temperature vs bending reinforcement 1

[chekre]

Hi guys,

when analysing a slab versus temperature, do u consider the enveloppe of the reinforcement required for temp and the one needed for bending or the addition should be considered?

In the paper written by Gilbert (the aci refers to this paper), It clearly states that the enveloppe should be considered while ACI in its proposed combinations, adds the effect of T to the gravity loads!

what is ur practise ?

thanks

 

[rapt]

It depends if the temperature effect is creating moments and shears in the member. If it does, then these should be included in the load combinations as per the ACI combinations and it could increase your reinforcement requirements.

There is also the requirement for shrinkage and temperature reinforcement. This is normally a minimum reinforcement to control restraint stresses in the concrete and is not normally added to the ultimate reinforcement.

 

[TXStructural]

There are two different requirements for minimum reinforcement of slabs, and while the values may be the same or similar, they are not the same.

Minimum flexural reinforcement is (currently) found in ACI 318-11 10.5. This value tries to make sure there is not catastrophic failure at the moment the slab cracks and all load transfers to the reinforcement. It also acts similar to T&S reinforcement.

Shrinkage and temperature reinforcement is transverse to flexural reinforcement, and requirements are found in 318-11 7.12.
As Rapt said, they are not added in the same direction. For some applications, you may want to use additional T&S on the face of a slab opposite the flexural reinforcement for serviceability or appearance (crack control) reasons.

 

[chekre]

ok, i agree. I think it shouldnt be added to the bending. Temperature will give more reinforcement if the structure is too rigid and isnt free to move. In this case (near core walls area), the temperature reinf't exceeds the one required for bending specially when the slab doesnt support any heavy loading. In addition, the T effects have been removed from the load combinations in ACI 11 and has been left to the engineer's judgment whether to apply or not.

we are studying a case where the slab will be cast and no shrinkage strip is allowed.

thanks

 

[rapt]

Ronyrony,

Is the slab prestressed?

 

[chekre]

No, solid slab. An analysis was conducted to determine the reinforcement required to resist shrinkage stresses. This value was taken into account in the mesh used.

Personally, i think, for such kind of elements, a shrinkage analysis should be done but it should be added to construction loads (say 150kg/m2 SDL and 200 kg/m2 LL). 2nd Analysis should be done for gravity loads (during life of the structure) and the enveloppe should be adopted since 60to 70% of the shrinkage must have been finished before service conditions. Temperature isn't a problem since the slab in question is located below ground. Therefore, no temperature is there (except some differential one between top and bottom fiber).

 

[rapt]

The effects would not be fully independant and additive. They both happen at the same time and affect the strain/stress conditions in the member.

You would have to do the section design for a combination of the bending stress and the shrinkage restraint induced strains and come up with a momnet/curvature profile for the section including both effects.

To siomply add the requirements for both effects calculated separately would be very conservative.

 

[chekre]

I agree.

 

[TXStructural]

You say "we are studying a case where..." Are you a student? If so: see above -> No promoting, selling, recruiting or student posting.

If you start adding reinforcement, be sure flexural members are still tension-controlled.

As I mentioned previously, shrinkage and temperature steel is only required in orthagonal directions to flexural reinforcement, since flexural reinforcement and the associated compression on the opposite face will negate the need for T&S. The minimum amount of flexural steel is the same as the minimum for T&S.

 

[chekre]

1-I am not a student. What i meant by studying is we have a case. Sorry if u got it wrong.
2- U can't count just on the flexural reinforcement to control the cracks due to temperature and shrinkage since when significant restraint is there, amount of reinforcement requierd for T&S may exceed the one required for bending.

regards