Concrete member subjected to combined biaxial bending and tension
Concrete member subjected to combined biaxial bending and tension
(OP)
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.






RE: Concrete member subjected to combined biaxial bending and tension
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
RE: Concrete member subjected to combined biaxial bending and tension
1/fPn = 1/fPnx + 1/fPny - 1/fPo
solving for fPn,
fPn = 1/(1/fPnx + 1/fPny - 1/fPo)
RE: Concrete member subjected to combined biaxial bending and tension
RE: Concrete member subjected to combined biaxial bending and tension
RE: Concrete member subjected to combined biaxial bending and tension
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).
RE: Concrete member subjected to combined biaxial bending and tension
If the whole member is in tension then the concrete area is ignored and it is just the reinforcement that you are checking.
RE: Concrete member subjected to combined biaxial bending and tension
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.
RE: Concrete member subjected to combined biaxial bending and tension
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.