Crack Control Bars

[slickdeals]

We are reviewing a project where the owner wants to add additional loads of 10-12 psf on an existing floor. The framing consists of PSI joists and soffit beams.

The joists are designed for a simply supported condition and we have provided #5@12" oc over the soffit beams as crack control bars.

I am calculating a negative moment capacity based on the steel in the slab and am ending up with a moment capacity of the order of 35 k-ft. This in addition to the positive moment of the joists makes up for the total static moment based on the additional loads.

Has anyone done something similar and are there any pointers?

Thanks

 

[civilperson]

If the joists are still simply supported after adding rebar to the top of the beams, there is no increase in the capacity of the joists.

 

[slickdeals]

The joists are supported on soffit beams with atleast a 4" bearing. I am assuming that this gives it some fixity.

 

[civilperson]

The 4" weld at the top chord is still a simple span condition. For fixity, the bottom chord must be extended until attached to the opposite bottom chord or to the bearing wall or deep beam.

 

[slickdeals]

@CivilPerson:
Timeout.....
This is a concrete prestressed joist. It is not an Open Web Joist.

 

[slickdeals]

http://www.prestressconcrete.com/joists.html

 

[hokie66]

You have to check the horizontal shear condition between the joist and the cast in place slab.

 

[haynewp]

"total static moment based on the additional loads."

When you mention static moment I think about having to check the maximum allowable redistribution of moment by ACI (I think 20%). From what you are saying, all superimposed loads will now be shared between positive and negative reinforcing?

 

[slickdeals]

Yes, all superimposed loads will be shared by Positive and negative reinforcing. I am using the negative reinforcing for flexure as opposed to crack control only as it was originally specified.

 

[rcd445]

Does the connection really provide fixidity to your joists? Can the beam support the torsion that the joists will transfer to it?

 

[slickdeals]

It is compatibility torsion, not needed for equilibrium. Right? Plus, it in an interior beam, not a spandrel.

 

[haynewp]

How far are you assuming the negative moment gets spread out in the slab with the 12" spaced bars?

 

[slickdeals]

The joists are spaced at 3'-6" o.c. I based my effective width based on joist spacing. I am not sure if that answered your question.

 

[haynewp]

OK, as long as that falls within the ACI max effective beam width for your joists.

 

[rapt]

slickdeals,

It is only compatability torsion if you are not relying on it to reduce moments in other ereas.

 

[slickdeals]

@Rapt,
You bring up a very important point.

For example, if I have a 30' one way slab on one side and a 20' one way slab on the other, then there will exist an unbalanced negative moment at the beam line. That negative moment will have to be transferred by torsion in the beam into the column.

One of my bosses says that even though these moments may exist, he brought up the question of how this beam would torque considering that it is tied with monolithic slab and into the column.

I could not answer the question. Can anyone point me towards any research in this? Do you design the interior beams for this torsion, which can often exceed the torsional capacity of a non-torsionally-reinforced beam.

Thanks

 

[ron9876]

The joist embedment results in fixity which is why the added top reinf is needed. How long are the added bars compared to the moment diagram. It seems like a reasonable assumption. Is there a chance to use the added load as part of the design live load or some other method to rationalize away the load?

The moments from both sides will impose a net rotation on the beam. Since concrete beams are not very rotationally stiff it will likely rotate and relieve the torsion. I haven't seen this as a problem.