Welded Truss Connections

Amirfa · Jan 2, 2017

If a truss is defined by having pinned connections and therefore theoretically its members only carry axial load, why are so many trusses welded in real life? Would a welded connection not mean that they are also transferring moment?

I realize some moment and shear has to be developed in these members since they are not weightless but ultimately aren't welded connections considered a "fully fixed" or moment resisting connection? If I were to analyze a welded steel truss should I not analyze it with moment connections (which would ultimately make it a frame)?

Thanks

Coty · Jan 2, 2017

Your question may be answered (with opinions, it seems) in an old thread here: Link

Yes, it does transfer moment, but the members of the truss will barely change as a result of fixing the web members versus pinning them. However, idealizing the truss with pinned-pinned connections for the web members will simplify analysis, which is why it is done in school, but I don't agree with it for practice. This was the way it was done in the past. With computers, you can simply model everything as truly built. Fix those connections and be sure to design the connections considering the moments transfer.

KootK · Jan 2, 2017

There's nothing perfect out there so a practical definition of a truss is a frame in which load is predominantly resisted via axial loading of the members. Welded joints usually increase joint fixity but don't necessarily change the balance all that much. And bolted joints can add significant fixity too. The choice of welded versus bolted connections usually has more to do with wconomics and aesthetics than it does with a desire to control behaviour.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

SlideRuleEra · Jan 2, 2017

Amirfa - In accordance with Coty and KootK's comments, there is a famous 1909 technical paper that revolutionized design of truss joints, for the reason you are questioning. The paper is by Mr. E.W. Pittman and is attached below. In essence, he recommended increasing thickness of gussets plates to permit use of smaller gusset plates with fewer (but more heavily loaded) rivets. Smaller gusset plates result in lower secondary stresses. Same principle applies for both bolted and welded joints.

http://www.SlideRuleEra.net