What are collar and tie roof

It is used when the roof span is more than 5.5 meters. It is a combination of collar beam roof and close coupled roof. The rafters are supported by purlins and the purlins rest on the ends of the walls. Collars and struts are used to support purlins and rafters. Its use is recommended when purlins are supported at ends with reasonable economy.

The size and type of the truss depends upon the roof slope, span, centre-to-centre distance of the trusses and the load coming over the roof. T-sections are best suited for use as principal rafters, whereas angle iron or channel section is used as struts. The tension members should preferably be of a flat or round section. The different members of the truss may be fabricated with two or more sections joined together. The members of a truss are joined by rivets or bolts or by welding the plates known as gusset plates. The minimum spacing of the rivets should not be less than 3 times the diameter and the maximum spacing is limited to 15–20 cm in compression and tension members.

The minimum number of rivets to be used at any joint should not be less than two. Gusset plates are designed for the forces coming at the junction but the least thickness should be adopted as 6 mm. The ends of the trusses are placed on bed plates provided on the walls. The bed plate maybe of stone or concrete. The ends of the truss are bolted down with lewis or rag bolts which hold down the truss firmly. The small trusses are pre-fabricated in the workshop on the ground and are then placed in the required position. The bigger trusses are pre-fabricated in smaller parts and then erected in the required position and fixed by gusset plate and riveting or welding.

The relative advantage of steel roof trusses over timber sloping trusses are as follows:

1. Steel sections forming the roof truss are light in weight and can be fabricated in different shapes and sizes. It suits the structural as well as architectural requirements.
2. Steel trusses being made of mild steel sections are free from the attack of white ants and dry rot.
3. Steel trusses are much stronger than timber trusses and they are equally strong in tension and compression.
4. These trusses have a greater resistance against fire and hence are especially suited where fireproof construction is desired.
5. Timber trusses can only be used up to a minimum span of 14 m or so, whereas there is no span restriction in case of steel trusses. Steel trusses are used for structures requiring large spans such as industrial buildings, large sheds, assembly halls, hangers and auditoriums.
6. The various sections forming a steel truss can be easily machined and shaped in the workshop and subsequently packed and transported to the site for assembling. Moreover, there is no wastage in cutting.
7. The erection of steel trusses from the rolled sections is very easy, rapid and economical.