Silly question? Loading capacity of structural slab. 9

[Localeng]

Here's a silly question.

A structural slab is designed for 50 psf of Live Load.
I want to bring a steel cabinet with 40 psf of empty weight.
Can I fill the cabinet only for weighing 10 psf more?

 

[TehMightyEngineer]

Yes and no.

Yes, technically the limit is 50 PSF.

However, if you were to run through the calculations it's probably fine as I doubt you're surrounding the cabinet with other 50PSF loads. For example a large fishtank is well over the 40 PSF live load most houses are designed for but we're not likely to be making rows and rows of fishtanks in a home, so this sort of variability is generally accounted for in the safety factor.

So, unless the floor is heavily loaded already I wouldn't loose sleep about it.

Also, what cabinet weighs the equivalent of 40 PSF? That must be one hell of a cabinet.

Maine Professional and Structural Engineer.

http://www.fepc.us

(Just passed the 16-hour SE exam, woohoo!)

 

[msquared48]

A lot of telecom equipment and battery cabinets are monsters.

Mike McCann, PE, SE (WA)

 

[MotorCity]

Generally speaking, uniform loads are assumed to be fully distributed over the entire floor unless some sort of load diagram is given or pattern load is specified.

So if your slab span is 6' and your cabinet is only 2' wide, you have only partially loaded the slab. Just calculate the capacity of the slab and compare it to your loads, moments, shear, etc.

 

[manstrom]

The slab is designed for moment and shear, not small point loads. The 50 psf area load is a means to the end of the moment and shear requirement.

If my 0.2k butt stands on one leg, the floor won't collapse. However, if my 50psf floor is loaded to 200psf, then we have a problem.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Localeng]

TehMightyEngineer
The cabinet height is tall, so it's heavy.
I will set 25 cabinets next to each other, so it can cause over-loading problem, don't you think?

MoterCity, manstrom
Let's assume four football players weighing average 310lbs are invited to a friend's house.
They were sitting in a 5'x6' room and the design live load is 40 psf.
Is this room over-loaded?
If not, why?

I really want to thank for your comments and discuss this issue more.
Real life situations differ from design requirement in many cases.

 

[dhengr]

Localeng:
Until just now, you didn’t say you were going to put 25 of these file cabinets in the same location on the slab. That is the really silly part of the way you asked your question. If you can’t ask an engineering question and provide sufficient engineering info. for a meaningful discussion, you deserve silly answers too.

 

[manstrom]

LocalE,

If I had a room that was designed at 100% stresses and filled it to 110% capacity, then I might have a problem.

In your example, the 5x6 room is probably not supported by walls at 5' and 6' below it. If it were, the members would not be 100% maxed out.

Extrapolate on your example. If I had a 25'x25' room with supports below the perimeter walls and filled it with 78 linemen (110% capacity), then yes it would be a problem.

It's the big picture that matters. Minor loads in small areas would only cause localized issues.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Signious]

Forgive me for being blunt here, but this seems like a really easy problem for any structural engineer to figure out.

-You have a concentrated load of x PSF over the area that you plan on putting the cabinets
-You have a 50psf rated concrete pad

1. Find maximum shear and bending moments for the slab under your concentrated loading from the cabinets as well as any additional live and dead loading you will encounter (people, desks, walls ext...)
2. Analyze the slab finding maximum shear and bending moments under a full 50psf load as per the slab rating
3.1 Compare the two, ensuring that the results from the 50psf loaning (#2) are greater than the results from the first step (#1)
or
3.2 If you know the construction of the slab (rebar, spacing, concrete strength & thickness) just run shear moment defln with that information under your modified loading from #1

On a semi-related note, should we have a 'Non-engineer' category for registration. I know I am relatively new here - but it seems every week or so there is a question asked by a non-engineer on this sub-forum who just puts in structural because that is the scope of their question. Having a non-engieer category for occupation would make it much more clear to people on how they should respond the question.

 

[TehMightyEngineer]

Short answer to the OP, with that many cabinets you will have to limit them to 10 PSF or get a local structural engineer involved to help you rate the slab specifically for this load.

Maine Professional and Structural Engineer.

http://www.fepc.us

(Just passed the 16-hour SE exam, woohoo!)

 

[Localeng]

dhengr
If I get an answer from other engineers, I intended to multiply to apply to my situation. Code requires using uniform live load and my approach to beg opinion was not that bad in terms of code-compliance approach. You may read the notes below and if you want to share your opinion, that'll be greatly appreciated.

manstrom, Signious, TehMightyEngineer
Thank you for your comments.
I know how to calculate the load carrying capacity of slab, but if I understood your postings, even though the live load is higher than the 'design live load', as long as the slab can support them, you feel okay. Did I understand you correctly?

 

[CANPRO]

A uniform design load is used because as designers, we often do not know the exact loading situation....so we apply a uniform that is appropriate for the intended use. If you sharpen your pencil and really look into it, the load rating can likely be increased. If you're designing a slab and the numbers dictate for a 50psf load you need 15M bars at 17.2342" on center, most reasonable people will specify bars at 16" on center. If you now know your exact loading scenario and as-built conditions of the slab, its just a matter of re-doing the analysis and making sure it works out.

 

[JAE]

Code requires using uniform live load ...

No - code requires both uniform and concentrated loads - see the IBC or ASCE 7 if you are in the US. If not, check with your applicable structural code provisions.

...even though the live load is higher than the 'design live load', as long as the slab can support them, you feel okay. Did I understand you correctly?

Not ever a good idea to exceed the live load of a structure unless an experienced structural engineer reviews all the aspects of the structural capacity, the level of knowledge pertaining to the actual structural elements, and the use of the space with respect to the load. In some cases, exceeding the capacity by 5% or so is permitted. Your phrase - "as long as the slab can support them" is pretty vague.
A slab can support a load up to the failure limit but I wouldn't recommend ever doing that.

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