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RE: loader surcharge at top of retaining wall
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- Subject: RE: loader surcharge at top of retaining wall
- From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
- Date: Wed, 15 Nov 2000 19:22:10 -0500
James Lutz wrote: > The force transmitted to the wall is equal to the momentum of the vehicle > (mass times velocity) divided by the time it takes to stop. I agree, this is why F = ma does take into account momentum: since "velocity divided by the time it takes to stop" is equal to the "deceleration", this resolves to the same equation: F = mv/t = ma. The key point here is that velocity becomes zero at the point of maximum deflection of the structure, thus there is no remaining kinetic energy at that point. Design of Welded Structures by Blodgett has a very good chapter on Designing for Impact Loads. It states that the impact force is F = ma = Wa/g. It also states that "the member will deflect slightly and allow a certain time for the moving body to come to rest, thereby reducing the impact force. Since the time interval is usually unknown, the formula cannot be used directly to find the force. However, it is usually possible to solve for this force by finding the amount of kinetic energy or potential energy that must be absorbed by the member." Thus F = ma is still valid "if" the time interval is known. The energy method is simply another means of obtaining the same result, which of course requires a more detailed analysis of member and material properties. It is certainly more accurate than "guessing" the time interval, but it does not negate the validity of F = ma.
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