Circular Column Minimum Bars

[Waqar Saleem]

salam everybody http://www.sepakistan.com/public/style_emoticons/#EMO_DIR#/smile.png

what should be the minimum number of bars in a circular column???can we use just one bar at center of column?some said that one bar at center cant be used,minmum six bars should be used.

[Badar (BAZ)]

a bar in centre will be useless as rft is needed for taking tension. One can use 4 bars and use rectangular ties.

[Waqar Saleem]

rft?why we can't use in center?

[UmarMakhzumi]

rft?why we can't use in center?

A column subjected to bending will have one face in tension and other will be in compression. Central bar will have minimal contribution.

[Ayesha]

rft?why we can't use in center?

A column subjected to bending will have one face in tension and other will be in compression. Central bar will have minimal contribution.

If a column with a single rebar is subjected to compressive or tensile force only, in that case rebar would be effective at center or NA. Consider it same as providing reinforcement at Beam Neutral Axis. its a waste. Universities should teach beam column in undergrad so that such concepts be more clear.

[UmarMakhzumi]

salam everybody

what should be the minimum number of bars in a circular column???can we use just one bar at center of column?some said that one bar at center cant be used,minmum six bars should be used.

reminds me of a question on mid term of masters exam, where a concrete wall had temperature of a -30 on 1 side and +18 on other with an udl on top and reinforcement placed at the center of the wall and the question to find the critical load for that given reinforcement and wall section.. that was pretty cruel !

[Waqar Saleem]

thank you everybody!!!!i have another question that with same reinforcement one column circular other rectangular, which will be better???

[WR1]

spiral reinforcement is better it has less capacity reduction factor hence more strength...but if you use circular ties it is same like rectangular ties..so in that case ties (rect or circular both are same)

[Waqar Saleem]

thanx @Rana Waseem it's ok spiral confinement is better than rectangular or circular ties,im of the view that if same reinforcement is used both longitudinal main bars and confinement but x-sections are different circular and rectangular in this case which column will have greater strength???

[WR1]

if you mean to say that we use different concrete section that is i) rectangular & ii) circular but keep the reinforcement and layout of reinforcement same what would be the effect rite?

it will not matter much at normal scale in a normal building unless you have huge dimensions but anyway, ofcourse circular shape is more desireable due to archtectural reasons and also due to structural issues like

circular cols will not have corner stress concentration

less material as compared to square/rect cols but at the same time same moment of inertia

equal capacity at every rotation of axis no matter from where the loading is coming

[Waqar Saleem]

thanx RAna now it is clear !!!! thank u very much for your valued time n knowledge sharing!!!

[UmarMakhzumi]

The picture below, is of Olive View Hospital, 1971 San Fernando Earthquake, CA, US San Fernando, California, Earthquake February 1971.

Here is another view

The spiral column displaced 18" from center and is still in good shape and taking load, where as the rectangular columns with ties behaved as below.

This difference in behavior is attributed to the greater confinement spirals provide resulting into a better "Poisson ratio" & Improved ductility of the system overall. FYI, this building had a soft story and also had hammering due to RCC Core Wall not separated well enough from adjacent part of building.For your question about rectangular or circular column, I would say Circular are good if required for use in high seismic zones and are provided with spirals instead of circular ties. Circular Columns are aesthetically beautiful too..

[Waqar Saleem]

thanx umar makhzumi thanx for this elaborated answer!!!!

[asadishaq]

why we use modifiers in ETABS like for beams we use 0.35 in bending and moment of inertia???what is the logic behind this?/

[asadishaq]

and we are using 4 in th lean under footing in SAFE,also we assign soil support to both lean and slab is it a good practise??

[Ayesha]

and we are using 4 in th lean under footing in SAFE,also we assign soil support to both lean and slab is it a good practise??

dont model lean as part of foundation. just model your slab and assign it supports.

[WR1]

lean is not part of concrete foundation modelling...unless you are doing a high fi phd scientific research...( i think)...and in ETABS we provide modifiers to account for cracking in concrete members as told in ACI chapter 10..

[Yasir Saleem]

The picture below, is of Olive View Hospital, 1971 San Fernando Earthquake, CA, US San Fernando, California, Earthquake February 1971.

Here is another view

The spiral column displaced 18" from center and is still in good shape and taking load, where as the rectangular columns with ties behaved as below.

This difference in behavior is attributed to the greater confinement spirals provide resulting into a better "Poisson ratio" & Improved ductility of the system overall. FYI, this building had a soft story and also had hammering due to RCC Core Wall not separated well enough from adjacent part of building.For your question about rectangular or circular column, I would say Circular are good if required for use in high seismic zones and are provided with spirals instead of circular ties. Circular Columns are aesthetically beautiful too..

Sir g in first Pic where the column is failing in the window there SHORT COLUMN effect is also visible ...what you think as we the length of the column is reduced due to the walls which is highly unfavorable in earthquake region because then the shear forces are high in that region and they cause such shear failure so this must also be kept in mind that the length of column is not restricted in that way that it produces short column effect......

[Yasir Saleem]

Sir g in first Pic where the column is failing in the window there SHORT COLUMN effect is also visible ...what you think as we the length of the column is reduced due to the walls which is highly unfavorable in earthquake region because then the shear forces are high in that region and they cause such shear failure so this must also be kept in mind that the length of column is not restricted in that way that it produces short column effect......

And sorry one more thing i like to mention here that i understand the concept which you explained to waqar i think but here it is important to tell him that such rft deatiling is very very poor for structures and he can see why it is poor .....Shear stirrups long. bars everything is not according to seismic requirements....what you say..

[asadishaq]

what is cracked and uncracked section analysis?????

[WR1]

for both of you above! if you want to start a topic which is not relevant to this category you can start a new thread so that its not confused with the original discussion!

[UmarMakhzumi]

And sorry one more thing i like to mention here that i understand the concept which you explained to waqar i think but here it is important to tell him that such rft deatiling is very very poor for structures and he can see why it is poor .....Shear stirrups long. bars everything is not according to seismic requirements....what you say..

Sir g in first Pic where the column is failing in the window there SHORT COLUMN effect is also visible ...what you think as we the length of the column is reduced due to the walls which is highly unfavorable in earthquake region because then the shear forces are high in that region and they cause such shear failure so this must also be kept in mind that the length of column is not restricted in that way that it produces short column effect......

Yeah, Short columns can be critical.. to summarize, "Stiffness attracts load"

[Ayesha]

what is cracked and uncracked section analysis?????

why we use modifiers in ETABS like for beams we use 0.35 in bending and moment of inertia???what is the logic behind this?/

Something to keep in mind when using cracked section properties.

Euler Buckling Load (Pe) = pi^2 *EI / (KL)^2

In your case EI = 0.35 * Igross

This is because you are using modified stiffness parameters.

This would result in a more "critical" buckling load.

**Note: ACI doesn't usually let you get above 75% of the theoretical value .**

That's pretty typical. I don't usually see this method as being capable of predict P-Delta amplification above about 70% of the theoretical elastic buckling value.

[Waqar Saleem]

a bar in centre will be useless as rft is needed for taking tension. One can use 4 bars and use rectangular ties.

i came to know in a book that in circular column there should be six bars used in minimum also one engineer said that according to ACI code six bars are required in a load bearing circular column .

[Badar (BAZ)]

i came to know in a book that in circular column there should be six bars used in minimum also one engineer said that according to ACI code six bars are required in a load bearing circular column .

Code does require us to use a minimum of six bars for spirals, it does not mention circular cross section in that specification. Code requires minimum of 4 bars for circular/rectangular ties.

But we can still use 4 bars in circular column as code also states, at many places, that if engineer is able to prove the safety of his design proposal in light of rules of mechanics,material behavior and serviceability requirements, than he can use it.

I answered your question keeping in mind that particular member is kind of dummy member, the one which is not an integral part of structural member.