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Working Platform design made of grating 1

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kaffy

kaffy

Mechanical
Jun 2, 2020
188
CA
Good evening fellow engineers,

I am designing a working platform for a maintenance technician for servicing the machine. Intent is to design a working platform from bar grating. I was going over the code and found the following design criteria.

A working platform shall be able to support in any position at least 450lbf, with a load concentration of atleast 225lbf over an area of 64in^2 with a factor of safety not less than 5.

The way I am interpreting the clause is:
-It should be able to withstand a concentrated load of 450 separately
-Another check should be carried out to see at UDL of (225/64) lb/in^2 with a factor of safety 5.

First question: Am I doing it right or should I consider both scenario at the same time?

Second question: If I have to consider both at the same time
One idea comes to mind is: I have the allowable fibre stress and section modulus per foot of width. I can calculate the allowable bending moment. Then I will create an equation
Bending moment due to concentrated (span L) + bending moment due to UDL (span L) = Total bending moment
and calculate the span.
Third question: Most of the grating load tables are unidirectional while in reality the load is acting over an area. are he deflections and stresses in other direction too small?


At this point, I am not worried about the beams supporting the grating

Thank You
Kaffy



 
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My guess is they intend a total load of 450 lbs with safety factor of 5, and also 225 lbs in 64 square inches with safety factor of 5, and those are considered separately. That is, the 450 lbs is two men, and the 225 lbs is one of those men standing on one foot. And I would assume the 5 factor is safety factor relative to failure, not relative to an allowable stress. The grating tables are based on 50% of yield, as I recall, or a factor of safety of 2 rather than 5.
Normally, for welded bar grating, the stiffness and stress crosswise would be neglected.
 
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Is a safety factor of 5 common for platforms?... except for mining stuff, I usually use the normal 1.5 live load factor for 100 psf live load or a concentrated point LL.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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IMO, below is the american industry standard used for grating design. What code are you using? Curious where the safety factor of 5 comes from and if it applies to me.

Screenshot_uz8cmh.png
 
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This is the elevator code. For the grating design, I interpret that as 225 lb/ sqrt(64 sq in) * 12 in/1 ft * (FOS 5)/(FOS 2.00) = Concentrated load in lb/ft found in grating tables. Note that FOS 2.00 is inherent in grating tables, and this is much greater than a typical structural engineer's working platform.
 
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@Jstephen: You are right. As per the code I am following, FOS is ratio of ultimate stress to working stress under maximum loading. I agree, It makes more sense to check both the cases separately.

 
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@GC, I am using B44 code which is for elevators. I found a manual with examples from the picture u showed. They have a good example of partially distributed load. What I am still not sure is that code says 225 lb over an area of 64in^2. They didn't mention if it is 64x1 or 8x8. I guess I can use the very worst case where only one bar is supporting all the load but seems overkill.
 
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@RPMG, So you are assuming that loading area = 8in x 8in. Based on that
Eq 1 = load / ft = 225 / (8/12)

you are multiplying that with fos = 5 to find your working load (Eq 2)

Eq 3 = concentrated load(as shown on grating table)*2
As the loading tables have fos = 2.
I have to compare (Eq 2 with Eq 3


Is that right?
 
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I think the grating should be designed for a UDL of 506 psf and allowable stress of 0.2 FY based on WSD method. But this loading is very high. If I understood correctly, you have to use heavy sections. Rods or flats will not work. ( FOR FOS 1.5 it is 0.66FY. For FOS 5 it will be 0.2FY)
 
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There are all types of heavy duty grating. I've used them for protective pit and drain covers for 747 type aircraft in pavements. Heavy duty rivetted grating.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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@Ajay..Thank You.
I agree. For a maintenance catwalk, Adding FOS = 5, seems really a overkill
 
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@Dik, I will check the grating but I am not sure if it is necessary to use the heavy duty grating when it is gonna be used by maintenance technician only.
 
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Here is the Nucor Steel grating catalog: They are a huge national supplier; and have load tables in their catalog.


Yes, you are correct about the loading. Bar grating is a 'one-way' element where you are simply supported at each end. There is no '2-way' load path. In fact, the grating has near zero strength in the transverse direction. The cross bars bend very easily.

The most common bar grating is Type 19-4. That means the bearing bars are spaced 19/16" apart (1 3/16") and the cross bars are spaced every 4" inches.
 
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The high safety factor may have merit as the catwalk can be utilized as the temporary staging area during equipment/plant maintenance. Also, the crew can drop heavy tool/part from above that causes an impact load.
 
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Don't bother checking... was an example and 'way overkill'... I'd use a load factor of 1.5 on a live load of 100psf, or sometimes 75psf.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Kaffy- What is your longest grating span?

Have a look at those tables... if you only have a ~4ft span between supports, then the grating has massive capacity. You can probably hit the 5:1 ratio no problem.
 
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I was converting everything on one side, so that the grating selection tables could be directly compared. That's the most useful way, but unintuitive. It would also be appropriate to say 225 lb/0.67 ft = the linear applied load. Now let's look at the grating span tables and divide them by 2.50. If you divide the allowable (FOS of 2.00) concentrated loads by 2.50, then you have met the requirement of an FOS of 5. This is equivalent to a mechanical engineer calculating the stresses in the bars that make up the grating and comparing it to the yield stress/5.

Also, to reiterate, this is an ASME elevator working platform, not a structural engineer's working platform. This is not a platform where you use the live load per the IBC and compare it using ASD. This follows the mechanical codes. No one should be talking about structural code live loads or how they're factored.
 
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@le99 Yes I agree but It is not intended for impact loading.
 
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