Pinned Connection details for concrete

[structech08]

Hello,

For concrete structure, how do you detail a pin connection between the column and foundation? Is it right to detail a moment connection in your drawings even if we assumed a pin connection in our structural analysis for foundation support? What are the consequence if do nott follow our support assumptions, say pin in the model but fix in the detail?

Thank you,

Structech08

 

[hokie66]

The simple answer is that a bit of extra fixity usually doesn't hurt. Pinned connections are often assumed for analysis purposes, but then reinforcement is provided to develop the inevitable and incidental moments which occur at the end of members. True pinned connections in cast in place concrete structures are complicated to build, expensive, and usually unnecessary. Exceptions to this general advice occur in designs where seismic detailing is a controlling condition.

 

[csd72]

The pin is normally at the soil interface rather than at the top of the footing. The soil will have some give over the additional rotational bearing pressure which will bring it closer to pinned than fixed.

 

[Teguci]

How do you detail a pin - sand pocket and centralized rebar (void the concrete at the base with sand which is later removed and replaced with concrete).
Is it right to detail a moment when analyzing as a pin? - Most of the time, yes. You will never have a full moment at the concrete to soil transition (soil will give). Assuming no rotational resistance is conservative and fairly standard.
Consequences - on a PT garage, when the strands are stressed the slab elastically shrinks. The columns attached to the slab are drawn to the center of rigidity. In some cases the top of columns can be pulled over several inches and a hinge will form. If the hinge is not detailed, you get cracks.

 

[hokie66]

Teguci,
If your building shortens by several inches, it is too long. Movement joints are required. And the elastic shortening by tensioning is only part of the story...normal drying shrinkage accounts for most of the shortening.

 

[structech08]

Hi Guys,

Thank you for your inputs. I appreciate it.

Regards,
Structech 08