bi-axial shear in concrete

huangyhg

bi-axial shear in concrete
i would like to know when i have to consider bi-axial shear; is it when vux > 0,2vuy (20%) ?. and is it correct to use: sqrt ((vux)^2+(vuy)^2)  <  2.65*sqrt(f'c) for shear provided by the concrete.
thanks

dear hyper
where abouts are you currently situated
i am from christchurch new zealand
there is no such thing as an international code but i may be able to help
let me know details
regards
mark
hi mark
the ideas comes from a book that considers an analogy with torsion to solve biaxial shear; where instead of using axial shear and 0.53*sqrt(f'c) uses 2.65*sqrt(f'c). but using the two shear forces. if we have vux=3, vuy=4 then vu=5; if the concrete resists 4.5 then we cannot do an axial design; therefore the shear resistance of the concrete should not be the same at some percentage of the two (vux and vuy).
regards.
hyper,
concrete theory is not one of my strong suits, but for a practical solution, this is what i would do:
i assume you are designing a concrete column or isolated beam loaded on both axes, and that the shear you are refering to is "diagonal tension" as opposed to "pure shear."   in either case a good design would include longitudinal bars and ties.
the shear resistance of a concrete member is provided by the contribution of the concrete (vc) and the contribution of the ties (vs).  
how the concrete contribute to the biaxial shear is not clear.   i have looked for some reference to biaxial shear with no success.    if we take the resultant of both shear as squareroot of (vux^2 + vuy^2), this resultant would be acting at an angle to the main axes of the section, and that would complicate the analysis.     i would use, instead, a reduced concrete strength for each shear (vcx and vcy), such that vcx = vc*vux/(vux+vuy),  and  vcy = vc*vuy/(vux+vuy).      notice that this would make vcx + vcy = vc.     
i would design and detail the ties independently for each shear (vux and vuy).   if there is only one tie around the section, each tie could resists both shears, since only the two sides of the loop parallel to the load are stressed.
this approach would produce a slightly conservative design.