104. The various steps to be pursued in designing a trussed roof and proportioning its parts, are as follows:
2. Determining the size of rafters and purlins.
3. Computing the truss loads and drawing the stress diagrams.
4. Computing the size of the truss members.
5. Detailing the joints.
The first step involves selecting the type of truss to be used, determining its shape, height and span, the spacing of the trusses and the manner in which the roofing, ceiling or any special loads are to be supported.
These points will be determined largely by the shape, size and character of the building and by the judgment of the designer, based upon previous study and experience.
The best "lay-out" will be that which is the simplest and most economical, while meeting the required conditions, but any lay-out showing a proper form of truss can be executed and given the necessary strength by using sufficient materials. The preceding chapters, if carefully studied, should enable one to lay out the roof and trusses with a reasonable degree of skill.
To compute the truss loads, a section through the roof must first be drawn, showing the slope of the roof, the outline of the truss with location of the purlins, if purlins are to be used, and the exact position of the trusses must be located on the plans. This will determine the roof area supported by the purlins and also by the trusses. The next step will be to estimate the dead load on the trusses, and to determine the allowance to make for wind and snow.
105. THE DEAD LOAD on a roof truss consists of the weight of all of the materials supported by the truss and also of the truss itself. Wind and snow are considered as live loads, because they are not always present. The common practice in figuring roof loads is to compute the roof area supported at each joint of the truss, and then multiply by the load or loads per square foot, for which the truss is to be designed. Ordinarily the dead load includes the weight of roof covering, sheathing, rafters, purlins and truss. If the rafters are supported directly by the trusses, the purlins are omitted, and sometimes the sheathing and rafters are omitted and the roofing, if of slate, tile or corrugated iron, supported directly on the purlins. It is therefore necessary to estimate the weight of roof per square foot in each instance.