The King post truss is one of the simplest and oldest structural frameworks used in architectural design. Commonly used in king post roof truss and bridge frameworks, this structural system has been around for centuries, dating back to medieval times. Its simplicity, strength, and economic efficiency have made it a preferred choice in various applications. This article digs deep into the design, applications, advantages, and limitations of the king post truss, providing a thorough understanding of this essential architectural structure.
A Brief Overview of King Post Truss
A king post truss is a triangular structure normally made from timber or steel, composed of two principal rafters and a vertical central post called the ‘King Post’. The central post extends from the apex of the truss to the midpoint of the tie beam, creating two triangles. It is one of the simplest truss work designs in terms of both structure and function, and it is used to support a roof by spanning short to medium distances.
History of the King Post Truss
The use of the king post truss can be traced back to the Roman and medieval periods, where they were primarily used in roof construction for churches and other significant public buildings. Timber was the primary material of choice, and the king post design was valued for its ability to span large open spaces without the need for internal support.
After the arrival of steel and iron, the king post truss began to be used in bridge construction. The ability of the truss to support load efficiently and economically made it ideal for small-to-medium-span bridges. Its simplicity means that it could be prefabricated and transported, making it a popular choice for railway and road bridges.
Components of a King Post Truss
1. Rafters: These are the sloping members of the truss work design that form the triangle’s sides. They bear the roof load and transmit it to the vertical king post.
2. King Post: This is the vertical central member, which runs from the tie beam at the bottom to the apex of the triangle. It is the key component that strengthens the truss and transfers loads to the foundation.
3. Tie Beam: This is the horizontal beam at the base of the truss. It helps resist the outward thrust from the rafters, preventing the roof from spreading outwards.
4. Struts: In some designs, diagonal struts can be added to support the rafters and increase load-bearing capacity.
Variations of King Post Truss
There are several variations of the king post truss, including:
1. Double King Post Truss: This variation features two vertical king posts, which allows for a longer span and a more even distribution of load.
2. Suspended King Post Truss: In this design, the king post is suspended from the rafters rather than being directly connected to the tie beam. This configuration is often used in bridge design.
3. Queen Post Truss: The queen post truss is a close relative of the king post truss but includes two vertical posts instead of one. This variation is used for slightly longer spans and distributes the load more evenly.
Geometry of a King Post Truss
The king post truss forms a simple triangle, one of the strongest geometric shapes in construction. The forces acting on the truss are tension in the tie beam and compression in the rafters, both of which are efficiently transferred downwards. King post truss dimensions are such that they support roofing structures, especially in small-span buildings like barns, sheds, and residential homes. The height of the king post roof truss and the pitch of the rafters can be adjusted according to design needs, but the essential triangular form remains.
Applications of King Post Trusses
1. Roof Construction
King post trusses are most seen in roofing. Their simple triangular structure makes them ideal for buildings that need large, open interior spaces without support columns. Residential homes, barns, sheds, and even historic churches have used king post trusses to support their roofs. For pitched roofs, this truss system efficiently carries the load and transfers it to the building’s walls or foundation.
2. Bridge Construction
In bridge design, especially in the 19th and early 20th centuries, king post trusses spanning short distances in timber or steel bridges were used. In these applications, the king post is important in carrying the weight of the bridge deck and transferring it to the piers. Although they are not as common today for new bridge designs due to modern engineering advances, they remain vital in historic bridge restoration and preservation.
3. Pavilions and Public Structure
King post truss details are used to construct open-air structures like pavilions, park shelters, or gazebos. The aesthetic appeal of visible wooden trusses makes them a favourite for designs that aim for a rustic or traditional look.
4. Aircraft Hangers and Industrial Buildings
Larger, more advanced versions of king post trusses, often constructed from steel, are used to construct large-span industrial buildings and aircraft hangers. Their lightweight design, coupled with their load-bearing efficiency, makes them suitable for such applications where internal space needs to be maximized.
Advantages and Disadvantages of King Post Trusses
| Advantages | Disadvantages |
| Simple to design and build, making it a cost-effective solution for many construction projects. | One of the key drawbacks of a king post truss is its limited span. |
| The use of fewer materials makes king post trusses less expensive to manufacture and install and the need for fewer skilled labourers further reduces costs. | The material choice, especially in timber king post trusses, can affect durability. |
| The triangular shape ensures that the load is distributed evenly, providing good support for the roof or bridge deck. It is efficient at spanning small to moderate distances without internal support. | Timber is subject to rot, pests, and weathering which limits the lifespan of the truss if not maintained properly. |
| King post trusses are aesthetically pleasing, giving a traditional or rustic look to the buildings and are used in barns, homes and public structures like gazebos. | It is not suitable for large-span structures without additional support. Complex trusses like queen post or pratt truss can be used for long-span applications. |
| It is versatile as it is made from materials like timber, steel, and concrete depending on the application and, therefore, is used in a wide range of structural designs. | As it can handle only moderate loads, it is not suitable for structures that need heavy load-bearing capacity, like high-traffic bridges or multi-storey buildings. |
Conclusion
The king post truss is a fundamental structural component used for centuries in roof and bridge construction. Its simple, economical design makes it an attractive choice for small and medium spans. Even with the limitations in terms of king post truss span and load capacity, its efficiency and aesthetic appeal keep it in use today in various applications. From rustic homes to historic bridges, the king post truss is extremely valuable in the architect’s toolkit. If you plan to build a barn, shed, or any space that might need king post truss construction details, contact the experts at Brick & Bolt for complete guidance. You can also get a team here to help you construct one.