ETABS:Two adjacent floors acting as a single span when secondary beams are pinned to primary beams

[Kel]

Dear Engineers,

I have modelled separately two different floors on ETABS and in between is a secondary beam connected to primary beams at the ends.

When I pin the secondary beams and do analysis and design,the floors seem to ignore the secondary beams by acting as a single span resulting into longer span with increased deflection and higher reinforcements required.I have attached a file showing this.

When I make the joints of the secondary beams fixed,the floors now act as expected;as two separate spans thus giving correct deflection and reinforcement results.

 I want to keep the pinned secondary beams but again I want the floors to behave normally(maintain separate spans).How do I fix this issue?

 

Kind Regards,

Kel

[Badar (BAZ)]

I think your reading of the results given by ETABS is not reasonable. Also floors is incorrect terminology to be used here, I reckon you mean two slab panels.

If two slab panels are connected by a pin-end secondary beam supported on fixed-end beams, you will get more deflections in that panel as compared to the model in which secondary beam can transfer end moments to primary beams.

[Kel]

Badar,

Is the increased deflection a practical representation because ideally the secondary beams should be shortening a long span and reducing the deflections? 

[Badar (BAZ)]

Have you compared the deflection with and without the secondary beam?

[Kel]

Yes,

With the secondary beam the deflection is 26mm.Without,it's 22mm.

With a secondary beam,the direction parallel to the primary beam becomes the shortest span. Without,the shortest span changes and the panels take loads directly to the primary beams. 

[Badar (BAZ)]

Can you show entire floor along with location of area where you are checking deflections. From your results of with/without slab and the partial snapshot, it appears as if it has one-way distribution. Your secondary beam might be framed in a way that is leading to an increase in deflection rather that other way around.

[Kel]

You are right.The slab modelled is ribbed.

When I have the panels distribute loads to primary beams directly(taking global-y as shortest span) and have the secondary beam only carry line load the deflection on the slab becomes lesser than having them distribute to secondary beams(taking global-y as shortest span).

The spans under consideration are 7.5mx6.0m

Got me wondering whether the presence of a secondary beam influences the direction of the shortest span or not.

Also,I think because of stiffness on slabs the impact of secondary beams on slab deflection is more prominent in larger spans