ACI Beam vs Column Definition 1

[zrck99]

I am designing a pedestal over a spread footing to support a cantilevered hss column. The column has fairly large moments and very light axial loads. I believe the pedestal falls into the ACI 318-19; 2.3 definition of a beam, "Member subjected primarily to flexure and shear, with or without axial force or torsion...". Based on this, I plan to use the less stringent beam reinforcement requirements rather than the column requirements.

Does anyone know of a part of the code that actual defines a ratio of axial force to moment that allows you to design the member as a beam? I've looked through older posts on here but not had luck tracking down a definition like this.

Thanks.

 

[HTURKAK]

Although the pedestal is defined as short column, the reinforcement requirements expressly stated in the code..

16.3.4.1 For connections between a cast-in-place column or pedestal and foundation, As crossing the interface shall be
at least 0.005Ag, where Ag is the gross area of the supported member.

So, i will suggest you to provide reinf. at least 0.005Ag or total area of anchor rods which ever is stringent and distribute the rebars evenly ( at least one for each corner )..

 

[JoshPlumSE]

Hturkak is right. I used to do pedestals for industrial equipment all the time. And, the 0.5%Ag was usually what controlled reinforcement.

FWIW, we still analyzed the pedestal as a singly reinforced concrete beam. Meaning that we ignored any axial compression and only considered the bars on the tension face. The thinking being that when you look at the P-M interaction diagram, we were in the portion of the curve where axial force INCREASES flexural capacity.

 

[rapt]

Not sure if ACI318 covers it.

Australian Code AS3600 has

10.3.2 Short column with small compressive axial force
Where the design compressive axial force (N*) in a short column is less than 0.1 f'c Ag , the
cross-section may be designed for bending only.

 

[JoshPlumSE]

The ACI code used to have almost the same provision that Rapt mentioned. That was specifically related to setting the strength reduction factor Phi (if I remember correctly).

Now, that reduction factor is based on the strain in the bars at failure. When you have a "tension controlled" failure the phi factor is 0.9, when it's "compression controlled" then the phi factor is 0.75. Anything in between is in a "transition region" where the phi linearly interpolates between 0.9 and 0.75 based on strain.

I believe my figure above was lifted from an older code where you see a kink a little bit above the yellow line where the phi changed between the column (compression controlled) and beam (tension controlled) phi factors. In today's ACI code there wouldn't be a sudden kink like that, but more of a gradual transition.