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Floor rating 120psf, I do not understand

Floor rating 120psf, I do not understand

Hello. I have a floor with a rating of 120psf (0.83psi). I do not understand why I do not go thru' the floor as I put down at least 20psi. Can someone explain this to me and also advise me on the maximum load (and point load) I can place on this floor. I would also like to know how to work this out - thank you very much!

RE: Floor rating 120psf, I do not understand

More information on the actual floor structure is needed to help you.

As to the 20 psi figure, this translates to a live load value of 288 psf.  In that the design load was apparently for 120 psf (normally 100 live plus 20 dead), what is the reason for the nearly 300 psf requirement if I understand you correctly?  Is this floor supporting heavy equipment?

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Floor rating 120psf, I do not understand

Check that, nearly 3000 psf.  I hate that when that  happens.  That's a lot worse.

There must be a mistake in your figures as no building floor I know of is designed for that.  The highest loading I have ever used was a 400 psf roof snow loading for a ski resort in the Cascade mountains of Washington.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Floor rating 120psf, I do not understand

My apologies, I did not perhaps phrase my question correctly.
OK, my floor is rated at 120psf, yet a 200lb man puts down 20psi - why does the man not go through the floor. Why can the floor support this load AND heavier loads, and how do I calculate this?
It's driving me crazy - thank you for the help!

RE: Floor rating 120psf, I do not understand

Weeriley, 120 psf (0.83 psi) over an entire floor area, tributory area for a support beam, or large section of a slab is not the same as 2880 psf (20 psi) on a very limited area, for instance a very heavy person's  foot print or a table leg.  You need to better explain your intended loading.  Better yet, hire an engineer to check out the ability of your floor and framing system to support your intended loads.

RE: Floor rating 120psf, I do not understand

The common design is for a uniform load and a point load applied under different conditions, and not at the same time.  

The man does not go through the floor because the plywood is strong enough to span the point load to the joists, and the joists strong enough to span the reaction from the plywood to the beams.  That's why the plywood is usually supported at 16 to 48 inches on center, depending on the thickness and the load rating.  

You can ususally spread out the point load on a 24" square area, so the actual local uniform load seen by the plywood is closer to 50 psf for a 200# man.  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: Floor rating 120psf, I do not understand

That floor was designed to support an uniform live load of 120 psf. For average person weigh 200 lb, and occupies a floor area about 1.5-2.0 sf, the space can be filled with standing people, shoulder to shoulder, and the floor would still be fine.

I think you have mis-conception on contact pressure and material stresses (shear, bending,...).

Check on the label on elevator, it indicates how many persons and the weight limit. Then divide the weight limit by the floor area to see what psf you get. Then divide the psf by 144, now I think you would never want to step into an elevator.

RE: Floor rating 120psf, I do not understand

100 psf is an approximation.  You couldn't physically pack people tight enough to exceed 200 psf and that would even be unusual. And while a man could apply 20 psi at the soles of his shoes, this load would tend to distribute over a larger area, depending on the floor material.
By necessity occupancy loads need to be round rough numbers.   

RE: Floor rating 120psf, I do not understand

Thank you all - great explanations, much appreciated.

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