Re: weak floor= 1st basement, ni<0.60, increase system rigidity Hello, you have defined the soil curtains in the basement as basement curtains. Therefore, the cross-sectional areas of the curtains in the basement are not taken into account in the weak floor irregularity control, and the sum of the curtain cross-sectional areas is 0. Analyze again by removing the sub-basement bulkhead option for bulkheads on a rigid floor. Although this is not required in your project, this option can be selected for earth curtains located above the rigid floor.