Post-Tensioned 2-way Flat Slab with drop panels

[ajk1]

I am checking an existing 30-year old post-tensioned 2-way flat slab with drop panels. The tendons are distributed about 60% in the column strips and about 40% in the middle strips (there is also some rebar that contributes about 20% to the moment of resistance; the negative top rebar is also distributed about 60% in the column strip). This does not conform to the 80% requirement for negative moment in the column strip, required by CSA A23.3 for 2-way slabs designed by the equivalent frame method.

Question: does the 80%/20% distribution of moment and reinforcent apply to prestressed concrete, or only to non-prestressed concrete?

 

[rapt]

Yes, it applies for all slabs, except 80% is a bit high, 75% is the normal column strip contribution. THis does not mean that exactly 80% of the tendons need to be in the column strip, just 80% of the total capacity at the supports.

ACI has a rule that ignores the distribution of moments for PT slabs, but that is a gross simplification and should not be applied to slabs with drop panels (as is pointed out by British Concrete Society TR43 on prestressed slabs) and also requires severe limitations on the applicability of the method due to the simplifications.

 

[ajk1]

rapt - ok, thanks