Provide detailed explanation for Gravity column design in RCDC.
Gravity columns is a choice that you as structural engineer have to make in your system. To reflect this correctly in analysis, ideally these columns should not participate in lateral load resistance in carrying shear and bending moments. These should be defined as ‘pinned’ ends in lateral load analysis. As per current limitations in E-tabs and STAAD, this is not easily manageable. Hence, we have allowed the users to select the required columns in RCDC and treat them as ‘Gravity’ columns. RAM software has the option to analyse the structure with combination of Lateral and Gravity columns.
As per code, the Gravity columns should be designed for the forces from analysis as well as effects of lateral displacement (known as ‘displacement compatibility’). This is done by considering moment due to Pu (Axial load from Gravity load combinations with DL +LL) acting at R*Delta distance away (Delta is displacement due to lateral loads). We would request you to go through Clause 11 of IS 13920 (2016) for more details on this.
In RCDC, we follow the above procedure in detail. Please check detailed design calculation report for Gravity columns for more information.
Please note that, in RCDC the member can be defined as ‘Gravity column’ only if that qualifies as column based on D/B ratio. Walls can’t be design as Gravity members. It can be either Non-ductile or Ductile. Further, the Gravity columns are designed for Vertical gravity loads with effect of later displacement due to lateral loads as explained earlier. Click on below link to understand the implementation of Gravity column in RCDC.