To calculate strength interaction diagrams using a spreadsheet, we first calculate the position of the neutral axis corresponding to the balance point (Fig. 6.15).
For values of c less than that balanced value, the steel will yield before the masonry reaches its maximum useful strain. Combinations of axial force and moment corresponding to nominal capacity can then be calculated, as can the corresponding moment.
Given the position of the neutral axis, c, less than or equal to the balancepoint value
Similarly, for values of c greater than the balance-point value, the steel will still be elastic when the masonry reaches its maximum useful strain. Compute the strain (and corresponding stress) in the steel by proportion, and find combinations of axial force and moment corresponding to each position of the neutral axis.
Given the position of the neutral axis, c, greater than or equal to the balance-point value:
