For IS800:2007 code, both for working stress method and limit state method, program first calculates the section class and then finds out the section capacity values (for LSD method) from which the allowable stress values are calculated (for WSD method). The section class is calculated based on Table 2 of the code. Width and thickness of flange and web element is required to calculate the ratio mentioned in column 2 of this table. So without proper sectional dimension, user defined sections cannot be designed according to IS800:2007 code.
When the user defined sections are created in Section wizard and exported as prismatic section or created in the user table as prismatic section, 6 sectional properties (AX, AY,AZ ,IX ,IY and IZ) are specified. These property values are the only necessary parameters for the analysis and calculate sectional deformation and corresponding forces. There is no information about the section geometry for STAAD to recognize elements of the cross section such as a flange, web which are critical for design and to find out the section class. For that reason, prismatic section design is not possible according to IS800:2007 code.
When the user defined sections are created in Section wizard and exported as general section or created in the user table as general section, program reads the 6 sectional properties as in prismatic section but apart from that, sectional dimensions of the equivalent I section are also available. In section wizard, program automatically finds out the equivalent I section dimension and same value is imported in STAAD. For User table section, along with the property values, options are available to specify the thickness, depth of web and flange of its equivalent I section using terms like D, TD, B, TB. STAAD then uses the rules for design of I shapes to design General sections.
For user defined section, if the section is welded section, STP 2 parameter must be specified to the member.