why did Eurocode use uncracked section instead of cracked section

[oloksy]

good day fellow engineers, I have a little doubt I want you guys to help me clear.

why did Eurocode use uncracked section instead of cracked section? while Indian, American and Australian codes are using cracked section

 

[JoshPlumSE]

You'll have to give a little more detail. Very few of us on this forum regularly deal with all those codes regularly enough that we quickly have the differences handy.

My quick thoughts, though:
a) Uncracked section is frequently used for initial stiffness calculations. Especially for things like vibration or certain types of service level deflections.

b) Cracked sections is used at strength level forces.... Especially for seismic.

c) Sometimes you will use a reduction factor on the uncracked properties to calculate things like P-little Delta effect or such. Because it is a lot easier to do this than to try to figure out a more accurate cracked property. Especially, since the cracked properties vary based on load conditions and such.

 

[oloksy]

@joshplumse so is it advisable for me to model my G+2 building with a cracked section?
Even when I’m using only dead and live loads (gravity load only)

 

[centondollar]

What do you mean by "model my G+2 building"?

Force follows stiffness in statically indeterminate systems, as I'm sure you know. If you have a simple structure, say, a simple span beam, then the stiffness of the member will not affect the internal forces that you design reinforcement against in the ultimate limit state. If you have a complicated structure, such as a multi-story building, then you have two options:

a) Determine, by some method, which members are likely to remain uncracked (and thus retain their full stiffness) and which members are likely to crack, and adjust the stiffness properties accordingly. Proceed to use results for ULS and SLS member dimensioning. Note that this method may not always be suitable or easy to employ in practice, and I have never used it.

b) Assign linear-elastic (uncracked) material properties to all members, acquire internal forces for linear-elastic (uncracked) members, and do the ULS member design using the results. For SLS, adjust the stiffness using a suitable method (e.g. interpolation method for beams (EC2) and/or adjusting the Young's modulus (shells, slabs)). This, according to my understanding, is how linear-elastic structural analysis of reinforced concrete is recommended to be performed according to EC2.

To be honest, I don't see a reasonable way to employ method "a)", since predicting which members are "uncracked" or "cracked" (and how much of the members are in such a state) during their service life is an almost impossible task and more of an academic exercise than an engineering task.

PS. Even if you have access to software that does SLS checks (deflection and crack widths using transformed sections and code formulas, or more advanced (suitable for computer software) methods) for you using some method, it is advisable to at least check the order of magnitude by a simplified hand-calculation, and - obviously - to read the user manual and understand the calculation method employed in the software.

 

[JoshPlumSE]

No enough information for me to answer. What sort of deflections are you looking at? What sort of members do you have?

I think it's probably more conservative to use cracked sections. But, I don't know enough to say for sure.

 

[HTURKAK]

The following is the copy and paste from the relevant code EN 1998-1 , 4.3.1 ( Eurocode 8: Design of structures for earthquake resistance )


4.3 Structural analysis 4.3.1 Modelling


(7) Unless a more accurate analysis of the cracked elements is performed, the elastic flexural and shear stiffness properties of concrete and masonry elements may be taken to be equal to one-half of the corresponding stiffness of the uncracked elements.



That is, the elastic flexural and shear stiffness properties are taken to be equal to one-half of the corresponding stiffness of the uncracked elements .( the moment of inertia and shear area of the uncracked section were multiplied by factor 0.5. Also the torsional stiffness of the elements has been reduced. Torsional stiffness of the cracked section was set equal to 10% of the torsional stiffness of the uncracked section.)

Better to read the code in detail...

 

[oloksy]

@HTURKAK I think this is when you apply only seismic loads. That’s why it was written in EC8. My concern is does it apply to gravity only loads?