Types of Roof Trusses and their Uses

Trusses are triangular steel sections which have been subjected to the essential axial forces because of the applied load externally. These triangular formations are used to span long lengths in exchange for solid web grinders. In this case, the loads of the building structures might lie on the three-dimensional plane, so the trusses are known as the space trusses.

Axially loaded members are used in trusses which can resist the load of building structures. On the roofs of single-storey industrial buildings and long-span floors, the trusses are usually used. The trusses are also used in the long span bridges which helps to carry the lateral load and gravity loads.

Different types of roof trusses

King Post Truss

These types of trusses are mainly used for short spans which are completely built of wood or wood combined with steel. In this case, steel rods have been used as the tension members.

Queen Post Truss

These types of trusses are also made with wood which is suitable for spans up to 10 m.

Howe Truss

The combination of wood and steel helps to form this type of truss. The vertical members of this type of truss are the tension members which are made of steel.

Fink Roof Truss

This type of truss is economical and suitable for ordinary buildings. In these types of trusses, the length of the compression members is small and the trusses are made with steel. This type of truss is also known as the French roof truss.

Pratt Truss

These types of trusses are generally used for flatter roofs which are made of steel. This type of truss is less economical as compared to the fink roof trusses.

Fan Roof Truss

The fan roof truss is the form of the fink trusses which is also made with steel.

Warren Truss

These are also used for the flatter roofs. In the case of corrugated roofing sheets which lead to leakage problems and gravel roofing, this type of truss is used. Generally, the span of this type of truss is between 10 m to 40 m.

Cresent Truss

In the case of requiring more headrooms and large spans, Crescent trusses are used.

North Light Roof Truss

These trusses are used in the case of factory buildings. These types of trusses help to avoid the direct glare of the sun.

Truss Used for Large Spans

For large spans including auditoriums or railway sheds, these types of trusses can be used. For large rooms, the series of this type of truss is used one behind the other.

Terms used for designing the roof trusses

Top Chord

This is also known as the upper cord of the roof truss which has been extended from one support to another through the apex.

Bottom Chord

This is the lowermost line of the truss members which has been extended from one support to another. This term is also called the lower cord of the roof truss.

Rise

This is the vertical distance that has been measured from the apex to the line joining support.

Span

Span has been referred to as the distance between the supporting end joints of the trusses.

Pitch

This is the ratio of the rise to the roof truss’s span.

Slope

This is the symmetrical truss which also refers to the ratio of rise to the half span.

Panel

This refers to the distance between the two adjacent purlins.

Purlin

These are the horizontal beams which have been spanned between the two adjacent trusses.

Sub-purlins

These are the secondary systems of purlins which help to provide support to the tiles or the state coverings.

Bay

This portion has been contained between any two consecutive trusses.

Rafters

The beams and rest on the purlins have been referred to as rafters. This portion helps to provide direct support to the sub-purlins.

Principal Rafter

These are the top code members which help to carry compressive forces.

Sheathing

These are the coverings reinforced concrete or boards which help to provide support to the roof covering.

Eaves

These are the bottom edges of the pitched roof surfaces.

Truss Spacing

The spacing of the truss has been varied from 4 m to 10 m span structures.

To learn more, watch the following video tutorial.

Video Source: Paul Kassabian

Sag Tie

This tie member has been provided for joining the truss peak.

Purlins Spacing

This has been referred to as the distance between two adjacent panel points.

Depth of a Truss

This component helps to determine the strength of the truss, the stiffness of the truss, the transportation cost and the subsequent erection.

Detailed designing procedure of roof trusses

There are several steps that need to be followed while designing the trusses which include:

➢ First of all, the selection of the general layout of the truss members and truss spacing should be done.
➢ The external loads that have been applied should be estimated. These types of loads include the self-loads of the truss, the weight of purlins and the roofs that have been covered together along with the wind loads.
➢ The critical loading should have to be determined while designing the trusses. The dead loads should be considered alone and after that, the dead loads and the imposed loads should be combined together.
➢ The framework should be analyzed to find forces among all truss members.
➢ Material and section of the trusses should be selected to produce a stress value in each member. Proper care should be taken while designing the trusses.