Design concrete Column for Axial only 2

[hx200]

I am designing a concrete column and would like to design it for axial only. we have enough shear wall to take the lateral load but in reality the column will be cast together with beams and dowel bars and there will be moment at connections. I want to assume pin connection in the model because the column sizes are limited, and in reality the bar will be yield if capacity maxed out and lateral load will be taken by shear wall.I don't know if i need to do a section analysis to ensure concrete crush will not be the failure mode (for moment and axial load in reality case) at the connection.

Thanks for any comments or input.

 

[Lion06]

If you have shearwalls you can ignore any lateral moments that might be seen in the beam/column connection, but I would design it for the gravity moments.

 

[hx200]

Thanks PAStructuralPE. Is there anyway I can get by for the gravity moments, like use a cross type dowels or use dowels in the middle of the column?

 

[ishvaaag]

In my opinion neglecting the gravity moments in a continuity scheme could sometimes produce nasty cracks, and we must have seen here already people in complaints for less than such thing.

If you want to entirely neglect moment, disrupt moment continuity by any practicable (and allowed) device whilst retaining the whole stability of the outfit. This, ordinarily, is one of the fields of prefabricated structures, but you can port what convenient to your more in-situ case.

 

[ishvaaag]

I may add that the kind of detail with X reinforcement in the joints fell out of favor here in Spain when one hotel having strangled hinges with such detail as support of arches at the noble floor crumbled down with significant loss of life (in the seventie's, I think). The designer was a construction chairman at the ETSAM and the author of one of the most renowned books here on concrete of the era, and the poor man later commited suicide, what no doubt later would influence the general distaste for the detail. Quite likely the foundation dislodged and broke the detail, but these things is that we are trained to prevent.

 

[Lion06]

You're really only talking about unbalanced LL moment on the column. Can you add some reinforcement or bump f'c to take care of it. A column has some moment capacity at the max axial load.

What kind of column section, loads, and reinforcement ratio are you looking at?

I'm just surprised that you can get a column to work for axial only, but are having difficulty getting a relatively small amount of moment to work with it.

 

[csd72]

No matter what you do, there will be some moment from the eccentricities of the applied loads.

 

[ash060]

hx200 - you mentioned a model in your post. Are you modeling the frame in a computer? If so, I would not think that there is a big deal to leave in the moment connections the computer is doing the analysis.

And designing a concrete column for moment and axial is just putting it in a spread sheet or MathCAD.

 

[rapt]

It is very hard to answer this without more information.

If you have any type of earthquake in the area, design for the moment the column attracts!

If no earthquake, a smaller column will attract a smaller moment. But is will still attract it. If the column is cracked, it is ok to use cracked stiffness so that it attracts less moment. But you should design for the moment it will attract. Redistribution is not allowed from non-ductile members and columns are usually not ductile.

And, as mentioned above unsightly cracking is a real possibility.

 

[hx200]

Thanks for everyone's comments. This is a slender column at a end bay and there is no column above. The amplified moment is giving me hard time to keep the desired column size. I agree not designing for the moment could have too much detail work and still probably be too risky.