Trusses and Purlins for Buildings and Bridges

Trusses and purlins are essential components of building and bridge construction. They provide the necessary support and stability to hold up the structure's weight.

A truss is a triangular framework of connected elements, typically made of wood or metal, that acts as a single unit to resist loads. Purlins are horizontal beams that run between trusses, providing additional support and helping to distribute the weight evenly.

Trusses can be made from a variety of materials, including wood, steel, and aluminum. They come in different types, such as Howe trusses and Warren trusses, each with its own unique configuration and load-carrying capacity.

Timber frame types can be broadly categorized into two main groups: post and beam and panelized.

Post and beam frames are characterized by exposed beams and vertical posts, often with a decorative element.

They can be further divided into traditional and modern styles, with traditional styles often featuring hand-hewn beams.

Modern post and beam frames often incorporate machine-made beams for increased efficiency and cost-effectiveness.

Panelized frames, on the other hand, consist of pre-assembled sections of wood, often including trusses and purlins.

These sections are typically made in a factory and then transported to the construction site for assembly.

Panelized frames can be more cost-effective and faster to build than traditional post and beam frames.

They also offer greater design flexibility and can be customized to meet specific building requirements.

In terms of materials, post and beam frames often feature solid timber, while panelized frames may incorporate engineered wood products.

Engineered wood products, such as glulam beams, offer improved strength and stability compared to solid timber.

A roof truss is a fundamental component of a timber frame roof, and its design plays a crucial role in determining the overall structure's integrity and aesthetic appeal. Trusses can be designed to support various types of roof coverings, including 2×6 tongue and groove boards, conventional or structural insulated panels (SIPs), and roofing.

The design of a truss typically involves two analysis models, either of which can be used to determine the truss's strength and stability. The choice of analysis model depends on the specific requirements of the project.

A well-designed truss can create a strong and visually appealing roof structure, with the addition of purlins adding a secondary layer and creating strong lines that run parallel to the ridge beam. Purlins can also support the roof's decking and provide a great place for placing LED lighting fixtures.

North Light trusses are traditionally used for short spans in industrial workshop-type buildings.

They allow maximum benefit to be gained from natural lighting by using glazing on the steeper pitch which generally faces north or north-east to reduce solar gain.

A truss running perpendicular to the plane of the North Light truss is typical to provide large column-free spaces on the steeper sloping portion.

This design can reduce the operational carbon emissions of buildings by increasing natural daylighting.

However, it's worth noting that the impact of North Light trusses on carbon emissions should be explored using dynamic thermal modeling.

By reducing the need for artificial lighting, North Light trusses can also reduce the risk of overheating.

But, as a trade-off, they can also increase the demand for space heating due to the increased volume of the building.

Trusses are a common choice for roof design, and there are two analysis models used to design them: either the truss with purlins approach or a different method altogether.

Trusses with purlins can reduce costs by allowing for increased on-center distances between trusses, requiring fewer trusses to be purchased.

Purlins can also create strong lines by running parallel to the ridge beam, making them a great option for traditional heavy timber roof framing.

Purlins often sit in pockets cut into the truss rafters, but can also sit on top of the truss rafter to create a secondary layer to the roof structure.

The span restrictions for purlins depend on their size and how close they are together, so it's essential to consider these factors when designing the roof.

Trusses can be designed using either of two analysis models, but the specific design approach will depend on the project's needs and requirements.

Wind Girder Design is an essential aspect of roof design, particularly when it comes to withstanding strong winds. A transverse wind girder is a key component in this design.

It's typically arranged as a Warren or Pratt truss, parallel to the roof plane. The chords of the wind girder are the upper chords of two adjacent vertical trusses.

These chords experience axial forces from both the vertical truss and the wind girder loading, which must be combined for an appropriate combination of actions. This can be a complex calculation, but it's crucial for ensuring the structural integrity of the roof.

A convenient arrangement is to place a transverse wind girder at each end of the building, allowing the longitudinal members to act only in tension.

Trusses are assemblies of tension and compression elements that work together to resist forces like gravity and shear. Under gravity loads, the top and bottom chords provide compression and tension resistance to overall bending.

The Fink truss is a type of truss that's commonly used for short-span high-pitched roofs, offering economy in terms of steel weight. Its members are subdivided into shorter elements to achieve this efficiency.

This type of truss is particularly well-suited for roofs of relatively short span, as it can provide the necessary structural support while minimizing the amount of steel used.

The queen post truss with straining beam is a configuration capable of much longer spans, with trusses developed for distances over 50 feet. This design is a great option for those looking for a strong and adaptable truss.

The straining beam between the tops of the posts creates a straightforward geometry and pleasing lines, making it a timeless and honest design choice.

Scissors trusses are a traditional design with many variations, perfect for small or large spans. They're not recommended for shallow roof pitches, so keep that in mind.

A complicated geometry with many angled intersections is what makes scissors trusses stand out. The added space from a higher center is a bonus.

Adding queen posts on either side of the central king posts is a common practice with scissors trusses.

The Warren truss is a great option for long span buildings, and it's commonly used in structures ranging from 20 to 100 meters in span.

One of the key characteristics of a Warren truss is that diagonal members are alternatively in tension and in compression, which is a clever design feature.

Warren trusses have fewer members than a Pratt truss, making them a more efficient choice for some applications.

They're also commonly used for the horizontal truss of gantry/crane girders, which is a testament to their versatility and strength.

A modified Warren truss can be adopted by introducing additional members to provide a node at purlin locations, which is a nice touch for added stability.

Bolted connections are generally preferred in steelwork construction for economy and speed of erection.

In these connections, it's essential to evaluate the consequences of slip and bearing deformation.

Pre-loaded assemblies are recommended to eliminate these consequences, which can lead to increased deflections.

Hollow sections are typically connected by welding, while open sections can be connected by bolting or welding, often involving gusset plates.

For Celsius 355 and Hybox 355 hollow sections, guidance on welded joints is available from Tata Steel.

Small trusses that can be transported whole from the fabrication factory to the site can be entirely welded.

Large roof trusses, however, require welded sub-assemblies that are delivered to site and then bolted or welded together.

In light roof trusses, bolted connections are less favored due to the need for gusset plates and increased fabrication costs.

Vierendeel trusses are rigidly-jointed trusses with only vertical members between the top and bottom chords, which are usually parallel or near parallel.

These trusses are subjected to bending, axial force, and shear, unlike conventional trusses with diagonal web members that primarily handle axial loads.

Vierendeel trusses are typically more expensive than conventional trusses, which limits their use to specific instances where diagonal web members are either obtrusive or undesirable.

Vertical members near the supports are subject to the highest moments and require larger sections than those at mid-span.

A roof truss is made up of several key components, each playing a crucial role in supporting the roof. The apex is the top-most point of a roof truss, where the top and bottom chords meet.

The bottom chord is the long horizontal beam that defines the lower edge of a truss, providing strong support for the rest of the truss. The top chord, on the other hand, is the top-most beam of a roof truss, often slanted and hanging over the side of the building.

Purlins, smaller beams that run perpendicular to the top chords, are used to support the roof's decking. They can also create strong lines by running parallel to the ridge beam(s).

The arch chord king post truss is a variation of the classic king post style with an arched lower chord, raising the structure and giving some extra volume below. This design adds a curvilinear element to the ceiling space, softening the lines and conveying a natural aesthetic.

A shallow radius cut is necessary to cut the arch out of solid sawn timber, but more semi-circular arches can be accommodated by using grain matched glue laminated arch material. This option is considerably more expensive than solid sawn timber.

The arch chord with queen posts timber frame truss is another variation that adds more vertical elements above the arched bottom chord with the extra volume below. This configuration often requires concealed steel connectors to resist tension and spreading along the arch itself.

We like to use a spline tenon to joint the 2 half arches to the base of the king post, creating a strong and sturdy connection.

Cruck Frame Bents are a type of traditional roof structure found primarily in the UK.

They consist of a pair of timbers, known as blades, which serve as the principals of a roof.

These blades are typically straight or curved and stretch from a level well down the side walls to the apex of the roof.

A true cruck frame is defined by its distinctive design, where the blades meet at or close to the apex of the roof.

A truss roof is a great option for many buildings, and it's made up of several key components. The apex is the top-most point of a roof truss, which can be the peak of a triangular roof or the top-center point of a flat roof.

The bottom chord is the long horizontal beam that defines the lower edge of a truss, providing strong support for the rest of the structure. It's essential to get this right, as it's the foundation of the entire truss.

A girder is a main roof truss that supports any secondary trusses framing into it, adding an extra layer of stability to the roof. This is particularly important in larger buildings or those with complex rooflines.

The heel is a joint in a pitched roof truss where the top and bottom chords meet, creating a sturdy connection between the two beams. This joint is critical to the overall integrity of the truss.

Purlins are smaller beams that run perpendicular to the top chords of each roof truss, supporting the roof's decking and adding a touch of elegance to the structure. They can also be used to create strong lines by running parallel to the ridge beam.

The top chord is the top-most beam of a roof truss, typically slanted and hanging over the side of the building. It's a key component of the truss, providing additional support and stability.

Webs include beams that join top and bottom chords in a triangular pattern, capable of carrying tension and compression stress. This helps keep the truss from bending and ensures a smooth, even roofline.

Pratt trusses are a popular choice for long span buildings, spanning anywhere from 20 to 100 meters. They're ideal for buildings where gravity loads are predominant.

The Pratt truss has diagonal members in tension for gravity loads, making it a reliable option for structures that bear heavy weight. This type of truss is commonly used in buildings that require a strong and stable roof.

In some cases, the diagonal members of a Pratt truss can be in tension for uplift loads instead. This is particularly useful in open buildings like aircraft hangers, where uplift loads are a concern.

You can add secondary members to a Pratt truss to create intermediate support points for applied loads. This can also help limit the buckling length of members in compression.

Here are some possible configurations for Pratt trusses:

Lateral stability is crucial for roof structures, and it's essential to consider the design of members in compression to prevent out-of-plane buckling.

For simply supported trusses, the upper chord is in compression for gravity loading, while the bottom chord is in compression for uplift loading.

The purlins and the transverse roof wind girder typically provide lateral restraint to the upper chord.

Additional bracing may be necessary to restrain the bottom chord, as shown in the illustration. This allows the buckling length of the bottom chord to be limited to the distance between points laterally restrained.

The diagonal members transfer the restraint forces to the level of the top chord, where general roof bracing is provided.

It's possible to create a horizontal wind girder at the level of the bottom chords, with longitudinal elements to stabilize all the trusses.