In STAAD Foundation Advanced, under Analysis Properties I see two Slab Thickness named Analysis Thickness and Design Thickness. What are these and should these be same or different ?
The Analysis Thickness is the one used for the stiffness analysis while the Design Thickness is the one used for reinforcement design. Generally you would use the same value for both. However if there is a situation where you would want the analysis to be based on a certain thickness but the reinforcement calculated based on a different thickness, you could use different values for these. For example you may have some areas in the mat which may be thicker and you may want to consider that additional thickness for the stiffness analysis but when designing the reinforcement, you may choose to use a uniform value for the entire mat.