Combined flexure and tension check - reinforced section

[alirama]

Does anyone know if there is a reduction factor required when combining flexure and tension in an RC section?

The specific example I'm working with is a slab (designed for bending/flexure). We also have an inclined column that is adding some tension into this slab. Do we need to add the bars required for tension to the bars required for flexure? (I.e. Do we need to reinforce for both cases independently?)

I would've thought that we could design the slab for flexure and then ensure there are enough flexural bars present to take the tension - or do I need a whole new set of bars for tension? Or is the answer somewhere in between?

I can't imagine a tension in a bar substantially "weakening" it's flexural capacity, but I agree that it will have some impact.

 

[bugbus]

Check the interaction diagram in the tension region

 

[chritsar]

In RC the sections are designed for the condition of "bending with or without axial force". The total tensile force will be the one from flexion + the one from tension. The required reinforcement will need to withstand the sum. This would be straightforward in the case of a linear element (beam, column etc).

In the case of a slab, in-plane forces can be dissipated through other mechanisms. I would leave it to others to explain it better.

 

[centondollar]

chritsar,

the in-plane forces are resisted by in-plane action also in a slab. Of course, a slab will tend to resist "pulling" or a catenary action (slab between columns) in two directions involving normal forces Nxx, Nyy and "normal force flow" Nxy according to plate theory, so it will be more resilient than a slender beam-type structure.

 

[KootK]

We also have an inclined column that is adding some tension into this slab.

I would've thought that we could design the slab for flexure and then ensure there are enough flexural bars present to take the tension - or do I need a whole new set of bars for tension?

You do not want to fool around with the bracing of an inclined concrete column. I vote for a whole new set of bars to serve as a drag strut to brace the column transitions. That system needs to tell a convincing story of how that drag load gets from the column-slab joints back to your lateral load resisting system (shear walls etc). Moreover, because this is a bracing thing, stiffness matters just as much, if not more, than strength. I'll typically oversize the drag bars relative to the raw strength demand as a way to limit the strain that I expect them to develop.