Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations waross on being selected by the Tek-Tips community for having the most helpful posts in the forums last week. Way to Go!

reduced southern pine design values 2

Status
Not open for further replies.
RFreund

RFreund

Structural
Aug 14, 2010
1,873
0
36
US
This may be old news to some, but new to me.
At a woodworks seminar today I learned that 'emergency' testing on southern pine wood has lead to the conclusion that there should be an immediate strength reduction for 2x4 SoPine due to more juvenile wood used in production.
Apparently the strength of a 2x4 SP will be reduced on 6/1/12. The official letter also 'strongly urges' to use caution when using other sizes as well as they may be reduced 25-30%

I imagine this being a real sh** sandwich for some design engineers, especially truss designers. I mean in a repetitive system it may not be so worry some, but for a 60' truss where one web member failing can cause a collapse, what are you to do uptil 6/1/12? It seems the grading or standard agency has put people in a tough place. They issue a statement saying values will be reduced for one size at a future date due to the fact they are currently understrength and future testing is needed for other sizes but they are also believed to be understrength.

I see this being tough for 'cut throat' truss engineers who will design with this knowledge in advance of 6/1/12, who won't?


EIT
 
I don't think it applies to all SYP, but refers to the testing methods used for some references. It is certainly a reminder that future allowables may be reduced because of the abandonment of the old historic accepted strength since the new growth may be lower than the old historic results compared to new growth test results.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.
 
Thanks RFreund for posting this as I was about to start the conversation too. I was at the same seminar and for about 1 hour there was a room full of about 50 very uncomfortable structural engineers discussing the topic. The whole room was electric with a sense of anger and nervousness. This was not a topic taken lightly for several reasons with the most expected one being -you are now informed, what is your responsibility/liability to the problem. In case you don't think 2x4 visually graded SYP lumber effects you and your practice then think of all the truss drawings you have reviewed or approved or specified in the last year.

The fact is that the values for SYP 2x4 Visually graded lumber are going to decrease. Now, does that mean that a switch just gets pulled in June and viola! we start having less strong wood. No, the values proposed are representative (due to testing) of the current (and past?) stock/supply of SYP. This is not a change where the lumber will be different starting in June. This is a change to reflect the reality of the actual test results. The wording of the release by the ALSC board of review is clearly meant to "inform" you and to use your best judgement with this information and in my opinion, they are attempting to limit their exposure to it. They are just following procedures... You, as the EOR or RDP need to be "responsible".

From the ALSC board of review minutes, Jan 5, 2012
"Although given the facts, circumstances and controlling authority of this particular matter, the Board did not approve design values for the other sizes and grades and has recommended a future effective date, it cautions all interested parties to take note of all available information in making design decisions in the interim. The values in the SPIB proposal represent approximately a 25-30% reduction. Many of the critics of the proposal acknowledged that some reductions were in order, albeit the magnitude of those reductions was disputed. All design professionals are advised in the strongest terms by the Board to evaluate this information in formulating their designs in the interim period."
(emphasis of original text (not mine), See attached file)

This is also not like there is now a new design method for wind design which is more accurate and several more chapters in ASCE 7. This is happening at the Strength side of the equation and it has been found to be the wrong values. Now we know about it but we are waiting until June to start "officially" using it. The question is, are you legally exposed to a failure?

Yes, Yes, there are a lot of conservative assumptions, statistical deviations, safety factors, redundant load sharing, blah blah blah. I have had my share of probability analysis and this issue is no different but I don't believe this has much of a place here. How many of you, when running the calculations of a joist, would look at the result "125% OVERSTRESS" and say, "yeah, I'm good with that" Knowing it applies to 90% or more of the members in the roof and move on.

To me this strikes at the heart of Life Safety. As for my part I know that I will be looking at truss drawings more carefully and I will be changing my truss specifications to use MSR only. As far as larger Lumber in SYP is concerned, I don't know. There are many decks with my name on them. The wording from the news releases implies changes may be forthcoming or are under review. I will have to wait and see. I want to believe the larger lumber sizes will be less affected due to what I believe the source of the problem: sourcing of smaller trees, utilizing juvenile and peeler core wood.

At the seminar someone mentioned that this entire issue came about because of an actual truss failure where they started to look at the strength values of the failed pieces. I have been unable to locate a source for this but I would be interested to see if someone else can shed light on the topic.

Other info

MAP
 
Currently it is specifically Southern Pine 2x4 #2 and lower grades. Higher grades are not changed. The other species are starting testing programs to see if they also need adjusted.

While the excuse of juvenile wood is being used, it hasn't been proven that is the cause.
 
I don't think the point of this discussion is to claim the sky is falling. But rather have a peer discussion. What are you going to do and did you know about this? We all assume risk here and it is part of the profession. We do our best to protect and give value to our clients and ourselves through experience and knowledge. We are human and have limits to both of these things.

I do most of my work in wood design and like to think that I stay up on things current. I knew something was going on since the end of last year but direct implication was not on the radar for me personally and wasn't really paying attention to it. The board or review meeting minutes of the ALSC are worded rather strongly and it appears that the magnitude of the value change took them by surprise as well as SPIB. Also interesting to note is the issue of timing and how to deal with that. Of ethical interest is the concern that a truss company may in fact try to push the market with getting it done before June 1 in order to save $$ for a project. As an EOR for a project would you let it go because the "official" value is still OK or would you impose the new values on the truss designer? Certain values are not "close enough" in my book and some are.

MiketheEngineer, can you honestly say that in the last 40 years you saw a strength reduction of 30%? Can you cite them? If this was steel or concrete would you be the same ho-hum? It appears that God didn't change anything but rather a lack of a feedback loop in the control system. The visual grade rules are still standing and it appears were all being done correctly. The problem stemmed from not doing much testing of the actual material and more to the point not having a trigger mechanism or feedback loop in place that flagged things being out of whack. That is why MSR lumber is not affected here. The other thing is that only the 2x4 were tested and are adjusted so the affected designs are mostly MPC trusses.

MAP
 
It seems like it would be a big deal for some people in the wood industry, especially designers in the wood truss industry. However for most SE's working as the EOR I think as long as they are aware it is not a big problem. You could specify all truss members to be MSR and save everyone the headache. Its just when the lumber is not specified who and a project is out to bid - who is going to use what values?

EIT
 
Yeah - probably about that (30%) Back in 1971 - values were appreciably higher and I KNOW GOD has nothing to do with it. But why does it take so long for us (the rule makers) to make up their minds???

Is it new growth wood - maybe - but we have been "harvesting" trees much as we harvest corn for at least the last 50 years. Weyerhauser et al plant WAY many more tress every year than we cut down!!

And we MUST use good values to protect the safety and welfare of the public - just quit moving the target!!
 
It could be just a factor of profit and/or better use of the lumber. As the in-grade values are adjusted by random testing of the lumber and the visual grading is based of what the graders are picking out for each grade. As the lumber companies look for more profit and/or selling what the customers want, they may be requiring the higher grades to be pick more often. These probably still meet the grading requirements, but are overall a lesser quality than what was picked for each grade thirty years ago. To really check this, a close look at the quality of the lumber used to test the grades thirty years ago would need to be look at. There were several attempts to look at (mainly in the wood truss industry that I know of to get better chord lumber) regrading lumber or to MSRing visual grades to get more of the higher quality lumber to use.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
 
Aren't design values selected as the 1/5 percentile values of the bell curve? That alone provides quite the buffer. SPIB's report on the adjusted design values gives good insight and provides much reason to suspect that not much is to worry about.

Personally, I'd be sweating more bullets if there were a report indicating that structural steel and high-strength bolts materials were compromised by 30%.

 
Just read this one, as I've been off the net for awhile, BUT the artical title is: "New Design Values for VISUALLY Graded Southern Pine Dimension". Isn't a lot of the lumber MSR rated now and wouldn't be affected by this? Don't truss companies use mostly MSR rated lumber?
 
From the Truss people at the seminar I attended it sounds like most still use visually graded lumber as it is still less expensive. However this may influence the use of more MSR lumber. I don't see much if any MSR lumber in the midwest, but then again we don't see a lot of Southern pine either.

EIT
 
I know of one company that has gone to DSS or MSR lumber for a majority of their trusses. They were spending way too much time culling and scrapping the other stuff. Paid off in many ways. No structural problems, no call backs, easier to setup, better replication, easier to install, etc, etc
 
Status
Not open for further replies.

Similar threads

dgengineering
  • Locked
  • Question
Different R Values
Replies
5
Views
6
Rsd789
Rsd789
dgengineering
  • Locked
  • Question
Collector Element Design 2
Replies
9
Views
21
alt0
alt0
Uralsk
  • Locked
  • Question
Feasibility of a Roll bender made out of very thick pine wood dowel?
Replies
1
Views
12
phamENG
phamENG
dgengineering
  • Locked
  • Question
Shear Wall Design for Wind 1
Replies
4
Views
25
lexpatrie
lexpatrie
Uralsk
  • Locked
  • Question
calculating the required force to break a single rod of a pine wooden dowel? 1
Replies
10
Views
39
3DDave
3DDave
Back
Top