Working Platform design made of grating 1

[kaffy]

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

 

[kaffy]

@RPMG & @Joel

Please see the attached calculations. I assumed three scenarios and it seems I should go with 36" Span.

Are we allowed to use interpolation on those values?

 

[JoelTXCive]

I'm not following your calculations 100%. Is grating width and span the same thing?

Because grating is a 1-way element, I would assume a tributary width of 1ft and then your span is whatever distance between supports is.

People above have brought up safety factors, LRFD loading factors, allowable stress versus ultimate loading etc.. They are all correct, but I'm going to argue that they don't matter. I'm assuming your are designing a small platform that is less than 10sq ft in size. You can get a quick and dirty solution that works. If you want to refine further, then you can....

Here's my solution........

"Platform Must support 450lbs with a safety factor of 5" - I think this is a very poor way of defining the uniform loading requirements. But it's okay because your point loading requirements are going to control and dwarf this uniform load.

"Platform must support a 225 lb point load applied over 64 in^2 with a safety factor of 5" - This translates into a 3.51 psi service level loading. Which can be further converted into 506 psf. If you hit that with a safety factor of 5. Then you need a grating from the table that can handle a point load of 2500 lbs.

Now go to the tables and if your span is 36" then a 2 1/4 x 3/16" grating will handle a 3197 point load, which exceeds your 2500 lb requirement. That same grating will also handle a uniform loading of 2132 psf (I would cease worrying about the 450lb total support requirement at this point).

Boom.....you're done. I realize I'm mixing safety factors here, but this solution works and we're not designing the Eiffel tower, and I'm assuming you have limited design budget.

Now you just need to make sure that whatever you are attaching the grating to can handle the 450 total loading or the 506 lb point load (with the factor of 5 safety).

 

[kaffy]

Thank You

 

[dik]

I just got a project on where the design LL for the platform is 75 psf...

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

-Dik

 

[RPMG]

A grating that supports a concentrated load of 844 lb/ft will meet your ASME requirements. That will limit the stress of your grating to 36 ksi / 5 = 7.2 ksi.

Just so we're clear, no one else is discussing the elevator code. They've forgotten that before they were structural engineers, they were students that could calculate stresses in a mechanics class. That's how you're required to think about your mechanical codes.

 

[Ajay Shankar]

In my experience, Gratings are designed only for UDL. Mostly industries buy Standard Gratings.

My advice- Take any standard grating and check for maximum permissible span. If grating can support an UDL of 502 PSF at Y Span max.

Design grating support runners at maximum at Y feet and design them.

CONCLUSION Grating can be any type ( one way supporting) only their span may vary if UDL is more than their design loading.

Kaffy can conclude if he is satisfied based on the all foregoing discussions.

 

[RPMG]

Kaffy has to comply with the elevator ASME codes, and not mix-match structural codes in some hybrid system for structural engineers to approve. The problem here is that this thread belongs in the mechanical section, and it doesn't matter how a structural engineer does things.

 

[kaffy]

Thank you very much guys