Circular Concrete Column Shear Capacity

[Bender888]

Hello.

I am a freshmen structural engineer - (12 weeks experience lol) so sorry if this is an obvious question

I have circular concrete column with a diamater of 600mm reinforced around the perimeter with 8 N20 bars - (20mm diam, 500Mpa)which can be seen attached.

This column is subjected to Axial Compression, Bending and Shear.

I used Rapt to calculate the ultimate capacity for the bending, compression interaction. Not sure how you guys do it but it takes to long by hand to plot the interaction curves lol.

I ran into a problem calculating the shear capacity of a circular section reinforced around the perimeter since there is no direct reference in the standard.

According to the Australian Standards 3600 the shear capacity of a rectangular section is a function of the distance from the extreme compression fiber to the outermost tensiles steel - (do). I am not aware of other design standards just make note i'm not refering to the effective depth - (d)

For an approximate analysis I have treated the the circular section as a equivalent rectangle and calculated The shear capacity (Vuc = B1*B3*bw*do*cuberoot((Ast*f'c)/(do*bw)).
I made the appropiate reduction for the axial force through the coefficient B3. I used the neutral axis from the rapt results to calculate my Ast which takes into account the prying action of the reinforcement.

The distance (do) I used can be seen in my approximate diagram.

Is this form of analysis acceptable?

I do not have exact details of loading conditions and section details with me but was wondering if anyone has some adive on the problem or some resources in the dealing with this problem.

 

[Ron]

Determine an equivalent rectangular section and compute from that. It will be conservative.

 

[Bender888]

Yeah that's what i did, as mentioned above.

But is this too conservative? Or is it the way it's done.

 

[asixth]

Bender888,

Welcome to EngTips. You must of started work straight out of uni, most companies give the graduates a bit of a holiday and bring them in after the new year. Anyways, it's good to see that there are still jobs out there for the graduates. When I graduated I was getting plenty of employment opportunities, however, I feel things have changed of the last 6 months.

May I ask what is the situation of your column (building/carpark) and is it forming part of the lateral force resisting system. Sect 10 in AS3600 doesn't mention checking columns for shear. Saying that, if your column is behaving like a beam, it must be designed for shear in accordance with section 8 of AS3600.

I recently had an example where I was designing a bridge pier which is subject to significant hydraulic forces during a flood event and as a result, I was required to bring the tie spacing closer together than what is required for columns to account for the high shear forces.

AS3600 doesn't mention circular column sections so you will have to source a copy of ACI318. Basically, the shear calculations for concrete break down to the following:

B1 => depth of the section
B2 => presence of axial forces (compression increases shear capacity). Only use this is you require the additional capacity. Remember, if your column doesn't have the compression force that you have accounted for, you won't have the additional capacity.
B3 => strut action from concentrated point loads (don't use this factor)
bv.do => shear plane
fc^1/3 => shear strength of the concrete
Ast/bd => contribution to shear strength from the reinforcement.

Basically, the ACI approach requires you to make a valid assumption on the shear area (0.8*D^2), and a valid assumption on tensile steel, you should be able to calculate how much steel is in the tension zone as you have mentioned above.

For a helix or circular tie, you take two vertical legs and the spacing of the shear reinforcement is either the pitch of the helix or the spacing of the circular ties. I don't think rapt can calculate capacities for circular beams so you may need to do some hand calcs for this one.

The relevant section in ACI318 are 11.3.3 and 11.5.7.3.

 

[Bender888]

Thanks heaps for the reply.

I'll check out those sections in the ACI.

Sorry, above I made a mistake. I meant to say I increased the capacity through the factor B3 which factors in the compression load not reduced.