Deflection calculation using simplified method of AS3600 (Australian Code)

[NTCONLINE]

Dear all,

I would like to have your opinions on how to deal with Ief in the calculation of deflection based on AS3600
when there are drop panels at the top of columns.

In the code, it says that we can use:
Ief (overall) = 25% of Ief left +50% Ief mid + 25% Ief right.

My question is: if the drop panels are present, should I take the drop panel into account to calculate the effective Ief
at the span ends? Certainly, if drop panels are included, Ief would greatly increase.

Thank you in advance.

NTCONLINE

 

[dik]

I don't know about the Australian code, but, if drop panels have sufficient depth, they have a real impact on how a slab behaves. I would suggest that they should certainly be included. There are a couple of Australian posters here that may have more specific information.

Dik

 

[rapt]

As the rule you are quoting is for beams, the normal assumption in developing a simplified rule for a beam would have been constant cross-section.

It would depend how far the drop panels extend. If they extend past the point of contra-flexure, then probably the increased inertia. If they are very short who knows.

Obviously, I would use RAPT to calculate it so that I did not have to guess, but I cannot say that as it would be regarded as advertising!

 

[Settingsun]

Given it's a fairly gross approximation for flat slabs as Rapt said, I suppose it depends on your appetite for risk (or how risky getting the deflection wrong is).

FWIW, the Warner et at "Concrete Structures" textbook says 'if extensive drop panels are employed it may be desirable to take the increase in beam stiffness into account". This is in relation to the use of the idealised frame method for flat slabs. It also refers to Simmons & Misic (1971) - ACI Journal - but it's not clear whether that is for deflection or for strength.

I would take guidance from the k3 factor of code clause 9.3.4.1. Drop panels permit ~10% reduction in slab depth when calculating deemed-to-comply span/depth ratio.

 

[NTCONLINE]

Thank you Dik, Rapt and Steveh49 for your feedbacks.

I am pretty sure that if the drop panel has sufficient thickness (and length) then it would certainly affect the deflection (i.e. less deflection as opposed to being without drop panels).

Hi RAPT, in your software, if I understand correctly, it uses moment-area method to calculate deflection ? does it include the effect of drop panels, when calculating the distribution of the moment along the slab ? (if it does, then I agree that the deflection is calculated including the drop panels, regardless its dimensions)

Hi Steveh, thank you for your comment, however, I really want to calculate the actual deflection rather than using the deem to comply method.

 

[Settingsun]

Hi NTC,
I meant that you could cross-check your final answer against the k3 factor. It seems to imply that drop panels increase Ief around 35%. That would be order-of-magnitude only.

If your drop panels locally double the I value at the columns, then Ief,drop = (2*25% +1*50% + 2*25%) = 1.5 times Ief without drop panels. Reasonable agreement.

 

[rapt]

NTCONLINE

It uses whatever concrete shape you have defined for both the moment distribution and the moment curvature calculations. It does not use the code simplified deflection methods , Ief and averaged Ief calculations, kcs ect. The section shape at any location is used at that location.