PT Slab: Ultimate Moment Capacity check

[cecil123]

I have been designing a PT Slab and am using hand computations first to get more involved and familiarised with the design procedure. I am using BS Code and the Concrete society handbook on Post-Tensioned Concrete Floors.I have the following questions:

1. I have computed losses and have arrived at the post-tensioning force in every span.

2. Now, I have also checked for service conditions and have arrived at the non pre-stresed steel required if the service stresses exceed the permissable limiits using the relevant codal clauses.

3. Now, I want to check at ultimate.First and foremost I want to compute the ultimate moment capacity, which according to the BS code is given by:


Mu=fpbAps(d-dn)

where:

fpb=design tensile stress in the tendons
Aps=area of prestressing tendons in the tension zone.
d=effective depth to the centroid of steel area
dn=0.45*x
x=neutral axis depth.

For bonded tendons, the value of fpb can be computed using, the Table 4.4 in BS 8110 Part I (section IV).
In the table, the ratio of (fpb/0.87fpu) can be computed based on the value of fpe/fpu.Im my problem I have,

fpe/fpu= 1200/1866 =0.6430
but, the codal values of fpe/fpu begin from 0.6 followed by 0.5 then 0.4.

How do I extrapolate (fpb/0.87fpu) for 0.6487 which is greater than 0.6?
Please help!!

 

[Ingenuity]

Given you wish to get more involved/familar with the PT design process and you are doing hand calcs, I would suggest that you determine the ultimate flexural capacity of the bonded PT section using strain compatibility, from first principles, like you do for RC sections. This will give you a 'feel' for the calculations. Also, since you have non-stressed mild steel in the section, your hand calcs can easily take this into account too.

Any text on prestressed concrete will have an example of ultimate flexural moment capacity from first principles.

Good luck.

 

[cecil123]

Thank you for the response.Yes, I want to do that.I have started for the Indian code, but for the British code I need the stress strain curves for concrete and steel and the relevant equations formulating these curves.Can you help me where I could find these stress strain curves and the corresponding equations for the british code?(for concrete, mild steel, high strength steel)