Designing for Impact Loading

[BreadTruck]

I understand that there are a couple of different ways that I can approach this type of design problem --applying an impact factor to the maximum force and use this new derivation in standard engineering calculation OR estimate the energy that is to be absorbed by the resisting member and design it as an energy-absorbing member.

My application is a moving object passing over a supporting member (or more specifically, a cover plate spanning a utility trench with vehicular traffic). I am avoiding complex non-linear membrane functions and treating the plate with simple beam theory. The vehicle is an unloaded fork truck travelling at a linear velocity of 15 to 25 ft/sec. The fork truck weighs roughly 9200 lbs [unloaded].

I know that the force will be somewhere between the lower limit of W ("W" = weight of fork truck) and 2*W. Can someone please share their method(s) in calculating the Impact Factor for my application [to get me in the ball park]? Also, is there a way to arrive at a similar value using the conservation of energy (i.e. both a kinetic [due to the linear velocity of the truck] and a potential energy component [due to the deflection in the resisting member, absorbing the impact], that would in turn be set equal to the total energy "U&quot?

Any help would be appreciated! Thank you in advance.

BreadTruck

 

[tincan]

Whats the trench width and wheel spacing of the fork lift? Will the fork lift be loaded and/or unloaded and the maximum wheel load? A short span with one wheel is one thing and a long span with the full fork lift truck loading is another? List the particulars and lets try tomorrow.

 

[SlideRuleEra]

BreadTruck - Give close consideration to the assumption that the fork truck will be unloaded. People do funny things, the day will probably come when someone drives a loaded fork truck over the trench cover. I would advise you analyze the trench cover loading under this condition (loaded fork truck), assuming a resonably low impact component (maybe 25% - reference Section 5, Paragraph A4 of AISC Manual of Steel Construction ASD, Ninth Edition). When compared to the results of your solution to the expected conditions (unloaded fork truck), the loaded fork truck analysis may govern. If it doesn't - little time has been wasted.

 

[BreadTruck]

The floor plate is 15 inches wide x 26 inches (span) long and 1/2 in thick. The wheelbase of the fork lift is say 54 inches (in the direction parallel to the direction of travel).

The fork truck can only traverse across the floor plate in the 15 inch direction and only three sides of the floor plate is simply supported -- the (2) 15 inch sides and (1) 26 inch long side (I know, I know...please, no flames, I have inherited someone else's mess).

I have considered the fork truck in the loaded condition with an impact factor of 30% which eventually trickles down to a maximum wheel load in the neighborhood of 9 kips. The floor plate was field retrofitted with a L3x3x1/4 stiffener [in the direction of the 26 inch span] on the fly by myself during field installation (time constraint and material available on-site).

There is 1/8" permanent deformation in the plate [along the free edge], after 3 months service. I have been asked to review this condition and give my opinion [and do it in a constructive way, without stepping on any toes].

Looking back, I'm wondering if my 30% allowance for impact was conservative enough and my focus has turned to evaluating the effects of an unloaded fork truck travelling in the neighborhood of 15 to 25 ft/sec, impacting the floor plate and in determining this magnitude of force. I was hoping someone could provide a set-up or means of calculating this force from a conservation of energy / "energy-absorbing" standpoint.

I sincerely appreciate your time and consideration.

BreadTruck