Transition between RC beam and beam/column? 2

[IJR]

Most codes have a limit for treating RC section as a singly reinforced section as is theoretically also known.

That limits gets lost in M+N case due to the little maze involved in deriving interaction equations.

If I have a little N and relatively larger M, should I go into interaction charts or simply ignore N and do my singly reinforced beam calculations. I tried to do both and I ended up with a section OK as a beam column but not as a beam.

Could someone do some theory here. Its been years since I left college.

Thanks and Nice Weekend

IJR

 

[Taro]

Members with axial load less than 10% of the nominal concrete compressive strength are generally considered to be "beams" (see ACI 318 Sections 10.3.3, 21.3.1.1, and 21.4.1).

Simplified analysis neglecting compression reinforcement is usually acceptable for members with very low axial force. If the axial force is larger than 10%, stress-strain compatibility should be used to develope the interaction diagram.

 

[IJR]

Thanks for the input Taro

The 10% is what I was looking for. Apparently if the 10% is exceeded, we will use the M+N interaction. Suppose now that moment is still larger than axial force though axial force is still greater than 10%:

The section will still be singly reinforced as far as moment is concerned. Do the interaction curve insure that the section is still capable of working as singly reinforced beam?

Just curious. I am already satisfied with your reference to ACI, and dont probably need more answers.

Thanks Taro

IJR

 

[JAE]

IJR

For the singly reinforced section, with axial load, you need to make very sure that the member will never be induced to bend in the opposite way from that assumed for the reinforcement.

Normally, even with a singly reinforced beam, we still include top bars to help carry stirrups and to somewhat reduce long term deflections.

 

[IJR]

Thanks JAE

You opened up another world to me, thanks for that. What I was actually looking for is a bit theoretical, from the fact that I lost track while learning interaction diagrams for M+N case. We detail columns with symmetrical arrangement without paying attention to moment. We pay considerable attention to M when we have a beam. We would first check if it can actually be singly reinforced or not, otherwise double reinforced. We never do this with symmetrically reinforced columns. I understood that also: The interaction curves make sure that with that arrangement of reinforcement the column section will carry M+N, THAT SIMPLE.

Sorry guys, I was probably led to wrong direction before posing this question. Got best answers from you guys anyway.


Thanks JAE

IJR