can you build on wood

[johnny2007]

I have seen joists resting on a line of bricks or blocks and then incorporated into the wall to go higher and never liked the idea, but to put a plate to rest them on is truly mad.
Take it out is my advice and build as standard.
BTW, don't you have BC plans for this project?
It should be detailed there.

 

[^woody^]

Mortar has a tensile strength of around 2.5MPa

I'm not sure what units you are referring to, but either way a Crawfords would be 3 if mortar is 2.5. Pre-dunking of course

And it would also depend on the mix ratio,plus additives.

 

[^woody^]

Brickwork is usually most efficient when subject to compression,eg vertical gravity loads, but far less reliable in tension.

Usually tension is only an issue in tall, thin brickwork panels subject to horizontal wind load. The panel will bow slightly, causing
some tensile stress in the mortar joints in the leeward side of the panel. How much you can allow for this depends mainly on the
water-absorption properties of the brick, low-absorption bricks giving slightly higher values of tensile resistance in the mortar than
very absorbant bricks.

In most traditional brick low-rise domestic buildings, the tensile strength of mortar is of no significance.

There is an argument that mortar in a wall will never (or at most almost never) be put in tension (not even in your bowing scenario), and that's why in all the specifications tensile strength is never referred to.

Either way it is the actual tensile bond strength that matters (ie the mortar and the brick/block) and not just the mortar.

 

[IT Minion]

WTF? A leaning wall or tower of bricks is held together by mortar due to adhesion and friction, not tensile strength.

Mortar has less tensile strength than a Crawford's wafer of the same thickness.

Isn't adhesion just another term for the tensile force applied by the mortar?

Adhesive strength is measured, first, by the tensile or pull-off adhesion strength of a test piece bonded with an adhesive to a substrate with set characteristics.

So should we now start quoting mortar mix on a biscuit scale?

 

[DIYhard]

I'm not sure what units you are referring to, but either way a Crawfords would be 3 if mortar is 2.5

mikeey84 said "Mortar has a tensile strength of around 2.5MPa according to ..... " What is the problem with those units? The same ones (MPa) are very commonly used nowadays to specify concrete. For comparison, a 1:1:2 concrete mix could have compressive strength of about 25MPa. Your Crawfords wafers must be pretty hard on the teeth.

Whenever I have taken down walls, bricks have usually parted with either a full 10mm of mortar or else with no mortar. Rarely, if ever, has the old mortar split to leave 5mm thickness on each brick. Clearly the tensile strength across the mortar bed has been substantially greater than the adhesion between mortar and brick.

CEMEX documentation says:
"Bond strength is required to withstand tensile forces due to wind, structural and other applied forces, movement of the masonry units and temperature changes."

Do our experts on this forum consider that CEMEX have got is all wrong?

 

[^woody^]

So should we now start quoting mortar mix on a biscuit scale

I think we should.

 

[tony1851]

DIYhard's point is correct, in that 'tensile strength of mortar' is meaningless.
It is bond strength that is important and this depends not only on the constituents of the mortar but also workmanship, and water-absorption properties of the bricks themselves. For a given mix, an engineering brick will give a higher bond strength than an old handmade stock brick.

 

[mikeey84]

I'm not sure what units you are referring to, but either way a Crawfords would be 3 if mortar is 2.5. Pre-dunking of course

And it would also depend on the mix ratio,plus additives.

MPa are Mega Pascal's, the unit of pressure.

Its defined as 1 Newton per square meter, so 2.5MPa is 2500 Newton's per square meter.

To be fair, it's a bit rubbish as far as tensile strength goes, being worse than almost all wood, metal and plastics, but it's disingenuous to say that it doesn't contribute anything.

Surely if there was no tensile strength in mortar, leaning against the top course of a wall would cause the bricks or blocks to move.

 

[tony1851]

MPa are Mega Pascal's, the unit of pressure.

Its defined as 1 Newton per square meter, so 2.5MPa is 2500 Newton's per square meter.

To be fair, it's a bit rubbish as far as tensile strength goes, being worse than almost all wood, metal and plastics, but it's disingenuous to say that it doesn't contribute anything.

Surely if there was no tensile strength in mortar, leaning against the top course of a wall would cause the bricks or blocks to move.

I think the unit MPa is equivalent to N/sq mm, so 2.5 MPa would just be 2.5 N/sq mm.

Although engineers tend not to put any faith in the tension capacity of the mortar/brick interface, in the real world it does contribute to a walls 'strength' to a small degree.

The same applies to friction; structural engineers say friction is never there when you want it, while mechanical engineers say friction is always there when you don't want it.

 

[tony1851]

To go back to the OPs issue with the timber built into the wall:

It is not normal practice for reasons which others have pointed out. There also appears to be no published data on the tensile strength - if any - of a timber-to-masonry bed joint, probably because it is never done in this way.

However, in the real world, it is unlikely to cause the OP much problem (other issues such as damp/insect attack etc being considered) because of the overall lateral rigidity of the extension. Demanding that the builder removes the timber and replaces it with blockwork
may mean that the wall becomes slightly less stable, if only because it would be a difficult job to do properly.
And would it help the client/builder relationship, which is important for the finished job?

 

[mikeey84]

I think the unit MPa is equivalent to N/sq mm, so 2.5 MPa would just be 2.5 N/sq mm.

I think, as I'm no engineer, that we are both correct, and they are 2 ways of expressing the same thing.

 

[tony1851]

In your original post, you said that 2.5MPa is 2500 Newtons per square meter. I think you will find that it is 2,500,000 Newtons per square meter - usually simplified to 2,500kN/sq.m.
In most structural calcs by convention, the units N/sq mm are used instead of MPa, though of course they are interchangeable.
Larger values are usually expressed in kN/sq.m.

 

[mikeey84]

In your original post, you said that 2.5MPa is 2500 Newtons per square meter. I think you will find that it is 2,500,000 Newtons per square meter - usually simplified to 2,500kN/sq.m.
In most structural calcs by convention, the units N/sq mm are used instead of MPa, though of course they are interchangeable.
Larger values are usually expressed in kN/sq.m.

Sorry, yes, got my Ms and Ks mixed up!

 

[tony1851]

Sorry, yes, got my Ms and Ks mixed up!

Easily done - I do it all the time, adding or missing off zeros.
(Now I realize why my beams keep falling down)

 

[John D v2.0]

This is where a sanity check come in handy, or a back calculation from the result back to the inputs.
Someone could even convert the units to Imperial and redo the calculations there if they were that way inclined!
Sometimes the sanity check is easy/obvious but often cognitive bias can get in the way.