Scissor Lift on Grating 4

[JRCEng]

Hello All,

New Member here, and I have seen a few of the other post about scissor lift on gratings but am still unclear on how to go about determining the load capacity of grating given a contact patch of about 4.5" (tire width) by 4" (tire length). The scissor lift is a Genie 1930 model, and weights 2748lbs, I'm assuming a concentrated load of 687 (one tire, which is not conservative, because tipping could occur, but even so it fails).

The grating is McNICHOLS (3/16 by 1 1/2 with a c-c distance of 1.1875"). Its Galvanized steel with an allowable design stress of 18,000psi, and Young's Modulus of 29E6 psi. The largest span that I measured was 76".

I utilized the Metal Bar Grating Engineering Design Manual found here

https://www.naamm.org/content/uploads/products/mbg_534_14_dec_2017.pdf

I used example 2 with the only changing being K ( I used 3.78947 whereas they used 10.1), there are only that many bars supporting load for the tire width given above 4.5".

Mw = FSw = 4796.08 lb in
C = 4Mw/L = 252.42 lbs (much less than 687lbs)
Dc = 0.398607"

For my load I just divided 687/252.42 because load/displacement linear relationship, and then multiplied Dc by this value to get

Dactual = 1.08485" (which is absurd)

I'm mechanical, not structural so I'm not sure if this math checks out, it makes intuitive sense to me, but any advice would be greatly appreciated. Thank you!

 

[IFRs]

The calculations are for allowable load with safety factors. Do your own calculations from scratch - the calculations are very simple and you will confirm the manufacturer's.

A 1 inch deflection in a 6 foot span under a 700 pound load engaging only 4 bearing bars seems quite reasonable to me. Why do you think it is "absurd" ? If you doubt the calculations, go measure it with a straight edge. If the actual is different from the calculated, do what you can to understand why and represent that in your calculations.

The grating deflection will engage cross bars which will share some of the load with adjacent bearing bars, it is sort of like a two-way slab with different stiffness and strength each way. At that deflection, I'd expect several bearing bars to be partially active on both sides of the load path.

Correct me if I am wrong but your experience shows that the grating deflects but does not yield? It has been used this way for some time and the grating is still flat? If the grating does not yield you still have some (perhaps small) factor of safety to yield and some more to ultimate (failure) strength. This could explain why over time it has seemed ok.

If the grating is used only as a travel pathway and the boom is never moved from its rest position, that is a much different scenario than using it up to the fourth floor. Clarity on this is very important. Stability of the lift with workders on it is a much different issue than damaging the grating.

Grating is cheap - put stronger grating where your calculations indicate the workers are exceeding the allowable load, confirmed closely by your hand calculations. If no one wants to spend the money (get that in writing), then propose that the unsuported span be cut in half with a support underneath. If that is also refused, get it in writing and proceed from engineering controls to management controls (signs, barriers, etc). If there is clear and present danger to the workers, make a clear and convincing presenetaion to upper management. If they tell you to go pound sand, start looking for another job where your skills and vision is appreciated.