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!

 

[phamENG]

Try it again using Example 4, Condition A. I haven't really looked into your numbers, but I think the procedure used in 4A is a better representation of your situation.

The condition doesn't really matter.

 

[XR250]

I would look very closely at deflection. Those lifts are sketchy enough on concrete surfaces.
1/4" deflection at the base could be 3" sway at the top which would not only scare the operator but increase the load on those wheels.

 

[humanengr]

You should be able to obtain the Grating Data Sheets (Catalog) that would provide the maximum allowable point load(s).

 

[JRCEng]

So it looks like for a = 4.5" and b = 4", where the a spans parallel to crossbar I get the following:

Mb = 3342.63 lb in (which I think is Mw in prior but its more conservative)

Du = 0.767976in --------- [687( (4/2)^2 + 76^3 - (4^2)(76/2) ) / 48EI , I is the same as above 0.199705 in4]

Its a bit more accurate, but still a little large.

 

[JRCEng]

The grating sheets specify load per foot length, so for 72" it supports a max concentrated load of 711lbs, and for 78" a max concentrated load of 656lbs. If I use an online interpolator, I get 676lbs for 76".

The way I read the chart, its as if they applied a ruler size force at the midpoint across the 10 or so bars. My load is distributed across 4ish bars (3.78bars). Would it be sufficient to say that since my load is 687, the grating cannot support?

Thank you everyone for the replies btw.

*Resource:

https://assets.mcnichols.com/content/dam/mcnichols/resources/PDF/LT-W-GW-GW2-CS-GV.pdf

 

[XR250]

FWIW, you need to add the 500 lb load capacity of the lift into the calcs and probably best to assume it is all on one side.
I really want to re-iterate the fact that deflection is as much as your enemy here as strength.
The lift is 2'6" wide and has a reach of 25 ft according to the Google
3/4" deflection at the base = 9 fn inches sway at the top!!!!!
It WILL likley topple.

 

[JRCEng]

XR250 I agree, I should have done it worst case scenario, I was trying to make it work bare minimum, was going to write a doc for operations that would place restrictions when the lift was in operation and moving (Example: No occupants or items inside while moving), but anyway I work the numbers the grating fails. They have been using the scissor lift up on the 4th floor of this building for many years, but I suspect there probably eating into the safety factor by doing so.

 

[JRCEng]

XR250 when you say "3/4" deflection at the base = 9 fn inches sway at the top" does it mean that if it dips(one wheel) 3/4" the structure when extended will swing out 9"?

 

[jayrod12]

Yes, he's saying that a wheel dip of 3/4" would equate to 9" of lateral movement at the top when fully extended.

And you've already pretty much determined that it won't work. But be mindful that generally speaking the wheel loads are not even. Many times, in lieu of more accurate information, I assume 3/4 of the weight (including the carrying capacity) goes to one side, and 3/4 of that goes to a single wheel. Therefore I generally plan for almost 60% of the total weight on a single wheel.

 

[dhengr]

JRCEng:
Why not add a couple steel plates under the wheels, maybe 1’x6’x.5” thk. Maybe 8” wide channels, 6-8’ long, with upturned flanges, would be even better than pl. This would add some weight and fiddling during machine movement, needing 4 pieces in total, but it would also distribute the load to an extra 8 or 10 bars in the grating, increasing the effective beam width. I agree with the above, that wheel deflection and basket translation at full extension, may be just as important as actual grating carrying cap’y. That quickly leads to instability on that narrow a machine. It seems to me that there might be plenty of times when you could have two wheels on the same effective width of grating, so look at that too.

Edit: The pls. or the channels atop the grating, become a gridwork of beams in terms of carrying and distributing the wheel loads. As a function of the relative stiffnesses of the pls. or channels and the grating, you want to match the deflections of the two systems, which then determines the load distribution to X number of grating bars. i.e., a stiffer pl. or channel will bring more grating bars into play. Also, check grating shear cap’y. in the case of a max. wheel load near a grating support. That wheel load is all on those 6 or 8 grating bars.

 

[human909]

No. No. No and No. (I know you've largely decided that this won't work but I can't emphasise "no" enough.)

Judging by the specs this is putting around ~4x the design pressure load onto that grating. And that isn't even looking at point load effects or DRY steering torsion on narrow bars. And then as others have mentioned you still have deflection....

The whole idea screams danger just to consider it.

 

[rb1957]

so "C" is the allowable load, being much less than the required load, therefore "NFG" ?

another day in paradise, or is paradise one day closer ?

 

[XR250]

XR250 I agree, I should have done it worst case scenario, I was trying to make it work bare minimum, was going to write a doc for operations that would place restrictions when the lift was in operation and moving (Example: No occupants or items inside while moving), but anyway I work the numbers the grating fails. They have been using the scissor lift up on the 4th floor of this building for many years, but I suspect there probably eating into the safety factor by doing so.

So you are saying that it will only be used for transport on the grating, not for having the lift extended? If that is the case and you can somehow control the use, then it sounds more reasonable.

 

[JRCEng]

dhengr
I see where your going, but is there a way to figure out how far you can stretch the concentrated load, if I can extend the load to 10 bars, then the deflection becomes 0.323728in(Updated I in denominator), the maximum allowable load becomes 672.78lbs(M = F*Sw, updated Sw) . Which again is a bit large but much better.

human909
I agree it seems dangerous, however it appears they have been using the lifts up in this building for quite some time. I'm leaning towards its not doable, unless we make structural changes, or purchase new lighter equipment, or even go back to using scaffolding.

rb1957
What do you mean NFG?

XR250
Its a possibility but I feel like it would be toeing the line (that is if my math is right).

Thanks everyone, giving me a lot to think about.

 

[phamENG]

To analyze a plate to spread the load, you'd have to look at the relative stiffness of the bars and the plate. Essentially, you'd have a point load in the center of the plate (+/- some reasonable distance) supported on several springs. The spring constants of the supports will be based on the flexural stiffness of the bars at that point. The difficulty here is that you're looking at a moving load. Unless you're going to lay plate over the entire walking surface (slip hazard, and heavy - the structure may not be able to handle it) or have a group of people picking up and moving steel plates from behind to in front as it moves, it doesn't sound like a practical way to solve the problem.

If you are unable to make sure that the unit won't be used while it sits on the grating, make sure you factor in the recomended lateral loads for construction work platforms in ASCE 37 (I don't have it in front of me but I believe it's 50#/worker or 2% of total load, whichever is greater).

 

[human909]

I'm just curious. Are they also extending this platform on the grating? (Overturning moments would significantly increase wheel load.)

I agree it seems dangerous, however it appears they have been using the lifts up in this building for quite some time. I'm leaning towards its not doable, unless we make structural changes, or purchase new lighter equipment, or even go back to using scaffolding.

Makes sense. Enough people by now how said it is a bad idea so I won't hammer it home. Certainly since they've already been using it for so long I can understand your desire to make the numbers fit what past experience has shown to be 'fine'.

A bit of time ago I was on the outskirts of a 'near miss' incident where past used showed it all to be fine (high load, multiple uses). A potential concern was raised by somebody the prior week and the the item was flagged for examination. A week later a failure occured that could have been quite serious. It is tough putting a stop to use, but it is much tougher if something happens and you haven't.

 

[rb1957]

it seemed to me that with a 252 lbs allowable (C) and a 687 lbs load … then your grating is "No Good", by a factor of almost 3.

just because they've done it for years, doesn't mean they should do it, and doesn't mean you should take responsibility for them doing it.

another day in paradise, or is paradise one day closer ?

 

[Ingenuity]

...deflection becomes 0.323728 in...

Really. Got enough significant figures to that deflection calc?

 

[dhengr]

JRCEng:
I agree with Jayrod12, I can imagine 50-60% of the total weight of the machine on one wheel under the worst conditions. And, the other wheel in that axle line (on the same effective width of grating) might be another 20% and only 2’ away, thus two point loads on the effective width of grating bars. Note also, re: stability, that their literature assumes this machine is run on a flat, hard surface (a conc. slab, heavy stl. pl?, or some such), so any relative deflection btwn. the two wheels, particularly side to side is really critical. Look in any number of Structural textbooks or Strength of Materials books for ‘gridwork of beams, grillage of beams, beams on elastic foundations.’ You write equations for the stiffnesses and strengths of the two perpendicular sets of beams (spanning members), apply a load to the total system, and want the deflections at the faying surfaces btwn. the two sets of beams to be the same.

You haven’t done a very good job of really describing the whole problem you are dealing with, and many of these details will alter the comments and possible solutions. The channels can likely be made to bridge the grating if needs be. Is the whole fl. system grating on stl. beams, just grating to be bridged at some locations, or what? The channels are stiffer than an equivalent wt. pl. The flgs. usually prevent the guy from driving off the track. The channels will likely take a good percentage of the wheel loads, as long as they span, over the grating, to solid reactions; as opposed to just being load distributors on the grating. While the Genie Lit. says the machine will turn on a dime, the channels won’t allow that. But then, I suppose they aren’t running that thing around like a radio-controlled toy race car either. You haven’t said, but it matters for a meaningful discussion and solution. I would expect you might need 6 or 8 channels, leapfrogging as you move fore or aft, maybe some heavier, wider pls. for turning directions, but I do not intend to have the whole fl. plated. These channels should be moveable by one or two people, although it sounds like a significant enough operation that the manpower might be available, its usually not a one-man operation. Don’t keep us in the dark if you want meaningful discussion, we are wasting our time when we have to guess.