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An average day

40 psf live load is called out IRC. I know it won't span at longer than about 8' at 12" O.C. because I calculated it as a 2x6 in BC Calc. The DCA 6 says not to go more than 6'. Going by that, if it's over the 6' to 8' range, I make them prove it will span.

Trust me, if you try to calc it, or use a joist table for a 2x6, it's not even close.
 
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BN4537 said:
40 psf live load is called out IRC. I know it won't span at longer than about 8' at 12" O.C. because I calculated it as a 2x6 in BC Calc. The DCA 6 says not to go more than 6'. Going by that, if it's over the 6' to 8' range, I make them prove it will span.

Trust me, if you try to calc it, or use a joist table for a 2x6, it's not even close.
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So, if is more than 6' horizonal and stringers are made from a 2x12 it needs to be engineered if there is no mid-span support?
Does it matter how many and distance apart the stringers there are or if they double the stringers?
 
Yes, pretty much. I would allow up to 8' if they are at 12" O.C. as borderline because it will span according to BC Calc.

I don't know how you are supposed to calc stringers. I size them as a single 2x6 beam with a 12" wide tributary area, spanning whatever the horizontal distance is. 40 psf live load, + 10 psf dead load. The dead load is admittedly high for a stair. I call it "conservative". The 12" tributary only truly applies to the middle stringers, but again, it is a conservative number that I can apply to the outer stringers as well.

The calcs assumed the worst grade of SPF 2x6 possible since notching a 2x12 down to 2x6 may affect the grade of the remaining board, and SPF is has a shorter span than DFL and SYP. I assume 12" O.C. spacing because you can't go wider than 12" O.C. for composite decking or you won't meet the 300 lb point load.
 
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BN4537 said:
The 12" tributary only truly applies to the middle stringers, but again, it is a conservative number that I can apply to the outer stringers as well.
Click to expand...
If the treads are considered stiff relative to the (cut) stringers, then the load on a tread will be spread approximately evenly across all the stringers. In which case the tributary width should be just the total width divided by the number of stringers.

Conversely, if the treads are considered flexible relative to the stringers, then the usual method of assigning tributary width to each stringer would apply.

I'm not sure which model is more realistic, but the stiff tread model seems plausible to me.

Cheers, Wayne
 
Mr. Inspector said:
So how do you know if the inside stringers comply with on 1x12" stringers.
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If you size them the way I demonstrated, you are covered either way because the inside stringers are the ones I size in that method, and just use the same size for the outer stringers. It is extra conservative, but if they don't like it, they can hire an engineer.
Mr. Inspector said:
Why is the code silent on stringers? Does that mean they all need to be engineered?
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I don't know why the code would be silent on stringer span. It's kind of a giant cop-out.

If the inspector does not believe in using alternate sources such as the DCA-6 limitations, calculations as noted above, or Inspector Gadget's method of using a floor joist table for the horizontal span, then an engineer would be required.
 
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