Thermal Movement Concrete Columns

[bookowski]

Looking for a sanity check to see if I'm missing something.

Concrete building, projecting balconies which are too long and too heavily loaded to work as cantilevers in current schemes/design (about 12ft with heavy landscaping). As a solutions the architect is proposing exterior columns which are clad to work with the building design. The columns would be clad but would still be exterior. My initial reaction is that the thermal movement of the columns will/could be an issue. It works out to about 0.6" per 100 Deg F per 100ft of column length. These columns would vary from 100ft. to 300+ft, resulting in some cases of a possible 1.5"+ movement.

I have said that this can't work because of this movement, but since then I am noticing a lot of tall concrete buildings that were built with perimeter columns outside face exposed as part of the facade. Are these working because they are only partially exposed and heat being sucked out through them is mitigating the movement? Am I missing something?

 

[racookpe1978]

But the problem would occur only if the tall (continuous ?) concrete columns with steel rebars grew, and the tall concrete and steel building did not grow - or the building grew under different temperature change limits than the half-exposed columns did, right?

Thus, a vertical but tall air-conditioned interior column would grow less vertically than an equal height exterior column exposed to the afternoon sun and air temperatures of 110 degrees F.

 

[KootK]

I find your thermal argument to be pretty persuasive. A couple of additional thoughts:

1) You'd think that this would also be somewhat exacerbated by the fact that the balcony columns would be lightly loaded and less prone to creep downwards.

2) If your concern came to pass it might be quite problematic as you might end up with a back sloping balcony.

3) I wonder if this might turn into a real problem if one were to use something like isokorb at the balconies and effectively prohibit thermal equalization.



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.

 

[bookowski]

racook - yes, but this is what would happen. Interior is conditioned and would remain relatively constant. The exterior would vary.

KootK - Yes on all 3. It seems problematic for a host of reasons - reversing drainage, would 1.5" seasonal movement create cracking, joints & attachments with building facade/railing, could fatigue even become an issue at some point, and in general who wants 1.5" of movement. I didn't think it made sense but thought maybe I was missing something. I think on these older buildings that I see with exposed concrete they must be energy hogs and the column doesn't quite get the extremes?

Isokorb is also a question on this one, a bit off topic but since you brought it up - have you used/had success with these? These are relatively new in this area and we've just started looking at them. For heavily loaded cantilevers they seem a bit scary but maybe it's just because we're not used to them.

 

[KootK]

The isokorbs have worked out well for me so far. They're new enough in my market that I usually get to table them and make architects thinks that I'm building envelope savvy. I know what you mean about the scariness. The detailing looks European. They take responsibility for the capacity determination and have their calculations done by a reputable Vancouver BC firm. Determining cantilever deflections with the isokorbs thrown in still makes me a bit nervous.

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.

 

[wannabeSE]

Just a couple thoughts:
1. Depending on temperature when constructed, the thermal movement may be +/- 3/4"
2. 3/4" would be similar to L/190 deflection on a 12' cantilever
3. Will the entire column experience a 100° temperature change or just the outer surface?