Design for Vehicular Impact 1

[Ntollinger]

I'm looking for recommendations for methods of analysis for design of an impact resistant cantilever security gate in the USA.

1. Two posts embedded in below grade concrete.
2. Steel WF section mounted across the posts and cantilevered to support a sliding gate section.
3. Vehicle impacts the WF section, imparting just about every force imaginable to the system, I would think.

It seems to me examining it as a static analysis would be simplistic...but what analysis method should be used? Any software packages that could model it effectively?

Thanks

 

[GregLocock]

Why not design it by looking at the moment of plasticity of the two posts, at ground level, and equating this to the kinetic energy of the vehicle? You cna then back out a load on the WF section.

Cheers

Greg Locock

 

[austim]

Greg is definitely on the right track (as usual).

Design for large plastic deformations, and equate the kinetic energy of your design vehicle with the strain energy of the plastic hinge(s ?) [strain energy=plastic moment*hinge rotation in radians, of course]. Beware of the simple error when calculating kinetic energy - it is 1/2*MASS*V^2, not 1/2*Weight*V^2.

Just a few pointers -

Your design process may well be the reverse of what you normally do. ie, firstly decide what is the sort of deflection that you are prepared to accept, (at various points - with impacts close to one of your supporting posts, tip of cantilever etc). From there you can calculate the impact force for each location (from strain energy=kinetic energy=1.0*force*plastic deflection [not 0.5*P*dist which is for elastic deflection]). Then you can select suitable sections to give plastic moments to suit the variuous loads.

If you are required to have any care for the survival of the vehicle occupants, then make sure that your deflection under impact is sufficiently large to limit the vehicle deceleration to an 'acceptable' amount. Hopefully a post to the Automotive Body Engineering forum may sort out just what is considered to be 'acceptable' deceleration.

Make sure that your major plastic hinges are in stubby sections, ie with b/t in the flange outstands less than 8, so that there is no flange buckling during plastic hinge action. Also avoid high tensile steel - a plain mild steel with a good yield plateau is what you need.

You might get some help from "The Steel Skeleton", by JF Baker and others, and read how they protected the populace of the UK during air raids with the plastic design Morrison shelter. Also look at thread507-13741.

If impacts are expected to be regular events, then you may need to consider making all components (including the main support posts) easily replaceable.

Good luck with this.

 

[wiktor]

The following should be addressed prior to the design of the gate:1. Design vehicle mass, speed and angle of impact. This should be determined by the prevailing traffic condition at the site and plausible collision scenarios. Typical AASHTO requirements for the design of the breakaway and non-breakaway supports for the sign, luminaries and similar posts assume impact to occur at typical bumper height, about 20 inches above the ground. Please refer to NCHRP Report 350 for data on the typical vehicles and collision test results. (

http://trb.org/trb/publications/nchrp/nchrp_rpt_350-a.pdf)2.

Assuming that the gate should be "impact resistant" it should be able to accommodate design collision forces within elastic range, so use elastic modulus to design the posts within impact zone and design the embedment in the concrete accordingly. 3. Consider whip lashing effect on the elements above impact zone i. e. posts, gantry and sliding gate.

Good luck with the design

 

[dlew]

Ntollinger,

The reaction force due to impact loads can not be accurately calculated because too many simplifying assumptions have to be made.

I am not sure that the method to calculate the reaction force of a ship berthing against a fendered dock could be used, without modifications, to calculate the reaction force of a vehicle hitting a gate. Ships have large mass and move at slow velocity, while a car or truck has a relative small mass and will hit the gate at a higher speed. Also, a compressible fender is used to absorb the energy of the ship, while the energy of the vehicle would has to be absorbed by the gate movement.

To calculate the reaction of the fender against the dock, the kinetic energy of the ship:
E = 1/2*mass*velocity^2
is equated to the energy absorption of the fender:
W = Integral of the variable force, F(s) over the total compression of the fender
where F(s) = k * s
k = spring function of the fender
s = total displacement or compression of the fender

In your case the gate displacement (at the point of contact) may have two component: the deflection of the gate structural framing plus the travel due to the gate posts rotating and compressing the soil.

The spring function of the gate structural framing within the elastic range is constant, and easily calculated (k = applied force/corresponding deflection). To calculate the spring function of the soil would require some simplyfing assumptions, since it would be a constant within the elastic range of the soil, but would become variable after the elastic range has been exceeded. But if you assume a constant k, then

Energy Absorption = Integral of {F(s) * ds } = Integral of { k * s * ds} = 1/2 * k * s^2
equating to the kinetic energy equation, the required displacement to absorb the impact energy is :
s = velocity * square root of (mass/k)
and the reaction on the gate is:
F = k * s
The following data would be needed :
1) Mass of the vehicle
2) Velocity of the vehicle
3) Spring constant of the gate at the point of collision

Maybe you could use this method to correlate the results of the other methods suggested on the other replies


Good luck!