Large Beam to Support Column dimension ratio

[TarikHKJ]

Hi,

What considerations should be kept in mind when dealing with big difference between column and beam dimensions?

for example in a ribbed slab
Main Beam dimensions: b=1000 mm , h=300 mm
Column dimensions: D=250mm circular

Thank you

 

[hokie66]

The actual difference in sizes is not what would concern me. The small column size is a worry, though. I would never even think of using such a small concrete column in a substantial structure. Is it a steel column?

 

[TarikHKJ]

It is for an existing 1 story home building, with a proposed second floor,
The second floor is designed to have a one way ribbed slab, with ribs going in the x direction in the picture attached.
The 2 columns C1' circled in the picture should be reduced in size as much as possible for architectural reasons.

https://imgur.com/a/3IlXpEq

The two beams intersecting at C1' are 1000mm width and 300mm depth.
The unfactored load on the columns is 140kN Dead(incl own weight, 100kN LL

 

[KootK]

General concern for the column being too small excepted, large discrepancy in member sizes is probably a boon to the column. If you think on it from a stiffness perspective, it means that:

1) Very little of the fixed end beam moment will make its way to the column and;

2) The column will be essentially fixed at the top in the plane of the beam.

One concern would be the lateral stability of the beam to column joint. I'd be looking for monolithic construction there and for the deep beam itself to be well braced by the floor system torsionally.

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.

 

[hokie66]

KootK,
It is a wide flat beam, not a deep beam.

 

[TLHS]

My only real worry would be punching, but that's easy enough to check.

I mean, obviously you won't get significant moment restraint from the column, but that's fine to design for.

Depending on your rebar arrangements and whatever you're doing to anchor the top of the column, it could also get mighty congested if you've got a couple of beams intersecting at the column lines, so you'll want to sketch that all out to scale to make sure it can be dealt with. I suspect your bars are pretty spread out, though, so maybe it's not a problem.

 

[TarikHKJ]

Yeah , punching is the first thing that comes to mind, what can be done to help with punching in this joint?

 

[KootK]

Main Beam dimensions: b=1000 mm , h=300 mm

So it is. Oops. That reverses most of my recommendations:

- no joint stability issues.

- lots of beam end moment making its way into the column and that should be considered in the design of the column and the column punching shear joint potentially.

- delicate issues at the ends as others have mentioned with punching shear and bottom bar support anchorage etc.

- less fixity at the top of the column

Good paper on the topic:Link. Depending where you practice, there's similar stuff in some codes. I've seen it checked a lot of ways, often concurrently:

1) Standard punching shear checks, three or four sided depending on how far you run the beam past the column.

2) One way shear check considering the effective shear width of the beam to be the column width + d.

3) Use shear ties running across the column to say that shear is being directed into the column from the sides by the stirrups, similar to conventional punching. At 300 deep, you're probably too shallow for stirrups or ties which, in it self, may represent a problem.



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.