Example $4-5$

Design the sag rods for the purlins of the truss shown in Fig. $4\mathrm{.}4.$ Purlins are to be supported at their one$-$third points between the trusses, which are spaced $21ft$ on centers. Use A$36$ steel and assume that a minimum$-$size rod of $\frac{5}{8}$ in is permitted. A clay tile roof weighing $16psf(0\mathrm{.}77k\frac{N}{m^2})$ of roof surface is used and supports a snow load of $20psf(0\mathrm{.}96k\frac{N}{m^2})$ of horizontal projection of roof surface. Details of the purlins and the sag rods and their connections are shown in Figs. $4\mathrm{.}4$ and $4\mathrm{.}5.$ In these figures, the dotted lines represent ties and struts in the end panels in the plane of the roof, commonly used to give greater resistance to loads located on one side of the roof $($a loading situation that might occur when snow is blown off one side of the roof during a severe windstorm$).$