News from VT

[Don P]

I got an email yesterday from Dr Woeste of VA Tech. He sent several pdf's and some news.

One that might be of interest is details for balcony design with dimensional lumber and wood I joists. Some of the detailing and thinking applies to any balcony though.

Next is an article he co-authored on lumber grading.

There was an APA update on shear decreases and remedies related to overdriven fasteners in sheathing, a chronic problem. I can post it if anyone is interested but the easy thing to remember is DON'T OVERDRIVE Fasteners in Sheathing! Adjust your gun to drive sheathing nails flush, it's ok to have to whack a few home with your hammer rather than sinking the majority too deep.

There is a new version of the Supplement to the NDS, Design Values for Wood Construction that was published in March. There have been some changes and I have not been through the new document but will update the calcs as I have time. After a quick look at the addendum I do have some issues with the direction they are going with some of the species combinations and complexity they are putting onto designers and inspections. I've sent my concerns to the ALSC in the past and wrote to Dr Woeste last night. On more than one job I've had multiple countries of origin for the same species. While helping another contractor get dried in not long ago I counted 5 countries of origin for #2 NSPR/SCP, each with a different set of design values. The contractor was unaware that he was not using SPF as the plans called for much less that he had that many different (weaker) substitutions going on. I've had as many as 9 countries of origin on one of my jobs. Designers, builders and inspectors are completely unaware of what they are working with. Not a good situation. We agreed. Hopefully the powers that be will listen and simplify things in the next update. One of my comments that was forwarded is this.

Grading and gradestamps are kind of irrelevant if it doesn't key the person back to a simply understood relatively small and straightforward set of load and span tables.

 For us I'll double check that our domestics remained unchanged. One thing that has changed on that front is SPF and SPF(s) have a new combined grade stamp, check your stamps and if you see that combination stamp be aware that it carries new design values.

[Don P]

Dr Woeste sent this link this morning, a related article that was published in JLC yesterday;
Special Report: Import Lumber Grades | JLC Online

[cib]

Good information Don thanks.

[Don P]

I've been thinking, and writing  :
I may send this in to JLC but I'm curious if anyone can follow my train of thought. Let me know what you think.

Back in "The Good Old Days" every sawmill produced lumber and in attempting to cater to their buyers needs, sorted into some form of top, middle and lower grades. This worked well enough as long as everyone knew what each other was talking about. But how could a far away designer know that the clients sawmill would produce lumber of the strength his plans called for? The sawmill near him might be selling a much stronger grade of framing lumber than the mill his client was talking to. That was the beginning of the call for some form of uniform lumber grading standards. In 1924 the American Lumber Standard was developed. Since that time Canada and the US have operated jointly under the oversight and grading rules of the National Lumber Grades Authority and the American Lumber Standards Committee. 

Base design values, the 8 strength, stiffness and density properties engineers use, are tied to strength limiting defects and species. A #2 piece of lumber of 2 different species may have the same limiting defects but because of the differing strength properties unique to each species they will be assigned different design values. This is why the various span tables have different allowable spans for the same grade of different species of lumber. For instance a 2x12 floor joist in #2 Dougfir-Larch can span 18'1" where the same 2x12 in #2 SYP is limited to a 16'6" span.

The grading rules also allow grouping some species of similar range and strength together and limit the design values based on the weakest species in that group.

Some species or species combinations carry different design values based on where they are grown or the limiting species contained in the group. Northern SPF (Spruce Pine Fir) carries higher design values than SPF(s) (South).

When European lumber began to be imported, country of origin led to many more different design values for each of the imported species. For instance #2 Norway Spruce in the current tables of design values now has 6 different sets of design values based on country of origin if stamped as a stand alone species. If grouped into various species combinations that adds 28 more possible sets of design values, for one grade. You read that right, a stamp containing some form of "#2 NSpr" has 34 possible sets of design strength values. The variability is considerable Fb 575 E1.1 in combinations to Fb 875 E1.3 in single species, based on species combination and country of origin. I have just picked one example species, there are several. The well understood and widely distributed species combination "SPF" of Canadian origin now may also be easily confused with a grade stamp SPF/SPF(s) containing lumber that originated in Canada, US, Europe or South America. The basic design properties range from Fb 875 E1.4 to Fb 575 E1.1. If the designer is working from an optimistic viewpoint and the supplier delivers from the lower end of the spectrum there is potentially a serious strength or serviceability problem. Using the 2x12 floor joist example from above and the design values just mentioned the maximum allowable spans would be 17'10" and 14'4" respectively. Misinterpreting those easily confused grade stamps could be a real problem.

Remember where all this began. There was a need for the designer of a building to know that the builders used wood of the same or better strength than what was being called for in the design. I can think of a couple of examples where I wandered around looking at the stamps. One house contained lumber from 9 different countries, another contained lumber from 5. The suppliers do not know which countries their lumber is going to come from, thus, the builder does not know what strength lumber is going to arrive on the job. In other words, we've lost sight of the original goal and are back where all this began. This is not working.

There are a couple of ways of addressing the problem. The first is to design very conservatively, use the lowest possible design properties that are listed. If imported wood might be used on a job, scan the tables and use the lowest design values. You'll never be wrong but will be limiting spans and driving up costs.

 Another solution: the designer can specify the minimum design values required rather than a species or species combination, grade, and country of origin. Lumber would need to be stamped with a species and grade but also with the design values, similar to the stamp on LVL lumber. Then the builder would order those design values from the supplier.  The supplier simply sends out stock that is stamped with those design values or better.

If the builder doesn't care for the working characteristics of a particular species and the supplier says he has what is needed in several different species, the builder can request the order be filled with the species of preference. Either meets the strength requirement, but each has unique working characteristics as well. This is the reason to retain the species stamp.