Keeping an eye on cumulative/global deflections

when designing roof trusses

On a recent site inspection to investigate reduced roof truss clearances over various internal walls, I came away thinking that if a bit more attention had of been paid to the potential for cumulative deflections at the time of detailing, (this particular job contained a girder truss to girder truss support scenario which is shown in Diagram 1) then changes to the girder truss design/truss layout could have been made, which would have improved the truss to wall clearance situation in the problem area. Now like most jobs the reduced truss to wall clearances were not solely because of the roof truss performance, but more a combination of differing wall elevations in conjunction with accumulating truss deflections. Nonetheless this article will focus on the conditions which can lead to less-than-ideal global deflections and the truss improvements which could have been made while detailing to minimise these long-term cumulative deflections in a girder truss to girder truss scenario.

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