Kyle Conway
Explore the benefits and drawbacks of analyzing timber floor joist design through manual calculation versus ClearCalcs structural design software. Determine the most accurate, efficient, and cost-effective approach for informed decision-making on your construction projects.
Designing a timber floor joist for a residential project in Australia involves following the relevant Australian standards to ensure the safety and structural integrity of the floor system.
We will compare the step-by-step process of analyzing timber floor joists using manual hand calculation versus using structural design software like ClearCalcs.
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In this example, we will design a timber floor joist for a residential room with the following specifications:
TIP: Get a practical guide on things to consider when designing timber floor joists and different types of timber floor joists commonly used in residential construction.
If the project is subject to joist size requirements, then it is possible that you would select the size first and determine the species and grade based on that size using the supplementary notes provided in AS 1684, and from there, we can find the suitable timber species, grade, and span based on the size of the floor joist.
However, most projects have flexibility with joist sizes, and builders typically are only able to procure certain timber types due to location or price.
It is good practice for engineers to ask the builder what their timber preferences are in terms of supplier, species, and grade to ensure that the design provided is procurable and buildable.
So, for this example, we will assume the builder has a preference to use "Hardwood – F8" as the timber species and grade, which is a common type in the "Structural Grade" timber from the Australian Standard AS 1720.1-2010 - Timber Structures, which provides guidance on timber selection based on the intended use and loading conditions.
Table 5.3 from AS 1684.2-2010 - Residential Timber-Framed Construction specifies the maximum allowed joist spacing based on the floor loading, span, and timber grade, as shown below.
Figure 1: Guidance on maximum allowed joist spacing based on the floor loading, span, and timber grade in AS 1684.2-2010. (Reference)
Joists spaced more closely together provide a floor with a greater strength capacity but often can clash with building services, and more joists cost more money.
For our example, let's assume the builder has requested a joist spacing of 450 mm (0.45 meters), a common spacing in the industry.
Given the room length of 5 metres, we would have 12 joists.
This can be adjusted later if we cannot determine a suitable section size for our joists.
The tributary area is the portion of the floor that each joist supports. It is generally half of the spacing in each direction for uniform loads.
To calculate the tributary area, consider the following.
Tributary Width (TW) = Spacing =
Tributary Area (TA) = Tributary Width (TW) x Joist Length (found from the room dimensions) =
For simply supported timber members, moment will always be critical to the design (rather than shear).
AS 1170.0 Chapter 4 (excerpt shown below) provides guidance on the load factors and combinations that should be considered for the design load.
Figure 2: Load factors and combination to be considered for the design load in AS 1170.0 (Reference)
We always take the most significant load combination factors and design timber floor joists (and all structures) in accordance with the worst-case scenario.
For a floor joist, we will consider the permanent and imposed action loading combination (as wind and earthquake are not relevant) and assign a load factor of 1.2 to the dead load and 1.5 to the live load.
Uniformly Distributed Design Load, =
Design Load, =
Moment Action () =
As per Section 3.2 of AS 1720.1 Timber Beam Design, we can solve for the minimum section modulus to achieve limit state design.
Using Limit State Design:
= design capacity in bending≥
v ≥ 2,565,000 Nmm
= capacity reduction factor (Clause 2.3 AS 1720.1) = 0.9 (refer table)
= duration of load modification factor (Table 2.3 AS 1720.1) = 0.57 (50+years)
= partial seasoning factor (Table 2.5 AS 1720.1) = 0.7 (seasoned timber)
= temperature modification factor (Clause 2.4.3 AS 1720.1) = 1.0 (ambient conditions)
= modification factor for strength sharing (Clause 2.4.5.3 AS 1720.1) =
= stability factor (Clause 3.2.4 AS 1720.1) = 1.0 (most conservative value)
= characteristic value in bending = 8 MPa (stress grade F8)
= section modulus of beam about the axis of bending
We can solve for the minimum section modulus to achieve limit state design.
By trialing different section sizes, we can find a suitable section that has a section modulus greater than the required above.
Let’s try a 240 x 45 section.
The minimum section modulus needs to be 711,000 cubic millimeters, so we will need to increase the grade of the timber.
Let’s try a grade of F27.
Using F27 grade timber, we find the 240 x 45 section size suitable.
The spacing of the joists can also be adjusted by making them closer together to achieve smaller minimum section modulus requirements. To adjust the joist size, return to Step 3 and complete the process again.
To ensure the serviceability of the floor, we must check deflection as per AS 1170.1-2002 - Structural Design Actions Part 1: Permanent, Imposed, and Other Actions. This standard specifies limits on deflection for different floor spans based on floor use.
In our example, the floor span is 5 meters, and we take a maximum allowable deflection of (where L is the span length).
Maximum Allowable Deflection
Second Moment of Area
The deflection is within the limits. The selected joist size is suitable for the residential project as per Australian standards.
Please note that this example provides a simplified outline of the design process, and in a real-world scenario, a qualified structural engineer should perform a comprehensive analysis considering all applicable factors and standards for accurate and safe design.
Always consult the relevant Australian standards and seek professional advice when designing structural elements for a project.
The Timber Beam Calculator to AS 1720.1:2010 enables you to do quick and powerful design and analysis for simple and continuous timber beams, with unlimited supports and loads.
The member selector tool, which has a library of thousands of common timber sections, allows you to get instant results for moment, deflection, and shear, which means that engineers don’t have to iteratively perform calculations as with the worked example by hand above which saves heaps of time!
From our example above, we have a joist length of 4000mm that is simply supported. So we'll just need to enter the total beam length and the support positions into the support table.
Live load: 1.5 kPa (15 kg/m²)
This includes the weight of furniture, occupants, etc. AS1170.1 Chapter 3 Table 3.1 details the imposed actions (live loads) that need to be considered for various building types. For a self-contained dwelling, a uniformly distributed action of 1.5kPa should be considered.
Dead load: 0.5 kPa (5 kg/m²)
This includes the weight of the floor finish, building services, subfloor, and other fixed loads.
Floor joists only bend about the major axis.
By default, the ClearCalcs Timber Calculator already has the Design Criteria, Member Properties, and Modifications Factors prefilled based on the inputs provided in the Key Properties and Loads.
Some of these can be manually changed, and clicking on each input will provide a description of the parameter as well as the relevant Australian Standard to refer to for further information.
For example, if you would like to know more about the Equilibrium Moisture Content (EMC) value used in the calculation, the calculator shows that it is based on Clause 2.4.2.3 of the AS 1720.1:2010.
The summary outputs provide immediate feedback for the design and detail the utilization of the moment, shear, bearing, and deflection capacity.
The member selector tool has thousands of standard grades and sizes of timber and provides immediate feedback on the utilization of their moment, shear, bearing, and deflection capacity.
Users can use this tool to provide the most efficient design possible, ensuring that members aren’t unnecessarily oversized (utilization <50%) or undersized (utilization>100%).
This can help reduce material costs for the builder and is also environmentally efficient.
In this residential project, the design called for an open-concept living space with multiple levels and a seamless connection between different areas. Timber floor joists were creatively used to achieve a visually appealing and functional interior.
Cantilevered Timber Floor Joists
A commercial office building aimed to be environmentally friendly and promote sustainability throughout its design. Timber floor joists played a crucial role in achieving this objective.
Glue-Laminated Timber (Glulam) Floor Joists
An old industrial building was repurposed into a modern co-working space that preserved its industrial charm. Timber floor joists were an essential component in the adaptive reuse of the space.
Exposed Timber Floor Joists
By applying these best practices and embracing innovative uses, timber floor joists can be utilized effectively and safely to create unique and sustainable spaces in various construction projects.
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