Precast elements are designed in stage wise. First stage is to check while to element is being casted. Second stage is pouring stage when the element is being casted at the site. The final stage is when the concrete is harden. In STAAD, you need to model elements stage wise and appropriate support conditions. For more details please contact me personally.
Basically it depends on client’s requirements. For structures that are located in the interiors, we use conventional steel structures because workmanship is a challenge. Good maintenance is also required else the structure may corrode, but in urban areas PEB structures are getting popularity. Apart from that when there is something like cyclone bracings need to be changed. Hence, it’s completely depend on the client. However, as a structural designer it it is our responsibility to make client understand all aspects for both PEB and conventional structures and we should always make sure that we understand the requirements of the client properly.