The program calculates the required reinforcement about each axis individually. The design results specify the design axis and also the reinforcement that is required for the design axis. The value of Asc is, therefore, the required reinforcement about the specified design axis. This is not the total amount of reinforcement required.
Different design codes have different approaches when bi-axial bending is applied. For the British design code - and codes derived from the British code - the program calculates an increased moment which is applied to a single axis independently. The program, therefore, shows the required reinforcement per design axis.
More modern design codes - the AS 3600 or the Eurocode 2, for example – specify that the design may be done for each principal axis independently. A force - moment interaction equation must then be satisfied for the design to pass.
(If a certain code does not give specific provisions for bi-axial bending (as with ACI 318 codes or NZ 3101) the British Standard BS8110-1997 provision is used.)
Although the program applies bi-axial bending to the column, the design codes simplify the design approach so that an effective uniaxial bending is considered. With all the design codes, the program calculates the amount of reinforcement that is required per design axis. The program calculates reinforcement for the secondary axis in the same manner as for the primary axis.
Note that if a uniaxial bending moment is applied, the required reinforcement is calculated according to the applied moment in the direction of that moment. Nominal reinforcement is required for the design axis perpendicular to the applied moment. It is possible, however, that the slenderness moment causes the column to be bi-axially loaded. The design will then follow the procedures described above.
When you enter the reinforcement in the Bending schedule tab, the Entered - and Required reinforcement is shown. When you enter corner bars, the value is applied to both the X-X and Y-Y design axes. The corner bars give resistance to both design axes.
Figure 1: Corner bars - supplied reinforcement for both principle design axes.
The middle bars, however, are only applied to the specified design axis (No. of middle bars about either X-X or Y-Y axis). The middle bars added about one axis do not contribute to the perpendicular axis. You can manually manipulate the middle bars in combination with the corner bars to enter the required reinforcement about the two design axes.
Consider the preloaded example: Unbraced slender column with bi-axial bending. The Design results are shown in figure 2:
Figure 2: Design results.
The amount of reinforcement required about the Y-Y axis is calculated as 2404mm2 (equal to 1.74% of the area of concrete, Asc). The required reinforcement in the X-X direction is calculated as 553 mm2 (equal to 0.4% of Asc).