Residential Floor System 1

[4thorns]

Hi All,

Given a simple residential floor system situation, joist ends sitting on a wood girder, dead load only...

2x8 SPF floor joists @ 16" o.c. weigh 2.2# +/- per square foot
5/8" OSB weighs 2.2# +/- per square foot

That's a total of 4.3# per square foot. I design at a minimum of 10#. I received some calcs for a girder today that used 15#. I ran the numbers on a smaller girder
to carry these joists @ 10# and it passed.

This seems like a situation where the engineer wasn't given privy to the finished floor materials and made sure that whatever went over the sub floor
wasn't going to make it fail. Seems to me that a little communication could have avoided extra cost for the consumer, a little less sweat for the builder
and no guesswork for the engineer.

In a situation such as this do you "Overkill" to make sure you're covered?

Doug

 

[strucguy]

Did you account for floor finishes (gypcrete, tiles etc), and weight of ceiling board. It is customary in our office to use 15 psf dead load for residential floor design taking into consideration that we as engineer don't have control on the final floor finish. I don't think a mere 5 psf reduction in design dead load will save a lot to the consumer. The real savings is in specifying the right materials.

 

[4thorns]

OK...stop laughing. I changed the 2x8 to 2.2# but didn't change the total. I really can add!

 

[4thorns]

strucguy,

In this case it took under 3# per square foot to make the difference. I have almost 6# to work with. Research has shown me that it takes quite a bit
of finished floor to make that up. Usually if the homeowner is going to have tile or the like for a finished floor that is insanely heavy, they make
it known during design.

 

[strucguy]

2.2 psf for joists
1.8 psf for OSB Sheathing
2.2 psf for GWB ceiling
3.5 psf for hardwood flooring

This adds up to close to 10 psf. I have seen cases where, the owner wanted to use 3/4" gypcrete with carpet finish instead of hardwood floor. In that case the total load reaches 13 psf easily.

 

[SLTA]

If you go back and calculate the savings for one size down of beam, it won't make a hill of beans of difference. I sometimes slightly upsize the beam or joists to ensure that the floor won't bounce, or for reasons of simpler constructability. Not worth worrying about.

Now, making sure your decimals are in the right place can make all the difference in the world. Last week I was lost to figure out why a beam was failing when I KNEW it should work, only to relook at my loads and realize that 100psf DL and 400 psf LL were a bit off. *sigh*

 

[TXStructural]

I'm not sure why the girder design used a load different from your own, but I think 10 psf is a bit light in any case. I always plan on remodeling which would not be accounted for by load factors - I seldom design for minimums where I can foresee significantly heavier loading without any change in occupancy.

 

[dhengr]

Also, all of the loads mentioned so far are long term loads, not short term. I agree with Slta... not worth a hill of beans, you want a stiffer floor system, designing that tight with loads you have no control over is penny wise and pound foolish.

 

[MiketheEngineer]

Stru has it right - what about somebody going back in tiling the floor. I always use a minimum of 10 psf dead load. What about a ceiling in the basement??

 

[4thorns]

OK guys. Thanks for the replies and I understand where you are coming from. I've attached a Woodworks Sizer report that shows what I'm referring to. It represents this girder which is in a small residential home. First floor loads are bedrooms and second floor is open area (loft). The highlighted portion is the dead load I used.. Given a small, 1 family, 2 bedroom residence with short spans, and a 1.5 creep factor shouldn't this girder be sufficient?

The more I think about it this should have been my OP.

Thanks,
Doug

 

[focuseng]

How much does the 3/4" slate floor tile in a mudset application weigh?
What is the deflection criteria?
When I have a "cheap" client (and I mean very simple house) I use 10psf as an average and hope for the best. In most of the homes that actually get an engineer involved, the finishes are all over the place. Any coffer beams in the ceiling below, High end marble flooring, Kitchen counters made out of 2"-3" thick stone. 15psf is not very conservative and the averages will even out a bit. Don't forget about compound deflections (a beam holding a beam holding a joist, which is supported by beam on the next level down...). I often don't get the luxury of a finish schedule at the outset of structural design.

If you know the dead loads then you know them - enough said and you go with it. If you are confident with 10 the that is fine. But if you don't then you must improvise with what you do know and I have seen some very heavy finishes put into a house.

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MAP