This practical guide for structural engineers explores lateral load-resisting systems for Australian homes. From wind load calculations to design strategies, ensure structural integrity and compliance with Australian Standards.
Lateral stability is a critical aspect of structural design, especially in regions prone to strong winds and seismic activities. In Australia, structural engineers familiar with calculating wind loads and earthquake or seismic loads according to the AS/NZS 1170:2021 series must design lateral stability systems (also known as lateral force resisting systems) to ensure the stability of the structure in resisting these loads in residential and commercial buildings.
This article focuses on various structural elements and systems used for lateral stability, exploring their benefits, disadvantages, design procedures, and considerations. All design examples follow the procedures specified in Australian Standards.
Braced frames are a common lateral stability system, providing strength and stiffness against lateral loads. They consist of diagonal bracing members that resist lateral forces through axial tension or compression.
Figure 1: An example of braced framing system in a multi-level carpark
Braced frames are suitable for a variety of building types and heights. They are particularly effective in low to medium-rise structures where architectural flexibility is not heavily compromised. Common use cases include:
Given building dimensions and expected lateral loads, determine the required bracing locations. Calculate brace forces and member sizes based on AS 4100 or AS 3600.
Step 1: Determine Bracing Locations 1. Perform structural analysis to identify critical locations for bracing. 2. Assume bracing at corners and midpoints of each wall.
Step 2: Select Bracing Members 1. Calculate brace forces using the tributary area method. 2. Assume a tributary width of 5 meters for each brace. 3.
Step 3: Determine Member Sizes 1. Use AS 4100 for steel structures. 2. Select a suitable section (e.g., SHS 100x100x5) based on axial capacity. 3. Perform the calculation checks a. Determine brace forces b. Check axial capacity of selected section
Step 4: Verify the member selected using ClearCalcs’ Steel Beam Design to AS 4100
Shear walls are vertical elements designed to resist lateral forces, providing both strength and stiffness to the structural system.
They are often constructed using reinforced concrete in commercial buildings and timber in residential buildings, depending on the amount of lateral load. An example for the timber shear wall will be addressed later in the article under the combined systems heading.
Figure 2: Timber shear wall
Figure 3: Reinforced concrete shear wall
Shear walls are often chosen for their ability to efficiently resist lateral loads in high-rise buildings. They find application in structures where architectural flexibility and optimal space utilization are crucial. Key use cases include:
Determine the shear wall locations based on structural analysis and calculate required thickness and reinforcement based on AS 3600 or AS 4100.
Given Parameters:
Step 1: Identify Shear Wall Locations 1. Perform structural analysis to determine optimal locations. 2. Assume shear walls at building ends or at internal lift shafts or stairs
Step 2: Select Shear Wall Thickness 1. Calculate total lateral force per wall using tributary area method. 2. Let’s assume a tributary width of 6 meters for each wall. 3.
Step 3: Determine Reinforcement 1. Use AS 3600 for concrete structures. 2. Select a suitable thickness based on analysis (e.g., 200 mm). 3. Calculate required reinforcement using AS 3600. Ensure both flexural capacity and shear force are checked.
Refer to the article on designing reinforced concrete beams and treat a 1m width of the wall as a 1m wide beam and apply the same procedure.
Moment-resisting frames (MRF) provide lateral stability by allowing beams and columns to resist lateral forces through flexural strength. They are commonly used in high-rise buildings and offer architectural flexibility.
Figure 4: Moment resisting frame example beam & column layout for a multi-level building
Figure 5: Steel beam-column moment connections
Moment-resisting frames are versatile and can be applied to a wide range of building types. They are particularly effective in structures where architectural aesthetics and design flexibility are paramount. Notable use cases include:
In some cases, a combination of lateral stability systems is employed to optimize performance and address specific design challenges.
Combined systems are employed in scenarios where a single lateral stability system may not fully address the structural requirements. This approach offers a tailored solution to complex design challenges.
Common use cases include:
Figure 6: Bracing & moment resisting frames are often combined with internal shear walls
Consider a 15m long, 8m wide, 2.7m high, residential building with a roof pitch of 22.5 degrees and timber joint groups JD5 in a Class N1 wind area as per AS 4055:2021. Design the necessary wall bracing to AS 1684.2:2022.
To do this by hand we would need to calculate the lateral load as per the Wind Load Design to AS 1170.2 article published.
Then, we would need to refer to the bracing capacities for various bracing types (bracing, wall lining/cladding, plywood, or hardboard) and determine suitable lengths for each bracing type to achieve the necessary capacity to resist the wind load. While this procedure is straightforward with basic calculations, it results in a lot of time-consuming iterative calculations to find a conforming design.
Using ClearCalcs can greatly speed up this process. First, we input the key properties from the problem into ClearCalcs. The wind pressure is automatically determined from the wind class and building dimensions.
Next we input the building configuration.
Firstly, we assess the bracing that resists the wind acting on the long side of the building.
As per Clause 8.3 of AS 1684.2, we can input different types of bracing applied across different lengths of the long side of the building. ClearCalcs automatically draws the bracing capacity provided based on the types stipulated in AS 1684.2, saving lots of time in design and allowing users to iterate to find the most efficient design while considering many different bracing types. ClearCalcs also allows custom bracing values if an engineer wants to design a non-standard bracing system.
For this example, we will trial two diagonally opposed angle braces with a plywood bracing panel (which acts as a shear wall).
Finally, we design the bracing to resist the wind acting on the short side of the building.
As per Clause 8.3 of AS 1684.2, we can input different types of bracing applied across different lengths of the short side of the building. ClearCalcs automatically draws the bracing capacity provided based on the types stipulated in AS1684.2, saving lots of time in design and allowing users to iterate to find the most efficient design while considering many different bracing types. ClearCalcs also allows custom bracing values if an engineer wants to design a non-standard bracing system.
For this example, we will trial two diagonally opposed angle braces with a plywood bracing panel (which acts as a shear wall).
The results of the specified bracing types are summarised instantaneously.
For an efficient but safe structural design, a system utilization of between 40% and 90% should be targeted. ClearCalcs users can adjust the bracing types chosen to achieve a more efficient or safer design, depending on the client's requirements.
Designing for lateral stability in buildings is a multifaceted process requiring careful consideration of various structural elements and systems.
Australian structural engineers, equipped with the procedures of wind load calculations to AS 1170.2:2021, steel structures to AS 4100, concrete structures to AS 3600 and residential timber framed construction to AS 1684 can now apply the principles outlined in this article to enhance the resilience and safety of residential and commercial structures for lateral loads.
By understanding the benefits, disadvantages, design procedures, and considerations for each lateral stability system, engineers can make informed decisions and contribute to the creation of robust and stable buildings that meet or exceed Australian Standards.
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