IRC 2009 Table 802.5.1 produces overstressed joists? 1

[ProLuke]

thread507-22997
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?

 

[msquared48]

40 psf seems high for the dead load on a flat roof.

Normally 12 to 20 psf.

Why so heavy?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ProLuke]

For the 24" spacing... I'm using 20 PSF * 2' spacing = 40 PLF

 

[msquared48]

So the live load is not snow?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ProLuke]

Please excuse my poor post... this is my first. Let me clarify:

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

 

[msquared48]

Same values in the IBC. Are you sure you used the correct maximum bending stress and shape factors for 2X12's?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[DLBennett]

I believe the tables include the joist weight in the dead load, as they should. Some analysis programs add the member load again as a default setting. Something to check.

DBennett
Lone Star Engineering, PC

http://www.lonestarengineeringpc.com

 

[wannabeSE]

ProLuke,
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.

 

[ProLuke]

Oh... so if I am using a non-snow live load, it would be considered a construction-duration live load, so for the calculations use 1.25 load duration factor? That would explain why I get 122% stress using 1.00 duration.

Thank you all for answering my question!

--Luke

 

[MiketheEngineer]

If all other things check out - I would agree with 1.25. You can even use 1.15 with snow based on your description of the deck.

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!!!