Cantilevered Concrete Slab Supporting Load Bearing Wall
Cantilevered Concrete Slab Supporting Load Bearing Wall
(OP)
I have a situation where due to existing structure I need to install bell piers a few feet away from a new load bearing CMU wall. I plan to design a cantilevered structural slab to pick up the load bearing wall and continue the structural slab back past the grade beam until the slab on grade/structural slab interface can withstand the resultant shear. I wanted to confirm my checks were adequate:
1. checking shear/moment strength of cantilever structural slab at grade beam
2. checking deflection at load bearing wall and back span deflection beyond the grade beam
3. checking shear at structural slab/slab-on-grade interface
Any other concerns I should look out for? See preliminary detail below:
1. checking shear/moment strength of cantilever structural slab at grade beam
2. checking deflection at load bearing wall and back span deflection beyond the grade beam
3. checking shear at structural slab/slab-on-grade interface
Any other concerns I should look out for? See preliminary detail below:






RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
Also, the foundation piers will see much more load from this condition than a simple tributary area condition.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
To me, this makes the system more rational and doesn't dictate the sequence of construction. The slab can then be placed after the roof is on and have control joints, etc. that are more typical slabs.
Maybe there's a reason you already ruled this out, but that's my first thought when I see your section.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
BA
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
Just place an offset pile cap on the pier and resolve the forces in tension and compression in the rebar.
Run a grade beam between these at the CL of the CMU wall above.
Your slab on grade is now just a typical slab on grade.
All this depends on how reasonable your loads are and if you can resolve the moment into that pier via tension and compression in the rebar.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
BA
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
Any chance that block wall is a fire wall that needs to remain standing if the new building burns down?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
This would be the most ideal and what I tried initially after posting here. The deep beam design is no problem, but I'm not sure I know a valid way to design a bell pier for moment at grade. Is it as simple as getting lateral resistance values from the geotech and turning the moment into couple forces?
At the moment, the designers are trying to determine if this should be a fire wall or just fireproof the corridor bay adjacent to this wall. Any particulars I need to look out for if this is a fire wall?
I've revised the detail to the below as it provides the simplest construction so far:
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
It complicates the rebar detailing a bit, but the bottom of the cantilevered portion of the beam shown in section could slope up toward the existing wall, if necessary.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
That's about how I do it. As BA suggested, it may be attractive to utilize your slab in tension for part of that couple. Unless you isolate things, that's probably where the tension will want to go anyhow.
This situation has caused me all kinds of grief when the wall needs to be able to free-stand after the new building collapses.
- high flexural demand on the wall acting as a cantilever. Often need a thicker wall or intermittent piers with horizontally spanning wall.
- the cantilevered block walls tax the grade beams pretty hard in torsion and everything else down the line in other respects. Particularly so if you're being conscientious and actually trying to calc out the rebar detailing in the joints properly (as opposed to just developing bars). In this one respect, your original solution may have some advantage. Relatively easy to dump the wall moment in slab uniformly. Easy... just not cheap. One could employ a hybrid solution where you use the grade beams for gravity but the slab for moment.
- In addition to resisting modest wind load as a cantilever wall, sometimes you need to design for a "pull down" breakaway load acting the wall as the adjacent framing pulls away per NFPA. Makes a tough thing even tougher.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
If a single wall is used, as it usually is, the best detail in my opinion, is to tie it securely from both sides and ensure that the structure on each side is capable of resisting the horizontal force on the opposite side if it should collapse in a fire. I agree with KootK that the fusible link or "melt away" connection cannot be counted on for reasons he stated.
BA
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
That's the misconception. Holding it up vertically isn't the issue. It's holding it up laterally.
Double wall is the Cadillac if the team can be brought around to that way of thinking. Two other good approaches:
1) Design for a rationally estimated lateral load on the cantilevered wall. NFPA has guidance.
2) Have the fire rating for the structure supporting the wall be the same as the wall. This is often overlooked and, for concrete systems, is often quite doable.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
It's one of those crappy situations where we've inadvertently created a race to the bottom of our own accord in an effort to try to create solutions cheaper than our competitors. My province recently went on a school building binge. Something like 22 new ones in a span of a few years. Owing to a unique bidding process, and my role in that, I had access to most of the designs. Every last one is single wall with some manner of utterly useless breakaway clip. Poor bloody children. One day, one of these things will be put to the test for real. The outcome of that will be great sadness, shame, and hopefully more fire walls that are doubled or connected to both structures as BA mentioned.
It must be sooo frustrating for the code folks. They move heaven and earth to make this stuff as prescriptive as possible so designers can't stick their heads in the sand and punt on account of saving project $$$. Then they leave this one part -- the connection -- up to our discretion and we screw it up. It's embarrassing really.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall
BA
RE: Cantilevered Concrete Slab Supporting Load Bearing Wall