Modeling truss frame in Etabs 1

[Ganesh Persaud]

Hello everyone,

What’s the rule of thumb when modeling trusses in Etabs with respect to the releases. Are the diagonal members pinned or fixed? Are all the members pinned or fixed. What the best way to determine this.
Thank you.

 

[Agent666]

I'd suggest looking at both, depending on the connection node continuity you could have either. I tend to look at both depending on the arrangement as true behaviour is probably somewhere in between fixed and pinned depending on the detailing.

It's probably more common to model as pinned and neglect the secondary moments, this would usually give you the maximum deflection.

Perhaps post a picture of your detail and you might get a response more directed to your specific arrangement.

 

[Lomarandil]

Agreed, for many steel cross sections used in trusses, modeling the diagonals as pinned and neglecting moments (you will get higher axial forces to compensate) is considered the standard practice and acceptable.

Apply engineering judgement as needed.

----
The name is a long story -- just call me Lo.

 

[Ganesh Persaud]

Below is attached the truss arrangement I was working on. I modeled all the members as pinned as this would have been a better case to see my deflections. Initially fixed conditions make it pass analysis and design. Did I go about this correct.

 

[CANPRO]

Really it comes down to how the webs are connected to the chords. If they're connected in a way that they can transmit moment, then you have some fixity.

How do you picture this connection being made? You have back-to-back angles for your chords and single angles for the webs. That is an ugly combination in my opinion. Maybe you could provide a sketch of how you picture the webs connecting to the chords.

 

[JoshPlumSE]

I almost always model truss connections as pins.... even when I've got HSS connections that are welded all around. It just makes the model and behavior simpler. I usually model the chord as continuous, but the webs and verticals as pure pinned connections.

If the truss doesn't work without moment connections (say virendeel) then sure I model it that way.

 

[ggcdn]

In Canada, S16 provides some guidance on this matter:

Simplified method (pinned) acceptable for atleast class 3 members and member length:width > 6:1.

Detailed method (accounting for joint fixity) must be used for panels adjacent to abrupt changes in chord slope, Vierendeel panels, for panels with abrupt changes in transverse shear, or fatigue assessment.

 

[ukbridge]

The Eurocodes recommend you model the top+bottom chords as continuous and the diagonals/verticals as pin joints i.e. provide releases at least in elevation.

If its say a truss bridge however you'll likely need to model the bottom chords and transverse beams as a vierendeel to provide a stiff load path for wind.

Don't release them in torsion, only in bending.

Also make sure you take into account secondary effects due to joint eccentricities.

From experience this is a lot easier to do in your model rather than calculate these manually, and even if doesn't make a difference for the member design it almost certainly will for the design of the connections. Don't be shy about adding a wall thickness size up to allow for this.

 

[Ganesh Persaud]

Thank you very much everyone. Please continue to provide knowledge on this based on your experience. I would have to do some research on this accounding to AISC.

 

[ukbridge]

No worries gl.

As always the more thorough the question the more we can help you. A simple sketch, what the "truss" is makes a massive difference.

90%+ of questions here I find fail to do this - whether its online or asking senior engineers a sketch goes a long long way.