RC column design - Fundamental Question

[mesapol]

How can I determine the N.A (neutral axis)of a rectanular column with bars in all 4 faces? It is different from beam, I cannot set C = T to evaluate the N.A, there is axial force, unlike beam? In addition, since there is axial force n bending, do I just refer to N.A or the plastic centroid still play a role? I am confused. I thought I only consider plastic centroid when the section is in uniform compression.

Thank you for your help!

 

[UcfSE]

Do a strain-compatibility analysis to find the stress in the rebars. You can assume plane sections remain plane. The NA will change as load changes. It is an iterative process.

 

[J1D]

Agree with UcfSE. You still use the three equations (force equilibrium, moment equilibrium and strain compatibility) to solve three unknowns: axial force (column load), depth of concrete in compression and strain of the most exterior rebar. You can design any member by these (beam is only an extreme state of beam-columns). If you give a column load first, the calculation will be straightforward (no iteration). But you need to watch the depth of concrete in compression every time, once it exceeds the section depth (means it is in “compression failure” mode) the strain equation should be modified. That is why in concrete column curve there are two branches.

 

[UcfSE]

One more thing to look out for is the resistence factor phi changes with the steel strain from compression-controlled section to transition range to tension-controlled section.

Try to get ahold of the PCA's Notes on ACI 318. It has some good information on column axial-bending capacities and will help you understand how to apply a strain-compatibility analysis properly. It's not exhaustive though so you will have work to do to understand more than just basics.

 

[mesapol]

UcfSE,

May I know what is PCA stand for. I think ACI 318 is American Code, right? When u mentioned it is a note, could you please tell which chapter or sections or pages, so I may find a copy from library in Australia.

Thanks alot.

 

[AlohaBob]

Thats the Portland Cement Association.