9 posts in this topic

Dear fellows!

I have observed that some senior engineers have not been using stiffness modifiers in their designs. What is the concept behind  i am not clear . As we design RC for strength, section is cracked and to cater for this cracked section, property modifiers are used as per

 ACI 318-11  10.10.4 (stiffness requirement)

 ACI 318-11  8.7 (stiffness requirement)

UBC97 1630.1.2 (modelling requirement)

 so why whole section is considered in design. is there any other logic which i couldn't understand.

 

Regards

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The RC members in a physical structure crack well below the ultimate load.Therefore using full MOI of the member in idealized model is not justified.

Though the use of full MOI will not affect the total value of bending (positive plus negative), the distribution b/w the members may change considerably.

It is not the absolute MOI that matter,rather it is the relative stiffness of members that must be considered.

In short, these modifiers (reductions in MOI) are used for the calculation of deflections i.e Serviceability checks...

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1 hour ago, EngrJunaid said:

Though the use of full MOI will not affect the total value of bending (positive plus negative), the distribution b/w the members may change considerably.

How is that possible?

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@Ayesha

Let me demonstrate that statement with a simple 2D frame in ETABS....

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See the Attached self explanatory picture.

Modifiers.png

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good comparison!! it means designing with modifiers will give more practical results and better moment redistribution. what is the effect of modifiers on lateral deflection?

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On ‎1‎/‎14‎/‎2017 at 0:35 PM, EngrJunaid said:

See the Attached self explanatory picture.

Modifiers.png

The factor of 0.35Ig is not applicable to beams with flanges. For the interior beam, the factor will be around 0.7Ig, and for the edge beam, it will be around 0.5Ig. Moreover, these factors are for un-braced frames; factors are different for braced frames.

 

On ‎1‎/‎14‎/‎2017 at 2:22 AM, EngrJunaid said:

The RC members in a physical structure crack well below the ultimate load. Therefore using full MOI of the member in idealized model is not justified.

 Please go through the commentary of the section 8.7.1 of ACI 318-08.

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On 1/14/2017 at 4:49 PM, waqar saleem said:

i found another related discussion

another document i found a somewhat good summary for modifiers

https://www.academia.edu/13211701/Stiffness_Modifier

Stiffness_Modifier.pdf

Lol, I was trying to write couple of articles 2 or 3 years ago and uploaded my draft (including the discussion of sefp) on scribd to download something as a trade-off. It was a draft, but if it has helped anyone, this is good to know.

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