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New Wood Allowable Shears

New Wood Allowable Shears

New Wood Allowable Shears

(OP)
It appears that the 2005 NDS allows shear values in Glulams up to 50% more than previous.

My question is this:  Is there any written guidance relating as to whether this is only applicable to new glulam beams, or is applicable to all glulams, say those made in the 60's, which would greatly affect remodels...  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: New Wood Allowable Shears

Mike:

My guess would be that you can't use the new shear allowables on old lumber or gluelam beams.  I think this is all tangled up in new lumber grading rules and new allowable stresses which came with that.  I think this all started back in about 95 - 99, with a major industry wide lumber testing vs. grading vs. actual failure stresses, 70 new adjustment factors, etc.  The testing program actually had a name and influenced the stress values in the NDS supplement from about 2001 and on.  I think the program was called the In-Grade testing program or some such.  Read the introduction to the supplement and some of the commentaries on the NDS changes from that era, they may give you some clues.

I've had some of the same questions as I compare the older NDS versions which I was accustom to designing with and the newer versions of the NDS, about 2001 and forward.  I was kind of out of the building design business, on a regular basis, for a number of years during that same period of time, so I wasn't paying close attention to all the changes and where they came from and their justifications.

RE: New Wood Allowable Shears

Mike:

My dates were wrong.  The Forest Products Lab in Madison, WI started the In-Grade Testing Program in about 1978.  The objective of the program was to come up with more reliable design values for existing grades of commercially available lumber, and to adjust grading rules to meet currently available lumber.  They were testing full size dimensional lumber members which had already been graded, not just small clear samples.  And, in the early 90's the new allowable properties started showing up in the supplement.  And, of course, they added reams of statistical  complexity to the whole process, so then they needed more size factors, and other adjustment factors, etc. etc.  Look for a good write-up in Wood Engineering text on wood properties and grading, that should give you some direction and understanding.  You might also look at the history of several of the ASTM standards on lumber grading and allowable lumber properties.

It makes me wonder how some of my older building are still standing, given how much I didn't know about the need for a 1.03 size adjustment factor and what I still don't know or understand about how much better the newer versions of the codes are for us all.  If only we would get paid for all the time we waste on this crap.  But then, I've never seen a building failure attributed to someone forgetting to apply a .96 or 1.09 adjustment factor; and have seen plenty of problems caused by stupid detailing or just not paying attention to how a structure really act and reacts to the way it was framed or detailed.  

RE: New Wood Allowable Shears

I know nothing about your NDS, but just a thought...have they redefined shear stress as V/bd rather than 3V/2bd?  Probably not, but that is 50% difference.

RE: New Wood Allowable Shears

(OP)
I thought about that, but I don't think so Hokie.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: New Wood Allowable Shears

Below is an link to an AWC article which explains the rationale behind the changes:

http://www.awc.org/publications/papers/WDF14-4-2005DesignValues.pdf

I'm not sure that it helps to explain things all that much though. According to the article it should be more like a 10% change in allowable shear for Glu-Lams.   

RE: New Wood Allowable Shears

(OP)
Well, just saw a set of calcs using the NDS values - 240 psi used where it used to be 165 psi for a 24FV4.  

That's one h*(( of a lot more than 10%!

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: New Wood Allowable Shears

I seem to remember a shift in methodology for calculating shear stress for ASD wood. I am having trouble finding it my bookcase, but the older method used an allowable stress of 87 psi x 2 whereas the current methods allow 175.

Can't seem to find the Design reference for the old, as the 2001 and 2005 NDS go with the 175 psi base value for my standby, Southern Yellow Pine.

This is of course for dimensional lumber, but I wonder if there was a similar swap in the GluLams?

What design spec are you comparing the durrent info to?


Thanks,

Daniel

RE: New Wood Allowable Shears

hokie66,

3V/2bd is the horizontal shearing stress at d/2. Why would this change?

RE: New Wood Allowable Shears

It wouldn't change.  I was just wondering if the academic boffins had tinkered with the design equations.  They have a habit of moving things from one side of the equation to the other.

RE: New Wood Allowable Shears

Hokie:
Do you always misspell buffoons, or did you just do that for the sheer fun of it?
 

RE: New Wood Allowable Shears

Didn't think of it, but I like your word better.

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