What is a Truss and How Do Trusses Work in House Framing Design?

When it comes to residential structural design, trusses can offer a number of advantages for house framing applications. But what exactly is a truss, and how do they work? As structural engineers, architects, and other professionals in the industry know, understanding truss structures is key to successfully designing safe and strong houses.

This blog article provides an overview of trusses, explores their different types, explains how they work, and discusses their benefits and limitations when designing safe and secure frames for houses.

What is a Truss?

A truss is a structural system made entirely of axially loaded members that consists of a collection of elements connected at pin joints or nodes.

The characteristics of a truss include the following:

• All straight members
• Members are connected together with pin joints
• Connected only at the ends of the members
• All external forces (loads & reactions) must be applied only at the joints

In theory, the pin joints provide no rotational resistance and behave as hinges.

What are The Different Types of Trusses?

Trusses are either plane trusses or space trusses.

A plane truss is a truss where all members lie in a single plane. As a result, plane trusses are treated as two-dimensional systems. Plane trusses are commonly used in different structures globally.

Some examples of commonly known plane trusses such as Flat Warren Truss, Flat Howe Truss, Fink Roof Truss and Howe Roof Truss are illustrated in Figure 1 (check out this article to find out more about the different types of trusses for residential structures).

Figure 1: Some examples of common plane trusses (Reference)

Conversely, space trusses have members not limited to a single plane. This means that space trusses need to be analyzed as a three-dimensional system.

Figure 2: An example of space truss frame (Reference)

Truss Terminology

Common terminology relating to timber trusses is demonstrated from the below extract of Australian Standard AS 4440 (2004) - Installation of nail-plated timber roof trusses for residential and light commercial structures.

Figure 3: Common components and terminology of roof timber truss (Reference)

• Timber Truss: A framework constructed with timber of uniform thickness and fastened together in one plane by connectors such as metal nailplates, bolts, plywood gussets, or other similar devices.
• Truss Overhang: The portion of top chord extending beyond the body of the truss to provide eaves.
• Eaves: The portion of main building outside the external wall line. Generally, it includes the truss overhang and also cantilevered portions if present.
• Truss Overhang: The portion of top chord extending beyond the body of the truss to provide eaves.
• Eaves: The portion of main building outside the external wall line. Generally, it includes the truss overhang and also cantilevered portions if present.
• Pitching Point: The intersection of underside of top chord and underside of bottom chord.
• Nailplates: Metal plate connectors manufactured predominantly from light gauge galvanized steel with teeth spaced and formed within the parent metal. Nailplates are normally pressed into opposite faces of the timber members to form a spliced or gusseted type of joint.

Truss Materials

Trusses are typically made out of timber and/or steel. Steel is used for longer-spanning trusses, particularly those in commercial buildings or bridges, while timber trusses are typically used in residential construction. Sustainably sourced timber trusses are more ecologically friendly.

Figure 4: Examples of timber and steel trusses (Reference)

Procurement and Cost of Trusses

Most roof trusses and all steel trusses are prefabricated and delivered to the site, which ensures quality workmanship and synergies from economies of scale. As a result, the lower amount of labor and plant required on-site greatly reduce construction costs.

Typically, timber truss suppliers and steel fabricators need at least 4 weeks of lead time to manufacture and deliver trusses to the site.

Timber trusses are typically cheaper and have less strength but have shorter procurement times due to standard sizes and simpler workmanship.

The costs of timber trusses vary significantly. The variables can include where you purchase them or if you opt for prefabricated trusses vs. trusses built on-site. In Australia & the US, you can expect to pay anywhere from $200 to$700 USD per truss (note most homes will need several trusses, usually between 20 and 40 as a rough estimate). Timber roof trusses for your home will cost approximately $9,000 USD to$20,000 USD.

It can be difficult to accurately forecast the cost of timber trusses in Australia and the US as they vary widely depending on several factors, including the size and design of the trusses, the type of timber used, and local market conditions. Additionally, the cost can be calculated in different ways, such as per cubic metre or per tonne.

Steel trusses are more expensive usually due to the price of steel and more labour required to construct the connections (bolts or welds) compared to the nail plated connections of a timber truss.

Steel trusses are typically twice as expensive as timber trusses as they are typically bespoke designs but the price is heavily dependent on steel prices.

Steel Trusses Tech Australia, Aussteel and Australian Steel Framing are the most common steel truss suppliers in Australia while timber trusses can be sourced from Timbertruss & Australian Timber and Trusses. In the US, Structural Timber Trusses and American Truss are long-time industry suppliers of timber trusses, while steel trusses can be sourced from US Steel Truss and Adams Truss.

Benefits of Truss System

The benefit of a truss is that the members are axially loaded, so there is no shear or bending. This means they are either in compression or tension or have no force (zero force members). This makes trusses a very efficient structural component.

Like any structure, a truss transmits externally applied forces through its elements and back into the supports or foundations of that structure. Because of the efficiency of their structural mechanics, trusses can span very large distances with relatively small section sizes. This lightweight nature of the member makes it quicker, easier, and ultimately cheaper to construct or install on-site. This efficiency makes them well suited in roofs in residential construction and bridges in civil infrastructure.

Trusses can span longer distances than beams or traditional frames, resulting in building projects that are less reliant on internal supporting structures such as load-bearing walls, beams, & columns. As a result, larger open spaces can be achieved by improving the aesthetics of a home. Buildings can also be future-proofed for different use cases, given there is fewer load-bearing walls and columns.

When there is fewer load bearing vertical members (walls and columns), there is more potential for future renovations to transform the existing structure into a building with a different use cases. For example, a building that uses trusses that have large spans to load bearing columns or walls on the perimeter of the property may only have non-load bearing walls internally. These walls can be demolished with ease and a residential structure could be transformed into a gymnasium, education center or any other type of building requiring large open spaces.

Trusses can also be prefabricated to minimize labor and plant on site reducing construction costs. The effectiveness of trusses is demonstrated by their frequent use to support long spans and heavy weights.

Limitations of Trusses

Members of a truss are slender and not capable of supporting large lateral loads and usually need to be used in conjunction with steel wire or braced framing to provide lateral support as per Figure 4.1 in AS 4440.

Figure 5: An example of bracing system for roof trusses (Reference)

Fortunately the lateral stability elements are simple to design and can be done so using ClearCalcs Truss Analysis and Steel & CFS calculators.

Conclusion

To recap, trusses can be a cost-effective option in house framing design. By understanding their different types and materials, as well as their benefits and limitations; designers have a wide array of options when engineering trusses for frame structures.

Trusses are a great option when the architect or builder is looking to have a horizontal member span over 8 meters. Trusses spanning up to 16 meters can be designed in accordance with the Australian Standard AS 4440 (2004), installation of nail-plated timber roof trusses for residential and light commercial structures.

However, trusses can be engineered to span much larger than 16 meters using Finite Element Analysis which is conveniently built into the Truss Analysis Wizard calculator. In addition, timber trusses can be preferable for spans smaller than 8 meters when steel prices are high.

Ready to start calculating with ClearCalcs?

If you’re considering undertaking a project that includes house-framing using a truss system or are curious about what it means, ClearCalcs is available to help! Our Truss Analysis Wizard makes frame design structurally sound without any calculations or guesswork involved - so you can rest assured knowing your project will be done correctly every time!