Using different concrete strengths for columns and slabs. 1

[HermanLJ]

Refer to image for example:

http://imgur.com/2ZaS3CG

Now I've been given conflicting advice by two different people who are both considered as structural specialists and are employed as such (to the extent that they are flown around globally to give advice on projects).

- The one says that you cannot design a column for a higher compressive strength if a small section (e.g. the slab) has a lower compressive strength.

- The other says that due to the confining properties of the slab, one can ignore the small section cast at a lower strength.

Who is right?

 

[JAE]

The second person is more correct if you are using ACI 318 (see ACI 318 section 10.15.)
Not sure about British, Canadian, other codes.

If the column f'c is not more that 1.4 times the floor f'c, then you design the column with the higher f'c strength.

If you are above the 1.4 limit, then you need to follow the provisions in 10.15.

 

[hetgen]

Hi HermanLJ

Confinement does increase bearing capacity of concrete; It is common to have a higher strength column combined with lower strength slab.

 

[HermanLJ]

Thanks! I don't have access to the code but does it make provision for downstand beams as well? Ie the column cast at the same strength as the downstand beams.

 

[hokie66]

Your first expert is wrong, and you should take his/her advice with a grain of salt.

Transmission of axial forces through floor systems is typically addressed by codes with provisions similar to those JAE referenced in the ACI Code.

Not sure what you mean by downstand beams, as they would typically be part of the floor system.

 

[dik]

JAE...

Canadian code allows for an increase in bearing for confined concrete. Similar to the ACI...

Dik

 

[kieran1]

See report number 296 on page 4 of the attached.

Kieran

 

[asixth]

I have used 80Mpa columns bearing thru a 40Mpa floor slab without requiring excessive confinement reo thru the slab zone. As long as there is a substantial diaphragm to confine the transition zone.

The confined concrete has capacity for 2*f'c where the column will have capacity around 0.5*f'c depending on the slenderness.

 

[TXStructural]

Common practice, where the columns are substantially higher strength than slabs, is to puddle the higher strength concrete at the column when the slab is placed. This provides continuity of concrete strength.

 

[hokie66]

TX,
That is common practice where required, but it is not necessary in most cases. As JAE and asixth have pointed out, confinement can make puddllng unnecessary. 1.4 to 2.0 x fc' of the floor slab can be tolerated, depending on the degree of confinement. Another way is to increase reinforcement through the joint.

 

[dik]

Unless you really need to... I'd stay away from 'puddling'... a real source for trouble, either punching shear with cold joint or 'forgetting' to do it...

Dik

 

[cliff234]

I agree. Avoid puddling if possible. Coordinating the placement of two different strength concrete mixes in the slab at one time is difficult - and impossible if the concrete is being pumped. ACI 318 allows designers to ignore the difference in strength as long as the column concrete is not more than 1.4 x the slab concrete strength. Engineering judgment must be used if you have drop beams, staggered floor slabs framing into the column with drop beams (i.e., a column between the split in the ramp of a garage with one-way slab and beam framing, etc.)

 

[BruceSPT]

Remember that corner and perimeter columns do not have confinement on all four sides of the column. Also columns around openings will not have slab or beam confinement.
Puddling can be done with experienced contractors but inexpereinced contractors usually avoid puddling.
Often contractors will pay the extra cost to increase the strength of the floor concrete to avoid puddling.

Although not covered in ACI 318, the response by ACI 318 committee to a question in ACI's Concrete International also indicates the same requirements apply (puddling) for higher-strength concrete walls and lower-strength concrete floors.

 

[aaronPTeng]

depending on the structural system, I have found typically with the Post tension concrete mix in my area it achieves a much greater 28 day strength that is specified. I would investigate this with your contractor as it gets me out of trouble without the additional cost of specifying a higher mix.

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[hokie66]

As JAE pointed out earlier, as long as the column fc' does not exceed 1.4 x the floor fc', no other measures are required. Above this value, confinement rules apply.

aaronPTeng, I think that is a slippery slope. If you need a higher strength, specify it.

 

[amecENG]

The Canadian Code (Clause 10.12.3) gives some interesting formulae for this condition:

The resistance of the column in the joint region shall be based on an effective concrete compressive strength f'ce equal to:
(a) for interior columns:
f'ce = 1.05f'cs + 0.25f'cc <= f'cc

(b) for edge columns:
f'ce = 1.4f'cs <= f'cc

(c) for corner columns:
f'ce = f'cs

where;
f'cc = column compressive strength
f'cs = slab compressive strength