Cantilevered Concrete Slab Supporting Load Bearing Wall

[spieng89]

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:

 

[jayrod12]

I agree, however if it is a bay over, the wall is likely robust enough to hold up the bay that didn't burn down. Besides a dual wall system like BA indicated, there's not really a great proven solution.

 

[KootK]

I agree, however if it is a bay over, the wall is likely robust enough to hold up the bay that didn't burn down.

That's the misconception. Holding it up vertically isn't the issue. It's holding it up laterally.

Besides a dual wall system like BA indicated, there's not really a great proven solution.

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.

 

[jayrod12]

I meant laterally, but I know where you're coming from.

 

[KootK]

Really? In my experience, there's hardly a wall out there that could resist this kind of loading without having been specifically designed for it.

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.

 

[jayrod12]

That's where the difference lies I think, I've always designed the firewalls to be legitimately free standing, I. E. If both sides burned down you'd be left with a 30 ft tall free standing wall. So it has plenty of out of plane capacity.

 

[spieng89]

I think what Kootk is hitting on is many engineers may design for free standing, but are they also designing for the weight of the joist/beam system that did not burn and is still tugging on the wall like a broken tree branch on a tree trunk

 

[KootK]

Precisely. Most folks in my area design for a reduced wind load and call that free standing. Given the BS nature of most breakaway connections, it should be that wind load plus the monstrous lateral "tug" at the top. I've been pulling the values for that from here: Link. And they are substantial.

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.

 

[BAretired]

It is evidently an issue which requires more serious thought than it has received in the past.

BA

 

[hokie66]

In school fires, students suffer because they lose their classroom, books, and materials. Firefighters are the people most at risk.