Truss Gusset Inducing Moment Into Chord 1

[BS2]

I am designing truss web connections for a 10' deep, 70' long truss with wide flange parallel chords and double angle web connections. The configuration is similar to a warren truss with verticals at the "V"s.

The question I have is in regards to the braces inducing a moment into the truss chords. In order to develop enough weld from the gusset plate to the beam, alpha bar for the connection becomes larger than alpha. This appears to induce a moment in the chord in accordance with the uniform force method special case 3. I do not think the EOR designed the chords for this moment.

I would appreciate any advice or insight into whether this moment is truly induced into the beam. And also, if there is a better way to make this connection.

Thanks!

 

[StrPE]

Assuming the workpoints of all elements coincide with the neutral axis' there is no moment applied to the member. The moment you are referring to is an internal connection moment resisted by the weld and gusset plate.

 

[hokie66]

If the EOR designed the truss with the assumption of pinned joints, the fact that the joints actually see a bit of moment just makes the truss slightly stiffer. Not a concern.

 

[WillisV]

StrPE is right on. The only way moment is actually generated in the members is if the workpoint does not coincide with the neutral axis, the difference between alpha and alpha bar is an internal connection moment for gusset plate design.

 

[MiketheEngineer]

Yes

 

[BS2]

Thanks for the quick replies!

Yes, the workpoints coincide at the neutral axis'. This makes me think that there should be no moment applied to the chord. And I agree that this is an internal connection moment. However, looking at the beam free body diagram in the AISC steel manual UFM special case 3, there is a moment, Mb, from the gusset interface that appears to apply to the beam. If the connection is relatively small, this moment should be small. However, if the connection is sufficiently large, this moment could be fairly large and it appears that it needs to be considered.

@MiketheEngineer - I assume you are referencing a better way to make the connection. If so, would you please elaborate?

I appreciate all of the excellent feedback and I apologize if this is a trivial problem... All of my training tells me that if the braces are coincident through the neutral axis then no moment is applied to the chord. But something is just not "clicking" as far as the gusset/chord interface moment.

Thanks!

 

[StrPE]

Mb is the moment at the gusset-beam interface. The overall moment on the joint, i.e.beam, is Vb*alpha - Hb*eb, which should approach zero.

 

[MiketheEngineer]

Nope - just agreeing with what was said above. The more engineers agree - you might be getting the best answer!!

 

[BS2]

I think it is making sense now. Thanks everyone for their input!

 

[connectegr]

I agree with StrPE...

Another way to look at it is using KISS method, or simple vector analysis. Consider the appropriate eccentricities in the welds and/or bolts, and prying and bending in the connection material. And provided the work point is at the neutral axis of the truss chord, there is no moment induced in the chord.

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