Twisting about constrained line of parallel purlins inter braced by sagrods under wind suctions.

Simply supported purlins inter braced by one line of sagrods in a parallel purlin system under wind suction are analyzed. The sagrods are assumed to be zigzag placed with 30–50 mm offset in adjacent spacing. The effect of this offset and the local deformation on the effectiveness of sagrod is analyzed. A purlin is divided into 3 segments by offsetted sagrods, and the general solution is found for such a purlin under wind suction. The solution is then extended to a parallel purlin system to determine the deformations of all purlins in the system. Examples are given to reveal the bracing effect of sagrods to purlins away from the sagrod anchor spacing. It is found that the sagrod bracing effect drops quickly as the number of spacings to the sagrod anchor spacing increases. The deformation of the last purlin in the 10 purlin system is close to a purlin without sagrod if the purlin thickness is 1.5 mm. FE analysis is carried out to verify the analytic results and excellent agreement is found. Simplified equations are proposed to find the tensile forces in sagrods, purlin deformations and internal forces easily. Practical meaning of this study may be formulated as follows: (1) the distance of two diagonally braced spacing should be smaller if the purlin thickness is small. (2) It is welcome to take measures to reduce the local deformation of purlin web at the sagrod points. [ABSTRACT FROM AUTHOR]