What is meant by moment release in a beam? Literature 1: The ends moment in the secondary beams are negligible as we move away from the column support. What do you mean by pinned and fixed support? The pinned support allow rotations degree of freedom while the fixed support doesn’t allow the rotations. The primary beam is supported by column which provides rigid (fixed) support. But in secondary beam there is no such thing. There is rotation at the ends of them as we can see also. The ends don’t resist the rotation. These moments are countered by causing twisting in the primary beams to which they are supported. Therefore for secondary beam ends act as pinned. Literature 2: Moment releases are used to model real world connection. Most likely you are looking at a simple structure with shear connections for your secondary beams. As a result, these connections do not transmit moment loads or provide fixity (actually they do provide some fixity and provide a little moment transfer, but this is typically ignored, since modelling partially fixity is quite a bit more complicated than the general conservative assumption of assuming all moments and torques are released). This is how most steel structures are built, with simple shear connections whenever possible. Using moment connections is expensive and slower to construct. Lateral stability is provided through the use of shear walls or braced frames. Of course moment connections and moment frames do exist, in which case, you wouldn't release the moments in the connections, and you would design the connection for the moments. Literature 3: Because the secondary beams are unable to get rotational restain from the beam it is connected to. The primary beam is unable to provode restrain because they rotate about their lonitudinal axis, i.e. in torsion. Note that in concrete design we consider all the beam column connections to be moment connection (idealization of partially fixed connection with high rigidity) , whereas in steel design it is left to designers discretion. While connection beam to beam in steel structures we provide simple shear connection to avoid any tortion in the primary beam.