An average day

[BN4537]

Horizontal span of the stringers was 14', so they added a midspan support.

 

[Mr. Inspector]

What section of the code tells you what the span of stringers are?

 

[e hilton]

What section of the code tells you what the span of stringers are?

It doesn’t, you have to use a tape measure on site.

 

[BN4537]

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.

 

[steveray]

Span is measured horizontally so it must have been 22' long?

 

[BN4537]

Span is measured horizontally so it must have been 22' long?

I think the stringer boards would have had to be cut out of a 20' board. I don't think they were quite 22'... but I only measured horizontally, so I don't know.

 

[steveray]

I think the stringer boards would have had to be cut out of a 20' board. I don't think they were quite 22'... but I only measured horizontally, so I don't know.

I just haven't seen sawn lumber over 16' in a long time....

 

[Mr. Inspector]

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.

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?

 

[BN4537]

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.