Moment for Moment connection design 1

[Ipetu]

I have a moment frame diagram with factored moments calculated. Please take a look at the attached frame and let me know what total moment the beam to column connections have to be designed for.

Thanks in advance.

 

[paddingtongreen]

That case is just for a horizontal load, from the right hand side if you draw your moments on the tension side.

If that is the only case, I would design all of them for plus or minus 68kip.ft.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[asixth]

Some steel codes require moment connections to be designed for a minimum of 50% of the structural capacity. For example, the Australian Steel Code requires:

"9.1.4.b. The minimum design action effects expressed either as the value of the factor times the member design capacity for the minimum size of member required by the strength limit state, specified as follows:

i) Connections in rigid construction-a bending moment of 0.5 times the member design moment capacity."

I would assume that the AISC of whatever code you are using will have a similar requirement.

The bending moment diagram that you have drawn is for lateral forces. The frame is going to have a negative moment at the knee joint from permanent loads which will need to be factored into the actions somehow. Australian codes require 1.2*G+Wu where G is dead loads and Wu is the ultimate wind event (typically 500 years).

You will need to ensure that the moment connection you are calling up will have the capacity to transmit the minimum design actions which I have indicated above.

Check whether a reversal of loads can occur, if they can then you will need to be specific about how you design the stiffeners.

Let us know if you need clarification.

 

[Ipetu]

asixth,

I appreciate your response. However, I want help interpreting the moment diagram that was attached to my original post. It appears you haven't looked at the diagram. Please review the diagram and let me know the net design moments between the beams and columns.

Thanks.

 

[paddingtongreen]

Ipetu, I noticed that some signs were omitted and that, as written, they don't add up. The diagram though makes it clear that the moments are caused by a horizontal load at the beam level. Neither the load nor the reactions are shown, and we don't know which convention they are using, have they drawn the moment on the tension side or the compression side, so we don't know which is clockwise and which is anticlockwise.

That was why I suggested designing for plus or minus. I suggested 68kipft. to make them the same, however you could do the outer beams for + or - 68kipft at the one end and 54kipft at the other, and the middle beam for + or - 40kipft at both ends.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[asixth]

Ipetu,

I made the assumption that it was a steel frame but it may be a concrete moment frame.

Looking at the moment diagram that you have shown, one beam is tranferring 94kip-ft into the column while the other is transferring -40kip-ft. Without knowing dimensions or stiffness of elements, I would expect the beams to transfer similar moments into the column, so I would expect that the beam moments transferrded to the column are 54kip-ft and 40kip-ft and the total moment in the column to be 94kip-ft.

If you could inform what the dimensions and stiffness of the elements are I can verify your results.

 

[hokie66]

asixth,
I think his diagram is labelled correctly. 94 is on the moment diagram, not on the member. They add up nicely, except perhaps for the sign convention.

 

[asixth]

Thanks, I can see that now. I thought at first glance because 54kip-ft was written next to the column that it was the column moment and not the beam moment.

Saying that, I don't see anything wrong with the bending moment diagram, it is correct.

Does your question relate to rigid and flexible diagrams? If it were a rigid diaphram and the columns are all equal stiffness then the bending moment would be the same for all columns. Because it isn't rigid and there is some flexibility the moments will be redistributed slightly and that is what has occurred for your example.

It doesn't matter whether the bending moment is drawn on the tension face or compression face, or whether the rotations are clockwise positive or anti-clockwise positive. Each to their own.

 

[paddingtongreen]

I have to disagree, asixth.

If the job of the structure is to resist a one direction force, we wouldn't want to design for a reversible force, and that is what we would have to do if we don't know which convention was used.

I regard the conventions as important because it is easier transfer information when you know which one is used.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[asixth]

I haven't come across a lateral force that is only applied in one direction. But if the structure is only subject to lateral force in one direction and it is not foreseen in the future that the structure will need to resist forces from the other direction than it would be feasible to design the structure for loading in that direction only.

 

[paddingtongreen]

Yes, one forgets that ones own experience might not be the mainstream, I spent a long time on oil refineries and power plants and they do have non-reversible forces.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Robert216]

Is the frame not mounted on the ground? Is the frame not resisting seismic loads?

 

[dik]

Assuming your beam is continuous across the top of the columns, the maximum beam moment is 68 K-ft. The interior column moment connection to the beam has to withstand 94 K-ft and the max moment between the column and the end is 68 K-ft. The diagram is reasonably OK with the interior moments being approx twice the end moments. They can vary depending on the relative stiffnesses.

The AISC has an excellent publication on the bolted connection of beams for moment connections. A matter of modifying the approach if you require the end span beam to sit on the column of if you can project the end column above the beam.

Dik

 

[dik]

I should have added that the Canadian code requires a minimum of half the section flexural capacity for splices if plastic design is used.

Dik