Modulus of Rupture (MOR) vs Bending Stress (F'b) 3

[Bld-Eng]

Hello,

Just hoping for clarity on Modulus of Rupture (MOR) vs. Bending Stress (F'b) as per Australian Codes.

I'm looking to hand calculate the bending strength of a 90 x 19mm Merbau decking board with joists 500cts apart.

By my calculations, to support a 1.8 kN point load, I need the decking board to have a minimum bending stress (F'b) of 54.8 Mpa.

Some examples of F'b available in AS1720.1 & Hyne software are:

- 90x35 KD-HWD-F17 (kiln dried) - F'b = 42 Mpa
- 100x25 F14 HWD (unseasoned) - F'b = 36 Mpa
- Wood Solutions list - Merbau - MOR = 147 Mpa (seems a high number)

Questions

1. Is MOR only used when the timber is in pure tension only, or

2. If MOR is another name for bending stress (F'b), why do the hardwood decking boards have such a high Mpa?

 

[bugbus]

MOR is the same as bending strength as far as I'm aware. Uniaxial tensile strength is different (and smaller).

I'm not sure about wood, but at least for concrete, my understanding is that MOR is always greater than uniaxial tensile strength because of the softening part of the stress-strain curve in tension. (I suspect it is the same reason in wood).

When the extreme tensile fibres reach the tensile strength of the material, there is still additional bending capacity in the member. The extreme tensile fibres see a gradual reduction in stress and the peak tensile stress shifts further towards the neutral axis. The MOR is the 'apparent' flexural tensile strength at the extreme fibres, even though the stress variation is no longer linear at the point of failure, which is why it appears to be greater than the uniaxial tensile strength.

Truly brittle materials like glass don't do this, they will fracture as soon as the extreme tensile fibres reach the tensile strength of the material.

 

[Bld-Eng]

Thank you bugbus

Another comment I received from a colleague is that the MOR would be tested on a piece of timber that is clear of defects (like knots, crossgrain etc) that also contributes to the MOR being much higher than the bending stress.

 

[Euler07]

Just some thoughts:
- Is the MOR a characteristic value (i.e. weakest 5% value)? This will need to be confirmed and I would be interested if you can post your answer from your research. Timber has a very flat normal curve which make characteristic values far lower than mean values. This is why sawn lumber has a far lower strength than LVL even though the average strengths may be similar (see the image below). For decking I would not be surprised if a mean value is used instead of a characteristic value.

- As far as I remember, Australia bending strength is using a 1 point load test instead of a 2 point load test, whereas other regions (Europe) may use 2 point. (Based off memory only and you will need to confirm). I'm not sure if this affects whether to use bending stess vs MOR as a 1 point test also contains shear stress in which case bending strength may not necessarily be a correct term. Also, decking may fail more as a result of bending and shear combined in which case MOR is a more suitable term?

 

[Smoulder]

MOR is on clear wood, so no defects. Not suitable for design. Also probably average not low end, so not suitable for design.