I do not look to web frequently..just see your post ..
Short answer to your questions;
- You can assume fixed base when you provide reasonable fixity for connection to pedestal and for the footing.. Compare the angular stiffness of joint and the foundation .. ( in most cases it is difficult and more costly to provide fixed conn. and rigid ftg. when weak soil ..........)
- You can assume pinned conn. and provide tension element or SOG could be designed to resist horizontal forces for the steel portal frames
Long answer,
- pls look to the following attached doc.
- Look to the ( Structural Steel Semirigid Connections: Theory, Design, and Software (By Faella, C., Piluso, V., Rizzano)
- You may try to post your specific question within new thread to get better responds...