topic Re: Deflection Problem in Robot Structural Analysis Forum

Deflection Problem

MUHANNAD.ABU_ABDO — Tue, 10 Jan 2017 09:02:06 GMT

I faced a problem while designing RC Beam with central span 5 m and two cantilevers at both ends 1.7m. i set all the perfectness and run the analysis after that this message is show up about deflection !!

My thoughts about ACI code ;
the required depth for both end continuous span equal = span/21 so 500/21=23.8 cm
the required depth for cantilever span equal = span/8 so 170/8=21.5 cm
So, the section depth is 25 cm which is satisfied ACI Deflection Limits !!

So where's the problem ??

Thanks in advance

Re: Deflection Problem

Artur.Kosakowski — Tue, 10 Jan 2017 12:23:08 GMT

I'm not ACI expert however the hand calculations you provided disregard both load and amount of reinforcement which seems a bit strange.

Re: Deflection Problem

MUHANNAD.ABU_ABDO — Tue, 10 Jan 2017 12:30:27 GMT

it's not strange !! i'm talking about deflection which is ( SLS ) Not (ULS) you can resist the deflection by depth or reinforcement .. i choose the section depth to meet ACI Code limits as in the pic

it's a normal procedure to find section depth ! it's not from my mind ??

as you can see there's no problem with deflection limit ?? so why is that message ??

Re: Deflection Problem

Artur.Kosakowski — Tue, 10 Jan 2017 13:06:40 GMT

My understanding is that the table you inserted gives you the allowable deflection limit and suggested minimal thickness of a slab or a beam. In general the formulas for deflections looks like this

As you can see there are two factors: load and stiffness. If so, when you increase the load you will increase the deflection and unless I'm mistaken this relation is not visible in your hand calculations. The same, when you increase amount of reinforcement you increase stiffness (both due to the fact you will reduce cracks and as E of steel is larger than E of concrete) so you decrease the deflection. I understand that they may be some guidance (for typical loads and reinforcement ratio) that you may use as the initial estimation of beam height to (standard) span length ratio but IMHO you can't tell that it works for any load.

Have you tried to calculate the real value of deflection for e.g. span according to ACI formulas with use of the cracked stiffness of the beam?

Re: Deflection Problem

MUHANNAD.ABU_ABDO — Tue, 10 Jan 2017 13:16:09 GMT

Yes you're right .. but i have to take this in my considerations when i'm dealing with special structures ! but it's ordinary building with normal loads and as double check i designed the same beam by using excel sheet and i didn't get any problem ??

Could you check the model plz ?

thanks in advance

Re: Deflection Problem

Artur.Kosakowski — Tue, 10 Jan 2017 16:20:59 GMT

i designed the same beam by using excel sheet

Could you attach this calculations?

Re: Deflection Problem

MUHANNAD.ABU_ABDO — Tue, 10 Jan 2017 16:31:18 GMT

Plz find the attached file

Re: Deflection Problem

Refaat — Tue, 10 Jan 2017 21:56:15 GMT

I think the value that should be compared with (RSA value) is (the value of long term deflection) which is considered the effect of cracking and the final amount of reinforcement.

Refaat

Re: Deflection Problem

MUHANNAD.ABU_ABDO — Wed, 11 Jan 2017 04:51:04 GMT

Thanks for your kind explanations , is it possible to neglect the effect of long term deflection during design ?

Re: Deflection Problem

Refaat — Wed, 11 Jan 2017 07:06:09 GMT

Definitely " No"

Refaat

Re: Deflection Problem

Artur.Kosakowski — Wed, 11 Jan 2017 07:49:02 GMT

If you want to see the value of 'uncracked' deflection then you may want to check the values obtained for static analysis of the beam model.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

Re: Deflection Problem

MUHANNAD.ABU_ABDO — Wed, 11 Jan 2017 15:10:13 GMT

thanks for your response .. i know that but my point is " is it possible in beam design to neglect it ? can i do it without Robot take the long term deflection into considerations ?

Re: Deflection Problem

Artur.Kosakowski — Wed, 11 Jan 2017 15:17:18 GMT

As long as you don't mark correction of the deflection by additional reinforcement you can just ignore these errors (assuming you checked the elastic deflections first and you consider this as sufficient enough). You may also increase deflection limits to avoid the errors to be displayed.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

Re: Deflection Problem

MUHANNAD.ABU_ABDO — Wed, 11 Jan 2017 15:28:59 GMT

Fair enough .. thanks a lot