Moments in Column

[WpgKarl]

I have a 3 level precast column, with corbels on one side, supporting precast beams at each level (The column is an end column in a bay). The columns are tied at each floor at the tops of the beams, and are braced in the other direction by the floor. Based of the column is assumed to be pinned, and the base of the beams cannot take any significant horizontal load, as it sits on a bearing pad.

Each floor level load will tend to bend the column outwards, so looking at the deflected shape of the 3 level column, would I have double curvature?

 

[shin25]

The column is pinned at each floor level. So, the maximum moment and compression will be at the lower level with an unsupported length equal to the height of the lower level.

 

[WpgKarl]

Thanks shin25. Actually what I wanted to find out about was whether the column would be in single or double curvature, as I need to calculate M1/M2 for the slenderness check, and M1/M2 is +ve for single curvature, and -ve for double.

Drawing it out, I can't see how it would be anything but double curvature....

 

[twinnell]

No double curvature since all connections are pins.

 

[PMR06]

I agree, WpgKarl. I think the columns between the upper floors will be double curvature, but the first level (assumung pin at foundation) will be single curvature.

Can you create a simple RISA model of the column with cantilevered pieces acting as corbels, and see how it deflects?

 

[shin25]

If I am not mistaken, for the lower floor, M1=M2, so Cm = 1.0.

 

[twinnell]

I am wrong. PMR06 is right.

 

[shin25]

I in fact said it wrong, since the base is pinned, M1 = 0 for lower level.

 

[WpgKarl]

Looking at it again, if the moments at each floor level are all in the same direction, I would guess double curvature - just by sketching out how the column would rotate above and below the floor levels and then trying to "connect the dots."

If the moments were of opposite sign at each floor, then they would act together to bend the column outwards in single curvature....

see sketch below - found it in one of my textbooks - thanks all for your comments

 

[haynewp]

Do you work for a precaster? I am just curious why are you trying to design the precast columns.

 

[csd72]

To get a rough estimate of the column moment diagrams.

1. apply the moment from each corbel to a section of colum 2 stories high pinned at top and bottom and reatrained laterally at the corbel.

2. superimpose these on each other to get an overall moment diagram.

this will be reasonable close to your actual diagram.

As noted above you will find they the bottom floor is in single curvature but many of the top floors are in double curvature.

csd