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# Column design for special moment frames

## Column design for special moment frames

(OP)
In all of the examples I have seen regarding column shear reinforcement for a special moment frames, they use a column that is not at the top of the building, they all have a column framing above and below it (ACI 318-11 R21.5.4). So when the calculate Mpr for the beams tying into the column, Mpr+ and Mpr- from the top beam and Mpr+ and Mpr- from the bottom beam (see attached). Mpr for the column is Mprcol = (Mpr+ and Mpr-)/2 and the Ve = 2*Mprcol.

If this was a column at the top floor, you wouldn't be able to assume the load goes above the top floor, so you would end up with more Ve at the top column and that seems wrong, is it?

Everything I have read, says for a column you Compute column shear force Ve associated with the formation of plastic hinges at the ends of columns and this force need not exceed that associated with formation of plastic hinges at the ends of the framing beams. So if I was calculating Ve for the top floor, if Ve from that associated with formation of plastic hinges at the ends of the framing beams is greater than that associated with the plastic hinge at the end of the column, I only need to use that associated with the column, right?

Any help would be appreciated

### RE: Column design for special moment frames

#### Quote (OP)

If this was a column at the top floor, you wouldn't be able to assume the load goes above the top floor, so you would end up with more Ve at the top column and that seems wrong, is it?

No, it's correct as you've described it so long as your hinge moment is governed by the beam.

#### Quote (OP)

So if I was calculating Ve for the top floor, if Ve from that associated with formation of plastic hinges at the ends of the framing beams is greater than that associated with the plastic hinge at the end of the column, I only need to use that associated with the column, right?

Correct so long as as your column plastic hinge takes proper account of any axial load in the column. Of course, axial column load at the roof is probably low.

Roof level joints are the one location where the plastic hinges can sometimes be allowed to form in the column rather than the beam. And it's for reasons just like those that you've raised here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

### RE: Column design for special moment frames

(OP)
when you say "so long as as your column plastic hinge takes proper account of any axial load in the column" you are referring to ACI-318-11 21.6.5.2 Transverse reinforcement: a) and b):

(a) The earthquake-induced shear force, calculated in accordance with 21.6.5.1, represents one-half or more
of the maximum required shear strength within lo;

(b) The factored axial compressive force, Pu, including earthquake effects is less than Agfc′ /20.

### RE: Column design for special moment frames

No. The point that I was trying to make is that, in some instances, increasing axial loads on your columns will improve moment capacity and delay the development of column plastic hinges. You can see this by examining any column interaction curve. from the bottom of the curve to the balanced point, more axial load is resulting in greater flexural capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

### RE: Column design for special moment frames

(OP)
Thank you very much, your help is greatly appreciated

### RE: Column design for special moment frames

You're quite welcome coeman. Best of luck with your project.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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