Temperature Steel

[BSVBD]

I've understood temperature steel to be installed to prevent cracking.

The more i research temperature steel, even in ACI, it seems to be only referenced and used in slabs.

What about walls?

Is temperature steel used only for slabs or horizontal surfaces?

Are not wall subject to cracking?

What am i missing?

 

[hokie66]

The requirement which is customarily referred to as "temperature and shrinkage" reinforcement is really to deal with cracks which form due to drying shrinkage, and restraint of that shrinkage. Drying shrinkage of course occurs in all concrete elements, but slabs being the most extensive in area, drying shrinkage cracks are most problematic in slabs. Walls are typically limited in height of placements, so horizontal cracking is due to flexure, and flexural requirements control. Vertical cracking of walls occurs due to restraint at the bottom of the wall, so horizontal reinforcement concentration at the bottom will tend to control that type cracking. Walls which need to retain liquids have special requirements for crack control, including additional reinforcement.

 

[Guastavino]

I think judgment applies. For walls, I use temp/shrinkage steel because a wall is similar to a vertical slab. Chapter 11 of ACI 318-14 which is now the "Walls" chapter commentary mentions temperature.

 

[rapt]

I have been having an interesting discussion with Ingenuity (offline) on this rule in ACI compared to the Australian code.

The Australian code would suggest that the ACI requirement would offer only very limited, if any, crack control in a situation where there is a high level of restraint to shortening. To achieve reasonably acceptable crack widths with high levels of restraint, the Australian code would require .6% reinforcement as a minimum and possibly more depending on the situation.

Beams are normally expected to be more heavily reinforced and minimum reinforcement to satisfy ultimate moment minimums should be higher than the slab S & T requirements as a T beam requires a high minimum than a slab.

In a flexure situation, the amount of reinforcement is normally reduced as one face is in compression (hopefully) and beams are normally flexural members, but technically, if there is high restraint, a beam should also require S & T reinforcement or be designed for the axial tension forces and it will probably be more than the ACI code minimum once you do the calculations.

I did the calculations for a slab requiring a high level of crack control in the transverse direction recently and required 1% reinforcement to achieve a .1 - .2mm crack width.

 

[JAE]

For the ACI 318, njlutzwe is correct (Chapter 14 in the older pre-318-14 codes).

There are requirements for T&S or minimum reinforcement in walls - both for vertical and horizontal directions.

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[MotorCity]

For all but the smallest structural elements, I tend to include t&s steel. All concrete shrinks when in dries. The more concrete, the more shrinkage. A wall can be very similar to a slab in terms of geometry, how does the concrete know whether it is part of a wall or slab?

In terms of how the t&s is distributed, I have seen 2 different methods. First, you can provide separate t&s bars and distribute as uniformly as possible, which is my preference. The other way I have seen engineers provide it is to simply add it to the flexural reinforcement. Although I agree for smaller members it can work, I think you need to distribute it mostly around the perimeter of the member where shrinkage cracks tend to manifest themselves.

 

[hokie66]

MotorCity,
All concrete does shrink when it dries, but unless the concrete element is restrained, it will not crack.

Requirements for flexural reinforcement and shrinkage reinforcement are separate, but not additive. The greater requirement controls.

Not sure what you mean by distributing shrinkage reinforcement around the perimeter.