System developers and designers continually optimize and refine the design of components to achieve a high degree of economy as well as satisfy various architectural trends. Roof is one of the most complex components in typical framed residential structures. Roof structures are mainly formed using roof trusses of various types in cold formed roof structures. Number of superior features are offered by cold formed light gauge steel which makes it an attractive material for truss system. Strength stiffness quality and light weight features of cold formed steel trusses makes them ideal for long spans. Complex roof geometric s can be easily achieved since various sections and sizes are available.
While trusses are structurally relatively simple, because of the complex sections used, numerous combinations of loads which need to be considered and the requirement for different roof trusses with different geometries in a house, it has become a common practice to use software programs like Tekla to analyze and design roof trusses. Roof systems using cold formed high tensile steel can be provided to suit various types of roof construction like hips, gables, dutch gables, mansard, etc. Variety of trusses can be used depending on the shape of the roof. Flexibility of roll forming of sections make to possible to introduce more sophisticated fabrication techniques, trusses with different cross sections.
The permanent forces acting on the structures include the self weight of all structural components of the roof and the weight of all permanent fixtures such as solar panels, water tank, air conditioning plant etc. Concentrated load should be considered at mid span of members and when there is an overhang, a load case should consider the application of the concentrated point load at a distance of 100 mm away from the edge. All roof systems components including the truss, battens, cladding and bracing should be designed to act together to transfer all forces to appropriate supports. The application of wind internal pressure needs to be carefully considered while designing trusses which support ceilings. The internal pressure will act on the ceiling plane if the ceiling is fully sealed.
Tekla like software are becoming widely available for analysis of trusses. Analysis helps to find the axial forces, bending moments, shear forces and deflections for all load components. The typical truss connection are those at the apex and heels. The connection between the top chords of a truss to the roof batterns is critical in maintaining the fully restrained condition for the top chord. Web bracing is one of the bracing items which is specific for trusses. This reduce the specific length of the web members. To transfer the horizontal wind loads top chord bracing are designed perpendicular to the span of trusses to the supports. They are required to provide restraint to the roof battens or purlins that act as lateral buckling restraints to the top chord of the trusses when they are in compression. It was emphasized that if a designer is not using computer program to carry out analysis and design they need to exercise a degree of engineering judgment to reduce the number of combination and checks to be computed. TEKLA training center in KERALA