I just calculated something and it looks wrong.

I measured the deflection of a simply supported timber beam 1.2 metres long under a central point load of 20 kg and got 2 cm deflection.

I measured the failure load when the same beam was 1.16 metres long and got 85 kg.

The timber beam has depth d = 14.5 mm and width b = 63.mm.

What is E (elastic modulus from deflection) and Fb (failure stress in bending)?

The formulas I used are E = P * L^3 / (4 * b * d^3 * delta)

and Fb = 3 * P * L / (2 * b * d^2)

mollwollfumble said:

I just calculated something and it looks wrong.

I measured the deflection of a simply supported timber beam 1.2 metres long under a central point load of 20 kg and got 2 cm deflection.

I measured the failure load when the same beam was 1.16 metres long and got 85 kg.

The timber beam has depth d = 14.5 mm and width b = 63.mm.

What is E (elastic modulus from deflection) and Fb (failure stress in bending)?

The formulas I used are E = P * L^3 / (4 * b * d^3 * delta)

and Fb = 3 * P * L / (2 * b * d^2)

Formula for E looks OK.

Unit problem? Suggest using kN and m throughout for E in kPa.

The Rev Dodgson said:

mollwollfumble said:

I just calculated something and it looks wrong.

I measured the deflection of a simply supported timber beam 1.2 metres long under a central point load of 20 kg and got 2 cm deflection.

I measured the failure load when the same beam was 1.16 metres long and got 85 kg.

The timber beam has depth d = 14.5 mm and width b = 63.mm.

What is E (elastic modulus from deflection) and Fb (failure stress in bending)?

The formulas I used are E = P * L^3 / (4 * b * d^3 * delta)

and Fb = 3 * P * L / (2 * b * d^2)

Formula for E looks OK.

Unit problem? Suggest using kN and m throughout for E in kPa.

Can you calculate it out please?

I’m getting 23 GPa for E, 59 MPa for Fb, and 760 kg/m^3 for density.
All those seem exceptionally high for timber, especially for Fb where I expected in the range 6 to 15 MPa.

The job is where I need to replace old timber that has rotted with new timber, and need to know how much bigger the new timber has to be to retain the original strength.

mollwollfumble said:

The Rev Dodgson said:

mollwollfumble said:

I just calculated something and it looks wrong.

I measured the deflection of a simply supported timber beam 1.2 metres long under a central point load of 20 kg and got 2 cm deflection.

I measured the failure load when the same beam was 1.16 metres long and got 85 kg.

The timber beam has depth d = 14.5 mm and width b = 63.mm.

What is E (elastic modulus from deflection) and Fb (failure stress in bending)?

The formulas I used are E = P * L^3 / (4 * b * d^3 * delta)

and Fb = 3 * P * L / (2 * b * d^2)

Formula for E looks OK.

Unit problem? Suggest using kN and m throughout for E in kPa.

Can you calculate it out please?

I’m getting 23 GPa for E, 59 MPa for Fb, and 760 kg/m^3 for density.
All those seem exceptionally high for timber, especially for Fb where I expected in the range 6 to 15 MPa.

The job is where I need to replace old timber that has rotted with new timber, and need to know how much bigger the new timber has to be to retain the original strength.

I agree with your E, and get 110 MPa for the failure bending stress!

You might assume a plastic stress distribution at failure, but even that gives 73 MPa.

Sure all the input numbers are right?

The Rev Dodgson said:

mollwollfumble said:

Can you calculate it out please?

I’m getting 23 GPa for E, 59 MPa for Fb, and 760 kg/m^3 for density.
All those seem exceptionally high for timber, especially for Fb where I expected in the range 6 to 15 MPa.

The job is where I need to replace old timber that has rotted with new timber, and need to know how much bigger the new timber has to be to retain the original strength.

I agree with your E, and get 110 MPa for the failure bending stress!

You might assume a plastic stress distribution at failure, but even that gives 73 MPa.

Sure all the input numbers are right?

Re-measuring everything.

b should be 61 mm instead of 63 mm.
d should be 15.5 mm instead of 14.5 mm.
Loads are within +-15%.
Lengths are correct.

That cuts E down from 23 to 19 GPa. Even allowing 15% off at 16 GPa, that’s still a heck of a high grade timber. But a density of 760 kg/m^3 is consistent with an E near 14 GPa, so it’s approaching the right ball park.

So if I ignore the actual measured failure load and assume a failure at near 16 MPa, then I can work from that in deciding which species and size of timber to replace it with.

Merbau has a density near 830 kg/m^3. E near 15.4 GPa. Bending strength near 115 MPa. Hey, that’s close!

mollwollfumble said:

The Rev Dodgson said:

mollwollfumble said:

Can you calculate it out please?

I’m getting 23 GPa for E, 59 MPa for Fb, and 760 kg/m^3 for density.
All those seem exceptionally high for timber, especially for Fb where I expected in the range 6 to 15 MPa.

The job is where I need to replace old timber that has rotted with new timber, and need to know how much bigger the new timber has to be to retain the original strength.

I agree with your E, and get 110 MPa for the failure bending stress!

You might assume a plastic stress distribution at failure, but even that gives 73 MPa.

Sure all the input numbers are right?

Re-measuring everything.

b should be 61 mm instead of 63 mm.
d should be 15.5 mm instead of 14.5 mm.
Loads are within +-15%.
Lengths are correct.

That cuts E down from 23 to 19 GPa. Even allowing 15% off at 16 GPa, that’s still a heck of a high grade timber. But a density of 760 kg/m^3 is consistent with an E near 14 GPa, so it’s approaching the right ball park.

So if I ignore the actual measured failure load and assume a failure at near 16 MPa, then I can work from that in deciding which species and size of timber to replace it with.

Merbau has a density near 830 kg/m^3. E near 15.4 GPa. Bending strength near 115 MPa. Hey, that’s close!

Why not just look up some span tables for australian timber?

If timber is advertised as “finger jointed” does that reduce the strength much?

“SpecRite 69 × 15mm x 2.7m Finger Jointed Merbau Screening”
vs
“SpecRite 70 × 19mm x 5.7m KD FJ Merbau Decking”

The second is sure to be strong enough. I’m not sure about the first.

Checks web: “Finger-jointing provides a greater degree of stability than single, large dimension lumber pieces which can in certain circumstances be prone to distortion. Most finger jointed timber is of a high grade because defects are removed during the manufacturing process. Finger jointing is an excellent method of upgrading low grade material into valuable clear straight-grained timber.”, so that’s a positive.

JudgeMental said:

Why not just look up some span tables for australian timber?

Because it never occurred to me. Looking some up now. They seem to be for thicker timber.

mollwollfumble said:

JudgeMental said:

Why not just look up some span tables for australian timber?

Because it never occurred to me. Looking some up now. They seem to be for thicker timber.

you’ll also get joist/rafter spacing for particular applications. eg, joist spacing for pine decking is closer than for jarrah etc.

There is a fantastic wood database (including lots of engineering properties) called WoodPro, but I can’t seem to find an on-line copy.

There’s a few sites with basic details like this you might find useful.

I’m wondering if the yanks should have a postal survey on guns.

Peak Warming Man said:

I’m wondering if the yanks should have a postal survey on guns.

You’d need a fair bit of beam bending to lever that one over the wall.

Rule 303 said:

There is a fantastic wood database (including lots of engineering properties) called WoodPro, but I can’t seem to find an on-line copy.

There’s a few sites with basic details like this you might find useful.

High rule303.

I did the end grain tests. In summary, the nail pulled out at 10 kg force on the claw hammer. The screw bent without pulling out at 17 kg force.

From the link above. The wood database.

Merbau (Intsia bijuga)
Merbau (Intsia spp.)
Common Name(s): Merbau, Kwila, Ipil
Scientific Name: Intsia spp. (I. bijuga, I. palembanica)
Distribution: From East Africa to Southeast Asia and Australia;
(primarily New Guinea)
Tree Size: 130-200 ft (40-60 m) tall, 4-5 ft (1.2-1.5 m) trunk diameter
Average Dried Weight: 51 lbs/ft3 (815 kg/m3)
Specific Gravity (Basic, 12% MC): .68, .82
Janka Hardness: 1,840 lbf (7,620 N)
Modulus of Rupture: 21,060 lbf/in2 (145.2 MPa)
Elastic Modulus: 2,310,000 lbf/in2 (15.93 GPa)
Crushing Strength: 10,650 lbf/in2 (73.4 MPa)
Shrinkage: Radial: 2.9%, Tangential: 4.8%, Volumetric: 8.0%, T/R Ratio: 1.7

I should have checked modulus of rupture before.

Australian Pine.
Specific Gravity (Basic, 12% MC): .39, .48
Modulus of Rupture: 9,340 lbf/in2 (64.4 MPa)
Elastic Modulus: 1,568,000 lbf/in2 (10.81 GPa)

mollwollfumble said:

High rule303.

I did the end grain tests. In summary, the nail pulled out at 10 kg force on the claw hammer. The screw bent without pulling out at 17 kg force.

Aaahhh… Well I have designed a very poor test of the fixtures in use.

For the screw threads to cut the fibres will require loading and unloading cycles, and for the nail to achieve full skin friction will require time for the timber to grab hold of the nail properly.

Mmmmm… I’m sorry if I have wasted your time by over-simplifying the conditions.