Concrete Column Unsupported Length 1

[amecENG]

I am double checking what the standard practice is for determining the unsupported length of a concrete column on the first storey of a building (without getting in to anything like horizontal soil springs, etc) .

I have a situation where my footing was lowered about 9' below the slab on grade to accommodate some piping. Is the unsupported length from:
A) the underside of the slab to the top of the footing
or
B) the underside of Level 2 slab to the slab-on-grade. The slab-on-grade is 5" thick with mesh and the soil is properly compacted.

 

[DaveAtkins]

I would consider the below grade member to be a pier, not a column--braced in all directions.

DaveAtkins

 

[PAstructural08]

Sounds like the pier is also the column; is it continuous from the footing to level 2?

If that is the case than both would be the unbraced length but worst case I think would be B. But you can only use the slab as lateral support if it is tied into the column with reinforcing.

 

[KootK]

I vote option 2.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[asixth]

I would actually go 3' below slab on ground as the point of lateral restraint. And I am saying this because I guess most of the lateral restraint of the footing would come from friction at the base more than the passive resistance of the soil.

And I would use 3' because that seems to be a common assumption where fixity is assumed for the lateral restraint of piles.

 

[amecENG]

Thanks asixth, I think that is a valid assumption

 

[KootK]

With the 3' below grade assumption, would that be considered a fixed base at that level (K~0.7)? Or pinned (K~1.0)?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[asixth]

I would use a K towards 1.

I would rely on the soil to provide the lateral restraint required but wouldn't try to create a rotational fixity of the column.

 

[hokie66]

asixth's assumption is similar to my thought process for this type situation. I wouldn't assume a brace point by the slab on grade.