Impact Load Analysis 1

[maneesha0702]

We were checking for a large lift where the load is heavier than 100 kips. The velocity with which this load is placed on the support is 20ft/min. How do we calculate the impact load. I tried to equate the kinetic energy to strain energy. However this produces an impact factor of greater than 1. So the total load applied on structure is greater than twice the weight. However as per AASHTO when I calculated that impact load is only 1/10 of the weight. What is the approach that should be taken in this scenario.

 

[SlideRuleEra]

maneesha0702 - The key to getting a reasonable answer is how much does the support deflect or compress when the load impacts. Also, the load may compress on impact, which helps too. I made calculations assuming a total of 1/4" deflection / compression and got an impact factor of 0.086 of the weight. This is based on a constant deceleration over the 1/4" distance, which is optimistic. The actual impact factor may be somewhat higher (maybe 0.2)
What is your estimated deflection / compression?

BTW, an impact velocity of 20 ft/min (4 in/sec) sounds high for such a heavy lift. If this is a lift performed by a suitable crane, I would expect an impact velocity an order of magnitude lower (say 1/2 in/sec), or less.

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[rn14]

I have designed for impact by equating kinetic energy to spring energy and it be very difficult. You get your load and refine your design which usually makes your stiffness go up, which then makes your load go up. If you go through the equations with V=0 you will find that your amplification factor is 2. If your velocity is high and your response needs to stay elastic it can be a loosing battle.

 

[WARose]

I don't know if you have Blodgett's 'Design of Welded Structures' or not.....but this is addressed in section 2.8.

 

[maneesha0702]

Sliderule
What were your assumptions in getting this .086 answer? Do you mind sharing this calculation? On a side note I need to thank you for all the rare information available on your web site.

 

[maneesha0702]

Slideruleera,
Thanks I figured out your calculation. Sorry all along I was thinking that deceleration was 32.2 ft/sec/sec.

 

[SlideRuleEra]

maneesha0702 - I have attached my calculations based on the assumed 1/4" deflection / compression. I prefer to address this type of problem using Newton's laws rather than energy. Too me, it is easier to visualize what is happening and the results should be identical. Also, notice that the magnitude of the load is not needed.

Thanks for the comment about the website.

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