Their are various types of trusses are used in the construction of house and building, etc.
Trusses are structures in which members are interconnected so that they form a steady state and they do not allow any rotation of the joints.
There are two fundamental types of trusses i.e. pitched truss or common truss characterised by its triangular shape.
It is usually used for roof construction, some common trusses are named in accordance with their “web configuration”.
The cord size and web configuration are determined by span, load, and spacing.
Different types of trusses:
These trusses are made of smooth plane by saw and jack planes, its desired joints are made for the purpose of connecting components.
The principal is then joined to the tie beam by a bridal joint or tenon joint.
These joints are fastened and strengthened by an iron strap, the two struts are fixed to the principal rafter by a slant and tenon joint.
The king post is fixed by the tenon joint in the upper edge of the tenon beam.
The heads of the king post are joined by mortise and tenon joint at the ends of the principal rafter.
These joints are fixed more tightly with the help of a three way mild steel strap on each side.
Purlins are placed at right angles to the principal rafters are connected through cogged joints and cleats.
In this types of trusses, there is only one vertical post but in the case of the queen-post there are two vertical posts called queen-posts.
A horizontal beam connected at the top of the queen-posts is called as a straining beam.
Each queen-post and principal rafter is subsequently joined by two interested members known as struts.
The two struts are subjected to compression are called compression members, while the two queen-posts are subjected to tension are called tension members.
The purpose of providing a straining beam is to keep the junction in a stable condition.
In addition, a tensioning sill is attached to the tie beam between the parts of the queen posts to counter the load coming from the struts , the queen-post truss is more suitable for span ranges from 8 m to 12 m.
These types of trusses are made by the combination of king post and queen post truss, for span up to 18 meters, it is more appropriate for combination of king-post trusses and queen-post trusses.
When the span exceeds 10 m, the wooden trusses become heavy and uneven, the use of mild steel trusses proves to be economical for large spans.
The safe work stress on mild steel is about 20 times that of ordinary structural wood under direct stress.
Steel trusses are light in weight and can be fabricated into various shapes and sizes as required.
It consists of thin parts of wood with a curved and light roof material at the top.
This variety may be suitable up to a maximum of 30 meters.
Properties of stell trusses:
- These types of trusses are tire proof.
- Free from attack by white ants.
- They are stronger than wooden trusses.
- Bow string truss or Bel-fast roof truss:
- The shape of such a truss is like a bow.
Mansard Roof Truss:
When steel trusses are launched, the mansard roof truss is a very rare used having two pitches.
One has a king-post truss from 30 ° to 40 ° at the upper-post and the other has a queen-post truss that varies from 60 ° to 70 ° at the bottom.
In this truss, there is more or less use of the combination of king-post on the upper side and queen-post truss on the lower side hence it is also called two-storey truss.
It provides economy in space and cost.
Since this type of truss has an odd shape, its use has become obsolete nowadays.
This type of roof is similar to the mansard truss but its top is flat with a slight slope to one side.
This is more appropriate when there is a need to have a roof between the posts of two queens.
Composite Trusses types:
When roof trusses are made of two materials such as wood and steel, it is called a composite roof truss.
In the case of the truss the members are subjected to tension and compression, so the tension members are made of steel and the compression members are made of wood.
If the tension members are made of wood, the wood section becomes heavy so it is necessary to make it in steel.
At the junction of steel and wood members, special fittings such as C.I. head, C.I. shoe, straps, steel angle, bolt, kingbolt, etc.
Uses of various types of trusses:
- Trusses are used in large spans to offer shelter and don’t resist superimposed ground loads apart from mud, rain and winds.
- It is used to support the roofs of auditoriums, cinema halls, sports activities stadiums, railway stations, airports.
- When the spans exceed a certain limit the stable beams turn out to be heavy and uneven, so trusses are used instead of beams.