For determining Plastic hinge moments in column, program first develops the uniaxial interaction diagrams separately about X and Z axes. It uses the φ factors specified for column overstrength on the Seismic Parameters screen for building this interaction diagram. Separate factors may be specified for axial force and moment. The default value of overstrength factors for bending as well as flexure is 1.3. However, user may specify other values for the overstrength.

Once the interaction diagrams with overstrength factors are determined, program goes through all the seismic load combinations. For each combination it determines the uniaxial bending capacity about X and then about Z for the axial load in the combination. These are the plastic hinge moment values about the two axes for that combination.

Program allows two options for users for plastic hinge calculations. If the first option is chosen, program will first go through the calculation of plastic hinge moments for each combination and then pick the largest value as the critical value for a column. It will then use that largest value among all combinations and use that for design of cap and footing.

The second option allows users to specify the plastic hinge value manually at top and bottom of each column.

Program uses the overstrength factors to arrive at column plastic hinge moments and it does not use the moment curvature at this time. In that case, user may compute the plastic hinge moments outside of the program and specify in the program on Seismic Design Parameters screen with second option selected.

For each seismic combination, program computes the plastic hinge moments at top and at bottom of column. However, if the plastic hinge is expected to occur at a different location, user may specify that on the Structure Model dialog.

Program calculates plastic hinge moment about Z at bottom check point in the same way as it computes X plastic hinge moment. Note that the controlling combination for plastic hinging about Z may be different than that about X. Program will report the combination number, axial force and the plastic hinge moment Mpz at bottom of column. Next program shall calculate plastic hinge moment about Z at top check point in the same way as it computes for bottom check point. It will compute the plastic hinge moment about Z axes which could occur for different combinations. For circular column, the plastic hinge moment would be same for two axes. However, for other column shapes will result in different values about two axes for the bottom of the column depending upon rebar pattern and column dimensions.