I think E' is supposed to be used. I'll Check when I get into the office, but I know that moisture content affects the stiffness and if you use E there is no way to account for that.
Asixth-
Do you typically account for shear deformations in wood beams/joists? I don't unless it's a very short, deep beam.
I am putting together a spreadsheet that enables shear deformations to be calculated along with flexural deformations. The timber code I use which is AS1720 (Australian) gives a characteristic modulus of Elasticity and says 'The average modulus of elasticity includes and allowance of about 5% for shear deformation.' I generally take this to mean that the designer is to multiply the short-term deflection by 1.05 and by doing so shear deformations have been accounted for.
Even if the shear deformations only account for 2% of the overall deflection, I still think it should be considered during the calculations.
There is also the statistical aspect which comes with estimating the E-modulus of a particular wood sample.
I agree with apsix, that the wording you used means that the E value has been adjusted to account for the shear deformations so that you don't need to do anything to account for it.
What does Clause 2.1.3 say? Does that give a method to calc Ebar or is Ebar what is given in the table? It looks like E is given in the table and 2.1.3 gives a formula for Ebar to account for shear deformations.
This is just my opinion, but the shear deformations for the typical span/depth ratios associated with wood construction would easily be less than the statistical differences between the actual E and the prescribed E. Even for a shear deflection that is 4% of the flexural deflection, you're talking about L/360 for LL compared to L/346. That's not a big deal, IMO, and if E is 4% higher than listed than the whole thing goes away.
Clause 2.1.3 just says to use the section property, average E and various modification factors when determining member stiffness. It doesn't mention shear deformation.
I think Ebar is a typo, it should be just E; both are defined as 'average modulus of elasticity'.
Paraphrasing the note from Table 2.4; 'E includes an allowance for shear deformation' and from Table H2.1; 'E contains the effects of shear'.
They basically say the same thing; shear deformation is already accounted for.
How is E' or Ebar defined in your code? I cannot see how it could take into account shearing deformation except in a very approximate way.
Deflection of a wood beam is affected by service conditions (wet or dry), duration of load (creep effects) and treatment factor (preservative treated incised lumber). Shear deformation is not normally considered.
Only E is defined, E' & Ebar are not used, except that E bar just appears in the notes of Table 2.4, as discussed above. As you say, shear deformation is allowed for in an approximate way.
There are also the factors to account for moisture content and load duration.
Clause 2.1.3 Member Stiffness: In determining member stiffness the design rigidity is the product of the relevant section property, the average modulus of elasticity and various modification factors denoted by 'j' that account for service conditions as given in Clause 2.4.1.2.
Clause 2.4.1.2 goes on to give modification factors that increase the deflection of the wood from creep for sustained loads. There are no modifications to the E-modulus that account for items which BAretired mentioned. The modification factors other than a load duration-creep factor are only accounted for in strength and joint design. I assume this differs to the NDS approach to E-modulus which the original post was questioning (E' opposed to E).
I will accept that shear deformations of wood is accounted for in the average E-modulus.
I was back-checking deflection calculations today of a widely used timber design program in Australia and was finding it to underestimate the deflection of common studs. Along with ignoring internal pressures and local pressure co-efficients IMO.
Shearing deformation does not follow the same curve as bending deformation. For a simple span beam with uniform load, shear is maximum at the supports and zero at the center whereas bending is zero at the supports and maximum at the center.
To account for shearing deformation by merely changing the E value would imply that shearing deformation is some constant fraction of bending deformation, which is not the case.
If the modified E value is intended to take into account shearing deformation, it must do so in a very approximate way. I do not know how the E' or Ebar value is defined, but I doubt that it is intended to take into account shearing deformation.
BAretired...I think you are correct about shear deformation being intrinsically different - and I would agree with your quoted statement above that with wood...it's all very approximate. Keep in mind the wood design values are generally developed through visual grading rules....a very approximate thing indeed.
Apologies for further hijacking the thread, but in response to the comments about the statistical variance of E - AS1720 has a recommendation stashed away in the notes of appendix B (note 3) that recommends adopting a lower 5th percentile value which corresponds to 0.5 E (hardwoods) for deflection sensitive elements. First time i have come across it...
