Compression Strength of two different grades of brickwork

[Abuh001]

Hi

If a beam rests on a wall consisting of two different strengths of brickwork, is it allowed to design the brickwork using the average strength.
Say the bearing length at 0.4 x the height of the wall under the pad-stone consists of 1.5m of brickwork of strength 1 N/mm2 and 1m of brickwork of strength 3.5 N/mm2 can I take an average to find a sort of "effective" brick work strength.

Thanks

 

[EngineeringEric]

It is based on stiffness for the load transfer. Are you dealing with 4"CMU and 4"clay? or something else. i do not believe the strength numbers you are talking about really will be your failure, which is concerning.

 

[BAretired]

Probably should take the lower strength for bearing assuming the pad-stone straddles two different grades of brickwork.

BA

 

[dik]

I'd use the lower one for design, and if checking the absolute maximum load, use the higher one with the reasoning that it would be supporting the load as the weaker wythe 'crushed'.

Dik

 

[dhengr]

Abuh001:
You know what you are looking at, but we can’t see it from here. A simple, well proportioned sketch, with a plan, elevation and section, some dimensions, loads, etc. would truly be worth a thousand words. And, you might get some meaningful, helpful, answers to your problem. I just don’t know what “the bearing length at 0.4 x the height of the wall under the pad-stone consists of 1.5m of brickwork of strength 1 N/mm2 and 1m of brickwork of strength 3.5 N/mm2" really means. Are these two wythes of brick making up the total wall thickness, or are these stacked atop each other making up a 2.5m high wall of one wythe, or are these one wythe of brick stacked end to end making up a 2.5m length of wall, with the beam atop them in some way? The “the bearing length at 0.4 x the height of the wall under the pad-stone,” is a real mystery. Your first, however many words, just don’t explain the problem. Your problem really depends upon how the beam and its bearing plate (pad-stone?) loads the brick below. The stronger/stiffer brick will likely take the majority of the load as a function of the relative stiffness of the two types of brick, and this might lead to differential movement (deflection, compression, shortening) of the two types bricks. If the two types of bricks are stacked atop each other, the beam reaction is distributed over a longer wall length as you move further below the beam bearing, so a higher bearing stress immediately below the beam, in the stronger brick, might be o.k. by the time you get down to the weaker brick below. Maybe the beam baring plate can be designed to accommodate/tolerate this brick strength difference. But, finally the brick strength/stiffness isn’t the only issue, you should test a panel of each brick and use the strength of the brick/mortar combination for your design values.

 

[concretemasonry]

Abuh -

How are you measuring the "strength of the masonry".

Most codes recognize the f'm, which is determined by testing of a prism consisting of masonry units and mortar. Masonry for code development is usually determined by testing wall sections (vertically loaded, shear and flexure) to account for the differences for the materials and the ability of the materials to work together. They have shown that the effect of mortar is minimal, which simplifies the structural analysis and supports the lack of requiring strength tests of on-site mortar.

Being a wall of different materials, it is best to design based a wall panel consisting of the materials being used. Obviously, site samples of a prism of masonry units and mortar is impractical and unreliable.

Take a look at the ACI 350 document since it has been adopted in many European and Asian countries.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dhengr]

Concretemasonry:
I’m doing a slight thread highjack here...,

Dick, would you please look at the thread titled “CMU Disintegrating,” thread507-435859, 23FEB18, and tell us what you think on the subject. I would be interested to hear what you have to say, have you seen this before, what causes it, etc. All conc. blks. don’t seem to be affected the same way, all the time, even though in the same location, and while 5,6,7 blks. are severely deteriorated.