Design of a multi-story building

[Ahmed A. Alamin]

Hi, i'm designing a concrete multi-story building. I've noticed that some of the columns at top stories fail while columns at lower stories at the same elevation don't though they have the same cross-section. Also, i noticed that moments under grivity loads increase in columns as you go up stories, can someone explain this?

 

[Lomarandil]

In low to mid-rise concrete moment frame buildings, column demand is often driven by flexure, and additional axial load (as you see on the lower stories) can be beneficial. Take a look at the bottom part of a column P-M diagram -- the weight of stories above "prestresses" the concrete axially and allows additional moment capacity.

At the very top level, the end moment of the beam or slab must all be carried by the column below (for lower levels, the end moment/rotation is split between the column above and below). This can also cause flexure in the very top column to be critical, especially when lateral loads are low (relative to gravity loads).



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just call me Lo.

 

[r13]

I think it is largely due to p-delta (lateral displacement) effect. The overall framing system is too flexible. Try increase column size, every few floors from the ground up, to stiffen the framing system, or add braces may do also.