Impact Force from falling object

[DEL2000]

Trying to determine the impact force from an object falling from multiple stories . . . we will have a wood platform that we need to design so that it stops the object. To refresh everyones' memory, the average force is called "impulse" and is equal to the change in momentum divided by the time that momentum takes to change. There is no good, easy way to determine how long it takes the object to come to a stop (exceedingly complicated), so it is next to impossible to get an acurate force with the impulse method. I was wondering if I could skip all of that and go straight from the initial potential energy (or final kinetic energy if you would prefer) and convert that energy into a moment, or a deflection, in the wood beam?

The other issue deals with using wood to resist this high impact force. Safety cages and air bags in cars are deformable and help lengthen out the time for the momentum to change, which reduces the force. A simple wood beam wouldn't be a material that would lend itself to high, energy absorbing deflections, but would break catastrophically. I was wondering if it wouldn't be a good idea to simply let the wood break (absorbing the energy), and have some heavy construction netting beneath the wood structure to catch/reinforce the falling object/broken wood beam.

I can't remember where I was (either in Las Vegas, NV or Vancouver, British Columbia), but I have walked adjacent to high rise construction on public sidewalks, underneath wood canopies that were placed to protect the general public from falling debris. I just was wondering about some input from somebody who has designed one of these protection structures.

 

[par060]

I went through a similar problem a year or so ago...and to make a long story short like you have mentioned the estimate of the applied force was based on the time it would take for the object to stop....fractions of a second..

with such a large force we decided to make the protective structure as strong as practical and reversed engineered what the resulting capacity was documened it and explained the physics to the client..worked out fine

not much help but thats my story

 

[VFF]

The approach I have used for flood debris impact loads on a bridge is to start with the mass of object (m), velocity just prior to impact(u) (in my case water, in yours based on the height dropped)and the allowable deflection or stopping distance (s) (which was given in the code - dependant on what the pier was made of).

Then, v^2 = u^2 + 2as where final velocity equals zero. This gives the deceleration required. Then simply F=ma and whallah! I had the force to design for.

To be honest, I'm not sure how this approach relates to your issue, but perhaps you could work out the deflection your timber could take and develop a deceleration distance from this?

 

[Tomfh]

Could you assume an elastic collision and equate kinetic energy to strain energy? Then you could get the equivalent deflection, and hence the forces in the beam.

I.E.

1/2 m v^2 = 1/2 k x ^2

k is easy to find, it is just the force per unit deflection for a point load.

x can then be back converted to an equivalent point load.



But please remember, I make no claims as to my qualifications regarding dynamics!

 

[desertfox]

Hi DEL2000

I agree that you could use the energy method and then use that to convert to a deflection of the protective cage or whatever.
How realistic the figures you get in practice compared to theory is another matter but i would use a good factor of safety and then you should be okay.

regards
desertfox

 

[pba]

We did something similar in the UK some years ago. Our crach deck was to protect pedestrians from falling debris from an old tower block. We used steel posts and beams fro the main frames. I recall that these were of small size as the total load was small. The impact load was resisted by a steel mesh and metal standing seam roofing. Under this was scaffolding planks and joists. The theory used was that the welded steel mesh could deform quite extensively and provided the ultimate protection. Smaller objects which hit the deck needed less effort to stop and could be taken by the scaffolding boards. The standing seam roof provided the waterproofing and gave a method of offsetting the steel mesh.

I never saw the final installation but understand it worked.