King Post Truss
A king post truss is a type of roof truss that is commonly used in small to medium-sized buildings, such as houses, garages, and barns. It is a simple yet sturdy structural element that consists of a vertical central post (known as the king post), two diagonal rafters, and a horizontal tie beam.
The king post supports the weight of the roof and transfers it down to the tie beam, which distributes the load to the supporting walls or columns. King post trusses are relatively easy to construct and offer a cost-effective solution for roofs with spans up to 16 meters.
They are also aesthetically pleasing and can add a traditional or rustic look to a building’s architecture. It is used for spans of up to 5m to 8m and it is not suitable for a long span.
Components of King Post Truss
- Central vertical king post
- Purlin
- Two inclined Principal rafters
- Common rafter
- Purlin cleat
- Tie beam
- Wall plate
- Ridge board
- Brick infill
- Two Strut
- Stone template
- Supporting wall
Analysis of Trusses
Truss members act as pinned joints.
They are joined together to transfer only the axial forces and not shears and moments from one member to another member. In analysis assumed the loads are to be acting only at the nodes of the trusses.
The trusses may be provided over a simply supported over the two end supports, single span simply supported over the two end supports, in which case they are generally statically determinate.
Such types of trusses can be analyzed manually
– Method of Joints
– Method of Sections.
Computer programs are also used for the analysis of trusses.
Economical Span
The economical span of the pitched-type roof trusses ranges from 6 m to 12 m. The Mansard trusses can be used in the span of up to 12 m to 30 m.
Design of king post truss
The design of a king post truss typically involves determining the span of the truss and the loads it will need to support.
The king post, diagonal rafters, and tie beam must be sized and spaced appropriately to handle these loads while maintaining the desired pitch and aesthetic look of the roof.
Various factors, such as wind and snow loads, the type and weight of roofing material, and the intended use of the building, must also be considered during the design process.
Design software and engineering principles are often used to ensure the structural integrity of the king post trusses. Ultimately, the design of a king post truss should provide a safe and efficient solution that meets the needs of the building and its occupants.
Manufacturing of Trusses
The computer design of light frame structure results in the generation of fabrication instruction.
That indicates the precise cutting pattern wise for each member and the size, location of plate connectors and type, etc. required.
The layout is made for each configuration to ensure that the trusses are exact in dimension.
In manufacturing, the various member is cut to the correct dimension and assembled in the correct position, which is located on a truss, floor, heavy table, or jig.
The following important information is
- Location of all supports
- Overall truss dimensions
- Web and chord sizes and grades
- Size of the plate, type of plate, and location for each joint
The webs and chords are connected together by the use of truss plates, which transfer the tensile force, compressive force, and shear forces.
Once the pressing of the plates has been completed, the truss is checked for size, gaps, dimension and species, plate size, positioning, and embedment and then moved into a storage area.
In some cases, the industry has a quality control program then they are checking the quality and production of each truss.
The completed truss is then wrapped together and placed in the storage station in preparation for shipping the truss.
Connection Used in Kingpost Truss
- Bolted connection
- Rivet connection
- Gusset plate connection
- Welded connection
King Post Truss Construction Details
- The triangular shape of the truss provides greater rigidity.
- In this truss, two inclined principal rafters meet at the apex of the truss.
- The central vertical king post provides support for the tie beam.
- In the king truss, the bottom chord member acts as a tie beam.
- This tie beam in the truss receives the end of the principal rafter and prevents the wall from spreading against thrust action.
- The principal rafter also provides support for the purlin, which in turn supports the common rafter of the truss.
- The two inclined members provided on both sides of the truss are known as struts. These struts prevent the principal rafter from bending at the center.
- The main difference between the principal rafter and the common rafter is that the common rafter provides support for the roof covering material.
- A vertical post is used to prevent the sagging of the tie beam at the center part.
- In the truss, both rafter slopes are the same.
- It is used for spans up to 5m to 8m.
- It is not suitable for long spans.
- The king post truss is used to support the weight of an expensive roof.
- It is also used for short-span bridges and simple rooflines.
- The truss does not provide storage space in the frame.
Read More: Lean-to Roof | Lean to Roof Design | How to Build A Lean-to Roof
Advantages
- King truss is mostly used in the construction of aircraft, and terminals to support the top cables.
- This truss is able to the maximum load in any direction.
- It is easy to construct.
- It is used for aesthetic purposes.
- It is easily available in any size and shape.
Disadvantages
- It is not suitable for a long span.
- It is not suitable for big projects like shopping complexes, business hubs, and commercial buildings.
Factor Affecting Selection of Roof Truss
- Span
- Load
- Roof covering material
- Ventilation
- Fire protection
- Energy efficiency
- Building codes
- Pitch
- Maintenance
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