Scissor Lift on Grating

[jreit]

Hello everyone.
We're trying to calculate if we can safely use a scissor lift over a grating.
We've calculated the uniform and concentrated load capacity of the grating, but I'm not sure if the wheel load will translate the same way. The gross weight + load of the lift is approximately 3500 lbs. The maximum wheel load of the scissor lift is 1200 lb as per the manufacturer but is that a straight shot comparison with the concentrated load capacity of the grating?
Also, the maximum tire pressure is listed at 133 psi. Wheels are 12x4.5x8 inches. It seems rather high and I'm not sure if that will cause any special localized effects on the grating.
Any idea on what the procedure should be? Thank you in advance.

 

[Bagman2524]

If you want to be very conservative just take the full weight of the scissors lift and put it at the center of the grating span and see if it passes. Otherwise you should check all possible load layouts based on where the wheels can be in relation to the supporting members of the grating. I don't see why you couldn't assume the 1,200 lbs. max wheel load is a concentrated load.

"Look for 3 things in a person intelligence, energy and integrity. If they don't have the last one, don't even bother with the first 2. W. Buffet

 

[ameyerrenke]

Considering that the small ones have no suspension, and its not uncommon to have a scissor lift balancing on 2 wheels at opposite corners of the machine when going over varying slopes I would say 1200 lbs would be unconservative. I would use the full weight as a point load.

 

[JLNJ]

If the grating is relatively flat I think you can distribute the load evenly. Using the max wheel load is reasonable.

Remember to look at actual bearing bars activated by the tires (not a 12" strip). Make sure you know how many continuous spans you have (probably a simple span). Check shear with the wheel(s) next to the support. The news may not be all that good for the contractor.

 

[TDIengineer]

A scissor lift normally figures on a floor with a design load comparable to a parking structure.. a lowly 50 PSF.

Most grating has a capacity well above that. Study the manufacturer's load table for the grating depth and span specified. Then as other's have noted, you can check the allowable grating plate point load by using the maximum uniform load in the design table x the span, and then half it.

 

[oldestguy]

If in doubt, consider overlaid layers of planking or similar.

 

[JedClampett]

Grating Pacific has load tables (

http://www.gratingpacific.com/metal_bar_gratings/heavy_duty_grating_selection.html)

for spans vs. fork lift size. Find the load and wheel size closest to your scissors lift.
Note that grating can hold a smaller fork lift than you think.

 

[racookpe1978]

So, weight of scissor lift + load = 3600 lbs.

Divide that into 4 wheels = 900 lbs per wheel.

You are considering using 1200 lbs/wheel from the scissor lift manufacturer. Apparently "conservative" but not unreasonable on the surface: This would assume the 3600 lb total weight load is on 3 wheels, and one wheel is in the "air". Since three wheels is the "only" stable configuration if the scissor lift is not rocking back and forth (which NOBODY is going to ride and work in!) that is a 1.0 safety margin.

That final 1.0 safety margin is NOT conservative though!

The 1200 lbs IS concentrated not on a square foot, but on 1/4 of one foot area (the thin rods and bars actually touching the bottom of the tire.) You cannot use a "per square foot" loading table but need to consider the "point load" values of 1200 lb/(1/4 sq foot) or 4800 lbs/ft rating problem.

A thick (stiff!) steel plate under the wheels will divide the load out to manageable lbs/sq ft basis. A thin steel plate will do nothing but camouflage the bending and yielding when it happens.

 

[PMR06]

I agree with others, you have to look at only the bearing bars that will be loaded by the discrete width of wheel. I would not count on the grating cross bars will not distribute load appreciably in a 2-way manner.

On one project, we analyzed the man-lift load on grating in an empty state for the case where it is being moved around the floor by remote. Then once the workers reached the location to do their work, we had them lay down plates under each wheel to distribute the loaded man-lift wheel loads out over a larger area.

 

[jreit]

Thanks a lot for your suggestions everyone.
The problem would be simplified if we had the manufacturer grating information available but I should have mentioned that we don't.
The grating and the supporting steel is in good shape but about 20 years old, so I used a different manufacturer's data (McNichols) to get a feel for the capacity based on the bearing bar spacing and size.
Comes to about 1100 psf for floor loading and 2500 lbs. for concentrated loads. Will check other manufacturer data also. Can't just use those values, so the issue is the appropriate reduction of those given values. Demand is about 300 psf and 1200 lbs. wheel load. It's a simply supported 4'6" span.
There was a guide from that I used to calculate the capacity but not sure if that's an airtight reference.

http://www.indianagratingsinc.com/PDFs/metal_bar_grating_engineering_design_manual.pdf

To clarify what racookpe1978 and PMR06 stated regarding the tire contact area; the tires are sized at 12.4.5x8. For a 12" diameter, and 4.5" width do I determine the grating area in contact with the tire and then use the 1200 lb point load to calculate the psf loading and determine if it's adequate? Is that the procedure you outlined?

 

[dcarr82775]

I have always looked at it as depending on the wheel width, the wheel will contact a certain minimum number or bars, and the strength of that # of bars is what I use. I assume a point load on the individual bearing bars instead of spreading is out over some small length. The cross bars do very little to spread the load until to start deforming the main bearing bars. Many lifts have a sliding platform so you can walk out well over the front. I take the total load as being on one axle.

If I need more bearing bars I will call for a steel plate overlay.

 

[dik]

We often use grating for mining operations where equipment much heavier than scissor lifts are used.

Dik