Wood Design - Deflection

[AlienMonkey]

In calculating deflection of a beam, is the adjusted Elasticity of the member, E', used?

 

[BAretired]

OzEng80,

I think you are misinterpreting the value of E in the reference you cited. I suspect that E_0.05 in the Australian Code corresponds to E₀₅ in the Canadian Code. It is the Modulus of Elasticity for design of compression members and is substantially less than E used for deflection calculations of flexural members.

BA

 

[OzEng80]

BA

Unfortunately the E value cited relates to flexural members – I have attached the rest of the Appendix for your consideration. Note that the appendix is only an informative ‘guideline’ which (it is my understanding) the industry ignores – first time I have come across it.
Given that this has become pretty much an AS1720 thread and the magnitude of the reduction I thought it would be worthwhile throwing some more wood on the fire...

 

[TERIO]

This seems to be a good reference on wood.

"Wood Handbook - Wood as an Engineering Material"

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

Chapter 4 on mechanical properties also suggests that E measured from bending tests includes the effect of shear deformation and can/should be adjusted to correct for this. I think the point is that say for instance if you measured using a simply supported beam with a point load in the center you would have something like

w=PL^3/48EI -> E=PL^3/48wI

The deflection w at load P inherently includes some deflection due to shear and will make E too small.

 

[BAretired]

OzEng80,

E_0.05 in your code is the lower fifth percentile estimate of Modulus of Elasticity. It varies from 0.5E to 0.85E, depending on the grade of lumber used.

It should be used in calculating an upper bound on deflection where clearances must be maintained, such as doors or windows jamming, but it is not used for routine deflection calculations.

BA

 

[asixth]

Is this common for wood design to look at upper bound solutions for deflection sensitive designs?

I am thinking of the application of this principle to other building materials such as concrete. I am unsure on ACI209 or Eurocode creep models but AS3600 (Australian Concrete Code) has a clause stating that the actual creep models may vary by +/-20%. Would a similar principle of calculating an upper bound solution be applicable to deflection sensitive concrete designs such as a slab supporting brittle finishes or a floor which is housing sensitive equipment?

Of course this application to concrete design is only a thought. I am in the process of writing a complete timber design spreadsheet to AS1720 and will be included the above modifications based on service sensitivity.