trusses - realease conditions

[Structures33]

Hey all - I'm modeling a building for analysis in RISA 3D. The roof is composed of triangular, CFMF trusses. For a typical CFMF truss, what would the release conditions be? I know the individual web members should be input as pinned. What are the release conditions of the top and bottom chords?
Thanks in advance.

 

[csd72]

If the web members are pinned then the chords can be modelled as continuous along each side.

 

[minorchord2000]

I model the trusses as suggested by csd72.

 

[msquared48]

Remember though, in some truss analysis programs, there must be some fixity at every joint or there will be no solution. This is the case at the supports and at the ridge, and at the bottom chord splice if there is one. The results will still be as if they were pinned connections though. No worries.



Mike McCann
McCann Engineering

 

[spats]

By definition, truss members only take axial load, and therefore, all members should be pinned. However, you must keep one member fixed at every joint for stability. Usually it's easiest to pin both ends of all web members, and pin only the start or end (as appropriate) on chord members.

 

[WillisV]

The answer is either depending on how you are going to design it:

1. I generally leave the top and bottom chords continuous and then design them to carry the combined axial load and moment. I do this because generally they are physically one continuous member and you can significantly reduce deflections of the system this way.

2. A true text-book truss does not resist any moments - so you would pin all of the members. In reality where the chords are actually one continuous member, at ultimate state since you did not design them for the moment they will form a plastic hinge and revert to the pin-pin system you designed for. No problem with this as it is not a collapse mechanism.

 

[Vinny7]

I also model trusses as suggested by csb72. if loads are applied between nodes on the top chord (from purlins etc.) there will be bending in the top member and as this member will be fabricated as a continuous member it should also be designed this way.