Concrete-filled reinforced hollow block masonry wall calculation

[nivoo_boss]

Hey everyone!

I'm designing a building that has a lot of masonry walls that are just a bit too slender to go without vertical reinforcement, according to Eurocode 6 at least.

The walls are all only vertically loaded. They are 190 mm thick and filled with conrete. The hollow blocks have characteristic compressive strength of 18 MPa, the concrete that they are filled with is probably C30/37 which has fk=30 MPa.

My questions regarding the calculation. Should I take the compressive strength here as 18 MPa (design strength would then be fd=18/1,5=12 MPa?). I'm planning using vertical reinforcement in the middle of the wall with a spacing of ~400 mm - how to calculate this section? I would probably look at it like concrete section with a width of 1000 mm and two D12 B500B bars as reinforcement. But what about the effective depth? Should this be then d=95?

Below is a little sketch of typical wall, here the slenderness isn't too much, but that's not the point.

 

[JLNJ]

Most methodologies use the prism strength (f'm), which is a composite of the masonry unit, grout, and mortar properties.

 

[nivoo_boss]

"Most methodologies use the prism strength (f'm), which is a composite of the masonry unit, grout, and mortar properties."

I think that's used when using calculation methods provided for masonry walls. But in my case I think I should consider this section as a reinforced concrete wall or column with a section of 190x1000 mm, perhaps to Eurocode 2.

 

[JLNJ]

I can’t speak for Eurocode provisions, but ACI 530 uses f’m in reinforced masonry calculations, whether it’s a slender or non-slender wall or a member which is primarily in flexure.

I have seen it written that the research shows that solid-grout units pushed to the very limit may shed their masonry face shells, and what remains carries the ultimate load as something similar to a reinforced concrete assembly, but that’s not normally accounted for in the design procedure.

 

[dold]

I think that's used when using calculation methods provided for masonry walls. But in my case I think I should consider this section as a reinforced concrete wall or column

But it is a masonry wall, right? Why would you design it as a concrete wall? The analysis is similar but the behavior/code checks/etc are different. Maybe I'm missing something?

 

[phamENG]

Filling masonry with "normal" concrete is not a good idea, and I've never met another engineer to disagree with me on that. Grout exists for a reason - use it. It's like using a screw driver to drive a nail with a hammer hanging on your belt.

For one thing I can tell you it is NOT a concrete wall - ACI, Eurocode, or wherever. Concrete walls don't have 3 dimensional cold joint patterns running through them. The analysis methodologies would be grossly inaccurate.

 

[nivoo_boss]

"But it is a masonry wall, right? Why would you design it as a concrete wall? The analysis is similar but the behavior/code checks/etc are different. Maybe I'm missing something?"

It is but it is too slender, the masonry calculations provided in Eurocode 6 cannot be applied as far as I know.

 

[phamENG]

The closest you could come would be a bunch of little concrete columns that match the cell dimensions (assuming the mason aligns them perfectly...). But then you'd have to reinforce each cell as a mini column with vertical bars, ties, etc. Good luck with that.

 

[phamENG]

It's about a 13' wall with 8" nominal CMU. Not that bad. If your code doesn't allow it, bump up to 10" or 12" block...what's that? 245mm or 295mm block? I'm not sure what your standard sizes are but I bet they're similar to ours.

 

[nivoo_boss]

"It's about a 13' wall with 8" nominal CMU. Not that bad. If your code doesn't allow it, bump up to 10" or 12" block...what's that? 245mm or 295mm block? I'm not sure what your standard sizes are but I bet they're similar to ours."

The 4 m there is not real, it's just an example. The real height is 6,1 m.

 

[dold]

It is but it is too slender, the masonry calculations provided in Eurocode 6 cannot be applied as far as I know.

Then your design will not meet code. Increase the wall thickness or brace the wall. You can't just choose an arbitrary code to use to get a design to work...

 

[phamENG]

I agree with dold there - what you're doing here is sort of like saying "this wood stud doesn't work, but a steel stud would...so let's drive a bunch of nails into the stud at 2" on center and call it a steel stud." An imperfect analogy, I know, but it feels apt...

 

[SobreroPeet]

Agree with dold and PhamENG on all counts, this sounds sketchy...

 

[nivoo_boss]

After digging into Eurocode 6, I found that section 5.5.1.2 ("Effective height of masonry walls") has formulas to take into out the effect of cross-walls/stiffening structures on the effective height. Turns out I can use a reduction factor of ~0,70 in this particular case, which brings the effective height to around 4,3 m.

 

[Brad805]

Masons use grout for practical reasons, so adding in a pump truck and concrete supplier might not be a great idea. That said, we have many posters on this forum from areas of the world that do things very differently. If you have slenderness problems why not pilasters? Very easy to do.

 

[sandman21]

Filling masonry with "normal" concrete is not a good idea, and I've never met another engineer to disagree with me on that. Grout exists for a reason - use it. It's like using a screw driver to drive a nail with a hammer hanging on your belt.

For one thing I can tell you it is NOT a concrete wall - ACI, Eurocode, or wherever. Concrete walls don't have 3 dimensional cold joint patterns running through them. The analysis methodologies would be grossly inaccurate.

The Core Method of Design, RMEH 7th ed. Amrhein. A number of buildings and/or building components use the face shells as forms. You may use either concrete provisions or masonry provisions depending on the assumptions you make. We will use the design for bond beams at proscenium openings.

 

[phamENG]

sandman - big difference between using a face shell as a form (by which I take your meaning to be to remove the webs; I've had cases where an architect wanted the masonry bond in a stairwell to continue uninterupted so we just had a face shell that was laid in and the concrete floor poured over the wall) and filling a standard masonry wall with concrete and calling it the same thing as cast in place concrete.

For the core design method, don't you use actual block as the forms - like building a pair of 4" CMU walls 8" apart and then filling the space with concrete? I don't have a copy of Amrhein's book but I recall hearing about it once before.

 

[dik]

From 45 years back, before there were any real code provisions I used to use the core dimensions adjusted for the difference in strength between the CMU and grout. Due to the absorption of water from the grout mix, the tested grout strength was significantly greater than that spec'd. For stability, I used the 7-5/8" block thickness... I've done several 18 storey buildings that way, and still standing... there was a permitting issue with another engineer at the time; he had 3 buildings that were constructed like that. The City agreed if he could get an expert to agree they would then issue a permit. A Dr. McCutchen (sp?) came in from Ontario, and in an afternoon gave his OK for all three buildings. The City still wanted to refuse the permit and after being threatened with a lawsuit, relented... part of my early history.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik