Brickwork masonry

[Ronnie101]

Regarding the British Standard numbered 5628, i.e. the “Code of practice for the use of masonry - Part 1: Structural use of unreinforced Masonry”.

Section numbered 19.1.6 concerning “Walls constructed of wide bricks” states that: “When walls are constructed with bricks having a ratio of height to least horizontal dimension of less than 0.6, the value of masonry strength (fk) should be obtained from tests” (and not from tables contained within the Standard).

My perception of the matter is that: taking 2 bricks which are identical in strength, length, and height, but one brick is wider than the other, then masonry constructed using the wider brick would be stronger than masonry constructed with the thinner (non-wide) brick.

The mortar strength would be identical in each case.

I appreciate the concept where, if 2 bricks have identical length, width, and strength, but one brick is taller than the other. Then masonry constructed of the taller brick will be stronger than masonry constructed of the less-tall brick, due to the ratio of height to width.

Additionally, it seems to me that the wide-brick masonry would be stronger/more stable, because it would have a lower slenderness ratio.

Is this correct, or am I failing to consider some fundamental aspect of masonry construction.

This may sound like an inane question. However, it is prompted by the wording of the Standard, which focuses on the word “WIDE” rather than “tall, as opposed to less-tall”.

Many thanks.

Ronnie101.

 

[concretemasonry]

Ronnie -

If you are forced to work with that standard, you will have to research further.

Take a look at the definition of "brick" and see if it does or does not include concrete masonry units. Also determine what the testing procedure is for the testing of the "masonry". Is it just for a masonry unit on top of another with mortar between or is it for a "prism" of prescribed dimesions constructed of the units with mortar. Most standards are developed using an assumption of a h/t ratio, with the option of testing other configurations with a correction factor, similar to ACI 530 and ASTM procedures.

You are correct about the aspect ratio of the assembled units since a wider unit in most testing will give a higher strength. You will probably find that the test results will permit more higher loads than the standard tables since the tables were based on "narrower" bricks.

In reality, the mortar strength is normally a minor factor in the compressive strength in the strength of a unreinforced masonry wall. The controlling factor is the strength of the masonry units.

 

[patswfc]

Ronnie

Dont get confused between characteristic compressive strength and load bearing capacity of a wall. The first is fk and the second is fk * area of wall (usually based on a 1m strip, I've ignored any reductions for slenderness, etc. to keep things simple).

Clause 19 deals with the compressive strength fk, which is based on several factors

1)strength of unit
2)mortar designation
3)shape of unit
4)bonded or unbonded construction
5)thickness of mortar joints
6)quality of construction

As you can see there are many factors that result in the fk value. In masonry wall construction the mortar joints are the weak point, thus if you have wide shallow masonry units, then you have more mortar joints and that is why fk reduces.

The values of fk are based on test results. When they did these tests they never used masonry units with a ratio of less than 0.6, probably because it was felt that the experimental test results wouldnt necessarily reflect real world construction, as the quality of wide mortar joints in the real world probably has a high degree of variability. Thus if you do use this type of construction i think you should get test panels constructed by those who will construct the walls on site and test these to find fk.

So if you use a brick say 440long by 215wide by 100wide the ratio is 2.15 (greater than 0.6) you will get a certain value for fk, if you then decide to lay the brick on its side the height will now be 100 and least width is 215 and the ratio will be 0.46 (less than 0.6) which will result in a small fk value.

As long as this smaller value is not smaller by a ratio of 215/100, then the load capacity of the 215 wall will be greater, as this is based on fk * area (other factors being ignored).
Other factors such as reduction for slenderness will be lower for the wider wall which will increase the load capacity of the wider wall further.

Hope this makes sense to you.

 

[Vinny7]

Looking at BS 5628-1 Table 2H
Fk for a block as described 440X215X100Ht(0.46) can be obtained for blocks having a ratio of 0.4 - 0.6.

I realise the other clause calls for testing but to simple construction could the values from the above table ont be used (at least for the block dimensions above).

7N (i) 5.6 N/mm2
(ii) 4.6 N/m2
(iii) 4.1 N/mm2

10N (i) 7.2 N/mm2
(ii) 5.9 N/mm2
(iii) 5.2 N/mm2

Any thoughts on this?

 

[patswfc]

vinny

you can use this table for the block size you have stated, but the original question was to do with bricks not blocks.

I know i used standard block dimensions in my explanation but that for my own simplicity (probably shouldnt have as it may confuse)