Truss modeling and connection question..another one

[faromic80]

I have some more questions about trusses and decided to start another thread because the one I started the other day is a little off topic. I have attached a scan of some questions I have on a truss I am analyzing. I got these shop drawings and have to verify everything works.
From what I understand, truss members have only axial loads, but this is not the case here. In the attachment I show the model i used for hand calcs: simply supported truss. Then I show the model with the existing columns coming up the sides. The top and bottom chords are attached to the existing columns at the sides. I also attached the RISA output for the top and bottom chords. The bottom chord has moment and NO axial loads?? The (2) C10x15.3 channels can't handle that moment. I'm thinking this is because of the connection to the existing columns? the top chord has both moment and large axial forces in it and I can't understand why? because of the distributed load on the top chord?
Also, the two center diagonal M15 and 16 have no axial force in them?
I don't exactly understand the splice connection at the top chord. I don't understand why the the two plates are welded on the the larger splice plate...is this for out of plane buckling reinforcement?

You can also see in the RISA model at the intersection of M8 and M9 the members don't coincide with the center of the top chord (see truss shop dwg) how can I model this in RISA?

As for the model I think it's pretty accurate because my hand calcs for the joint forces is within +-10 kips (attached)

 

[kslee1000]

Just for fun, try the following if you haven't done it yet:

- No release on the top & the intermediat column joints.
- Release vertical & moment constraints on the bottom chord members immediate adjacent to the columns (member end on column side).
- Release the rotational restraint on the remaining truss members.

 

[kslee1000]

Final words:

Sorry missed your sketch shown loading, the uniform load will at least causing moment on the top chord members. A truss usually takes concentrate load at the panel joints, not uniform load, through some arrangement, such as purlins/girts...etc. For your system, if you have continuous top & bottom chords, it acts as a deep beam rather than a typical truss.

 

[faromic80]

Something interesting also....
when I model the truss with dummy members and then model it with the actual members I get approximately the same reactions in the members. Actually the chord axial forces decrease by 15 kips with the acutal members being modeled..?

 

[rb1957]

that sounds odd to me ... the truss-beam doesn't look that redundant, implying the internal loads should be reasonably independent of the element areas ...

how are you accounting ofor beam column effects on the upper chord ? consider the middle span/element ... it's in compression and has a transverse distributed load

 

[faromic80]

I am designing the top chord as a beam column element. I check the spans between the splices and then the splices for combined loading. I've attached a sketch of my configuration. Speaking of the splice connection. I'm concerned about how the load from the diagonals is transferred through the gussets. Is there a moment (in plane torsion) induced on the gusset plates or is there not because the connection is symmetrical??? I just can't seem to visualize this connection.

 

[kslee1000]

Suggest to draw a free body diagram at the notes with results from your model analysis, it provides better idea.