During construction , why do we add steel reinforcement in concrete columns, when columns are always subjected to compression ?

You can not perform columns without minimum reinforcing at least, because the collapse of the  concrete is sudden  collapse (without indicators) therefore is added rebar so that gives us a warning that the column began to collapse

Because the steel also resists compression,

During the course of the study pivots

There are areas with compressed steels

but still even when we take a purely centric loaded column.  With absolutely no moments.

Why can't we have the column then without reinforcement?  

Reinforcement is meant for TENSION forces only, or?  

How comes that you still fine in a column the rebars staying under compression.

Otherwise why would we need the stirrups around them if they would be in tension only?

 

Somehow it appears to be the we need the stirrups (links) becasue the bars are getting compressed? 

Strange....

 

Often you see horizontal cracks in columns at the spots where the crank is (overlapping) because hte bar got compressed there, why?

 

 

Increase streghth , buckling , connection to other elements , and all horizontal forces like wind & earthquakes affect on all building inculuding columns so there is tension because of moment.also concrete not enough for compression so with steel  kind of cost.

 

To prevent brittle failure*

To resist the moment due to eccentricity *

To reduce the section area of column*

 

all horizontal forces like wind & earthquakes affect on all building inculuding columns so there is tension because of moment.

also concrete not enough for compression so with steel  kind of cost.

other reason transfering load .

Increase streghth , buckling , connection to other elements , transfer forces to all clo.

the tensile streth of steel will transfer to concrete

although columns are compression members , They are also subjected to bending moment when connected with beams ( it can be distributed by stiffness ratios between column and connected beams by frame method analysis ). Also if the column is long column it is subjected to additional moment .  

we add steel reinforcement in concrete columns, when columns are always subjected to compression to support the efforts of thermal expansion and buckling

Because column is subject  also for other forces such laterall, bending,buckling.