Using different concrete strengths for columns and slabs.
Using different concrete strengths for columns and slabs.
(OP)
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?
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?






RE: Using different concrete strengths for columns and slabs.
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.
RE: Using different concrete strengths for columns and slabs.
Confinement does increase bearing capacity of concrete; It is common to have a higher strength column combined with lower strength slab.
RE: Using different concrete strengths for columns and slabs.
RE: Using different concrete strengths for columns and slabs.
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.
RE: Using different concrete strengths for columns and slabs.
Canadian code allows for an increase in bearing for confined concrete. Similar to the ACI...
Dik
RE: Using different concrete strengths for columns and slabs.
Kieran
RE: Using different concrete strengths for columns and slabs.
The confined concrete has capacity for 2*f'c where the column will have capacity around 0.5*f'c depending on the slenderness.
RE: Using different concrete strengths for columns and slabs.
RE: Using different concrete strengths for columns and slabs.
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.
RE: Using different concrete strengths for columns and slabs.
Dik
RE: Using different concrete strengths for columns and slabs.
RE: Using different concrete strengths for columns and slabs.
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.
RE: Using different concrete strengths for columns and slabs.
"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
RE: Using different concrete strengths for columns and slabs.
aaronPTeng, I think that is a slippery slope. If you need a higher strength, specify it.
RE: Using different concrete strengths for columns and slabs.
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