column size reduction over height (multistory) RC building 1

[suliman ali]

dear engineers

i have inquiry regarding the reduction of columns over height of reinforced concrete frames

is it for strength of architecture or economic purposes ??

and what is the preferred condition for earthquake design is it reduction or maintaining the same size over height


thank you

 

[Lomarandil]

Column and moment frame actions decrease as you climb a multi-story building.

As such, column sizes (sometimes) also decrease.

Architects like this, because it gives them more usable floor space on the upper stories.

From a material standpoint, a smaller column uses less concrete, so it saves money that way.

From a design engineering standpoint, there is some detailing needed at joints when the column size changes, so we try to be strategic about when to change the size (typically not changing the size at every story).

From a construction engineering standpoint, it is also more efficient with formwork, etc., to keep the column size constant. This is why some buildings do not change column size at all, despite the fact that they (structurally) could. (These buildings often change the amount of column reinforcement instead).

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just call me Lo.

 

[KootK]

1) I agree with Lo that the cost is all about the formwork so the construction tendency is to maintain a particular column size for the entire building or, at the least, a number of stories at a time. I was once told to hold the column size for ten consecutive stories at a time on very tall buildings. I don't know that there was any magic to that number though.

2) For rectangular columns, and depending on the formwork system being used, there can be merit in decreasing one side dimension of a column while holding the other. See the sketch below.

3) Particularly in low to mid-rise buildings, the uppermost columns can be some of the highest demand columns at any level. That, because bending moments tend to be similar at all levels and, at the highest level, you don't have much axial load to kind of prestress the column and reduce reinforcing tension demand.

4) Unless there is an occupancy change part way up the building, it is often of little architectural consequence whether or not column size is decreased. Whatever accommodation has been arrived at for a 15th story condo unit is often equally applicable to an 30th story condo unit.

5) For tall buildings, a popular strategy is to use high strength concrete at the lower storeys, hold the column size, and reduce the concrete strength up the height of the building. This saves on concrete costs without incurring offsetting formwork costs.

 

[KootK]

and what is the preferred condition for earthquake design is it reduction or maintaining the same size over height

For seismic performance, it is usually considered optimal to have all stories be of about the same lateral stiffness so that yielding and energy dissipation will be roughly uniformly distributed up the height of the building rather than aggregated at a few stories that then experience a ton of displacement demand. In this respect, for stories of equal height and uniform lateral system proportions, it would be desirable to maintain a consistent column size. That said, I haven't seen much of an attempt to enforce this strategy for "gravity" columns in practice in the markets in which I've worked.