purlin location as a restraint poin

huangyhg

purlin location as a restraint point
in the design of portal frame rafters or trusses, can we assume purlin locations as a restraint point, if so what are the limitations.
please send in a detailed reply asap.
anthy555...purlins are not true restraint points, though they provide some benefit.  obviously, they are a means to transfer loads to the frame, usually vertical, but lateral as well.  their connection method typically precludes anything other than linear load transfer.  for this reason, they can be considered as lateral bracing to the frame members.
in simple analysis, they are sometimes ignored except for the general check of lateral bracing (a form of restraint), and for their locations as points of load transfer to the frame.
as the analysis gets more elaborate, their effects can be modeled in a variety of ways when a single frame is isolated, or they can be included completely in a full structure model.  purlins are usually just simply supported beams.