Extending columns above roof

[mes7a]

In the column interaction diagram.. zero axial load would correspond to the lowest moment capacity because there is almost zero axial load to counter the tension side of the column. What would be the effect of extending the column 3 meters above the roof to create axial load to create more compression effect on the tension side of the columns just below the roof joints? There will be no beams above the hanging columns above the roof.

 

[KootK]

1) The project architect will have you shot.
2) You won't accrue enough axial load to make much difference.
3) You would improve the strength of the slab/column or beam/column joint quite a bit.

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.

 

[mes7a]

The weight per cubic meter of concrete is 23.56 kN/cu^3.. the height of the column above roof is 3 meters. The size of the column is 0.5x0.5 meter. So 23.56 x 0.5 0.5 x 3 = 17.67 kN.

Won't 17.67 kN be enough axial load to make a difference as far as compressing the tension side of the column right below the joint is concerned?

 

[KootK]

Nope. That load is peanuts for a huge column like that. Maybe on the order of 2%f'c? Do you not have an interaction diagram you can look at for comparison?

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.

 

[mes7a]

Nope. That load is peanuts for a huge column like that. Maybe on the order of 2%f'c? Do you not have an interaction diagram you can look at for comparison?

There is supposed to be another floor above it (total of 3 storeys). But we didn't continue for lack of budget (so imagine a 2 storey with hanging columns above it.. the beam spans are 5.5 meters longest at the lower floor). Would there be negative effect if you make the columns so huge and didn't finish the designed 3 storeys? I don't suppose this would make the tension side of the columns underneath poor. Also the column at ground floor is supposed to be 0.5x0.5 meter.. at 2nd floor 0.3x0.3 meter.. at roof 0.25x0.25 meter.. but we make all 0.5 x 0.5 meter size all the way for ease of construction to avoid rebar bent or placement error.

 

[KootK]

I don't foresee any structural problem with 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.

 

[mes7a]

I don't foresee any structural problem with it.

Imagine 9 columns with length of 3 meters and 0.5x0.5 meter diameter above the roof (already built) that are just hanging (it is supposed to hold another floor but we didn't continue). We will use light rafters and square bars on top of the columns for the lightweight roof. But this would make the 9 columns practically not braced with rigid beams. During seismic loading, would this make the 9 columns sway back and forth enough to rotate the column-beam joint below them? Do you suggest to put huge rigid I-beams on top of the columns to avoid sway or they are not necessary and light rafters are enough?

 

[structSU10]

depends what the analysis suggests. If the as built lower levels have capacity to have a light level above, with the columns acting as cantilever columns above the current roof, its fine. If they cannot do that, you may need to add supplemental bracing. It comes down to the numbers to determine what you can / cant do.

 

[TehMightyEngineer]

In industrial buildings I'll often see columns oversized for planned additional floors above the existing. Really will help out the next engineer a lot if you can do this but I'd only do it for an industrial building where columns should be oversized anyway for the industrial abuse they're likely to receive.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[mes7a]

depends what the analysis suggests. If the as built lower levels have capacity to have a light level above, with the columns acting as cantilever columns above the current roof, its fine. If they cannot do that, you may need to add supplemental bracing. It comes down to the numbers to determine what you can / cant do.

The columns were built to support another floor. But we didn't continue and just planned to put light weight roof. However I heard open ended columns can act like pendulums.. would this strain the joints below? Because if it does.. we would need to spend a lot to put very rigid I-beam bracing instead of just square bar beams that is size 250mm x 150mm x 6mm thickness.

 

[mes7a]

The above is the column detailing. There is no lap splice since we use 14 meters straight from foundation all the way to third floor. It's residential with beam span of 6 meters. The stirrups were spaced 70mm at 1 meter above and below and 150mm at midspan. What do you think of the columns? Would it sway and instead of using lightweight roof rafters.. we need to use heavy I-beams?

 

[Ron]

Stub columns above the roof surface are common for anticipated future expansion, but not considered as a help in the design as KootK notes. Further, they increase the necessary flashing area on the roof, must be protected from weathering since their cover is likely set for an interior condition, and create more maintenance for roofing purposes.