Bending Moments in Slab after introducing the secondary beam

[AniMhj]

Dear everyone,
I have a query regarding the bending moments in slabs after adding secondary beams.
As per the conventional approach, the bending moments in the slab near the secondary beam should be the hogging moment. But in ETABS, I am getting sagging moments near secondary beams which seem to be odd. Can anyone explain this?

 

[Luceid]

Is the slab stiffer than the beams?

 

[AniMhj]

No. Beam is stiffer than slab. The slab thickness is 125 mm, while the depth of beam is 400 mm.

 

[jayrod12]

But what's the span of the beam in comparison to the slab span without said beam?

 

[Klitor]

Try putting in stiffenes modifier for the beam (say x10 stiffness in bending) and see what happens

 

[Luceid]

An image of the layout would help.

If your slab is sagging at the beams - then the beams likely aren't stiff enough to support the slab. More depth does not mean more stiffness. EI/L

Imagine a 300ft long beam with a perpendicular slab spanning 10'. The slab is probably going to "support" that beam more so than vice versa

 

[AniMhj]

The span of the secondary beam is 5 m and the panel size of the slab is 5m x 6.5m.

 

[Luceid]

Are there beams in the left-right direction? Many of these secondary beams are unsupported if not

 

[AniMhj]

Yes, there are beams along both directions. Columns are connected by beams on both directions.

 

[Luceid]

Alright - well you can see a change in the typical midspan moment, it's no longer a nice parabolic shape. The beam is contributing some stiffness and crimping the moment diagram. I would test increasing the secondary and primary beam depths independently to study if one or both of these elements are undersized to provide any real help to the slab. You still haven't provided a thoroughly stiff enough load path for the slab to perform as you desire, not that either is necessarily "better", but you need to be sure that the beams are being accounted for properly in your model, and the best way to do that is by some quick tests.