Cantilever Transfer Slab, Would you design it this way? 5

[I-I]

I have uploaded a sketch below showing the project I am designing, This transfer slab is going to support 5 stories above. Because there are bedroom above and I couldn't shift the columns above inwards so I have to put them on external and being supported by this transfer slab. The loads of each column above is roughly 200kN (Dead plus Live). Although I am not the one to design the PT transfer slab, I am not comfortable with this arrangement. But at the same time, I cannot shift the columns above 3.2 m inwards. (The columns below cannot move outwards they are on the external already, outside of them are roads)
What is your thoughts on this?

 

[KootK]

Or this version if one is considering the wallumn approach.

 

[r13]

I think even the architect will shake his head on the first, the second looks very sturdy and traditional.

 

[KootK]

I doubt it. Calgary's new library is framed similarly (not me) and I can't recall a building receiving more architectural accolades. Granted, the perimeter setback is less.

 

[MIStructE_IRE]

Not far from here in the 60s we had the Ronan Point collapse which kickstarted the discussion on robustness and disproportionate collapse in this part of the world.

A woman’s fridge blew (oddly enough!) and took out a load bearing wall, causing the entire corner of the building to catastrophically fail. We now have sufficient vertical and horizontal ties to give us a situation where the collapse will not be disproportionate to the cause.

In this case however, if your single column goes due to vehicle impact, poor construction or a fridge explosion... I don’t see how you stop the storeys above from spectacularly collapsing.

 

[MIStructE_IRE]

I also just read “outside of The lower columns are roads..”!!

So every day we have trucks passing the only column holding this thing up?!

If this was dragons den - sorry guys, I’m out!

 

[KootK]

That's the stock, fear inducement photo but, then, it's precast. I'd consider that to be a much more fragile situation.

 

[KootK]

So every day we have trucks passing the only column holding this thing up?!

It's not the only column holding this thing up. It's effectively one column of many holding it up that happens to be at an acute corner.

In major buildings, columns adjacent to drive lanes is a pretty common scenario. Design 'em for impact and/or take some reasonable protection measures, and carry on.

It's not as though a column's being inside the envelope is necessarily a guarantee of it's safety. This is actually one of my EIT projects.

 

[KootK]

Although I'm sure that these columns are inside the envelope. All the better.

 

[MIStructE_IRE]

Yes, to be fair, I’m wrong saying this is the only column holding it up.

This section through this particular gridline just doesn’t sit right with me.. coming back to our earlier teeter totter analogy!

Anyway, what was OP’s question again?!

 

[r13]

I think it needs super heavy/thick slab at the core to counter the loaded perimeter cantilever slabs. The design will be difficult even for the gravity load alone.

 

[KootK]

And this doesn't feel substantially better?

You know that I share your general concern. But, then, you also know how it goes here: most questions wind up being a pile on of smart people pointing out valid problems with whatever's been proposed. OP's must often wind up with the impression that there's no room for creativity in structural engineering.

As anyone who's been in the game a while will know, successful practice as a structural engineer requires being a solutions focused optimist. Pessimists need to find business angles that do not involve architects, developers, or contractors. I'm just trying to probe this a bit to make sure that we've vetted it sufficiently before giving it the heave ho.

 

[Celt83]

KootK: your sketch there is basically a highway which if this 5 stories of residential flat plate pt probably sums up to about highway loading so not beyond possible.

I have trouble grasping how PT for this transfer level will work. I'd image you need to pattern the column reactions and without patterning you'd almost want an upside down tendon layout. Assume staging of the tendon stressing would also be required. I'd really hesitate to just punt this design to a third party without vetting it a little bit myself,

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[MIStructE_IRE]

I’m also curious, and I’m certainly no PT guy, but in the out of balance load case below, is there a kind of stress reversal case where compression is lost in the slab?

I realise I’m sounding like an awfully conservative pessimist here. I’m generally an ‘anything’s possible For the right money’ kind of guy but something’s niggling me on this one.

 

[MIStructE_IRE]

I do however like the haunched wallumn and transfer beam. I could probably live with that.

 

[KootK]

Or, what if one explored milking an upper level suspension system for a little OT resistance redundancy?

 

[KootK]

Something like this might be the more natural, and less invasive, way to get some OT redundancy.

 

[KootK]

your sketch there is basically a highway which if this 5 stories of residential flat plate pt probably sums up to about highway loading so not beyond possible.

A fine analogy my friend. Thanks for tabling that.

I have trouble grasping how PT for this transfer level will work.

Sure, let's talk Turkey. Does what I've shown below, in very broad terms, speak to you at all? Banded - Banded with some infill distributed as needed. Obviously, you can only provide balancing for one load case so you'd have to pick what matters to you most and deal with the rest in mild reinforcing.

I'd really hesitate to just punt this design to a third party without vetting it a little bit myself,

Ditto but that's how it's done in some parts of the world so I'm content to turn a blind eye to that here.

 

[KootK]

I realise I’m sounding like an awfully conservative pessimist here.

You're really not. We're just playing off of one another on different sides of the argument.

I’m also curious, and I’m certainly no PT guy, but in the out of balance load case below, is there a kind of stress reversal case where compression is lost in the slab?

I'm having trouble seeing it. With an eye towards the sketch below, can you elaborate at all? Obviously, one would need to be a bit careful in detailing the column to slab connection to transfer the unbalanced moment successfully. But then that's just, you know, the usual, ridiculously complex STM & anchorage stuff that is our bread and butter.

 

[BAretired]

So far, this wins my vote:

I would send it back to the architect and tell him to start again.

BA

 

[Celt83]

Gut reaction, assuming the load on transfer slab other than the columns is typical residential loading, I would anticipate the whole slab to be in negative bending so the PT wouldn't have a traditional low point between columns. To really balance out the transfer reactions the PT is more or less going to want to be draped in a way to effectively add load to the central span. You'd also really want a low point out at the edge of the slab to help lift up the free end but not sure about the anchorage detailing most of my resources more or less dictate seating at the cross section centroid.

The reverse drape of tendons to a loading vs unloading shape in the span I think would make it hard to get service stresses to check out without staging the stressing to go along with the transfer level construction.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer