Creep in Concrete Structures

[sybie99]

Hi Guys

When desihning say a 8 storey RC Structure, say flat slab floors and shear walls for stabiliy. I basically know how to design all the elements in the structure. Where my experience and knowledge falls short is the effect creep will or may have in the design and whether is should be taken into account, and if so, how.

If anyone is experienced in the design of RC structures, could you please give me some information on why or why not it is important to consider creep effects. I know what creep is, but except checking for the possible "shortening" of a multi storey structure from the creep in columns due to permanent compressive stresses, for what reason would creep have to be taken into account?

When designing a simple beam, I would not look at creep. But it seems in big multi level strucures some people do look at this. Does it affect deflections say deflections at midspan of flat slabs? Is there a simple way of checking for creep, I would be designing to the British Standards.

I hope to get your opinions and assistance on this topic.

Thanks

Seb

 

[Lion06]

I've really only dealt with creep for deflections. It is a huge factor - usually much larger than the initial deflections. Check out the Strength and Serviceability chapter of ACI 318 for the factor that gets applied to the initial deflection for creep.

If you don't count on any compression steel, the creep factor is 2, which means that your final deflection will have 3DL + 1.6LL (the 1.6LL is just my own determination of 30% LL being sustained). That's a lot more than DL + LL.

 

[JedClampett]

IMHO, unless your span/depth ratios are over the ACI minimums or you have permanent large live loads (library, filing cabinets, etc.) you shouldn't have creep issues. Even when I was suspicious, I've checked it using ACI methods as StructuralEIT mentioned and it didn't exceed code deflection maximums.

 

[apsix]

If using BS8110-1 creep is already accounted for in serviceability checks when using the span/effective depth ratios in Section 3.
If using BS8110-2 creep is accounted for by using the creep
coefficient found in Section 7.3.

 

[IDS]

Does it affect deflections say deflections at midspan of flat slabs?

Creep has a major effect on deflections of any flexural member. If you are calculating deflections you certainly need to include creep, and also shrinkage and make sure you allow for the cracked stiffness.

It may be that you don't need to calculate deflections if you follow the span/depth requirements of the code, the code should make that clear (and I'm not familiar with that aspect of BS 8110). But if you do need to calculate deflections, you certainly do need to include creep. The code will give you a suitable method.

Calculating the effect of shrinkage is actually much more problematic, and if you are using lightly reinforced beams or slabs you need to look at that in detail, if deflections may be critical to your application.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[rowingengineer]

Detrimental results in RC structures due to creep:
a) Increased deflection of beams and slabs.
b) Increased deflection of slender columns (possibly leading to buckling)
c) Gradual transfer of load from concrete to reinforcing steel in compression members.
d) Loss of prestress in prestressed concrete.
e) retaining walls on one side of buildings creep will ensure a eccentric loading



An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[asixth]

Gradual shortening of prestressed members which can imposed loads on restraining elements (e.g prestressed highway girders).