A Practical Guide to Timber Floor Joist Design in Residential Construction

Introduction

Timber floor joists are essential structural elements in residential construction, providing support and stability to the flooring system.

This article aims to provide structural engineers, building designers, and architects with a comprehensive understanding of timber floor joist design, specifically focusing on Australian standards and building codes.

Table of Contents

• Timber Floor Joists Design Considerations
• Relevant Building Codes and Industry Standards in Australia
• Design Process of a Residential Floor Joist to Australian Standards
• Exploring Different Types of Timber Floor Joists

We will explore design considerations such as load-bearing capacity, span length, deflection, and vibration. Additionally, we will delve into different types of timber floor joists commonly used in residential construction, including traditional solid timber joists and engineered timber I-beam joists, as well as briefly discuss alternative engineered floor joist systems.

TIP: Check out the this example of timber floor joists design and compare the process between manual hand calculations and timber beam calculator.

Figure 1: An example of timber floor joist in a new house construction

Timber Floor Joists Design Considerations

Designing timber floor joists requires careful consideration of various factors to ensure structural integrity and safety. The following key considerations must be addressed:

Load-bearing capacity

Determining the maximum loads the floor joists will bear is crucial. This includes both dead loads (weight of the structure itself and permanent fixtures) and live loads (occupant loads and furniture).

Span length

The span length refers to the distance between supports and affects the joists' dimensions and spacing. Longer spans require larger and stronger joists.

Deflection

Excessive deflection can lead to an uncomfortable or unstable floor. Australian standards specify maximum allowable deflection limits based on the floor's intended use and occupant expectations.

Vibration

Proper floor joist design should consider mitigating vibrations caused by occupant movement or equipment. This is particularly important for floors with sensitive equipment or activities.

Relevant Building Codes and Industry Standards in Australia

Compliance with building codes and industry standards is essential in timber floor joist design. In Australia, the following codes and standards are applicable:

• The National Construction Code (NCC): Part 3.1.2 and Part 3.5 of the NCC provide requirements for structural design, including floor joists, in residential buildings.
• Australian Standard AS 1684: Residential Timber Framed Construction: This standard provides detailed guidelines for timber framing, including design procedures, materials, and connections.
• AS 1720.1: Timber Structures: Design Methods: This standard offers comprehensive guidance on the design of timber structures, including floor joists, with specific provisions for different load types and environmental conditions

Design Process of a Residential Floor Joist to Australian Standards

1. Load Calculation

To calculate the total design load on the floor joist, we need to consider both the live load and the dead load and the critical load combination.

For determining the critical load combinations and structural design actions for a timber floor joist in Australia, you should refer to the Australian Standard AS/NZS 1170:2002 - Structural Design Actions.

This standard provides general principles and requirements for determining loads and actions that need to be considered in the design of structures, including timber floor joists.

AS1170.1 Chapter 3 details the dead loads and live loads that need to be considered, AS 1170.2 details wind loads, AS 1170.3 considers snow loading and AS 1170.4 considers earthquake loading (snow and earthquake are typically not applicable to floor joists).

AS 1170.0 Chapter 4 (shown below) provides guidance on the load factors and combinations that should be considered for design. Generally, in timber design, we always take the most significant load combination factors and design timber floor joists (and all structures) in accordance with the worst-case scenario.

Figure 2: Section 4.2.2 of AS/NZS 1170.0:2002 outlining the load factors and combinations for structural design. (Reference)

Additionally, you should also refer to the specific requirements and guidelines for timber structures as outlined in AS 1720.1:2010 - Timber Structures, Part 1: Design Methods. This standard deals specifically with design methods for timber structures, including floor joists, and provides detailed information on material properties, member design, load combinations, and more.

2. Joist Size Determination

To determine the required size of the floor joist, we need to consider the span length, load, and the appropriate design criteria specified in AS 1684 and AS 1720.1. AS 1720 can be used to design timber members from first principles and a detailed example of this will be provided in a subsequent blog post.

To speed up the design process, the supplementary notes provided in AS 1684 provide span tables, and from these we can find the suitable timber species, grade, and size of the floor joist. Take a look at the example below for grade F27 seasoned hardwood with a floor load width of 3600mm.

Figure 3: An example of span table for floor joist provided in AS 1684 supplementary notes.

3. Connection Details

There are several types of connections used for timber floor joists, each serving different purposes and providing varying levels of strength and stability. The choice of connection depends on the specific structural requirements, loads, and other factors of the construction project. Here are some common types of connections for timber floor joists:

1. Nailing: Nailing is one of the simplest and most common methods of connecting timber floor joists. Joists are typically nailed to beams or other supporting elements using nails or screws. Toe-nailing and face-nailing are two common nailing techniques used for securing joists to beams or headers.
2. Joist Hangers: Joist hangers are metal brackets designed to hold the ends of the floor joists and attach them to beams or headers. They provide strong and reliable connections while allowing the joists to be flush with the supporting member.
3. Ledger Strips: Ledger strips are horizontal wooden members attached to the side of a beam or wall. The ends of the floor joists can be secured to these ledger strips using nails or screws.
4. Timber Connectors: Timber connectors are metal brackets and hardware specifically designed for connecting timber members. They offer enhanced strength and stability compared to standard nailing or screwing.
5. Through-Bolts: Through-bolts are long bolts that pass through the entire depth of the joist and connect it to the supporting element. They provide a very strong connection but may require pre-drilling holes in the timber to avoid splitting.
6. Notched Connection: In this method, a notch or groove is cut into the supporting beam or header, and the end of the joist fits into the notch. This type of connection creates a flush surface and enhances stability.
7. Bridging or Blocking: Bridging or blocking involves adding small timber pieces between adjacent joists to provide lateral support and prevent twisting or buckling of the joists.
8. Metal Plates or Gussets: Metal plates or gussets can be used to reinforce connections between timber members. They are typically secured with nails or screws and provide added strength and durability.
9. Adhesive Bonding: In some cases, structural adhesive can be used along with mechanical fasteners to enhance the connection between timber members.

The connection details between the floor joists and supporting members, such as bearers or walls, are crucial for structural integrity. AS 1684 provides guidelines for the appropriate connection methods and fastener sizes.

For example, the connection between the joist and the bearer could involve using a specific size and type of nails or screws at designated spacing.

It is important to consult AS 1684 for the specific connection requirements based on the selected timber species, grade, and size of the floor joist.

4. Calculation Verification

Engineers should always verify their design to ensure accuracy. Structural design and analysis software like timber beam calculator with built-in presets for common timber structures like floor joists, floor bearers, and floor lintels can speed up the calculation and verification process.

For example, structural engineer Matt Ward uses common presets like deck joists and deck beams for his residential design to make his workflow more efficient, saving valuable time.

Additionally, users can input their loading conditions and assess the adequacy of a range of joists from known Australian manufacturers using the member selector tool instantaneously.

This can ensure the selected timber joist is the most efficient member and can be procured by the builder.

Figure 4: A database of timber joists from common Australian manufacturers showing how they are automatically checked to asses their loading condition in ClearCalcs floor joist calculator.

Exploring Different Types of Timber Floor Joists

Traditional Solid Timber Joists

Traditional solid timber joists, commonly made from dimensional lumber, have been used for many years in residential construction.

Key characteristics and applications include:

• Characteristics: Solid timber joists are typically made from softwood species such as pine or spruce. They are available in standard sizes, with rectangular or square cross-sections.
• Applications: Solid timber joists are well-suited for residential floors with shorter spans and lighter loads. They are cost-effective and widely available.
• Benefits and Limitations: Solid timber joists offer simplicity in design and construction. However, they have limitations in longer spans and may be susceptible to warping, twisting, or shrinking over time.

Engineered Timber Joists

Engineered timber I-beam joists have gained popularity in residential construction due to their superior performance and design flexibility.

They are constructed from various types of engineered timber products, such as laminated veneer lumber (LVL), oriented strand board (OSB), glue-laminated timber (Glulam) or laminated strand lumber (LSL). They are manufactured by arranging layers of lumber pieces with their grain running parallel and then gluing them together under high pressure and heat. Figure 3: Material properties are aligned to achieve greater efficiency in engineered timber members. (Reference)

This process creates a continuous, strong, and stable member that can be custom-shaped to meet various design requirements.

Key benefits and advantages include:

• Enhanced strength-to-weight ratio: Engineered I-beam joists utilize a combination of timber products, adhesives, and steel fasteners, resulting in high strength and stiffness with reduced weight.
• Consistency and predictability: Engineered joists are manufactured with precise specifications, ensuring consistent performance and eliminating variability often found in solid timber joists.
• Resistance to warping and shrinking: Engineered joists are less prone to warping, twisting, or shrinking compared to solid timber joists, ensuring a more stable and durable floor system.
• Design flexibility: Engineered joists can accommodate larger spaces as the better utilise the material properties of the timber and creative architectural designs due to their longer spans and ability to carry heavier loads.
• Environmental considerations: Engineered timber products often utilize sustainable sourcing practices and contribute to reducing waste in the construction industry.
• Common use cases: Engineered I-beam joists excel in applications that require longer spans, such as open floor plans, vaulted ceilings, and areas where mechanical systems need to pass through the floor.

Other Types of Engineered Floor Joists

While the focus of this article has been on solid timber and I-beam joists, it's worth mentioning alternative engineered floor joist systems, including trusses and open-web joists. These systems offer unique features, benefits, and specific use cases:

Trusses

Trusses consist of interconnected structural members and are commonly used for larger spans, such as roof structures.

Trusses offer clear spans and make installation of plumbing and electrical services easy. The key advantage of trusses is that their members are loaded axially, thus eliminating any shear or bending. In other words, they are either in compression or tension, or have no force at all (zero force members). This makes trusses highly efficient as a structural component and enables them to achieve greater spans than solid timber joists.

Figure 4: Example of timber and steel trusses. (Reference)

For more resources on truss design, check out these other guides and resources:

• Types of Roof Trusses and Their Applications in Residential Design
• What is a Truss and How Do Trusses Work in House Framing Design?

Open-Web Joists

Open-web joists, also known as metal-web joists, utilize a combination of steel and timber components. They offer design flexibility and ease of installation that allows for efficient routing of mechanical services. They also offer greater spans than solid timber joists.

Figure 5: An example of open-web joists. (Reference)

Conclusion

Timber floor joist design in residential construction requires a thorough understanding of load-bearing capacity, span length, deflection, and vibration considerations. Compliance with relevant building codes and standards, such as the NCC, AS 1684, and AS 1720.1, is crucial.

Traditional solid timber joists are suitable for shorter spans, while engineered timber I-beam joists provide enhanced strength-to-weight ratio, design flexibility, and improved resistance to warping.

Alternative engineered floor joist systems like trusses and open-web joists offer additional options for specific applications.

By considering these factors and employing sound engineering principles, structural engineers can ensure the safe and efficient design of timber floor joists in residential construction projects in Australia.