ACI 318 Questions

[ahmadramadan]

Hey all

Hope everyone is ok,I have couple of questions to ask

1. Where is the threshold in the code to distinguish between beam- column and column in design, or in other words what is the biggest axial force that we can ignore and consider the beam-column as a beam
2.Generally speaking if we ignore torsional in design the building will stay safe with some cracks(taking only compatibility torsion not equilibrium torsion ) is this right???
3. after Etabs distribute slab force in beams, how can I calculate the redistributed force on beam ???


Thank you all

 

[rapt]

Answering your second question

You can ignore compatibility torsion. Not all torsion. But you have to be consistent when you ignore it and make sure that you do not rely on it in other areas of the design.

You should not ignore any torsion in a sway frame as the sway stiffness is relying on it!

 

[BAretired]

1. The column interaction diagram should provide all you need to know. As you can see, some axial load is actually helpful when a column is more beam-like than column-like. See attached.

BA

 

[RUStructural]

Agree with BA, small amounts of axial compression actually increases the section's moment capacity as can be seen in a typical P-M diagram. Running a hand calc on the moment capacity of a concrete section and comparing to an SPColumn run for that same section will provide identical vales at 0 axial load. To my knowledge there is no ACI specified threshold of axial force which allows it to be ignored (ie considered as only a beam) however if the axial force is small it is conservative to ignore it. I guess the difficulty is in defining small without a P-M diagram to reference, something like compression values below 0.10*f'c*Ag would usually be on the safe side. I would do a quick P-M run on any column section I was designing to firm tho. Any tension in the section needs to be considered as it can drastically reduce bending capacity.

 

[struggle66]

RAPT,

Does it mean if we use equivalent column in slab design for one way beam & slab system, we have to design the beam for torsion?

Thanks

 

[Enhineyero]

Hi ahmadramadan ,

To answer your questions:

1. Element can be designed as a beam if axial stress is less than 0.1fc' or 0.1fc'(Ag) in terms of force

2. Generally, RC beams in buildings will not resist any torsion. unless it is applied by an external load (e.g. cantilever slab without a back span)

3. Moment redistribution in the ACI code, as with all other codes, is a clause to redistribute the moment from the support and transfer it to the midspan moment. The purpose of this is to reduce the rebar requirement at supports, inorder to avoid rebar congestion at beam/column joints.

 

[JAE]

2. For torsion - you must consider compatibility torsion for some cases - i.e. perimeter beams receiving load from an interior span as you mentioned in your original post.

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