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Special RC Moment Frame Condition

Special RC Moment Frame Condition

Special RC Moment Frame Condition

(OP)
Dear All,

It would be nice to get some clarity on seismic design of frame elements which are not designated as part of the seismic force resisting system. My seismic design category is D.

I have a frame structure with a three bays. The outer bays are each 16'-0" while the middle bay is 35'-0". Based on architectural requirements, I am unable to satisfy ACI clause 21.6.2.2 for the columns which support the 35'-0 beam. That clause states that the flexural strength of the columns at the joint should be 20% greater than that of the beams.

Initially I thought that I could simply consider the 35'-0" beam as a collector between the frames on each side for lateral loads. Clause 21.6.2.3 however states that I must ignore the columns which support the beam as well. Based on this clause, my understanding is that I cannot use a special moment frame unless I make the column supporting the 35- beam really massive. Is my interpretation correct? Is there any way around this?

Thanks

RE: Special RC Moment Frame Condition

Does the column have to be "massive" or just more highly reinforced?...or a combination of both?

You might also include the version of ACI 318 that you are using.

I don't see any way around the restriction....as you say - 21.6.2.3 allows an out by ignoring those two interior columns. This would be done by including a flexural pin in your frame analysis at the top of the two interior columns, thus negating their participation in the lateral resistance. You would be left then with a 67 ft. wide moment bent. This would perhaps require more rebar in the short beams, driving up the resistance needed for the end columns.


RE: Special RC Moment Frame Condition

(OP)
Thanks JAE,

The architect wants a maximum column size of 16x16. Since our beam is going to be about 30" deep I knew it would be problematic and the column would be too highly reinforced.

I am using ACI 318-11.

Ignoring the middle columns and having a 67' wide frame would probably require even bigger columns.

I probably need to look at using shearwalls instead of moment frames.

RE: Special RC Moment Frame Condition

I'm presuming that you must use special moment frames per the seismic area you are in?

In other words, is there a way to bypass Chapter 21 by using an R=3? In a high seismic area this may not be possible.

RE: Special RC Moment Frame Condition

(OP)
Yes JAE,

I am in Seismic Design Category D.

RE: Special RC Moment Frame Condition

16" deep columns and 30" deep beams will obviously violate the "strong column / weak beam" philosophy.

RE: Special RC Moment Frame Condition

What depths are your 16' beams?

The 35' beams only need to be detailed per the requirements of 21.13.

So, can the 35' beam not have continuity with the 16' span beams? Then you'll minimize the negative moment capacity of the 35' beam at the interior column which is summed with the positive moment capacity at the support of the 16' beam.

Since you're attempting to make these 35' beams not part of the lateral system, they need to be detailed with the ductility to 'go along for the ride' but not cause intended load paths. So, the 35' beam shouldn't have many, if any, requirements for the longitudinal bar sizes other than positive moment strength.

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