Concrete member subjected to combined biaxial bending and tension

[pplbb2011]

When concrete member subjected to combined biaxial bending moment and load, which code could be used for the design? I didn’t find the solution in ACI318.

Can I use the following formula as checking basis?

N/Nu+eN/Mu<1

Thanks in advance.

 

[sumnerdave]

Assuming a short column, the CRSI "Universal Column Formulas" from the CRSI Design Handbook, 8th Edition (1996) are the best design for bi-axial bending and axial load with load factors from ACI 318 Section 9.2 or Appendix C.

Using the Bresler Load Contour Interaction Equation:
S.R. = (Mux/fMnx)^1.15 + (Muy/fMny)^1.15 <= 1

if Pu < 0.1*f'c*Ag

 

[sumnerdave]

for Pu >= 0.1*f'c*Ag, using the Bresler Reciprocal Load equation:
1/fPn = 1/fPnx + 1/fPny - 1/fPo
solving for fPn,
fPn = 1/(1/fPnx + 1/fPny - 1/fPo)

 

[pplbb2011]

Thanks Sumnerdave. You may misunderstand my point, this is asking the solution for combined bending and tension, but not compression. Is there any way for it?

 

[delagina]

model in staad or risa LOL

 

[Lion06]

I would never want to do a biaxial bending check by hand. I would use PCA Column, which is now SP Column (SP is for Structure Point).

This is no different than a member with biaxial bending and compression except that you're going to be below the P=0 line instead of above it. The moment capacities of members in tension are greatly reduced (as you can see if you look at any interaction diagram).

 

[csd72]

Yes, the steel stresses in tension members are reduced to limit cracking. I expect the design factors are different because of this but otherwise the philosophy is still basically the same.

If the whole member is in tension then the concrete area is ignored and it is just the reinforcement that you are checking.

 

[JoshPlumSE]

When performing a hand calculation:
1) Use first principles to find the uni-axial moment capacity for each direction when the column is subject to your applied tension.... Noting that the presence of tension will reduce the flexural capacity of your column.

2) Now, all you have to do is figure out what kind of bi-axial interaction equation to use to interpolate between the two uni-axial cases.

2a) The simplist method (which would also be conservative would be Mux/Phi*Mnx + Muy/Phi*Mny < 1.0. Where the Mn values were calculated in step 1. This interaction equation basically draws a straight line between the two uni-axial cases and says that any load point within that straight line would be considered okay.

2b) The reality is that the interaction diagram shouldn't be a straight line, but should be more of an ellipse. There are a number of methods (PCA Load Contour, Bressler) that you could use to interpolate between the two uni-axial cases. I prefer the PCA Load Contour Method. But, that doesn't mean it's any better. It's just the method that I know best.

3) For the most part, you won't be able to get the "true" solution with a hand calculation. You'll want to use RISA, or PCA Column or a program like that do do it for you.

 

[pplbb2011]

Thanks guys.

I believe you are talking the classic contour of P-M, which could be covered by PCA software, etc. However, if the tension load is heavy and out of the contour line scope, what it will be?

Probably, I may not make right description, i am now designing a beam with heavy tension + biaxial bending inside, this is not column characteristic concrete member, i don't think software like PCA could cover such instance.