End Support Condition for Concrete Beams in Etabs vs Actual Detail

[e104909]

Id like to seek your opinion on this case. I am struggling to convince myself with this assumption from a senior engr.

The concrete joists (that connects to the girder) in ETABS were released in M22, M33, and T. The actual connection detail of the joist to beam is a standard hook which is fully develop in tension. The ETABS model assumes the slab to be a membrane element.

With the Pin end condition, the number of rebars at negative moment is 3-#8. If I make this fixed (w/o releases), then of course the number of bars in the negative moment are short. The argument is that the moment will be redistributed to the positive region and if the positive moment is capable of handling the redistributed moment then it should be fine. This assumption is only at the end spans of a continuous joists and only at the end where it connects to primary beam which makes the joist to be a secondary beam.

Is this assumption valid?
Will the concrete have to crack first in the negative region before it redistribute it to the positive region?
Since this is concrete, shouldnt we stick to traditional fixed end condition and let the joists be designed for negative moment as well?







Regards,
E104909
Civ-Str P.E.

 

[hardbutmild]

Is this assumption valid?
Will the concrete have to crack first in the negative region before it redistribute it to the positive region?
Since this is concrete, shouldnt we stick to traditional fixed end condition and let the joists be designed for negative moment as well?

Short answer: yes, yes and no.
Long answer:
1) I think that this assumption is usually valid, but this depends on the amount of reinforcement and detailing
2) I believe that not only should concrete crack, but reinforcement should yield to form a plastic joint.
3) I don't believe that full fixed moment can ever be developed, because this causes torsion in the support beam and it should have low torsional stiffness at high loads (it will probably crack at high loads and if I'm not mistaken, cracked section has 5-15% of the uncracked stiffness for torsion).
I would however ensure that the primary beam can resist the maximum expected torsional moment.

 

[Enhineyero]

Is this assumption valid?
Yes it is valid if your design allows for it (i.e. cracking on top region of joist)

Will the concrete have to crack first in the negative region before it redistribute it to the positive region?
Yes, unless specifically detailed as pinned-ends (i.e. precast griders stop/start at piers).

Since this is concrete, shouldnt we stick to traditional fixed end condition and let the joists be designed for negative moment as well?
Most designers do this. Unless the cracking is not an issue, which for 99% of structures is. For conventional RC frames, I wouldn't re-distribute the moments more than what the code allows, which is mostly done to minimise bars at joints and prevent congestion.