Modify RC column stiffness

[FreshMan2020]

I have this flat slab sitting on top of forest RC columns. A couple internal columns fail in minor axis under gravity loads unless I increase the width of the column, which the client is not happy to do. My question is, can I modify the column stiffness (say 0.35 Ig) to let it 'crack' so it will attract less bending moments and thus pass? (I will design the slab accordingly and the column wont be used for lateral purpose) If yes, what the crack will be like, will it lead to any failure and if there is any particular connection detail required for column-slab? If no, why can we modify the out-of-plane stiffness of RC walls?

 

[r13]

I think it is fine, but wonder why the column fails under gravity load case.

 

[Ganesh Persaud]

Firstly, what software are you using?

Secondly, instead of first increasing the size of the column, check the reinforcement that was specified for the analysis and design of the columns. I’m an ETABs user so I’m familiar with Some of the errors that you might get. Like retired13 said, not sure why your column would fail under gravity loads? That’s weird! Model might be corrupted.

You can also perform some basic hand calc using the tributary area method and check that specific column in your design.

Walls are generally not designed for out-of-plane bending to avoid excessive longitudinal reinforcement. In this case, use a small modifier so numerical instabilities could be avoided.

 

[JAE]

Hard to crack a heavily axially loaded column. Too much P/A compressive stress to overcome - of course this depends on the relative axial load vs. bending.

 

[Celt83]

If you are failing weak axis bending then your column could likely check out as slender, make sure you also check the amplified moments and not just the slab reactions on the column. I'd push to increase the column size.

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[BAretired]

Increasing column size attracts more moment.

BA

 

[jayrod12]

I'd be more inclined to provide a higher concrete strength, or perhaps you need to play with the reinforcing layout, providing more of the 1-4% of reinforcing on the side faces instead of the ends to improve the weak axis bending capabilities. Many programs just default to equal reinforcing on all sides, but that doesn't mean you have to follow that. You should be able to specify whatever layout you would like.

 

[r13]

Are there unequal spans, with one much longer than the other, in the weak direction of the column? Or excessive load at one span?

 

[rowingengineer]

Can we please get a sketch or screen shot. The description isn't great as to why this would be an issue to only a few columns.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[KootK]

1) Similar to retired13's comment, I feel that an important aspect of this is identifying the reasons for your column moments being as they are. Can you elaborate?

2) What are the dimensions of your column and is it strong or weak axis moments causing the issue?

3) To gauge how much moment we're talking about, what is your load eccentricity? e = M/P?

4) You can tinker with stiffness as long as you're approach to that is realistic and, as JAE mentioned, gives due account to the presence of axial load.

5) You might shed moment from your column by stiffening up the beams / slabs delivering moment to your columns.

6) As Celt83 alluded (I think), your situation might benefit from a column design that accounts for slenderness and P-Little-Delta amplification by way of more advanced procedures such as baking that stuff into your initial model. The classic code provisions for slender columns can be quite conservative. Here are a couple of good articles to get you started along the path to righteous, slender column design:

a) StructureMag
b) The Late/Great MacGregor

7) One "cheat" that I know of is to design your column as K=1.0, pretending that it's pin-pin, but then:

a) Use seismic capacity design factors on your material factors and;
b) Keep your axial load level below the balanced point on your interaction diagram.

This usually ensures that your rebar would yield in tension (ductile) before your concrete would crush in compression (less ductile). Unfortunately, this procedure in and of itself usually leads to a pretty stocky column.

 

[KootK]

If yes, what the crack will be like, will it lead to any failure and if there is any particular connection detail required for column-slab?

I wouldn't attempt any special connection detailing to try to encourage pin behavior. I would, however, design the slab for punching shear assuming full restraint between the slab and column to ensure a safe punching shear design.