Hi,
Thanks for your question and thanks for including your data file.
The subject of biaxial shear in Eurocodes has always baffled me as there doesn't seem to be any clauses that cover this (maybe this has changed as I have an old version of EN1992).
The software tries to deal with any shaped arbitary concrete section with biaxial bending and shear, but the interpretation of the results can be very misleading. The shear calculations are dependant on an effective depth and this is measured from the most compressive fibre to the centre of the tensile force - perpendicular to the neutral axis. The NA is determined my resolving the moments and the shear perpendicular to the NA is also calculated based upon the moment NA angle.
The code also requires an effective shear breadth, which for clause 6.2.2 is the smallest width of the tension zone. If a rectangular section is rotated round from the horizontal then the default smallest width is in fact zero - so this does not make sense. This is why you are seeing a small value for bw. It is not zero because the program splits the rotated section into slices and determines the smallest slice width at the centre of the slice. You can of course change this in the parameters form for shear calculation (as well as the effective depth if required). Also, with biaxial shear, real shears must be used as the resolution of the shear force will be different if absolute shears are used.
So, for a shear design of a rectangular (or near rectangular) section I do not consider biaxial shear in a single case. I generally create two load cases, one for My and Vz load effects and a second for Mz and Fy load effects. This then allows me to design the shear link requirements in the two orthogonal direction separately. Then, although this is not part of the Eurocodes, I use clause 5.6.5 of BS5400 part 4 for biaxial shear in walls, to check that:
V_Edy/V_Rdy +V_Edz/V_Rdz <= 1.0
I'm not sure if this is helpful for you but this is the way I would consider your section.
There is a problem with the software as your section with the given reinforcement does not converge for the Mz Vy case where the NA is fixed at 90 degrees (previous problem). It seems that the solution cannot converge because of the large out of balance My moment being created due to the NA being fixed. I got round the problem by setting the tolerance for My to 3000 so that it does not fail. The resultant solution in this case seems to be logical.
I hope this has provided you with enough information to solve your problem and if so please marl my reply as a solution so that others may benefit. Thanks
Kind regards
Dave Geeves
