Reverse Deflection in Cantilevered Balconies? 3

[MikeManning]

Hello,
I work for a large general contractor, and we do a lot of high-rise residential concrete construction.
I'm working on a bit of a guide to good balcony design (for waterproofing purposes) in these buildings, and I've got a theory, which might be totally out to lunch (which is why I'm checking).
My theory is that, in buildings with suspended concrete slab construction and cantilevered balconies, deflection of the slabs due to dead load and occupant loading inside the building might actually cause cantilevered balconies to deflect "up" (I'm sure there's a proper word for that).

The reason this would be meaningful is that balconies are often poured with a very slight slope outward, presumably for drainage, but we often find balconies don't drain properly, and I'm wondering if this type of reverse deflection might be part of the reason.

I've done a quick sketch of what I mean.

If this is out to lunch, let me know!
But if it's not out to lunch, then how much extra slope should we put on the balcony slab to allow for this deflection? 2%?

Thanks

 

[EZBuilding]

It is typical in our market to have a 3/4 inch depression in the slab. Exterior walls or storefronts are placed on this depression, which serves as a secondary mechanism for preventing water intrusion into the building. The idea being that if water is able to breach the enclosure and get behind the wall, it would need to "go-up" the slab depression to get inside of the building.

 

[FE_struct1]

@MikeManning,

Good guidelines ! I went back to re-read your post and noticed that you mention your projects being high-rise. How tall are your buildings typically ? If 40 storeys or more, column shortening might also be affecting the overall deflections.

A quick and dirty way to check - Are the sloping issues more common on the upper storeys than the lower ones ? I ask because column shortening effects are more pronounced in the upper third of any tall building.

 

[FE_struct1]

You have room for a book on your desk!

You got me ! No room for any pesky books here. Also, not sure if I can really call it a desk

 

[dik]

same situation here...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[GeorgeTheCivilEngineer]

In the UK you would need to be a thermal break cast into the slab which means you could just align that to take out any upwards deflection.

 

[MikeManning]

@FE_struct1

Really interesting idea there with column shortening.
Most of the high-rises we deal with are in the 30--40 story range - and I think generally we've seen leaks on lower floors.
Would columns near the interior of the building be more likely to shorten than those around the exterior?

 

[BridgeSmith]

Would columns near the interior of the building be more likely to shorten than those around the exterior?

The columns with the highest permanent load stress will shorten the most.

Rod Smith, P.E., The artist formerly known as HotRod10