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Engr Waqas

Relation between Temprature reinforcement and grade of steel

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Assalam o alaikum dear fellows,

Why we need more temperature steel ratio for lesser grade of steel as shown by following relation?

For Fy = 40 ksi 

As(min) = 0.002bh

For Fy = 60 ksi

As(min) = 0.0018bh

 

Moreover, According to 7.12.1.2, it says

 
7.12.1.2 — Where shrinkage and temperature movements are significantly restrained, the requirements of 8.2.4 and 9.2.3 shall be considered.
 
And in its commentary it says
 
R7.12.1.2 — The area of shrinkage and temperature reinforcement required by 7.12.2.1 has been satisfactory where
shrinkage and temperature movements are permitted to occur. Where structural walls or columns provide significant
restraint to shrinkage and temperature movements, the restrain of volume changes causes tension in slabs, as well as
displacements, shear forces, and flexural moments in columns or walls. In these cases, it may be necessary to increase the
amount of slab reinforcement required by 7.12.2.1 due to the shrinkage and thermal effects in both principal directions (see
References 7.7 and 7.16).
 
Kindly tell how to increase this amount? Thanx.
Edited by Engr Waqas

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As you said, according to R7.12.1.2, first of all, reduce the modulus of rupture by half i.e. from 0.62√f'c to 0.31√f'c.

Second, increase the reinforcement as required for axial tension (T=phi As fy) in addition to what required for moment.

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Waqas,

The statement is asking you not to rely on temp and shrinkage steel values provided for cases where analysis shows higher amount is required. You can carry out the temperature analysis and add to that the amount of steel required due to shrinkage. You can calculate extra steel due to shrinkage based on how much strain would be caused by shrinkage or any other method if you are aware of.

Thanks.

Thanks.

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On ‎12‎/‎3‎/‎2016 at 6:23 PM, Engr Waqas said:

Why we need more temperature steel ratio for lesser grade of steel as shown by following relation?

For Fy = 40 ksi 

As(min) = 0.002bh

For Fy = 60 ksi

As(min) = 0.0018bh

Kindly give your value able comment over this also. thanks.

Is proposal of temp. steel by ACI based on the thing that this temp steel may have to bear temp stresses upto yielding and higher steel strength can support more portion of concrete in temp stresses so requiring less steel and vice versa?

If this steel ratio is quite higher as compared to yielding of steel then why cant we have same ratio for both of grade?

Edited by Engr Waqas

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5 hours ago, Engr Waqas said:

Is proposal of temp. steel by ACI based on the thing that this temp steel may have to bear temp stresses upto yielding and higher steel strength can support more portion of concrete in temp stresses so requiring less steel and vice versa?

What does the commentary say?

Thanks.

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    • By Engr Waqas
      Assalam o alaikum dear friends,
      It is quite basic level question but I am wondering how steel resists temperature and shrinkage cracks? Co-efficient of Thermal expansion for both steel and concrete is nearly same. Which shows if concrete contracts at an amount steel would also contracts at the same amount. So I m wonder which exactly phenomenon is involved in resisting shrinkage cracks?
       
      Thanks in advance.