tendon as compression steel in calculating stress at tranfer for pretensioned beams

[kww2008]

Is it possible/correct to include prestressed tendon as compression steel near the compressive face of pretensioned beam when calculating concrete stress at transfer ? I am aware that the tendon in compression is usually ignored in this calc.

I came across a person who suggested that the flexural stiffness of pretensioned beams can be improved by including the prestressed tenson as compression steel in the section analysis to determine compressive stress at transfer. The reason provided is that since the prestressed tendon is bonded to the concrete, the tendon is capable of resisting compression (through reducing tension), and this is different to applying a compressive strain to a non pre-tensioned tendon which obviously has no stiffness.

 

[yakpol]

It will be equivalent to using transformed section properties (concrete + steel) instead of just concrete. All design codes allow this approach...

 

[kww2008]

Thank you for your response, Yakpol. My question is about whether it is correct to model tendons in the compressive portion of a section as "compression steel" in the section analysis. Normally we ignore them.

 

[yakpol]

kww,
Are you talking about strands used for pre-tension or tendons used for post-tension?
In pre-tension case use transformed section properties and PS jacking force to calculate stresses at transfer. That's a proper way to account for steel in compression or tensile zones. Alternatively you can use only concrete section properties and calculate prestress losses due to elastic shortening. The results will be identical.
In post-tensioning case during transfer consider duct holes and don't include tendons since they are not bonded yet.
Textbook may describe it better.
Regards,

 

[rapt]

If the prestressing steel is bonded to the concrete, its stress will reduce when the concrete is in compression, just as it increases when it is in tension. So it should be included. Not including it can give un-conservative results as it may reduce the section capacity due to its position relative to the neutral axis!

Just make sure near the ends of the tendons (last 2-3m) that you consider the reduction in effectiveness of the bond due to the development length of the strand.

 

[kww2008]

Thank you Rapt for the discussion. I agree that when calculating section capacities, both steels (reinforcing and prestressed) should be included. I am interested specifically in the calculation of stresses at transfer. Assuming all the tendons are close to the bottom of a simply supported pre-tensioned beam, ignoring these tendons when calculating the elastic stresses at transfer is conservative as these stresses will be smaller if they are calculated including the tendons as "compression steel" (since the section is stiffer in bending).

Whether to include the tendon as "compression steel" will likely have to be based on how a pretensioned beam behaves at transfer.