IRC 2009 Table 802.5.1 produces overstressed joists?
IRC 2009 Table 802.5.1 produces overstressed joists?
(OP)
thread507-22997: Rafter Ties and Reduced Rafter Spans IRC (code)
I am designing a house with a flat roof, sloped ceiling (so the ceiling is attached to the roof joists).
The span is 19'. From IRC table 802.5.1 I can use SP #2 2x12 @ 24" O.C.
When I calculate the stress of a Southern Pine (2-4" thick, 12" wide) 2x12, repetative factor, 40 lbs dead and 40 lbs live, I get overstressed 122%
Is there something I am doing wrong? Or has the IRC just allowed roof trusses to be overstressed because they probaly will never see 20 PSF?
I am designing a house with a flat roof, sloped ceiling (so the ceiling is attached to the roof joists).
The span is 19'. From IRC table 802.5.1 I can use SP #2 2x12 @ 24" O.C.
When I calculate the stress of a Southern Pine (2-4" thick, 12" wide) 2x12, repetative factor, 40 lbs dead and 40 lbs live, I get overstressed 122%
Is there something I am doing wrong? Or has the IRC just allowed roof trusses to be overstressed because they probaly will never see 20 PSF?






RE: IRC 2009 Table 802.5.1 produces overstressed joists?
Normally 12 to 20 psf.
Why so heavy?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
I am calculating for 20 PSF dead and 20 PSF live (no snow load)
The span is 19'.
The lumber is going to be Southern Pine #2.
When I look at the IRC table R802.5.1 for roof joists with 20 psf dead and 20 psf live, at 24" O.C. spacing, it says I can span 2x12 Southern Pine #2 19'-3".
When I calculate the stress using my beam program (EnerCALC) I get that the roof joists have a stress ratio of 122%. I've verified the program calculates to the NDS 2005 methods properly.
I checked the floor joists in the IRC and they match what my beam software outputs... it is just the roof joists that are too stressed.
Is there some reduction factor you can use for roof joists, beyond the repetative member reduction? What do you calculate the stress to be in that 2x12?
I am tempted to just go with my calculations, but I'd love to go with the code tables to allow my clients to use less lumber and save money. Any thoughts?
--Luke
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
DBennett
Lone Star Engineering, PC
www.lonestarengineeringpc.com
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
You don't say what bending stress and deflection you get with calcualtions. We can only guess. My first guess is that you are using 1.0 rather than 1.25 for a load duration factor.
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
Thank you all for answering my question!
--Luke
RE: IRC 2009 Table 802.5.1 produces overstressed joists?
BTW - DO NOT MAKE IT FLAT - give it at least a 1/8'' per foot angle - 1/4'' is better.
FLAT ROOFS LEAK AND WILL ALWAYS LEAK!!!