Min Net Tensile Strain reqs in new ACI318-08 Version

[pa4912]

I have a torsional question for those who are using the new ACI 318-08 version.

It states (R.10.3.3) that to use 'moment redistribution', the beam must be designed for a Minimum Net Tensile Strain of 0.0075. Does this mean if we want to use compatibility torsion, our edge beam must meet this requirement? Or is it just refering to the secondary beam framing into it?

Or both?

 

[Lion06]

I believe this requirements only applies for continuous beams built integrally with their supports, not for the supporting members. It would apply for the supporting members as well, but completely independently of the supported beams.
For your situation noted above, you could provide for moment redistribution of the beam framing into the edge beam and only that member (not the edge beam) needs to meet the requirement for a tensile strain of 0.0075. If, however, you want to redistribute moments in the edge beams then you will have to meet that limit in the edge beams as well. You don't have to meet the limit in the edge beams to redistribute moments in the supported members.

 

[rowingengineer]

I agree with StructuralEIT, however I will make the following comment: compatibility torsion is an assumption to help in the design of concrete structures in a quick and efficient method. While these assumptions have been shown to provide buildings with appropriate service life, this is still a simplification of the real situation by codes.

Thus I think it prudent that the designer remember: The beam will still see torsion, even tho we are ignoring it, thus the beam will elongate due to torsion and as such it is prudent that longitudinal steel be provided to minimum levels to ensure ductility of the system.

When in doubt, just take the next small step.