During truss fabrication, the truss member are usually connected by welding does behave like fixed joints not pin joint so how will that behave like truss without pin connection?
The geometry of the trusses are usually long narrow members carrying axial loads,connected to each other in triangles. And the connections are designed to let the axis of truss members pass trough a common center on the connection plate or connection bolted bracket.
Because of this geometry there is no moment at the connections, even if they are rigidly connected. Therefore the connection can be assumed to be a pin connection.
There may be very small tertiary moments due to heat expansion or deflection but those are well within the tolerance range of the truss.
That doesn't mean if a truss will be designed for extreme temperatures or large deflections the effect can be ignored.