Steel Roof Trusses
Steel roof trusses – These are triangulated rance frames which carry purlines to which the roof coverings can be fixed. Steel is stronger than timber and will not spread fire over medium and long span roofs. The rafters are restrained from spreading by being connected securely at their feet by a tie member. Struts and ties are provided within the basic triangle to give adequate bracing. Angle sections are usually employed for steel truss members since they are economic and accept both tensile and compressive stresses. The members of a steel roof truss are connected together with bats or by welding to snapped rates called gussets. Steel trusser are usually placed at 3.000 to 4.500 centers which gives an economic purlin size.
“Truss in connection with roof trannes, is used in the sense of defining the action of a triangular not framework where the spread under load of sloping rafters is resisted by the horizontal to member, secured to the feet of the rafters, which trusses or ties them against spreading. The small section, mild steel members of the truss can be cut and drilled with simple tools, assembled with bolted connections and speedily erected without the need for heavy lifting equipment.
The small section, steel angle members of the truss are bolted to gusset pares. The end plates of trusses are bolted to columns and purlins, and sneering rails are bolted to clears bolted to rafters and columns respectively, to support roof and side wall sheeting. The considerable depin of the roof frames at mid span provides adequate strength in supporting dead and imposed loads and rigidity to minimize deflection under bad. The structural frames and their covering provide basic shelter for a variety of uses.
The basic functions of sheet coverings used in conjunction with steel roof trusses are to: –
1. Provide resistance to penetration by the elements
2. Provide restraint to wind and snow loads.
3. Provide a degree or thermal insulation or not less than that set out in part L of the Building Regulations.
4. Provide resistance to surface spread of frame as set out in Part B of the Building Regulations.
5. Provide any natural day light required through the rood in accordance with the maximum permitted areas set out in Part L of the Building Regulations.
6. Be of low self weight to give overall design economy.
7. Be durable to keep maintenance needs to a minimum.
Pitched Trusses – Spaced at Suitable centres to carry purlins to which the roofs coverings are fixed. Good rainwater run off – reasonable daylight spread from roof lights – High roof volume due to the triangulated format. On long spans roof volume can be reduced by using a series of short span trusses.
Flat Top Girders – Spaced at suitable centers to carry purlins to which the roofs coverings are fixed. Good rainwater run off. Reasonable daylight spread from roof lights – High roof volume due to the triangulated format – on long spans roof volume can be reduced by using a series of short span trusses.
Flat Top Girdens – Spaced at suitable centers to carry purlins to which the roof coverings are fixed – low pitch to give acceptable rainwater run off – reasonable day light spread from row lights – can be designed for very long spans but depth and hence roof volume increases with span.
Pitched Trusses – These can be constructed with a symmetrical outline with or with an asymmetrical outline. They are usually made from standard steel sections with slope wedded or bolted connections, alternatively they can be fabricated using timber members joined together with bolts and timber connector or formed as a precast concrete portal frame.
North Light – Spaced at suitable centres to carry purlins to which roof shattering is fixed. Good rainwater run off – if correctly oriented solar glare is eliminated – long spans can be covered by a series of short span frames.
Monitor – girders or cranked beams at centers to suit low pitch decking used. Good even daylight spread from monitor lights which is not affected by or installation of building.