What is 'shrinkage bar' in concrete beam??? 2

[zakmuh]

Hi everyone. Hope you all are okay. I am new in here and new to structural engineering as well. I am working in mid east as a structural design engineer. Adding shrinkage bars to concrete beams is a must practice but I don't exactly know why we add shrinkage bars and how to provide bars. Do we have to do additional calculations for this? I work on BS code by the way. Can anyone explain this procedure for me please?

Many thanks in advance mates

 

[transmissiontowers]

I don't know much about concrete design, but from what I remember from school, there is a minimum area of rebar required (I believe it is sometimes called temperature steel) so the concrete does not crack as it hydrates or expands with temperature. The ACI code has formulas for minimum steel required. IIRC, you find the area of the beam and there is a small percent that the rebar must be for the beam area. I believe you put it in the top and bottom of the beam. When you design the beam for bending, you determine the steel required for stress and then you see if that steel area is above or below the shrinkage area required. If it is below, you add more bars.

Some of the real concrete designers may correct my faulty memory, but you should read the ACI or whatever code is used from the British Engineers who organized your part of the world.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[frv]

Temperature and Shrinkage reinforcement is placed perpendicular to the main flexural reinforcement in slabs and its purpose is exactly as its name implies: restrain cracking due to shrinkage (form hydration) and temperature changes.

I've never heard of or seen temperature reinforcement in beams, unless you mean the portion of the slab above the beam, which happens to function as a flange. In that case, there will be T&S reinforcement in that part of the slab, but it's a requirement for the slab, not the beam.

 

[MrHershey]

Is this for a post-tensioned (PT) beam?

For mildly reinforced beams, shrinkage and temperature reinforcement in beams is usually not considered. You'll generally have more than required in the long direction and generally the beam is thin enough width-wise that you aren't restrained from shrinking in the short direction(s). Note, however, that at least in the US codes you'll need side-skin reinforcement in the long direction if your beam is deep enough (10.6.7 in ACI 318-05, don't know if there is a similar requirement in BS codes or where it would be).

For PT beams, you'll need to account for shrinkage in your prestress losses. Whether you need additional bonded reinforcement for shrinkage or not will depend on the amount of post-tensioning you have.

 

[zakmuh]

First of all I do apologise for this trillion years late responce, SORRY! I really appreciate all the replies you guys have made. Thanks!

I am not working with post-tentioned just in-situ. So..what I understand from this is, so long as we've given area of steel is more than what is required, then thats adequate? Cheers

 

[bpstruct]

The ACI code requires skin reinforcement in beams deeper than 36" (0.91m) near the tension face to control cracking, but the requirement is not for shrinkage. The provision can be found under 10.6.7, and the spacing is dictated by 10.6.4. Unless you can get hold of an ACI code, these section callouts don't mean much. I don't have a BS code. You also need to provide a minimum amount of flexural reinforcing steel per code (ACI).

 

[frv]

So..what I understand from this is, so long as we've given area of steel is more than what is required, then thats adequate?

In a beam? Yes. In a slab, no. You still need to provide T&S reinforcement in a slab.

 

[hokie66]

bpstruct,
Why do you say the side face reinforcement is not for crack control due to shrinkage?

 

[zakmuh]

FRV
Quote (frv)
'In a beam? Yes. In a slab, no. You still need to provide T&S reinforcement in a slab. '

Thank you very much for your response mate. Can you also tell me about T&S reinforcent for slabs please? Do you do a seperate calculations for this?
Cheers

 

[bpstruct]

to hokie66:

My understanding for the skin reinforcement in beams is to control crack width due to tension, not shrinkage. Hence the requirement only near the tension face. Since we are assuming cracked section, we want to restrain those cracks from becoming excessive. I could be misinterpreting....take a look at the commentary to 10.6.7 in ACI (08 referenced here).

 

[hokie66]

bpstruct,

Thanks. I have now read the section your referenced, which states that it was changed in the 2005 version. It says that if the side face reinforcement is not there, cracks in the web can be wider than cracks at the main steel. To me, that means the provision guards against overly large cracking due to shrinkage induced tension in the web. Restraint exists at the top and bottom due to big bars, so an unreinforced web section between will tend to shrink more and crack more. Maybe just semantics, as it is a largely empirical provision, but I think tension due to restrained shrinkage is the main issue for these side face bars.

 

[TXStructural]

"Can you also tell me about T&S reinforcent for slabs please? Do you do a seperate calculations for this?"

Temperature and shrinkage reinforcement is used to restrain development of cracks and decrease crack width due to both initial drying and curing shrinkage, and those due to thermal cycling and similar phenomena. It is used perpendicular to flexural reinforcement, since the flexural reinforcement near the tension side of a slab (or beam) will serve this purpose in one direction, and members will have compression on the other face, thereby controlling cracks.

The usual method is to use a minimum value from the applicable code. In most slabs, the minimum ratio is multiplied times the gross cross sectional area of the member to get an area of steel to be placed orthogonally to the flexural steel. It should be evenly distributed near the exposed surfaces (just below cover). For structures which are intended to be watertight (tanks and similar), and those requiring less visible cracking, use twice this minimum amount.

 

[hokie66]

TX,
2 x (.0018Ag) is usually not enough for watertight structures. .006Ag is more like it.