Design of irregular shaped columns 1

[benjaman]

Hi,

Could anyone please shed some light on the design reinforced concrete irregular shaped column design, e.g. hexagonal, L-shaped ?

 

[ishvaaag]

It will depend on the case. Some codes give enough guide to properly treat the buckling problem, then you from acquired forces at the section may proceed to design.

You may model the thing to include P-Delta and in-member P-delta by segmentation of the column members at some proper reduced stiffness in a 3D FEM model, then use K=1 or lesser from non-sway chart to get the solicitations at the sections for the checks.

Respect design of longitudinal rebar you may design it from a compatibility of deformations at the section, that would give you either the rebar required or if some provided is enough, in more of how the section is behaving for the solicitations. Once you have that you need care for shear design, for which its interaction with longitudinal forces should be cared for in every case since it may bring some concrete strut to failure; reading the sections of the codes about shear in concurrence with axial forces or bending action will help to decide what is a sound design. For most cases, where shear forces are relatively weak, the usual procedures with some inscribed section will be enough but not without the engineering judgement on interaction previously named.

Also, some sections are close enough to others that most would design them just the assimilable; most surely most designers would use an inscribed circular section as substitute of the hexagonal.

 

[Lion06]

If the section is symmetrical about both axes, it's pretty straightforward. I agree with ishvaag tha tmost engineers would inscribe a circle in the hexagon and use the circle for the design calcs. When you get into an L-shape, it's not as straightforward. This is similar to a single angle compression member for steel, and buckling is more of a consideration. I don't know that modeling a stick column of a steel angle (or an L-shaped concrete column) is going to do you much good other than getting out accurate second order moments. It won't provide you with buckling information, and that's something that should be investigated with a section like that. For the strength check of an L-shaped column, I would determine the principal axes. AISC pretty much does this for you with single angles, but to do it yourself you'll need to go through a little bit of math, including finding the product of intertia, Ixy. When you have that information, you can recompute your moments to be about these principal axes instead of the geometric axes and proceed with the strain compatibility calculations to get 4 or 5 critical points on the strength interaction diagram. You will likely end up with moments about both axes which makes the interaction "surface" a little more difficult to visualize since it will now be 3-D instead of 2-D. After the strength considerations are worked out I would do a buckling check with rz, or rmin, by developing a DE for your specific beam-column or checking to see if one that is in Timoshenko's "Theory of Elastic Stability" fits your case.

 

[benjaman]

Thanks a lot for the responses.

Regards,
Benjaman

 

[Jaweedmd]

The requirement is very specific.."Design of irregular shaped Reinforced concrete column"......

The software called IRRCOL, a part of PCACOL from portland cement association PCA is most suitable for RC columns of any shape. It has the options of design / check of the column. Even slender columns can be designed.