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Southern Pine design values and the IBC

Southern Pine design values and the IBC

Southern Pine design values and the IBC

(OP)
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.p...

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?

RE: Southern Pine design values and the IBC

Why not use them anyway - they are conservative.

RE: Southern Pine design values and the IBC

(OP)
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.

RE: Southern Pine design values and the IBC

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.

RE: Southern Pine design values and the IBC

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

RE: Southern Pine design values and the IBC

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

RE: Southern Pine design values and the IBC

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

RE: Southern Pine design values and the IBC

(OP)
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-lett...

RE: Southern Pine design values and the IBC

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.

RE: Southern Pine design values and the IBC

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?

RE: Southern Pine design values and the IBC

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.

RE: Southern Pine design values and the IBC

(OP)
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.

RE: Southern Pine design values and the IBC

"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?

RE: Southern Pine design values and the IBC

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.
 

RE: Southern Pine design values and the IBC

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

RE: Southern Pine design values and the IBC

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

RE: Southern Pine design values and the IBC

"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...

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.

RE: Southern Pine design values and the IBC


"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.




RE: Southern Pine design values and the IBC

"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.

RE: Southern Pine design values and the IBC

What do you mean , a jerk?

The internet is not like conversing in person.

Sorry for any misunderstanding, English is my second language, altho I live in the USA>

RE: Southern Pine design values and the IBC

"What do you mean , a jerk?

The internet is not like conversing in person.

Sorry for any misunderstanding, English is my second language, altho I live in the USA> "

I've seen your posts numerous times and you have perfect english so now I'm not sure if you're straight up trolling me. Whatever.

RE: Southern Pine design values and the IBC

OIC, you though I was being sarcastic - when I re-read my own post, it did seem so. No intent of sarcasm at all, I was thinking that the FoS gives me much more confidence in my wood design

RE: Southern Pine design values and the IBC

It is not trolling, cool your jets. I have lived the USA a long time but French is my preferred language.

RE: Southern Pine design values and the IBC

Also I am very dyslexic and I leave out words, a few typos.

But I am saying that I was totally not realizing I sounded so sarcastic - I can see that now re-reading my post,

No harm intended.

RE: Southern Pine design values and the IBC

also, I used to be known in these fora as "SacreBleu"

RE: Southern Pine design values and the IBC

Haha well I'm glad I just misunderstood you. All good

RE: Southern Pine design values and the IBC

The values for the wood tested are the values for the wood that are being purchased, used, and loaded. It is therefore prudent to design based on what will be used. I would not wait for building code officals to release a statement that they now only accept new NDS values... do you think the lumber suppliers will sell product as pre-Code decreased #2 SYP or post-Code #2 SYP? The wood is what it is and it is our job to state what load to wood can withstand.

Now i will state that when analyzing older SYP i would use the old NDS value (say 2003 construction) but new members for repairs will be for new reduced values.

And Thank you AELLC, I have ended up numerous online conversations coming off as a jerk when i did not intend. Now i do not feel alone anymore :)

RE: Southern Pine design values and the IBC

LOL Eric,

I started this online stuff way back on AOL in one of those chat rooms (actually the Architectural chat room)
Have a hundred war stories there - and we baited and tortured rude newbies mercilessly there.

RE: Southern Pine design values and the IBC

Back on topic, IMO we can take a lot of liberty with stress allowables, but deflection is real and doesn't give you any reserves, if any.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

RE: Southern Pine design values and the IBC

@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."

Yes, because, in my experience, serviceability seems to control most designs in wood construction.
SYP is rarely used for walls or roof framing (in our area anyhow). Usually only for floors and beams. That is because SPF is lighter and straighter.

RE: Southern Pine design values and the IBC

Not that i disagree with you all, but i just ran some live roof valleys (they are SYP here, if not LVL) and they actually governed by moment not deflection at a span of 10.5 (true span 15.6). So as with all things, i think all aspects need to be looked at... But we all do that anyways right?!?

RE: Southern Pine design values and the IBC

Oh the days of AOL and Dial up..... so worth the 90 free minute CD install!

RE: Southern Pine design values and the IBC

I can see that for a hip or valley. I read a HUD design manual once that said you did not need to check deflection of hips and valleys due to folded plate action making them much stiffer than they are alone. I check it anyway.

RE: Southern Pine design values and the IBC

(OP)
It looks like we may be using spf wall studs more often here as well now that we do not get much help from Southern Pine design values. Although, the irony of this is that So Pine values have been reduced due to their rigorous testing protocol. My understanding is that other lumber species are not tested with this frequency, and may exhibit similar loss in design properties (it just has not shown up yet). I predict we will see a reduction across the board (at least with pine species) in the next year or so. In the meantime, it will appear as though So. Pine is weaker in comparison to SPF.

RE: Southern Pine design values and the IBC

FYI, around my part Mid-Atlantic US:
SPF for 2x4 and 2x6
SYP for 2x6 and up

RE: Southern Pine design values and the IBC

Most framers I've talked with hate using SYP and the few times I framed houses, it was a PITA to work with because it was twisted, knotted, excessively crowned. We never called it out before due to these reasons. We've always used Spruce. The design value changes pretty much reinforce this decision.

www.idecharlotte.com

RE: Southern Pine design values and the IBC

SPF, Hem-Fir, and all their variants.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

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