Beam/Column Moment Capacity Ratio 1

[BONILL]

While designing special moment frame in ETABS, I have the following situation:

I have 20"x20" reinforced concrete columns supporting 16" Wide x 36" Deep beams on the top floor. Since there is no column above the joint, and at least 3 beams frame into my column at every joint, the Beam/Column Moment Capacity Ratio is exceeded. I would need too big of a column section to get it to work. Please advice as to any other possible solutions.

 

[BAretired]

What is the problem? You either increase the size of the column to make it work or you satisfy yourself that a crack in the column is not the end of the world.

BA

 

[Ron]

BA...right as usual

 

[BONILL]

Thanks for your response. But is a crack in the column the only result of not complying with the Beam/Column Capacity Ratio?

 

[a2mfk]

I think BA is saying make it bigger bro, and it sucks that you have to tell the architect to bump his wall out a few inches but sometimes we have to do that, even if we already committed to asmaller dimension....

Did you play around with the beam size at all?

Though I don't think its recommended, fixing it at the base and designing the foundation for moment is another option...

 

[ron9876]

If this is a gravity condition then maybe a column with a smaller I would reduce the moment to an acceptable level.

 

[CTSeng]

I'll often go to pinned top of the column at the roof level by using minimal dowels centered in the column and some bearing pad material to form a cold joint. May require a little more reinforcement in the slab/beam structure but better than telling an architect that the top floor columns drive the overall column design.

 

[PUEngineer]

Are you running the full amount of required positive moment steel at midspan through the beam-column joint?

 

[PUEngineer]

Regarding the post about cantilevered columns...I'm not sure that's allowed by the code where a special moment frame would be required, I could be wrong. And the literature I've read says it's typically required to hook all longitudinal column bars at the tops of the columns, because the axial demand of columns at the roof basically makes the column act more like a beam than column. So, a hinge will want to form right under the roof framing in the column.

NIST has a good paper on special moment frames.

http://www.nist.gov/manuscript-publication-search.cfm?pub_id=861617

 

[BONILL]

I'm not too comnfortable with the idea of assuming a pinned condition at the top but will sure give it a try. PUEngineer I am running the full amount of required positive moment steel at midspan throught the joint.

 

[PUEngineer]

In cases where the joint exceeds the 6/5 moment ratio, why not try cutting off some of the bottom steel prior to the joint. At the face of the column, your positive moment capacity only needs to be half of your negative moment capacity.

 

[BONILL]

Thank you all for your recommendations.

 

[BONILL]

On that same note, what is the difference of having a concrete column below and a steel column above the floor, and a concrete column below and no column above the floor. ETABS does not run the Beam/Column Moment Capacity for the first condition but does so for the second condition. Why is that?

In addition to cutting of some bottom steel prior to the joint and increasing the size of my columns, wouldn't increasing the steel area of reinforcement at each face of the beam help as well?

 

[BONILL]

I meant at each face of the column to increase the moment capacity of the column.

 

[PUEngineer]

I'm not an ETABS guy, so I can't really comment on the first. I can say that they sound like they should be treated similarly, because I wouldn't count the steel column above into the strong column / weak beam equation.

Increasing the column strength would help. I assumed you'd done this prior to trying to increase the column size. But the column vertical bars need to be developed according to the beam requirements.

Did you download that NIST paper that I referenced? It's very good, deals with detailing procedures also.

 

[FinnB]

What not redistrubute some of the moment from the connection i.e. top moment into the mid-span of the beam. I would have no problem reducing the beam top moments at the support by 30 percent and then adding 30% moment into the beam bottom steel as a trade off.

You obviously have to calculate the redistrubtion a bit more accurately than I have outlined. Will reducing all you beam end moments at the column by 30% be enough to get the column to work for the size you have chosen?

Your columns seems quite big for roof loadings. Do you have very long beam spans or cantilevers?

 

[BONILL]

FinnB,

I am currently working with the moment redistribution.

Thanks all for all your helpful comments.