Supports in STAAD.PRo are specified as PINNED, FIXED, or FIXED with different releases. Pinned Base : is a base which resists Tension and Shear Fixed Base: is a base which resists Moments along with Tension and shear, A pinned support will have reactions for all forces but will resist no moments, (See Fig. (1)). A fixed support has restraints against all directions of movement. Note: In reality the action is always between the two i.e neither does a FIXED BASE nor PINNED BASE exists in reality. When you should use a FIXED base or PINNED base:
When working with your design model in Staad Pro, you have to define the support types wither Fixed or Pinned and assign to nodes where your structure will be supported, at this stage you don't have sufficient informations about whither you will have bending moment on the support base or not, When checking the design handbooks, you can't find specific information about when you shall consider the Support Pinned or Fixed. When defining the Support type in Staad Pro, and you decided to choose the Fixed Support as you may assume that you have Wind Load and this wind load will be increased with the structure height | Fig.(1): PINNED BASE Fig. (2): FIXED BASE Fig.(3): Beam under Wind Load |