## Vehicle Impact Loads on Stationary Structures

## Vehicle Impact Loads on Stationary Structures

(OP)

I am working on a design of a vertical lifting steel open-framed security gate positioned between reinforced concrete posts with the requirements that it not "fail" the impact of a truck (50 kips weight) traveling at 50 mph. (I am not designing the posts). Knowing that force is dependent on time and mass, I am having trouble finding any documentation showing calculations/formulae for this type of situation. Some of my random thoughts - A. How long does the impact last?-This governs force B. Dampen load with hydraulic bumpers between gate and posts? C. How much of the load does the truck's frame absorb? This should reduce force on gate.

Are there any "proven" methods to simplify this to a static problem? dynamic to static load ratio? Thanks.........

Steve

Are there any "proven" methods to simplify this to a static problem? dynamic to static load ratio? Thanks.........

Steve

## RE: Vehicle Impact Loads on Stationary Structures

## RE: Vehicle Impact Loads on Stationary Structures

## RE: Vehicle Impact Loads on Stationary Structures

It's really an energy/work dissipation problem Work (energy) a function of force and distance. This has to equal or exceed the energy of the truck (a function of mass and velocity squared).

You could simplify things by making the overly conservative assumption that the vehicle and the posts absorb no energy (just your gate does), but I think you'll find that unacceptable.

I suggest you check out crash tested systems such as the Dragnet impact attenuator, which can be installed so it lifts out of the way on two poles:

http://www.energyabsorption.com/products/permanent/DragnetTower.htm

## RE: Vehicle Impact Loads on Stationary Structures

I have thought about defining an allowable amount of deflection. this would permit calculation of time- 73 ft/s to 0 ft/s in my allowable deformation. knowing time, I can come up with a number of force on gate. Pretty high number and basic approach :(

Have also read about treating the system as a spring with stiffness based in elastic limit of structure.

Sent request for info to energy absorption company. thanks.

The search continues............

## RE: Vehicle Impact Loads on Stationary Structures

In a truck the engine and chassis continue right up to the bumper, so don't figure on too much crush in the truck, say 500 mm into a 'reasonable' structure, and maybe 1m into a concrete block

If you can accept plastic deformation (you better had) then look at how Armco fencing works for the same job. if you know how far apart the gateposts are then your job is to design a concertinaring system that straightens out as the truck hits, to absorb the KE of the truck.

Hydralic dampers won't really do the job, you can use them to take the spikes out of the initial impact, or for gentle impacts, but in the proposd scenario they'll just lock up.

In cars the crash energy is mostly absorbed by forming hinges in columns or beams and then plastically deforming the hinges, by telescoping the column or bending the beam. You'll often see corrugations pressed in tothe front longitudinal beams of a car for this very reason.

remember, it really does come down to 1/2mv^2=Deformation energy of gate+deformation energy of truck+1/2 mv2^2

where v2 is the bounce back speed, figure on 0

Cheers

Greg Locock

## RE: Vehicle Impact Loads on Stationary Structures

## RE: Vehicle Impact Loads on Stationary Structures

It is probably conservative by a factor of at least 2 to consider a truck with a trailer as being non-trailered.

## RE: Vehicle Impact Loads on Stationary Structures

Irwin

## RE: Vehicle Impact Loads on Stationary Structures

## RE: Vehicle Impact Loads on Stationary Structures

I did take their advice to heart (and calculator), and performed my analysis based on plastic/energy absorption. Turns out the manufacturer's design was inadequate. Of course they asked me to fix it. My task, thus, continues.

Taking it a step further, as was said before, selecting an "acceptable deflection" limit is key. My criteria is stopping the vehicle within a certain distance. U.S. Government requirements come in varying levels, for example mine is 3'-20'...a wide range, so I have to limit my system as well. Once the vehicle impacts the cross-beam, the plastic hinge forms quickly. Deflection continues and energy dissipates further as the vehicle is slowed to a stop.

The problem I am up against is that once the hinge forms, and deflection increases to the limit Code, and I, have chosen...there seems to be no good way to determine whether rupture, in bending, will occur before all the energy is dissipated. Plastic theory does not specifically set that "magic" limit when fracture of the steel occurs. Tensile elongation limitation is one thing in carbon steel, but I have yet found nothing defining rupture failure in bending. "Austim" also seemed to be searching for this type of info. in his thread last year,Thread (507,13741).

It's fascinating stuff, but seems to always generate more questions than answers. Looking forward to hearing your results.

Also, be thankful you don't have to design the posts. The foundations for absorbing the energy these impacts require are tremendous, let alone the effects of soil springs.

Good luck!

Nathan C. Tollinger

## RE: Vehicle Impact Loads on Stationary Structures