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?

 

[ChorasDen]

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

Just so it's clear, NDS requires a load distributing element between the members to use the repetitive member increase, and I'm not sure how that would be achieved with drip through decking and no sheathing.

 

[Eng16080]

ChorasDen, I might not understand your point above, but in my judgment I would use the repetitive member factor increase for this project if the joists were instead spaced no more than 24" o.c. (and assuming nominal 2" to 4" thick joists). I would consider the connection between the decking and joists as meeting the requirement for "load distributing elements."

 

[TLHS]

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.

Do you guys in the US not have a code provision like in Canada where for areas accessible to both people and snow you design for the worst condition but not the two combined (except in certain conditions), because you physically can't have both things there at the same time from a practical standpoint. Combined live and snow exist, but not on the same surface (i.e. you'd do roof deck snow plus occupancy live from internal floors, but not count occupancy on the roof deck at the same time as snow on the roof deck)

 

[ChorasDen]

Eng, I was interpreting that there is no decking/joist, the OP wanted to only consider flat structural members. It's completely possible I am misinterpreting, however, and need corrected.

 

[ChorasDen]

Do you guys in the US not have a code provision like in Canada where for areas accessible to both people and snow you design for the worst condition but not the two combined

In the IRC, I believe there is this provision, but IRC is limited to 70psf snow load, so it wouldn't apply here. Besides, has anyone ever been to a college town? Thinking that people won't stand on a deck or balcony where there is snow load on is well outside of my comfort zone, I would personally design for the 0.75SL + 0.75LL condition.

 

[DoubleStud]

But if one stands while there is a lot of snow, the snow will distribute the load to several decking member . Use AASHTO load distribution for truck load?

 

[ChorasDen]

I know that was in jest, and I do appreciate it, but just for brevity, I would say the snow would compact, which would violate the NDS note about unacceptable deflection .

 

[Eng16080]

Do you guys in the US not have a code provision like in Canada where for areas accessible to both people and snow you design for the worst condition but not the two combined (except in certain conditions), because you physically can't have both things there at the same time from a practical standpoint. Combined live and snow exist, but not on the same surface (i.e. you'd do roof deck snow plus occupancy live from internal floors, but not count occupancy on the roof deck at the same time as snow on the roof deck)

I don't know if there's a specific code provision addressing this, although I think a lot of us here in the US probably design decks exactly as you mention.

What I typically do is start by assuming that live and snow loads occur simultaneously and if that results in a reasonable design, I leave it at that. If, on the other hand, the resulting design would be more difficult to construct than what is typically done, then I'll perhaps take a closer look and consider whether the live load and snow load on the deck are really likely to occur at the same time.

One other point: for decks, especially elevated ones, I have no problem over-designing them somewhat, especially considering that this is often the highest risk part of most building structures.

 

[Eng16080]

I was interpreting that there is no decking/joist, the OP wanted to only consider flat structural members. It's completely possible I am misinterpreting, however, and need corrected.

I was interpreting the design as having flat decking with joists spaced at 3 ft on center with the decking connected to the joists. In my above comments, I was considering the load duration factor, CD, for the joists, not the decking. For the decking, I would use CD = 1.0 (no increase). I think we're in agreement.

 

[BridgeSmith]

Before you get too crazy with combining snow load and live load, you might want to check the depth of snow it takes to produce 100 psf.

 

[jayrod12]

Yeah I can't imagine there being a 100 psf worth of people standing out there in 5 feet of snow.

 

[SE2607]

DoubleStud - Is the decking spliced at every support? If not, perhaps you could use continuity to help you out.

 

[lexpatrie]

That 250/300 pound point load isn't for an occupant, it's for a fire-fighter in gear.

 

[XR250]

Yeah I can't imagine there being a 100 psf worth of people standing out there in 5 feet of snow

Even if there was, it would certainly arch over the deck boards.

 

[dvd]

3 ft joist spacing doesn't make as much sense from a lumber viewpoint, there are a lot of site-cut deck boards - maybe go with a 2x 19.2" = 38.4" spacing, which works for 16' lumber.

 

[XR250]

3 ft joist spacing doesn't make as much sense from a lumber viewpoint

12 ft. boards?

 

[phamENG]

12 ft. boards?

Don't you try to impress us with your superior use of mathematics!

 

[dauwerda]

When the owner wants to replace the deck boards with a composite deck in 5 years they are going to be upset that they have to add a bunch of joists as well.