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DontRememberMuch2015

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Picture a series of I beams with a steel plate welded on top and bottom. The dimensions of this big steel rectangle are 41' x 11'' and it has a weight of 125,570 lbs. Imagine it being suspended off the ground (by magic) at 1' height. It is sitting over a 4" reinforced concrete slab (Class A concrete where K=7.443 N/m and E=3,410,000). Suddenly the steel rectangle drops out of the air and falls onto the ground (currently trying to calculate both elastic and inelastic impact). I am trying to find the G-Force that the steel rectangle experiences. (In reality there is something on the steel rectangle and we need to make sure it didn't experience too high of a G-Force) ...really struggling with this one.

I will post some knowns as well as directions I took. If anyone knows where I made a mistake or what to do to find a G-Force I would appreciate it.

Impact Velocity: V=√2gh where g= 9.81m/s and h=0.3048m, therfore; v= 2.445 m/s

KE= 0.5mv^2 where m=56957.6kg and v=2.445 m/s, therfore; KE= 170,134.629J

F=ma where a=9.81 m/s^2, therfore; F=558,754 N

This is where things get hairy. I looked up some things online but the units just didn't make sense. I think I want to find the deflection of the concrete and the deflection of the steel from impact and somehow integrate them and use that to find the time where the velocity goes from 2.445 m/s to 0 then I can get the acceleration (or deceleration) of the acceleration which is the G-force. From my understanding the G-force is the 4th derivative of d... dx/dt, dv/dt, da/dt, then G-force... da'/dt.. if you will? I also thought I should look for Jerk force as well as relate the stiffness and deflection of the material but I am coming up empty. Anyone have leads? Can anyone walk me through the process or provide a step by step. This isn't homework its work related so I don't have a lot of time to go through this back and forth. Any suggestions will be MUCH appreciated.

Chose graduate because none of our engineers can figure it out. Might be high school knowledge, but we have lost it haha!

Edit: This was not measured with an accelerometer, we are trying to analyze an accident that happened so I am more concerned with the first initial impact G-force and trying to ignore the dampening effects.