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Base Plate Moment Connection

Base Plate Moment Connection

Base Plate Moment Connection

(OP)
In the attached configuration, a column will be designed to transfer bending forces to the CIP pile footing below. Mu is relatively low (about 11 k-ft). Due to some space concerns around the base of the column, I am attempting to place the bolts inside the column flanges. Although, I know that this is typically assumed to act as a pinned connection, I don't see any problem designing it as a "fixed" connection to transfer moment to the foundation. In my mind, the mechanics of the problem are no different than a "flush-end moment" connection in a steel frame. As long as the AB design is adequate, the column is properly attached to the base plate, and the base plate has sufficient rigidity to transfer the forces from the flange to the ABs.

Any thoughts? Concerns?

RE: Base Plate Moment Connection

You'll likely get more deflection and rotation that you would expect out of a traditional fixed base plate. In flush end plate connections they address that with the gamma_r factor. Essentially, increasing the required capacity of the connection to limit the inelastic rotation at lower load levels.

But, conceptually, these base plates can certainly take some moment. It's just a matter of how much you are comfortable with. I remember doing this some years ago with a VERY lightly loaded column. I was a junior engineer at the time and we had an existing footing (and anchor rods) where the loading on the column changed slightly so that there would be moment in the plate. We tried this in an attempt to save some $$ for the client. The connection would have worked, but I believe we eventually got over ruled by the EOR who went with a more traditional Anchor Rod arrangement.

RE: Base Plate Moment Connection

You are getting more away from a fixed assumption which further redistributes moments. If I felt comfortable the above structure can take the movement required to engage the bolts then I don't see why not. Based on that detail, it should be modeled as a spring to check what happens to the above structure if you are not assuming it pinned in the analysis.

RE: Base Plate Moment Connection

I agree the moment is quite low......

Three things affect the actual "Fixity" of the base......
1) Elongation of the anchor bolts
2) Bending of the base plate
3) Rotation of the pier/footing

Although there is a way to calculate the actual "Fixity" using equations developed for each of these mechanisms, I generally take the conservative route and design the connection for FULL FIXITY even though we know that it will be somewhat less.

The question you must answer is what to design the column for......

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