Structural Engineering: RE: Cross-grain Compressive Strain in Timber

The industry published values for allowable compression perpendicular to
grain stress are based on test with a 2x2x6 in. test specimen with a 2
in. load head placed in the center on one side and a 6 inch bearing on
the other. This doesn't give a true strain in the traditional sense.
With that said, however, the currently published values are based on a
deformation of 0.04 inches (approximately 2% strain). Values produced
in this manner are typically above the proportional limit for
compression perpendicular to the grain. Older values were based on a
deformation limit of 0.02 inches (approximately 1% strain). The older
values were close to the proportional limit stress. This value can be
estimated as 73% of the current design stress.

Jeff Linville
AITC

-----Original Message-----
From: Thor Tandy [mailto:vicpeng@telus.net]
Sent: Monday, September 14, 2009 9:54 AM
To: seaint@seaint.org
Subject: RE: Cross-grain Compressive Strain in Timber

Hi Jeff.

I was doing an exercise over the weekend on base plates that are
connected
to timber elements. (Yeah, I'm still looking for my life) E.g. guard
rail
side-mounted onto a deck edge beam. This is a typical base-plate
problem
but the material is timber instead of concrete.

I used first principals to determine a pressure distribution across the
plate (in this case assumed very stiff but that might not be a problem
on
timber), looked at using concrete section concepts and then at strain
compatibility methods. For concrete, there are factors that are not
applicable to timber and the yield behaviour of timber in bearing is
different from concrete. That said, by (wise??) juggling of the factors
etc
I arrived at section properties in all 3 methods that produced anchor
requirements and restraining loads within 10% of each other. I'm not
claiming the results are correct. Timber in bearing squashes the fibre
(X-grain) and as deformation occurs the sides of the plate are also
supported by shear across the fibers. So the pressure bulb has to be
different from concrete. But it's probably simpler than concrete being
either triangular or possibly reverse parabola (I'm just guessing). The
deformation is very MC, and species, dependent but I figure there had to
be
an average equivalent strain modulus that I could use to do the
calculations. From Borg Madsen's book on timber testing and behaviour I
extracted a strain of about .02 at a stress of about 2.5MPa.

So I was seeking an opinion on what strain to use to determine the
stress
distribution across the plate-timber section.

Thanks

Thor

-----Original Message-----
From: Jeff Linville [mailto:linville@aitc-glulam.org]
Sent: Monday, September 14, 2009 8:06 AM
To: seaint@seaint.org
Subject: RE: Cross-grain Compressive Strain in Timber

I might be able to help you with a number, but I am not clear on what
the number means. How are your example numbers determined?

I'm also not clear on the application.

What species of wood are you talking about?

Jeffrey D. Linville
Director, Technical Services
American Institute of Timber Construction
(303)792-9559
linville@aitc-glulam.org

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-----Original Message-----
From: Thor Tandy [mailto:vicpeng@telus.net]
Sent: Friday, September 11, 2009 6:36 PM
To: SEAInt
Subject: Cross-grain Compressive Strain in Timber

I'm doing some casual analysis of metal support plates bolted to the
side of, say, a built-up 2x10 beam. If I try to use, eg, concrete
section analogy, I need to know a strain modulus for the timber (yeah, I
know timber doesn't behave like concrete). What cross grain modulus
might I use?

Eg: conc = .0035, masonry = .003, steel = .002 I could see a virtual SE
= .007? I know that ASTM tests to develop bearing stresses and lengths
invoke the concept of strain = del L/L and I also am aware that the
value must be very variable espec with MC.

Again, if I use strain compatibility analysis, I have to know what value
to use for the strain modulus .

Thanks

Thor A. Tandy P.Eng, C.Eng, Struct.Eng, MIStructE Victoria, BC
Tel: (250) 382-9115

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