PT Concrete - Bilinear Moment

[dougantholz]

I have a question about the ACI Code - 1995.
ACI 318-95 18.4.2 (c) & (d) refrences the allowable tensile fiber stress in a pre-compressed beam. (c) allows for 6 root f'c, but (d) allows for 12 root f'c if the "..analysis is based transformed cracked sections and on bilinear moment -deflection relationships shows that immediate and long-term deflections comply with requirements of..."
What is meant by bilinear moment-deflection relationships?
and Does modern PT software like PT Data or ADAPT account for this?
Thanks
Doug

 

[JAE]

Doug:

I believe that the bilinear moment-deflection relationship is taking into account the fact that as a concrete member begins to receive moment, it deflects on a linear basis - essentially based on the gross moment of inertia, Ig. Once you hit the cracking moment, your Icracked kicks in (Chapter 9) and you now have a different relationship between moment applied vs. deflection.

Thus, you have two (bi) theoretical linear relationships between moment and deflection - one based on Ig and one based on Icracked.

 

[dougantholz]

Great. Thanks JAE. I thought that might be it. Based on that, and the fact that most popular PT software can handle this design, then I can design for the higher allowable.

 

[Ingenuity]

dougantholz,

Check the capabilities of your PT software, particularly in the estimation of long-term deflection calculations - several programs just use a multiplier of the short-term deflection to "guess" the long-term delfection, and this can often be very inaccurate. You may wish to hand check that the cracked section analysis is correct too.

Also, re the hiigher allowable flexural tensile stress limits - 12 x SQRT (f'c) is NOT applicable to 2-way flat plates.