Understanding House Steel Truss Roof Types and Applications

Steel truss roofs are an incredibly popular choice for homeowners due to their strength, durability, and cost-effectiveness.

There are several types of steel truss roofs, including the Pratt truss, the Howe truss, and the Warren truss. Each type has its own unique characteristics and applications.

The Pratt truss is a type of steel truss roof that is commonly used in residential construction. It features a series of diagonal members that are connected to form a triangular shape.

Steel truss roofs are ideal for homes with large spans or irregular rooflines, as they can provide the necessary support and stability.

A well-designed steel truss roof can last for decades with minimal maintenance, making it a practical choice for homeowners.

Suggestion: Flat Roof Type

A house steel truss roof is essentially a triangulated system of interconnected structural elements, typically made up of straight steel beams.

These elements are connected at nodes, where the connections are often assumed to be nominally pinned.

For more insights, see: Steel Building Roof Insulation

The principal force in each element of a truss is axial tension or compression, which is the primary way these elements work together to support the roof's weight.

In a house steel truss roof, all the members and applied forces are generally in the same plane, making it a type of plane or 2D truss.

Steel truss roofs come in a variety of forms, each with its own unique characteristics. A Warren truss, for example, is commonly used in long span buildings, ranging from 20 to 100 meters in span.

The top and bottom chords of a truss provide compression and tension resistance to overall bending. In a Warren truss, diagonal members are alternatively in tension and in compression.

Warren trusses are also used for the horizontal truss of gantry/crane girders. They have equal length compression and tension web members, and fewer members than a Pratt truss.

Suggestion: Life Span of Roof

Steel truss roofs are a popular choice for buildings that require long spans, such as airport terminals and sports stadia roofs. They're also great for carrying heavy loads and can be used as transfer structures.

In typical single storey industrial buildings, trusses serve two main functions: to carry the roof load and to provide horizontal stability. This is achieved through a combination of columns and truss connected to form a frame.

Trusses are widely used in these types of buildings because they're efficient and can span long distances without the need for internal supports. They're also relatively lightweight, which makes them easier to construct and transport.

Here are some common truss building arrangements:

In some cases, trusses are used in conjunction with a portal structure, where loads are applied to the truss by purlins and side rails. This arrangement provides resistance to global bending moments and helps to distribute loads evenly throughout the structure.

Overall, steel truss roofs offer a number of benefits for building designers and constructors, including efficiency, flexibility, and cost-effectiveness.

The Warren truss is a great option for long span buildings, capable of spanning up to 100 m in length.

This type of truss has a unique design where diagonal members alternate between tension and compression, making it efficient and effective.

Warren trusses typically have fewer members than a Pratt truss, which is a plus for builders looking to minimize material costs.

They're also commonly used for the horizontal truss of gantry/crane girders, showcasing their versatility and strength.

A modified Warren truss can be used to provide additional support at purlin locations, making it a great choice for complex building designs.

The saw-tooth truss is a variation of the North light truss, commonly used in multi-bay buildings. It's a popular choice for its ability to provide natural light and ventilation.

A saw-tooth truss typically includes a truss of the vertical face running perpendicular to the plane of the saw-tooth truss, similar to the North light truss.

The Fink truss is a great option for short-span high-pitched roofs. It offers economy in terms of steel weight.

This type of truss is commonly used to construct roofs of relatively short span. It's a popular choice for many builders.

The members of a Fink truss are subdivided into shorter elements, making it a cost-effective option. This design allows for more flexibility in terms of arrangement and subdivision of the chords and internal members.

The Fink truss is a versatile choice that can be adapted to different building projects. Its simplicity and efficiency make it a favorite among builders and architects alike.

Vierendeel trusses are a type of rigidly-jointed truss with only vertical members between the top and bottom chords.

The chords in Vierendeel trusses are normally parallel or near parallel, which affects the way the elements are subjected to loads.

Elements in Vierendeel trusses are subjected to bending, axial force, and shear, unlike conventional trusses with diagonal web members where the members are primarily designed for axial loads.

For your interest: Parallel Chord Scissor Truss

Vierendeel trusses are usually more expensive than conventional trusses.

Their use is limited to instances where diagonal web members are either obtrusive or undesirable.

The vertical members near the supports in Vierendeel trusses are subject to the highest moments and therefore require larger sections to be used than those at mid-span.

Designing a house steel truss roof involves careful consideration of various factors. Two analysis models are commonly used for trusses: one that considers axial force only, and another that takes into account bending moments as well.

For smaller spans, tee sections are often used for chords, paired with angles as internal members. These members can be bolted or welded together.

When selecting truss members, it's essential to consider factors like internal forces, ease of connections, aesthetics, and the need for prefabricated truss sections. You should also think about the out-of-plane buckling resistance and resistance under reversed loading.

Here are some common types of truss member sections and their uses:

Connections in steelwork construction are often either bolted or welded. Bolted site splices are preferred for economy and speed of erection, but pre-loaded assemblies are recommended to eliminate consequences of slip and bearing deformation.

For smaller spans, tee sections are frequently used for chords, with angles used as internal members. The internal members may be bolted or welded to the tees.

Back-to-back angles or channels may be used for longer spans or heavier loads. This approach is often necessary for transfer trusses in buildings.

Nodes in trusses are usually welded, especially in cases where members are rolled sections, typically UC sections. Bolted splices within the length between nodes are used to complete necessary connections.

To avoid slip at the splices, pre-loaded assemblies are generally specified. This helps ensure the truss remains stable and secure.

Hollow sections are chosen for many exposed trusses due to their structural efficiency and aesthetic appeal. They are often welded in the workshop as part of the truss fabrication process.

In steelwork construction, bolted site splices are generally preferred to welded splices for economy and speed of erection.

Bolted connections require evaluating the consequences of slip and bearing deformation in connections, which can be eliminated by using pre-loaded assemblies.

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

Guidance on the design of welded joints for Celsius 355 and Hybox 355 hollow sections is available from Tata Steel.

For large roof trusses that can't be transported whole, welded sub-assemblies are delivered to site and either bolted or welded together on site.

Entirely bolted connections are less favored in light roof trusses due to the requirement for gusset plates and their increased fabrication costs.

Profile shaping of tubular sections is necessary for joint fabrication.

A longitudinal wind girder is necessary in buildings where the roof trusses aren't "portalized". This is a key consideration in design and construction.

The general arrangement of a longitudinal wind girder is similar to that of a transverse wind girder, with a Warren or Pratt truss configuration. Chord members are typically provided from hollow sections.

Posts, if required, are the upper chords of consecutive stabilized roof trusses. This is an important detail to keep in mind when designing a longitudinal wind girder.

In buildings with braced gable ends, a longitudinal wind girder is necessary to provide additional support and stability. This is a crucial aspect of ensuring the structural integrity of the building.

Here are some key features of a longitudinal wind girder:

By incorporating these features, a longitudinal wind girder can provide the necessary support and stability to withstand wind loads and ensure the safety of the building and its occupants.

All the purlins connected to a roof bracing can be considered as lateral rigid support points.

In designing truss members, it's essential to consider that they are subjected to axial force, but may also experience bending moments, especially if the chords have been modeled as continuous members.

Intermediate purlins can also be considered as a rigid point of support if the roof behaves as a diaphragm (Class 2 construction according to BS EN 1993-1-3).

Lateral support points are provided to the lower chord by additional vertical bracing elements between trusses.

These design considerations will help ensure that your truss members are strong and stable, and can withstand various forces and loads.

For another approach, see: What Are Roof Purlins

Modelling is a crucial step in designing a truss, and it's essential to get it right. For standard buildings, it's common and usually justified to work with 2D models, such as a portal, wind girder, or vertical bracing, rather than a global 3D model.

When modelling a truss, it's often convenient to work with restricted models. For example, you can model a truss without its supporting columns if it's articulated to the columns. This approach can help simplify the analysis and make it more manageable.

If separate models are used, you may need to combine the results of several analyses to verify the resistance of certain elements. For instance, the upper chord of a truss that also serves as the chord of the wind girder may require this approach.

A global 3D model can be used, but it's essential to provide appropriate member releases to avoid 'parasitic' bending, which can create an illusory precision of the structural behaviour.

You can model a truss as either continuous chords with pinned internals or pinned joints throughout the truss. This will depend on the specific requirements of your design and the type of truss you're working with.

Here are some common modelling approaches to keep in mind:

All purlins connected to a roof bracing can be considered as lateral rigid support points, which is a crucial consideration when designing a house steel truss roof.

Intermediate purlins can also be considered as a rigid point of support if the roof behaves as a diaphragm, specifically in Class 2 construction according to BS EN 1993-1-3.

Lateral support points are provided to the lower chord by additional vertical bracing elements between trusses, which helps to distribute forces evenly throughout the structure.

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Two analysis models are commonly used for trusses: one considers axial force only, while the other accounts for bending moments as well.

When designing truss members, it's essential to consider that they can be subjected to both axial force and bending moments, especially if the chords are modeled as continuous members.

The type of support provided to the truss members can significantly impact their behavior. For instance, purlins connected to a roof bracing can be considered as lateral rigid support points.

Here are some key points to consider regarding lateral support:

In designing roof trusses, it's crucial to consider the type of loading they will experience. For example, the upper chord is in compression for gravity loading, and the bottom chord is in compression for uplift loading.

To ensure lateral stability, it's essential to design members in compression against out-of-plane buckling. This can be achieved by providing lateral restraint to the upper and bottom chords.

Here are some common methods for providing lateral restraint:

Loads play a crucial role in determining the structural integrity of a building. Dead loads, such as the weight of the roof sheets and ceiling, are typically applied at the top and bottom chords of the roof truss.

The self-weight of the roof truss is checked during the design process, and the superimposed dead load is calculated as 0.5 kN/m and 0.833 kN/m at the top and bottom chords, respectively.

Live loads, on the other hand, are temporary loads that can vary depending on the intended use of the building. However, in this case, we're not provided with specific information about live loads.

Wind loads can be a significant consideration in designing roof trusses, especially in areas prone to high winds. For this example, we're using the SkyCiv Load Generator to calculate the wind loads acting on the roof trusses.

Curious to learn more? Check out: Load Bearing Roof Truss

The calculated wind pressures for each zone are the same, so we'll consider two cases: the positive (or max) case and negative (or min) case. The positive wind load is 2.168 kN/m, and the negative wind load is -1.508 kN/m.

Snow loads can also impact the design of roof trusses, particularly in areas with heavy snowfall. For this example, we're assuming a balanced roof snow load of 0.23 kPa.

The snow load on the purlins and roof trusses is calculated as 0.766 kN/m for the balanced case and 1.399 kN/m for the unbalanced case. To convert the snow load from a horizontal projection to an inclined load acting on the top chord of the roof truss, we need to divide the load by the cosine of the roof angle.

Here's a summary of the loads considered in this example: