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Floor Deflections

Floor Deflections

Floor Deflections

My office has a questions regarding floor deflections:

We have a building where, due to wall layouts, we are not able to run columns continuously to the foundation in one area.  Therefore, in this area we have a column supported by a beam on the 2nd floor.  

My question is this:  When looking at the deflection of the beams on the 3rd floor that are supported by this column, for code purposes, do we have to add the deflections from the 2nd floor support beam, to the individual beam deflections on the 3rd floor?  This particular case is an interior column.  The ceiling below is a drop ceiling.  Based on my condition, I dont think I would need to do this, but are there any conditions where this would be appropriate?  (Say at an exterior wall with a large window supported by the 3rd floor beam).   

RE: Floor Deflections

I always think about this in terms of relative deflection.  Is there a component that will experience additional relative deflection because of the secondary beam's deflection?  Is there a long line of sight that could be affected?  Etc, etc, that's how I handle problems like that one.

And watch your vibration criteria, since you absolutely must consider the effects as cummulative for vibration.

Just my two cents...


B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Floor Deflections


It is a loading condition on the slab above. The support settlement has to be allowed for in the design and detailing of the floor slab for strength and serviceability including deflections.
A couple of possible scenarios-
- The roof above this column settles an extra 50mm when the transfer beam deflects causing much more severe ponding on the roof.
- The beam deflection causes the moments in the floor above to be tension on the bottom at the settling support instead of on the top where the reinforcement has been provided but all the bottom reinforcement is not continuous at the column.

The latest Australian code limits total long term deflections for transfer members to L/500 if the effects of the deflection are allowed for on the structure above and L/1000 if they are ignored.

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