Change in concrete column size 1

[AhmedJ.J.]

Hi guys,

Suppose there is a 9 columns (50x50cm) in a grid of 3x6m horizontal by 3x6m vertical. The building is 5 stories(obviously, these 9 columns are not the only columns in this building, but extracted only as an example here). The last upper slab (roof slab) is wanted to be ribbed slab because the middle 3 columns are removed. When analysing the columns of the roof, the edge columns (the remaining 6 columns) require more steel (larger section of about 70x70cm) to resist the large moment, while the columns of the other stories are ok to remain 50x50cm. In general, should the columns of the last story to be the same size or smaller compared with the lower columns? In the said case, could the columns of the last story be larger than those beneath them?

Thanks

 

[Lomarandil]

It's not entirely uncommon for the top columns to require more steel (relatively) because while the flexural demands are similar to lower levels, there is less reliable compressive force available.

This is amplified in your case because you also have the effect of the larger slab spans on the top story, which is creating larger moments in your frame due to gravity forces.

Analytically, it can be OK (and correct) to have a top story column larger than the column below. In this case, the detailing at the bottom of the top story column requires special attention.

Economically, it may or may not be viable to change column size (depending on details of your formwork and slab system). But that requires more information about your region and construction practices to determine.

 

[AhmedJ.J.]

Thanks for your reply, Lomarandil. Yes, exactly as you said and I looked for such detailing where the column gets larger in size but I couldn't find anything.