ACI 318-02 Strength Reduction Factors for columns

[reeves111]

For members subjected to axial compression and bending, in the transition zone the code allows a linear variation of the phi value from .70 or .65 to .90 .However I am unclear on one point. If I use the equations under Fig R9.3.2 at what point can I begin the transition? It appears that you can start from the balanced point and vary it right up to .90. In the old code you had to compare Pn with with .10F'cAg or Pb and would start varying it after the lower of those two values. Am i correct that there is no longer a limiting point at which to start? thanks

 

[UcfSE]

The transition is based on the tensile strain in the steel. The code defines a compression controlled section based on the steel strain. On the other end is the tension controlled section that begins I believe at et=0.004. Between these two values of strain is the "transition" zone. You interpolate between the phi at the compression controlled section and the phi at the tension controlled section based on the tensile strain. Note also that minimum strain for a tension controlled section is 0.004 (verify) but minimum to use phi=0.9 is et=0.005 and you also interpolate in between these numbers based on the strain specific to your design and load effect.

 

[reeves111]

So since a tensile strain of .002 corresponds with the balanced failure condition, then my assumption would be right then that it varies from the balanced point?

 

[UcfSE]

You don't have to assume where to start, it is in the code itself. Tensile strain of .002 means the steel is yielding (60ksi), not necessarily that you have balanced reinforcement. I think the answer you are looking for is in chapter 9, read through it again. I don't have my code with me to help you look it up right now but I believe there's also a graph showing the variance of phi with strain. You interpolate between the code defined compression controlled section and the code defined tension controlled section, based on the steel tensile strain, key being code defined.

 

[reeves111]

Perhaps I should clarify what I am working on. I am developing a spreadsheet to generate the interaction diagram for any concrete column. I want to vary the phi value on the spreadsheet in the transition zone, although presently I am using the conservative value (.65or.70). Thanks for your help I have found my answer.

 

[UcfSE]

Your welcome, I came across this already when I worked on my own spreadsheet for the same thing. Good luck with it!

 

[yakpol]

reeves11,
Developing spreadsheet you must be running strain compatibility analysis that will show strains in all the reinforcing bars. Resistance factor is a function of maximum strain in rebar and has a Z-shaped diagram. Simly interpolate:

IF MaxStrain <= 0.002 THEN phi = phi_cmp (0.7)
IF MaxStrain >= 0.005 THEN phi = phi_bend (0.9)
IF 0.002<MaxStrain<0.005 THEN phi = phi_cmp + (phi_bend-phi_cmp)*(MaxStrain - 0.002)/(0.005-0.002)

For implementation see my spreadsheet ShortCol at

http://www.yakpol.net

Hope it helps!

 

[reeves111]

thanks for the spreadsheet link.I was taking a look at your spreadhseets values and comparing it to mine.Our sheets differ only in the balanced Load and Moment values. I also noticed that by selecting 99 or 02, these change. I think this is where the difference is between our sheets. My question is what was changed in the code from 99->02 that changes the calculation for balanced values??Sorry if this is an obvious question but I am not familiar with codes earlier than 02. thanks

 

[yakpol]

reeves111,
The theory stays the same for both codes, the only difference is the resistance factor. I compared my spreadsheet with PCACOL (99 code), and it was right on the money.

 

[reeves111]

Right, they were changed to accomodate the load factor change. Thanks