Cantilever Block wall? 1

[RD2EN]

I have this blockwork stair core as below. The architect wants it to be cantilever at landing. I am wondering if the blockall can be cantilever? If so can I simply use the bottom reinforcement Top reinforcement in cantilever wall 'A' to support wall 'B' (like stru-tie deep beam method?) Or do I needs bond beams under walls 'A' to cantilever and pick up the wall 'B'? The design axial force in wall 'B' is 300kN including the loading from the slabs on first to third floors.
Thank you.

 

[Lomarandil]

Forgive me if I'm missing something basic, but I would expect you would at least need elements:

[ul]
[li]To transmit the portion of wall B (bottom middle) which will not naturally arch to walls A as a deep beam[/li]
[li]To carry the floor 1.5 landing and portions of wall A directly above.[/li]
[/ul]

I do think that much of your design load could find a rational load path through deep beam action

----
just call me Lo.

 

[JLNJ]

Determine and follow your load path. I would think the landing can't "cantilever'.

If the side walls carry the load, then there will be a force concentration at the "notch" where the sidewalls step in. Seismic or wind will exacerbate this effect.

 

[RD2EN]

So do you think using bond beams under wall A with top reinforcement lap into back span sounds okay? I think it can be done if it is RC, not sure about blockwork.

 

[RD2EN]

Can I use bond beam under wall A to cantilever then? Is it possible?

 

[BAretired]

Has anything been built yet? If not, why not use concrete walls for A and possibly for B as well.

How are you framing the stairs and landing between Second and Third floor?

Any chance of convincing the Architect to extend the 2600 x 5000 rectangle down to Ground Floor?

BA

 

[Enhineyero]

I think you are in the right track OP, though I'm a bit uncomfortable relying on a very deep strut-tie model. Personally, I would place bond beams spanning the whole stretch of wall A on each level(including the top-most) to distribute the load of wall B. I would design wall B each level as a strut and tie supported by bond beams at wall A.

Also consider the effects on your lateral load system. Wall B might attract some loads and check if you have enough vertical bar anchorage or appropriate load path for these.

 

[masonrygeek]

While this isn't the exact same thing as being discussed here, here is an example of a cantilevered masonry lintel at the corner of a wall.

 

[KootK]

I am wondering if the blockall can be cantilever?

Absolutely. We just gotta think through the details a bit.

Or do I needs bond beams under walls 'A' to cantilever and pick up the wall 'B'?

Probably, but only to pick up the block shown in black below. Little baby loads nowhere near 300 kN.

 

[KootK]

Or it might just be simpler to do a whole ring in concrete beams. In my estimation, CMU can do anything concrete can do with one exception: complex joint rebar detailing. So the strategy would be to use the concrete only where you have to to make up that deficiency.

 

[KootK]

Or even this is you're feeling super bad-ass. Once the ring beam goes low-shear, flexural only detailing, it can be block as easily as concrete.

 

[hokie66]

If the whole block wall is reinforced and grouted solid, certainly it can work without any formed concrete or "bond beams". But you have to make sure shoring under the wall stays in place until strength is achieved.

 

[blihpandgeorge]

as i was reading this i was going to write exactly what kootk and hokie66 have said. lots of examples of this type of core wall structure.

you dont say whether the wall is a acting as a stability / shear wall element? and if so whether it is primarily wind or if seismic is also required. If a stability element, i would initially ignore wall B above first floor if you are using block unless you can justify the shear lag around the corner.

 

[KootK]

If a stability element, i would initially ignore wall B above first floor if you are using block unless you can justify the shear lag around the corner.

I was thinking something similar, as shown below, for strong axis bending of the shaft: a stress "shadow" in the flange above and below the offset. Obviously things get a lot uglier for weak axis shaft flexure in the offset wall.