Douglas Fir Grade Bending Stress from before 1990

[davab]

Hi everyone,

I have an old as-built drawing that shows the bending stresses, but it is worded differently than how it is worded now so if anyone can chime in based on his or her experience, it would be much appreciated. See the attached image for your reference.

In today's standard per NDS, the bending stress values are different between Visually Graded Dimension Lumber 2"-4" and Visually Graded Timbers Beams and Stringers (5"x5").

So will the 4x14 DF purlins @ 8'-0" oc in the roof fall under 1300 psi as "Beams" or 1500 psi as "Joists" because it is 4" wide?

Is there any online resource that clarifies this? I will not take a chance on this so I am inclined to using more conservative value but what are your thoughts?

 

[Wallache]

Which option makes them calc out to 99.9% stress? Seriously, there was no extra capacity in those old panelized roofs. I probably get called out to develop a fix for a cracked 4x14 once a month here in the Southwest.

 

[jayrod12]

I disagree with the above. I find often there's residual capacity in joists and purlins.

I'd have to check my 1963 design book but I feel anything 4" wide qualified as a beam. joists were limited to 2" wide. It has to do with the uncertainty in the thicker lumber that there isn't an imperfection in the section somewhere.

 

[hawkaz]

The 1962 NDS defined a beam as: "Lumber of rectangular cross section, 5 or more inches thick and 8 or more inches wide"

My experience is the same as Wallache.
I tend to find some extra capacity in the girders, but not the purlins (4X14) or sub purlins (2X4)

 

[jayrod12]

Can confirm it is 5" and thicker for beams. so 4" would be considered a rafter, joist or plate.

 

[JoshPlumSE]

My first impression is the following:

Any 2x4's used in floor or roof framing would be 1750.

Post and timbers (1200F) would be anything 5x5 or bigger that is reasonably square. e.g. 8x8 or 10x12, but not 8x12.

Beams and stringers (1300F) would be anything 5x or bigger that is not very square 6x12.

Rafters, Joists and Plates (1500F) would be any 2x or 4x that don't fall into the other categories. 2x6, 4x12.
​

At least that's the assumption I'd start out with. If it doesn't work then I'd find a NDS or UBC from around the time the building was constructed to see if we could back out the bending stresses from the values listed there.

Note: I suspect that for things like the rafters and such, they've already applied a repetitive use factor.

 

[Deker]

Agree with everyone above...4x would be 1500 psi. For reference, take a look at the WCLIB Grading Rules (excerpt below). The most recent edition can be found here: Link. The USDA Wood Handbook is also a great source for investigating these types of questions: Link.

Also agree with Wallache that most of the panelized wood roof buildings we come across are almost as bad as PEMBs in terms of ability to handle additional loads. Much easier to retrofit, though.

 

[davab]

wallache:
So 1300 psi gives me 97% stress in bending, which leads me to believe that 1300 psi was used. With 1500 psi, demand capacity ratio (DCR) is 85%. As a side note, no issue in shear.

jayrod12:
I haven't seen much residual capacity in my area, especially in tilt-up warehouse building roofs.

hawkaz, Deker, & JoshPlumSE:
Thanks for your input. I will review the document that Deker posted. It would be a great resource in the future too.