. Practical structural design; a text and reference work for engineers, architects, builders, draftsmen and technical schools;. rtical reactionline. The point ofintersection is thenconnected to thefast end by a Hne,which gives the di-rection of result-ant U\. At (6) is the force diagram. First draw a load Une parallelto the wind resultant and lay off the amount of wind at eachend joint and at each joint on the truss. From the ends drawlines parallel to the two reaction lines. This forms a trianglealj, the side jl being equal to R<i, and the side al being equal toU\. The remainder of the dia
. Practical structural design; a text and reference work for engineers, architects, builders, draftsmen and technical schools;. rtical reactionline. The point ofintersection is thenconnected to thefast end by a Hne,which gives the di-rection of result-ant U\. At (6) is the force diagram. First draw a load Une parallelto the wind resultant and lay off the amount of wind at eachend joint and at each joint on the truss. From the ends drawlines parallel to the two reaction lines. This forms a trianglealj, the side jl being equal to R<i, and the side al being equal toU\. The remainder of the diagram is readily drawn, all the lineson the windward side being parallel to the members of the truss,with the load line (the wind) inclined. In Fig. 154 is shown the method to use when the wind is blow-ing from the free end towards the fast end. No explanation isrequired for this figure if the explanations given for Fig. 153 areunderstood. The free end of a truss may rest on steel plates instead ofrollers. The only difference between this method and that whenthe free end rests on rollers is that the reaction under the free. Fig. 153. — Wind Pressure on Roof-wind on Fast Side. 232 PRACTICAL STRUCTURAL DESIGN end is inclined at an angle of 18 degrees from the vertical awayfrom the wind, this being practically the angle of friction of steelon steel. Trusses are of course designed for the maximum stresses,and with the majority of trusses the maximum stresses occur with the wind from?^/ the fast side. Ana- lyze the truss withwind from either sideand then proportioneach member for the•greatest force it isexpected to resist,the two sides of thetruss being alike. Concentrated Loads Sometimes a rooftruss must be de-signed to carry atrolley at some designer doesnot always know inadvance on whichpanel the trolley willbe carried, the owner of the building wishing to be free to changesuch things at pleasure. Instead of a trolley it may be a shaftfor machinery, or a heavy pipe.
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