Long span roof structures and materials are designed to cover large areas with minimal support, making them ideal for big-box stores, warehouses, and sports stadiums.
These structures are often used in areas with high ceilings, such as 30 to 120 feet.
The primary function of a long span roof is to provide a clear, unobstructed space for various activities.
Long span roof structures can be made from a variety of materials, including steel, wood, and concrete.
Steel is a popular choice for long span roofs due to its strength, durability, and low maintenance requirements.
Wooden long span roofs are also used, particularly in areas where a more traditional aesthetic is desired.
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Types of Long Span Roofs
Long span roofs can be categorized into several types, each with its unique characteristics and applications.
One of the most common types is the Truss Roof, which is a popular choice for buildings with large open spaces. Truss roofs are made up of triangles that provide excellent structural support.
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The Beam Roof is another type, consisting of horizontal beams that span long distances without intermediate supports. This type is often used in industrial and commercial buildings.
A Rafter Roof is a type of long span roof that uses sloping rafters to support the roof's weight. Rafter roofs are commonly used in residential buildings.
A combination of beams and trusses can also be used to create a Hybrid Roof, which offers the best of both worlds in terms of structural support and aesthetic appeal.
A Cable Roof is a type of long span roof that uses cables to support the roof's weight, often used in large public buildings and stadiums.
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Roof Structures
Roof structures play a crucial role in supporting the load of a long span roof. Fabric structures are extremely strong in tension and can span large areas with minimal material.
They can be pushed into tension by supporting structures, pulled by structural cables, or inflated by air pressure. This makes them a great option for large roof areas.
Fabric structures are also translucent, providing good natural daylight. This can be especially beneficial in areas with limited natural light.
Pitched trusses are another popular option for long span roofs. They are made from steel sections connected together with bolts or welding to shaped plates called gussets.
Steel truss members are usually angle sections, which are economic and can accept both tensile and compressive stresses. This makes them a durable and reliable choice.
Trussed rafters can be designed for very long spans, ranging from 15-45 m. They are usually fabricated from timber or steel and are spaced at suitable centres to carry purlins.
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Warren Girder
The Warren Girder is a type of roof structure that uses parallel chord trusses to minimize fabrication costs for very long spans.
This is achieved by having webs of similar lengths, which reduces the complexity and cost of construction.
In addition to this, a modified Warren girder with extra verticals can be used to curtail the unsupported length of compression chord members.
This design modification is particularly useful for structures where the compression chord members need to be supported in a specific way.
The Warren Girder is a popular choice for roof structures due to its efficiency and cost-effectiveness, making it a great option for builders and architects.
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King Post
The King Post is a crucial element in roof structures, especially in truss designs. It belongs to a central vertical post that is utilized in architectural or bridge design.
In a King Post Truss, this central post functions in tension to give support to a beam underneath from a truss apex above. This design provides stability and strength to the entire structure.
The King Post is a key component that helps distribute loads evenly across the roof, ensuring it can withstand various weather conditions.
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Pratt
The Pratt truss is a type of roof structure that's particularly effective at withstanding gravity loads. The web members are arranged in such a way that the longer diagonal members remain under tension.
As you might expect, this creates a strong and stable structure. The shorter vertical members undergo compression, which helps to distribute the weight evenly.
The Pratt truss is a great option for building roofs, as it's able to handle significant loads while minimizing the use of materials. This makes it a cost-effective choice for many builders.
Howe
The Howe truss is a type of roof structure that's perfect for light roofing. It's designed to handle wind load stresses, where the longer diagonals undergo tension.
It's suitable for spans that vary between 6-30 meters, making it a versatile option for many building projects.
Vierendeel
The vierendeel truss is a type of structure that's quite unique in its design. It's characterized by rectangular openings that aren't triangulated, unlike some other truss types.
This design allows for fixed joints that can transmit and withstand bending moments, making it a sturdy option for certain applications. The vierendeel truss is a frame that's specifically built for this purpose.
It's worth noting that the vierendeel truss is designed to handle bending moments, which is a key factor to consider when choosing a roof structure.
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K-Type
The K-Type truss is a great option for roof structures. It's particularly useful for very deep trusses.
You can also use K patterns for very shallow trusses to achieve the exact inclination of the web members. This is a game-changer for designers who want precise control over their roof's geometry.
The K-Type truss is a reliable choice for many applications, thanks to its versatility and ability to handle various loads.
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Portal Frame
Portal frames are a type of structural frame that can span large distances with a reduced sectional size.
The joints between the beam and columns are made 'rigid' to transfer the bending moment to the columns, allowing for longer spans.
This is typically achieved by adding a haunch, bracket, or deepening the section at the joints.
Portal frames are commonly fabricated from steel, reinforced precast concrete, or laminated timber, also known as 'glulam'.
The 'rigid' joint enables the beam to be reduced in size, making it a more efficient option for certain roof structures.
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Space Frame
Space frames offer greater design flexibility compared to space decks. They're lightweight and rigid roofing systems made up of connectors that join chords and bracing members together.
Their strength comes from the rigidity of the triangle, where loads are transmitted as tension and compression along the length of each chord. The use of structural steel or aluminum alloy tubes is common in space frame fabrication.
Buckminster Fuller pioneered the use of space frames in the 1960s for his geodesic dome structures.
Suspension Structures
Suspension structures are incredibly versatile and can be used in a variety of applications, including bridges and roofs.
They work by using wires or cables that are fastened to supports and then suspended elements are attached to them, taking the local stress.
Plane structures, which are horizontal, use a wire that's fastened to supports and then suspended elements are attached to it, making it a great option for roofs.
These structures are used to distribute loads evenly, making them a great choice for areas with high foot traffic or heavy loads.
Suspension structures are designed to withstand extension forces, which means they can stretch but won't break under normal conditions.
They're often used in combination with other structural systems to create a robust and durable roof structure.
The cables in suspension structures behave like simple suspension elements, while the roof structure behaves like a normal load-resisting unit, subject to moments, shears, and other kinds of action effects.
This unique combination allows suspension structures to provide excellent support and stability, even under windy conditions.
Rafter
Pitched trusses use rafters to form the top edge of the truss, which are connected at their feet by a tie member to prevent spreading.
Rafter spacing can make a big difference in the overall structure. In pitched trusses, rafters are spaced at suitable centres.
The type of rafter used can also impact the design. Steel truss members are often angle sections, which are economic and can handle both tensile and compressive stresses.
In trussed rafters, the depth of the rafter increases with the span, which can be a consideration for building design.
Saw-tooth roofs often use a combination of steep and shallow pitches to maximize natural light and minimize roof volume.
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Frequently Asked Questions
What is the best long term roofing?
For long-term durability, consider clay, concrete, or slate roofing materials, which are known for their exceptional lifespan and resistance to wear. However, their suitability depends on various factors, including your location and personal preferences.
What is the roof covering preferred for large span?
Steel is the preferred material for large span roofs due to its high strength and fire-resistant properties. It's a popular choice for its durability and safety benefits.
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