Effects of member offset on story drifts 3

[teach_me]

I noticed that applying offsets on beam to column connection on STAAD reduces storey drift values compared to leaving connections as it is without offsets. Is that normal or i am missing something?

 

[MotorCity]

How much of an offset? Which direction is the offset? For a relatively small offset, I would not expect a drastic change in story drift (assuming your fixity of the beam to column remains the same). Take a look at the entire deflected structure before and after the change. More often than not, this can highlight inconsistencies in the model.

 

[teach_me]

@MotorCity around 250mm, in my example (500x500 column connected to 300x500 beam), two beam spans both applied offset on each ends. Deflection is also reduced when offsets are applied apparently. Bending and shear reactions are like up to 10% lesser compared to the results when offsets are not applied. Is there a command to check beam-column fixity?

 

[Celt83]

Offsets, rigid offsets, create two conditions, assuming the beam ends have been offset:
1. They shorten the beam span by the offset dimension. This will reduce the beam end shears.
2. They introduce moment into the columns

If the moments resulting from the offset create bending counter to the lateral forces this will result in a reduction in sway deflection.

 

[JoshPlumSE]

That is totally normal, but shouldn't be done with steel frames. For concrete, use of the offsets may get you closer to the "real" elastic drift. For steel, you technically need to account for panel zone shear deformation (which few people do). Thankfully, most people will accept the idea that ignoring panel zone shear deformation and rigid offsets approximately cancel each other out.