehicles will pile into the back of it and kill loads of people.

Even worse, your structure could collapse and land on a load of cars which crash into it and results in a major pile up. You will get the blame.

MADNESS

SO I think if you're estimating steel think of something pretty reasonable and quadruple it. Also your back stantion needs to be close to 45 degrees and your main beam good enough to be a abridge in its own right.

Or don't bid.


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