2x decking span for drip through deck 3

[DoubleStud]

Hi guys, I have a project that has 100 psf snow load. Instead of using typical deck joists with 16" o.c., the architect wants to use large lumber spaced farther apart. I calculated how far a flat 2x can span simply supported with 100 psf snow load. It can span 4 ft and meet L/360 LL deflection criteria with the snow load (I kept duration factor to 1). How about point load from someone standing? What kind of check shall I do? Shall I maybe check 300 lb point load in the middle of the flat 2x6 perhaps? If I do that check, it will be closer to 3 ft o.c. Maybe I should suggest using 3x decking? What are your thoughts?

 

[WesternJeb]

People loads on roofs are covered under the 20 psf roof live load. Analyzing a singular person is overly conservative; the 20 psf number has withstood the test of time to work so I would just go with that.

Plus your system will be repetitive, analyzing only one member at a time is very conservative.

 

[DoubleStud]

WesternJeb, I agree with you, however it is not a plywood. I feel like the load is not being spread from one member to another. You would really feel the bounciness? What do you think?

 

[Deker]

Consider specifying tongue and groove decking if you are concerned about lack of load spread.

 

[WesternJeb]

Well it seems like you're set on using a person load. That being said, I would use 250# for a construction worker with tools on him. This is represented in the ASCE 7 section on "grab bar" loads.

What type of ceiling is it? Check with IBC table 1604.3. l/360 is a little strict for roof members if it isn't a stucco ceiling. If it is ceiling tile then l/240 is more appropriate.

 

[phamENG]

Take a gander at this beaut': WCD2: Tongue and Groove Roof Decking

 

[DoubleStud]

This is a drip through deck. Sorry I should have explained earlier. T&G is not an option.

 

[WesternJeb]

Oh, even better. If it is a drip-thru deck then you likely don't have a ceiling so the deflection on roof members can be l/180 for roof live loads per table 1604.3 in IBC!

 

[DoubleStud]

I am not sure I agree WesternJeb, you would want better deflection since you will be walking on it a lot.

 

[WesternJeb]

Is this a floor or a roof? I might have misinterpreted this as a roof since you mentioned snow load.

 

[DoubleStud]

This is a deck outside. So I am checking both snow and live load.

 

[Deker]

It seems like WesternJeb is under the impression that this is a roof deck. A drip through deck doesn't make much sense to me for a roof. Can you clarify DoubleStud?

I would consider a point load check on a single 2x as a separate check from the uniform live load requirements. Maybe it's from a table leg, piece of furniture, a person standing on their tip toes, etc. I'd probably use 200# or 250#. You can consider continuity of the decking if you're trying to squeeze every last drop out of the design.

 

[DoubleStud]

Thanks Deker. If I assume 250 lbs in the middle of the span, I say the spacing of the joists need to be around 3 ft o.c. without fixity at support and without considering the thickness of the joists. OK, I just want to see if I am on the right path.

 

[BridgeSmith]

If the architect wants the spacing at 4', I would suggest adding a 2x4 across under the decking at midspan to distribute the live load across several of the deck boards.

 

[WesternJeb]

Apologies DoubleStud, didn't mean to go that far down the wrong path. That makes sense as to why I thought you were way overdesigning, LOL!

Agree 4' is too far for the required loading for normal people to be used to and 3' makes more sense.

 

[phamENG]

Keep in mind that all of the joist load tables out there assume repetitive members. Not sure if by 'larger lumber' you mean just deeper joists. At 3 or 4 feet on center you don't get that 15% boost.

 

[DoubleStud]

No worries WesternJeb, my fault for not explaining better.
BridgeSmith, he wants to expose the large timber joists for aesthetic reason, adding 2x4 below the deck will probably make it look bad. He did not specify the spacing, he was relying on me to figure out what spacing we should do. So I will tell him 3 ft max with 2x decking, more if he will do 3x.

 

[ChorasDen]

How are you analyzing, ASD or LRFD? If ASD, what code year? If ASD, where is your DL + 0.75L + 0.75S? When I run that, I'm getting 120psf from the info you've provided, so that should cover your situation, and I wouldn't personally be concerned with a moving 'person' load. 120psf live load likely also means that your spans are not going to meet what the architect wants.

 

[ChorasDen]

Since this is a deck, does the 100psf include the effects of sliding snow and/or drifting?

 

[Eng16080]

These would be my main concerns (some already mentioned above):
[ol 1]
[li]Depending on the lumber species and grade used, I think it should technically be possible to make a 4ft span work by code assuming a minimum 3 span condition. I don't believe the code requires a concentrated live load be applied in this case. With that said, I think it's very reasonable to use a concentrated load to account for the weight transferred through the foot of a well fed person. I would use a magnitude of 300 lb applied over a 2"x2" area, which is the same concentrated loading requirement used for stairs. (I assume this is to account for the weight of a person standing/stepping on one foot.) Ultimately, I would not want to use 2x decking at a 4 ft span even if it does meet code. I'm sure it would feel bouncy. I think your decision to limit the span to 3 ft is very reasonable.[/li]
[li]I would specify on the plans that the decking must be continuous for a minimum of 3 spans and that adjacent pieces be staggered.[/li]
[li]I would not use decking wider than 6" nominal, so either 2x4 or 2x6.[/li]
[li]Considering that the joists will be spaced greater than 2 ft o.c., you won't be able to use a repetitive member factor increase for the joists (as mentioned by phamENG).[/li]
[/ol]