Debonded Strands 1

[dalauder]

What is the benefit of debonded strands in reinforced concrete beams?

Pretensioned steel strands are commonly used to prestress concrete members. These are bonded along the lengths where the strength is required and often debonded in the remaining region.

Currently my girder design leaves all strands bonded their entire lengths, but most examples debond interior strands at the end. I need to calculate the bursting resistance, but must first determine:
1) Whether strands at the top (parabolic shape that are at the bottom at midsection) count toward number of end strands.
2) How many, if any, strands I might want to debond at the ends.

 

[rapt]

delauder,

You information is unclear. If it is pretensioned, how are you providing strands with a parabolic shape?

Debonding of bottom strands is normally done to satisfy transfer prestress conditions at the end of the free ends member. It is no use elsewhere.
Providing strands with a harped profile shape using hold-downs, normally a 2 points can also be used to achieve this.
You need to read a good text on prestressed concrete design before you go any further if you do not already know this.

 

[dalauder]

Thanks. I'm not designing the prestressing or the girder, just the stirrups and needed to know how many strands to use in calculating bursting resistance.
The designer typically doesn't use debonded strands and I wanted to check why they are used since Aashto Bridge design guidelines required differentiation between bonded and unbonded strands.
And my bad on the parabolic shape. The strands are tied to the midsection in a straight light that runs diagonally upwards at both ends: 3 straight lines emulating a parabolic shape.