Load path in staad pro

[alfaxeng]

Hi all, i am a praticing structural engineer and while recently analysing a steel floor frame i came across an arrangement as show in the attached image.A platform is cantilevering from the main beam ( There is a floor frame behind the beam and this provides torsional stability..i have hidden this).
The problem is i want the pfc beams to act as cantilevers and want the end angle to be supported on these beams.
But unfortunatly, staad has taken the stiff pfc beams as propped cantilevers inducing vertical forces to the angle creating a critical moment to the angle, the angle is supported at end cantiliver beams only.
This is quite strange. Is there any way i can direct in staad the load path? Because i want the angle to be continously supported on to all the cantilevering pfc's.

 

[JAE]

I would guess that the stiffness of your two interior cantilevers is much less than the stiffness of the two outer cantilevers since the outer are closer to the end supports of your main beam.

As the load is applied, the two inner ones are "wet noodles" and simply rotate downward much more than the two outer ones, giving you a weird moment curve in your angle.

You typically can't dictate load path in programs like staad - that's what you "hired" staad to do for you - show you the accurate, correct load path.

 

[alfaxeng]

Thankyou for your response. I intially thought of this, but when comparing with the angle,the internal cantilevers will be more stiffer? Also, in the real case senerio do you think that the rigidly connected cantilever pfc's will support on to the small angle resulting failure of the angle?

 

[canwesteng]

You would use releases at the ends of the angle to get the behavior you're describing. Line elements in FEA underestimate the torsional stiffness of open sections, so debatable if this is more accurate or not anyway, but since most of the model is missing hard to say what the actual restraint is here.

 

[BulbTheBuilder]

JAE (Structural) hit right on the nail!

The rigidity of the moment connection isn't counted for and only get wet noodles. I don't know the end goal, but you can decompose your load by hand and apply them to element you want to study.