Which Moment for Design ? 2

[palk7 EIT]

Hi,

In case of fixed connections with beam's to the opposite sides of a column, the moment gets cancelled as they are in oppos. direction to each other and the resultant moment that goes to the column is minimal, however in reality its not necessary for the beam to be equally loaded on both sides as one beam might be loaded and other might not be loaded & under this circumstances the beam to column moment will be significantly higher. So which moment is to be taken for the design of beam to column connection?
1). The one where the end of the beam is higher (39.4 Kip.feet)? or
2). The resultant cancelled moment due to opposites sides which is very minimal 4.431 (Kip.feet).

Attached the pic, its not a actual building.

Thank you very much

 

[GastonLagaffe]

I think most building codes will specifically say that you need to consider the case with full live load on both sides (max axial load), and the case with live load on only one side (less axial, higher moment). Snow & wind will have similar provisions as well, though you might have a 25% on the other side or something along those lines.

I notice that you're specifically asking about the moment for the design of beam-to-column connection. In that case you'll want to look very closely at your load paths. That'll likely dictate what moment you should be applying - the other span may very well not balance out your moment at the connection.

 

[phamENG]

Gaston is correct. You have to consider both. The column will have maximum axial load with full loading within its influence area, but will have maximum moment during a skip loading scenario. The beams would also have different moments at different locations, which is especially important if you're looking at reinforced concrete.

 

[palk7 EIT]

GastonLagaffe, Yes for the snow & wind there are partial loading & full loading, I am not sure about the live loads for that matter but I will check it. Could you please help me understand what you mean by "look closely at load paths. That'll likely dictate what moment you should be applying" I guess for the specific design of beam to column moment connection it should be like whatever moment is to the left of the column is taken for that beam to column connec. design (left part) and whatever moment is to the right of the column is taken for beam to column connec. design (right part) and forget the about the balancing stuff as it only affects the column moment and not the connection part, is that so?

 

[palk7 EIT]

In that case is it practical to design the beams as simply supported for gravity loads and design them as moment connection for lateral loads alone so that it will be easier from analysis part and the moment connection capacity can be designed for the maximum moment due to the lateral load's?

Thank you

 

[phamENG]

That's called a "flexible moment connection" in the AISC parlance. (Page 11-2 of the AISC steel construction manual 14th ed. if you have a copy.)

That may make the analysis easier, but it makes the detailing a little harder. You have to detail your connections to behave the way you model it.

 

[SWComposites]

The only thing that gets canceled out is the effect on the column itself. Each beam to column connection sees the entire moment from the beam.

 

[palk7 EIT]

Yes realized it later, overlooked it. Thank you

 

[HTURKAK]

You are expected to consider the combination producing maximum effect . In this case, if the effect is mid column moment due to gravity loading , the beam having large span shall be loaded with LL while short span only DL. If you are looking mid column axial load effect, you shall load the both spans with LL.. etc..

I will suggest you to look the subject of INFLUENCE LINES in any structural engineering book.