Uniform Load Question

[jpw2913]

Please help me to understand something that I know I should understand better.
Floor live loading in most areas of a residence is to be 40psf per IBC 2009. Yet I can think of many examples of everyday items that would cause loading greater than 40 psf. A bed that sits on 4 feet for instance, or a heavy dresser, or a piano. IBC gives no concentrated load to design for for residential floors. Can someone give me some insight here so I better understand how the minimum uniform loads were determined? Thansk.

 

[KootK]

It sounds as though you understand it perfectly jpw3913. There are all kinds of common situations that exceed residential loads. Waterbeds, kitchen islands, pianos... Still, experience has taught us that bad things generally do not happen in these situations. I'd like to think that 40 psf was the result of rigorous study and statistical analysis. I'm not even sure that's true however. Hopefully someone else here knows genesis story.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jike]

Most furniture is against walls and not in the middle of the floors. Much of the floor is actually unloaded or minimally loaded. It is all about how the load averages out. We have similar situations in commercial, institutional and industrial buildings except heavier loadings.

If you are really concerned, then do a study with actual loads.

 

[dhengr]

Jpw2913:
Imagine 200lb. people (+/-, on avg.) packed into a room, with each person occupying 4 or 5 sq.ft. That’s your 40 lbs./sq.ft. You don’t often find people that closely packed, that’s just really socially uncomfortably close, but possible. Then consider the possibility of people movement or even dancing, jumping after a TD, or some such That LL has been around for a long time, and has generally lead to acceptably stiff fl. systems. Over time, some codes have allowed 30 lbs./sq.ft. in upper level bed rooms. As for the types of loads KootK mentioned, pianos, kitch. islands, water beds, and the like; these are all extra heavy loads which you should ask about during the early stages of the structural design. Water beds can weigh more than 40 lbs./sq.ft. on 4 or 5 joists, and well out into the middle of the room span; kitch. islands are the same and have people around them; and I usually double up a few of the joists in these areas of the fl. system. While a grand piano may not weigh much more than 40 lbs./sq.ft. over the area it covers, it causes very large point loads on the sub-floor sheathing, maybe mid-way btwn. two joists. So, check the floor sheathing for these point loads, and check one joist for these point loads, maybe space the joists at 12" o/c in this area. On residential floor systems, deflection, vibration and bouncyness are usually the biggest complaint, not out-n-out failure from overload.

 

[manstrom]

The 40 psf load is a means to get moments and shears over a structural member. Once you start looking at the details of a 500# fishtank or a 1000# piano, things get weird. It's micro vs. macro, and it gets complicated.

We often get questions like "can I put this concrete planter on our roof deck?" It may weigh more than 40 psf, but will it increase the moment and shear on the floor to more than its capacity?

Other considerations are punching. Will the leg from a grand piano punch through a subfloor? What about deflection.

I typically respond to these questions with - Is it something that you would normally see in a residentail setting? If so, it's probably fine. Piano? Sure. Engine lift? No.

 

[Signious]

I had similar concerns when I first started in structural. We had a house with some extreme sagging in the floors (~14cm deflection on a 5m span) where the floor designer used 10dead/40live. Their was a large kitchen island in the middle with a 1mx2m granite countertop right smack dab in the middle.

I started to treat 'special' loadings as surcharge loadings on top of the 40psf live. I wouldn't count a bed, or a large sectional couch - but a permanent 100gal fish tank would definitely fall outside the code minimums.

 

[JStephen]

If nobody had ever built a floor before, the question would be more meaningful. But generally, whatever is used for a floor, is going to be adequate for people, so you can figure maybe 400 lbs on space 5"x12" or so when a heavy person walks. If the floor can handle that okay, it's going to be good for a lot of table legs and refrigerators, etc.

Kind of a side note, I happened to be in San Jose this last week and toured the Winchester Mystery House. Very interesting tour. But it occurred to me, here's all these residential stairways and small rooms and when they have tours, they DO have people pretty much jammed together on them at times. Made me wonder if they had ever had to beef up some of that stuff.