Southern Pine design values and the IBC

[RWW0002]

I know there have been several posts concerning the reduction in Southern Pine design values over the past year, but varying agencies and sources seem to be at odds on whether NDS Supplement 13 (with an effective date of June 1 2013) has already taken affect for all new IBC-based design or whether local jurisdictions must adopt the change.

The following from Southern Pine Inspection Bureau implies that enforcement varies by jurisdiction (see Q11).

http://www.southernpine.com/media/QA_NDVs_060113.pdf

This "non-mandatory unless approved" sentiment is echoed in several other sources, but I was under the understanding that since ALSC (American Lumber Standards Committee)approved the change per IBC referenced ASLS PS 20, the values take affect June 1, 2013 and should be used for design without specific jurisdictional approval. (Sorry for all the acronyms)


Bottom line, if the local jurisdiction (under IBC 2006 or 2009) has not mandated the use of the new design values and has not approved any amendment to the prescriptive design sections of the IBC, do the updated design values have to be used?

 

[AELLC]

Why not use them anyway - they are conservative.

 

[RWW0002]

That is the approach I would recommend, but it may be difficult to convince contractors/architects/owners that Southern Pine is suddenly "weaker" than SPF without specific code backing.. Also, if it comes to battles with truss manufacturers (which it appears you are becoming all to familiar with AELLC) I would like to know where I stand and what code backing there is.

Mostly I am curious if all engineers are adhering to these values or not.

 

[AELLC]

I can't say because I don't use SP.

I just have my ways, it depends on whose ax to grind, who has to pay extra for what.

My battles with truss mfrs all boil down to how truss software works, and the local absurd practice of requiring me to show truss-to-truss hangers on my dwgs, despite what it says clearly ion the IRC, and the Bldg Dept knows that.

If I had to value-engineer a house for a tract homebuilder, using SP lumber, my advice would be opposite if doing a high-end custom home.

 

[msquared48]

We had to do the same thing back in the late 80's, early 90's with the '91 and '94 code changes. Lots of stresses were decreased based on recent testing then.

A lion does not change it's spots.

Mike McCann
MMC Engineering

 

[AELLC]

But all of the sudden, the shear allowables shot upwards.

 

[msquared48]

Yep. The same thing happened under toe 2010 NDS with other species.

The shear in DF/DF glulams used to be 165psi. Now it is over 250 psi from what I recollect, maybe even higher.

Mike McCann
MMC Engineering

 

[wannabeSE]

msquared48,
Did you mean the shear stress increase in the 2001 NDS Supplement? There is an article on AWC's website that includes an explanation. See page 2 of

http://www.awc.org/publications/Papers/NDS2001article.pdf

 

[RWW0002]

Although the effect may be similar, this change is significantly different from the '91 and '94 design value changes because it is in between code cycles. The values in the 2001, 2005, and 2012 NDS have been revised through an approved supplement as opposed to the IBC directly referencing a new version of the NDS. Additionally, no one seems to be sure whether the new values are automatically enforced or not.

The battle we fight is still the same, convincing others that wood members sitting in the lumberyard have changed properties overnight, but this time it is more difficult to blame it directly on the building code.

For those of you in the southeast who have been regularly specifying So.Pine as a comparable alternative to Douglas Fir, are you using reduced values? changing to MSR lumber or another species? or using the old values until you are directly forced to do otherwise?

By the way, here is the link to a SBC magazine article concerning this issue.

http://sbcmag.info/news/2013/sep/sbcas-public-letter-alsc-awc-nahb-sfpa-and-icc

 

[AELLC]

It seems to me that since wood design has a FoS of 3.5 to 4, a reduction of 30% in values could easily be overlooked.

 

[AELLC]

Are you taking advantage of all the usual "value-engineering" tricks?

Taking stud height as floor to ceiling height, or are you taking actual cut height of stud?

 

[XR250]

I use the new values although the strength aspect only seems to affect really short span, heavily loaded members. I feel the new E values are a welcome change. This fast grown lumber seems to sag alot more than the numbers would dictate.

 

[RWW0002]

Why do the strength reductions only seem to affect short spans? The fb values saw a significant reduction (20-25%).
This should affect long spans more than short unless you are saying that these spans are deflection controlled anyway.

I would think exterior stud walls would see the greatest impact, especially tall gable end walls. Contractors already look at me funny when I make them use full height gable studs instead of splitting the wall at the plate. Now when I tell them they have to use an even tighter spacing than before they are really going to think I am overkilling it.

The other thing that makes this situation strange is that a jurisdiction that has not accepted the amendments to the IBC prescriptive design tables would still allow the old span tables to be used per the IBC but would "punish" engineered design assuming they must use the reduced design values.

 

[AELLC]

"use full height gable studs instead of splitting the wall at the plate."

I have always taken for granted the gables get full height studs. Wouldn't there be a hinge otherwise?

 

[Triangled]

In the late 80s early 90s as M2 noted, west coast species, notably Douglas fir, experienced a major code adjustment for the allowances. This was a result of the extensive "in-grade" testing program whereby very very many samples were selected from lumber already visually graded, thus "in-grade", and tested to destruction. The results were reviewed graphed amassed dissected inspected highlighted and formed the basis for a lot of struggle in the wood industry at the time. Combined with the spotted owl preservation movement and the recession de jour, the wood industry was set back a bit.
I think yards with wood in stock, and contractors and truss yards who could by outdated grade lumber cheaper protested the most. Everyone liked to point to the exact date that the new code went into effect, but the fact of the matter is mother nature's wood strength didn't change the day the code went into effect. The code simply codified more accurately the actual strength of the lumber. Remember when tension in lumber was the same as tension in the extreme finer in bending? And then all of a sudden, oops, better reduce that by 50%. The lumber didn't really change, but our understanding improved.
I think the new SYP values are the state of the art. I kinde prefer that the engineering profession lead this implementation and not leave it to the building officials. But, a contractor with a buck to make, can be a challenge.[pre][/pre]

 

[AELLC]

If it is a tract home, I would take a lot of liberties with the calcs. Typical tract homes have very few hotspots except loads at very large girder trusses and the bending in tall gable end walls where the clg is high due to a volume clg

 

[AELLC]

^^^...the studs are high due to...^^^

 

[Jerehmy]

"It seems to me that since wood design has a FoS of 3.5 to 4, a reduction of 30% in values could easily be overlooked."

according to this, the factor of safety is a bit higher than 3.5-4.

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_05.pdf

wood rupture properties are quite high. Variability and imperfect members cause the safety factor to be quite a bit higher as these are average rupture properties. I'm sure the factor of safety is 3.5-4 for the bottom 2-3% of wood in a normal distribution scale, but closer to 10 for the average piece of lumber.

 

[AELLC]

"I'm sure the factor of safety is 3.5-4 for the bottom 2-3% of wood in a normal distribution scale, but closer to 10 for the average piece of lumber. "


I am glad you pointed that out, because quite coincidently, I attended a wood design seminar last week, and they explained that the Code-writers did that for the EXACT reason. Whew.

 

[Jerehmy]

"I am glad you pointed that out, because quite coincidently, I attended a wood design seminar last week, and they explained that the Code-writers did that for the EXACT reason. Whew."

It's the internet so I'm having trouble detecting if your being a jerk or not. Seems that you are.