Tensile cracking limits in Class T post-tensioned beams

[grantstructure]

I"m doing my first PT beam designs since the creation of Class T, and the increased limits on allowable tension. In going through the example in the ADAPT manual, they set the beam limits to 9 /f'c. I was going to use the full 12 /f'c, but now I'm wondering. Is there some reason to hold the limit to 9?

 

[rapt]

I think the whole "class" system they introduced is a backward step.

Class T is a real worry. They say it is cracked. But there is no requirement for crack control or bonded reinforcement. For any design with stresses above class U and possibly from about half of the class U limit, there should be a requirement for minimum bonded reinforcement at the tension face and at a maximum of about 12 - 15" centres.

 

[cooperDBM]

Beyond class T, 12 sqrt(f'c), is class C which represents a cracked section. Class C sections do have to be detailed to control cracking. Allowing for classes is just a way to formalize the use of partial prestressing. An engineer, or client, is free to limit stresses to class U or T if they want the extra durability or improved appearance. Now there are more options.

 

[rapt]

CooperDBM,

The "options" are simply meaningless limits. You can now put a name on something that is completely meaningless.

And they got the class T limit wrong. Bonded reinforcement is required to control cracking well below this limit!