Vehicle Impact Loads on Stationary Structures

[Lakelander]

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

 

[jheidt2543]

From Physics, F=ma

 

[LPPE]

I dont know of a dynamic to static load ratio (sonds nice, though), but your equations of kinematics should help. You can find them in your college Physics books.

 

[manual90]

This problem is more complicated than it seems because the load on your gate will depend heavily on its stiffness and how it compares to that of the vehicle hitting it and the posts.

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

 

[Lakelander]

Thanks for the input everyone. I have been reading in Roarke's stress strain book on impact loads. It has some equations to use (chapter on dynamic & temp stresses). The author notes, however, that it is only an approximation and there are many variables that exist during the impact such as angle of impact, rigidity of the structures and how the kinectic energy can be disipated.

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............

 

[GregLocock]

50 mph from a 25 ton truck is a big ask. You don't need to worry too much about angle of impact, head on will be the worst case.

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

 

[austim]

You may find that thread507-23317 has some pointers for you. (You are clearly not the only one concerned with security structures now)

 

[ludvik]

Does a 25 ton truck have a trailer? If so, the trailer would swing out, or "buckle" laterally under the extreme condition of a 50mph head on collision. In fact, it wouldn't be difficult to work out what the "buckling" load of the trailer if you knew a few basic things like coefficient of friction of the tires and weight of the trailer.

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

 

[Irwin]

I do not believe that you can analys a strongly dinamic srtucture with static way! If your model is stiff enough in this case I would try to analys the loads from energy!

Irwin

 

[LPPE]

I'm interested to know what design you came up with. I did some preliminary calcs a while back with a truck in a truck stop running into a light pole at 5 MPH and was rather surprised with the forces. I can only imagine what 50 kips at 50 MPH will do.

 

[Ntollinger]

As Austim said, Thread(507,23317), may give you some ideas to consider. As the originator of that thread, I found I received excellent direction from all who responded.

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

 

[vmirat]

As an engineer with the Department of Defense, I have had this problem come up several times. One idea that I have toyed with is to use the same concept as an aircraft arresting system which uses a cable connected to a fluid to dissipate energy. This arrangement would be adjustable to meet varying threat environments.