Tension stfiffening in RC beam 1

[mesapol]

When there is steel (bottom layer) in a concrete beam,the steel is in compression while the concrete in tension when the section shrink. So, how does it form tension stiffening in the beam when cracks start?

How does tension stiffening help to reduce cracking moment?

Thank you for your help!

 

[rapt]

The bottom steel is in tension at midspan due to bending and at each crack location, the concrete in the bottom face carries no stress.

Between the cracks, the concrete can still carry some tension stress, developed by the bond between the concrete and the reinforcing.

In strength design this is ignored and the concrete is assumed to carry no stress when it is in tension. For deflection and, for some codes, for crack control, the tension in the concrete between the cracks can be taken into account.

This is called tension stiffening.

The shrinkage will modify the stresses in the concrete but the tension face steel will still be in tension.

 

[mesapol]

I now understand the term tension stiffening. But, how does tension stiffening reduce cracking moment?

 

[apsix]

Tension stiffening is a term used for the additional bending stiffness the uncracked portion of the concrete provides to the beam after it has cracked (as explained by rapt), by definition it doesn't exist before cracking, and therefore doesn't reduce the cracking moment.

The cracking moment is reduced by any pre-existing tension stresses in the beam, such as those caused by concrete shrinkage.

 

[mesapol]

Thanks apsix. I still have problem to understand how the initial induced tensile stress by shrinkage can help to reduce cracking moment? Doesn't it help to increase cracking moment since the bottom fibre of concrete is in tension due to bending and there is tensile stress initially produced by shrinkage?

 

[rapt]

Mesapol,

No, if the concrete is free to shrink, there is no stress induced.

The reinforcement inside the concrete restrains the shrinkage and induces tension stresses in the concrete. If the reinforcement is near one of the faces of the concrete then this tension stress will be greatest on that face. Whether this is considered to be an increase in the applied load tension or a reduction in the cracking moment is another matter. Because it is an internal effect and easier to include in this way, the Australian code treats it as a reduction in the tensile strength of the concrete and thus a reduction in the cracking moment.