huangyhg

shear modulus for wood? inexistent?
should i consider shear modulus for wood in design? or is it inexistent?
thanksa lot
if you mean "horizontal shear", where the layers of wood slide over one another at a bearing point, then yes, you should consider the published values. this type of failure will always occur before a tearing across the grain itself, for which there are usually no published values, at least in us practice.
in the australian code the values given for e, the elastic modulus, include an allowance for shear deformation. if you only want it to calculate deflections that should be enough.
what do you want to know for?
shear modulus of elasticity, g, exists for all materials. it is calculated as:
g = e/(2+2v)
where:
e = modulus of elasticity
v = poisson's ratio
however, i have not had occasion to use it in wood design.
i assume you're trying to check deflections. i've never had occasion to find shear deflections of wood
i'm pretty sure that the 'encyclopedia of wood' by stirling publications has shear modulii for various species; will check my book tomorrow and let you know.
the shear modulus g in timber design in the uk (to bs:5268) is taken to be e/16.
the central deflection due to shear on a beam from a udl is calcualted using
wl*2/(8ag) then multiplying by a form factor, c (1.2 for a rectangular section) this is then added to the deflection due to bending.
my references agree with taro
however i would be interested if anyone knows how this equates to e/16 stated in b.s. 5268 (part 2 clause 2.7)
alan
don't know how the following table will format(?)
species glr/el glt/el grt/el
balsa 0.054 0.037 0.005
yellow 0.074 0.068 0.017
birch
douglas 0.064 0.078 0.007
fir
spruce 0.064 0.061 0.003
sitka
sweetgum 0.089 0.061 0.021
walnut 0.085 0.062 0.021
black
poplar 0.075 0.069 0.011
yellow
from table 4.1 in encyclopedia of wood
as you can see, there's quite a mix. 'l' is short for longitudinal, 'r' for radial and 't' for tangential glr is the modulus for the 'longitudinal-radial' plane. if you need the tabular information, i can scan it and email it to you.
taro:
wood, being orthotropic, has 6 values for poissons ratio. doug fir for example has values 0.29, 0.45, 0.39, 0.37, 0.04 and 0.03 for ulr, ult, urt, utr, url and utl, respectively.

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