huangyhg

impact load
if i drop a mass of 100 lbs from a height of 5 ft to a 10 ft simply supported beam (at the middle), how do i find out the actual moment/effect due to impact? i would appreciate help. i know it has something to do with impact loading, but i am not sure how to analyze.
there are a bunch of threads about this. search the forum. biggest problem is determining the de-acceleration either in time or deflection. if you have either of these you can find what you need.
testing is often the best way, i have heard that .3 0r .2 seconds is often used.
you actually have a classic single degree of freedom system there so the answer is straightforward. work out the spring stiffness, then you can just match pemass_initial=kemass_at_impact= elasticenergy in beam to get a max deflection.
m*g*h=1/2*k*y^2
then drop the deflection y back into your beam equation.
obviously this ignores the local stresses due to impact.

cheers
greg locock
greg's way is best since it elegantly sidesteps all the deceleration period nonsense.
it is worth noting that greater flexibility is your friend in this case as the more the beam deflects the lower the acceleration.
a short column shape beam with a high strength/stiffness ratio would be best (unless there are other factors).
why does acceleration matter?

what csd72 is referring to is deceleration, and it matters because f=ma.
if you know the deceleration of the object, you can calculate the force it imparts on the beam.
the energy method, as stated before is an alternate method. i haven't thought through the method thoroughly, but i see no reason why it wouldn't be fine.
in reality the rate of decceleration does not affect the beams capacity, so why pretend it does?
tom,
you have obviously forgotten your physic fundamentals:
as f=ma, if you reduce the acceleration and you reduce the force.
frv,
deceleration is just acceleration in the opposite direction, as far as physics is concerned there is no difference other than the direction.
reducing the acceleration does reduce the force, but it also increases the duration of the force so that the same amount of work gets done.
but the beam doesn't care if the load duration is 5 or 10 or whatever milliseconds, it is the magnitude of the load that dictates if the beam is adequate.

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