concrete column

[sayed]

i'm having an issue on this problem. I am being told that this condition leads to a "pinned" connection of the column to the stem wall. (there are also ties along length of column)

My issue with this is that a) wouldn't the couple forces within the column inherently make this moment resistive? b) would section b-b be equal or greater in strength than section a-a?  surely one can't assume a pinned connection anywhere along the column.

assuming you can only consider this pinned, how would the rebar placement need to be changed in order to make the column/stem-wall interface a moment resistive element?

 

[TheStructuralEngineer]

Based on my understanding, the connection is capable of transmitting moment provided that the dowels are corresponding to column reinforcement and have adequate development length. 

 

[tj_PE]

yes, it looks like the dowels (based on their apparent scale) are only for stability/shear resistance. you would have to provide a lap between dowels and vertical reinforcement within the column, and as tse said above, the dowels would have to have adequate development into the foundation as well. 

 

[MA_PE]

Typically lapped bars are shown with a little space to show they're two separate bars.  That being said, the cold joint between the two members may not provide sufficient bearing to facilitate transfer of the compression side of the moment with out some local spalling so it wouldn't be the same as the continuous section at A-A, although assuming there is sufficient lapping of the reinforcing bars it would certainly have some moment capacity.  However, for design purposes it is conservative to assume the connection is pinned and not fixed.

 

[sayed]

Typically lapped bars are shown with a little space to show they're two separate bars.  That being said, the cold joint between the two members may not provide sufficient bearing to facilitate transfer of the compression side of the moment with out some local spalling so it wouldn't be the same as the continuous section at A-A,

that's kinda hitting the nail on the head. I knew corners were weak points, but i suppose this weak link is what makes the pinned assumption a good one

 although assuming there is sufficient lapping of the reinforcing bars it would certainly have some moment capacity. 

well isn't there a required lap length by code?  i figured you couldn't call out a lap like this without conforming to required length (i think about 42D ?)

is the moment resistance entirely dependent on the lap length and nothing else? Is there no configuration of rebar that 100% makes it a fixed condition?

 

[EBAT75]

Old post, but new forum member, so let me see.

If the schematics and proportions are correct, this is an eccentrically loaded case.

At the column/wall interface, there would be a moment in the stem wall equal to column load times eccentricity. If this is a pin connection, can there be strain compatibility between the column base and the stem wall? I don't think so. Wouldn't a column base plate be needed for that?

Just R/F and comp development length in the stem wall and splice length into the column is for non-eccentric loads. Moving on down, the stem wall and the footer themselves would have eccentric loading.  Assuming a pin does not make it behave as one and prevent say spalling. An assumption must be compatible with how the structure behaves under the load conditions.

Unless constraints such as setbacks, grades limit the choice, this kind of eccentric arrangement may not be the better option.