Is it acceptable to decrease (phi) from 20mm to 12mm in the upper column? 1

[ibrahemnasr]

I wanna ask is it acceptable to reduce the diameter reinforcement in one column from 20mm to 12mm in the next upper story?
or i should use to successive diameter (from 20mm to 18mm and fro 18mm to 16mm) is this true?

 

[briancpotter]

This should be fine, provided you meet all code requirements regarding steel % (min. 1% in ACI 318), cover, and development length.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[asixth]

No reason why it wouldn't work. I would keep the min 1% reo.

Saying that, for a column supporting 1 or more levels I would provide a minimum 20mm bar diameter. Only for lightly loaded columns would I use 16mm bar. I wouldn't use a 12mm bar in anything really other than a small retaining wall or small pier which doesn't have much load. A 20mm bar just gives me a bit a confidence, it's a good lump of steel and feels good, a 12mm bar can be bent rather easily (almost by hand) so I don't like to use them in columns. These are not code rules, just my rules I like to go by.

 

[ibrahemnasr]

Thanks for all

mr asixth
just to complete the idea if i use 28mm bar in the ground story can i jump to 20mm bar in the first story ? or i should use bar diameter near to the 28mm say 25mm
what i wanna say is there any code requirements to use successive bars when you need to reduce reinforcement diameter from story to another ?

thanks again

 

[asixth]

Not that I'm aware of

 

[TXStructural]

There is no restriction about transition from bar size to bar size in ACI 318, but I don't know about other codes. When lap splicing, you need to lap for the smaller bar, but make certain that all bars are fully developed or lapped before they reach a section requiring a developed bar (as where you use a short lap, but the larger bar needs to be developed for column flexure below the lap.) If you design for mechanical splices (couplers), be sure the coupler accepts both sizes.

However, unless you have a maximum bar spacing requirement to meet, the best practice it to reduce the number of bars rather than the diameter. In typical, non-seismic applications, columns should generally have the largest practical bar size. Obviously development and hook considerations could also limit the maximum bar diameter.

Constructibility and economy are seldom maximized by designing purely for least weight of materials. Consistency in formwork and reinforce allows faster construction for most projects. In most areas of the developed world, formwork costs account for about half of the cost of a complete reinforced concrete frame structure. Keeping reinforcement consistent reduces lay down area needs and reduces errors and inspection costs (if all columns are the same, it's harder to mess them up and they can be constructed in any order using a "typical" column reinforcement cage and any available set of column formwork.)

 

[ibrahemnasr]

thanks mr TXStructural
i have another question
I have to reduce the column cross section in the upper levels What is the maximum practical distance to reduce from story to another? and is it practical to make reduction every two story in 12 story building?

 

[JAE]

No one has mentioned strength here. I'm assuming that the reduced bar size or smaller number of bars still provides the necessary strength based on analysis.

I've found in many concrete buildings that the required column reinforcement actually gets higher the further up the building you go due to higher bending moments and less axial load (i.e. you drift down and further right on the typical interaction curve).