SHEAR STRESS IN RC COLUMNS

[neukcm]

Hi everyone!

What is the best way to overcome failure of rc columns due to excessive shear under seismic loading? Normally column design is governed by axial and bending moments but in my case it is the shear stress at lower columns (first storey) that needs some attention.Any advice/idea will be appreciated much.

Thanks in advance.

 

[KootK]

1) Add shear resisting ties.

2) Increase the concrete section if your design allows you to use the concrete for shear resistance.

3) Adjust your lateral system to shift some of the shear away from the problem column. Shear walls etc.

4) Add additional, reliably present precompression to the columns.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[neukcm]

Thanks KootK, i'll keep those options and try to re-analyze again my structure.

 

[bowlingdanish]

4) Add additional, reliably present precompression to the columns.

Oooh. Are you able to elaborate on how this helps, and by how much? And what does the detail look like, like a PT strand that is tensioned down the centre of the column?

 

[KootK]

Are you able to elaborate on how this helps

Push one of your hands tightly against the other and then try to slide them past one another. Just like that. Compression usually helps frictiony situations

and by how much?

You'll want to consult the prestressed concrete section of your local concrete code for that. For simple friction and shear friction, the improvement in capacity basically scales up linearly with the level of compression. It's less intuitive, and less beneficial, for diagonal tension which, as you know, is usually the main concern in concrete.

And what does the detail look like, like a PT strand that is tensioned down the centre of the column?

Could be. It's a lot rarer in vertical elements than horizontal but some real world examples include:

1) Precast, prestressed columns.

2) Vertically post-tensioned shear wall systems.

Note that shear improvement is not the primary reason for prestressing in either of those situations. Just a side benefit.

The more common example is simply the presence of plain old dead load. Designers often don't take advantage of it because:

1) It's scary.
2) It's sometimes not allowed for seismic.
3) It's an accounting hassle.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deker]

Is the column part of your lateral system or is it a gravity column being detailed for deformation compatibility? Axial load can help but once you reach about 0.3 f'c Ag you limit ductility in the column, which can be counterproductive. I would start by increasing the column size if possible. After that you can look into using more shear reinforcing / high strength shear reinforcing or increasing the concrete strength (assuming axial load is high enough to permit using shear reaistance of the concrete in the hinge region).

 

[neukcm]

KootK/Bowlingdanish,

Actually i tried to worked out for Options 1 & 2, i don't have any idea about prestressing columns, will the strands be eccentric? or will it be distributed along the periphery of columns with concrete cover? If so then how will be the main longitudinal bars be distributed/placed? Sorry really don't have any idea about columns with prestress but i'm interested about that topic, maybe will do some research. Thanks

Deker,

Thanks, yes it is part of the lateral system and will also be exposed to high axial load due to unsymmetrical framing. I will try to increase my column and check proper orientation (strong-weak axis) and see whether i can control the excessive shear due to seismic. Thanks