Lateral Stability in Residential Design: A Guide for Australian Projects

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

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

Use Case

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:

• Warehouses: Braced frames offer efficient lateral stability for large open spaces typical of warehouses.
• Industrial Buildings: In industrial structures, braced frames provide stability without obstructing large working areas.
• Residential Buildings: Braced frames can be employed in mid-rise residential buildings, ensuring stability while allowing for flexible floor layouts.

Benefits

• Efficient in resisting lateral loads.
• Simple design and construction.
• Suitable for a wide range of building heights.

Disadvantages

• Limited architectural flexibility due to brace placement.
• Potential for interference with interior spaces.

Design Procedure

• Step 1: Identify bracing locations based on structural analysis.
• Step 2: Select appropriate bracing members and connections.
• Step 3: Ensure compliance with AS 4100 for steel structures or AS 3600 for concrete structures.

Example

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.

• Building Height: 20 meters
• Lateral Load: 100 kN
• Bracing System: Diagonal braces in steel
• Material: Steel with yield strength, $F_y = 250MPa$

Design Procedure

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. $F_{brace}=\frac{Total Load}{Number of Braces}=\frac{100kN}{8}=12.5kN$

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 $F_{brace}=12.5kN$ b. Check axial capacity of selected section $A_g=2*(100-5)*5=950mm^2$ $P_n=0.9*F_y*A_g=0.9*250MPa*950mm^2=213.75kN$ $F_{brace} \lt P_n \longrightarrow \text{Therefore the design is safe.}$

Step 4: Verify the member selected using ClearCalcs’ Steel Beam Design to AS 4100

Shear Walls

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

Use Case

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:

• High-Rise Residential Towers: Shear walls are commonly used in high-rise residential buildings to provide stability and maintain a slim structural footprint.
• Commercial Towers: In office buildings and commercial complexes, shear walls help resist lateral forces while accommodating various office layouts.
• Mixed-Use Developments: Shear walls are suitable for mixed-use buildings that combine residential, commercial, and retail spaces.

Benefits

• Efficient in resisting lateral loads, especially in high-rise buildings.
• Allows for architectural flexibility as they can be integrated into the building envelope, such as located in lift walls or stairwells.

Disadvantages

• May require careful detailing to prevent undesirable cracking.
• Challenging in retrofitting existing buildings.

Design Procedure

• Step 1: Analyze building layout to identify shear wall locations.
• Step 2: Select appropriate materials and reinforcement based on AS 3600 or AS 4100.
• Step 3: Ensure compliance with detailing requirements to prevent shear failure.

Commercial Building Reinforced Concrete Shear Wall Example

Determine the shear wall locations based on structural analysis and calculate required thickness and reinforcement based on AS 3600 or AS 4100.

Given Parameters:

• Building Height: 15 meters
• Lateral Load: 80 kN
• Shear Wall System: Reinforced Concrete Shear Walls
• Material: Concrete with characteristic strength

Design Procedure:

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. $F_{wall}=\frac{Total Load}{Number of Walls}=\frac{80kN}{4}=20kN$

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

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

Use Case

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:

• Architectural Buildings: Moment-resisting frames allow for creative architectural designs in buildings where aesthetic appeal is a priority.
• High-End Residential Structures: In luxury residences, moment-resisting frames provide stability without compromising the desired architectural features.
• Mixed-Use Developments: MRFs are suitable for buildings that combine residential, commercial, and recreational spaces.

Benefits

• Suitable for a variety of building geometries.
• Can enhance architectural aesthetics.
• Efficient in resisting lateral loads.

Disadvantages

• Requires careful detailing to prevent weak-story mechanisms.
• Potential for increased construction costs.

Design Procedure

• Step 1: Analyze building layout and identify suitable locations for MRF.
• Step 2: Select appropriate beam-column connections based on AS 3600 or AS 4100. Consider welded end plate connections for steel-beam column connections when seeking to design a MRF. This can be designed in accordance with AS 4100 and AS 1554. When designing a reinforced concrete MRF refer to the article previously published on Detailing of Reinforced Concrete Structures to AS 3600 for guidance.
• Step 3: Consider detailing requirements to ensure ductility and prevent weak-story mechanisms.

Combined Systems

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.

Use Case

Common use cases include:

• Mixed-Use Complexes with Varying Heights: Combining braced frames and shear walls can address the varying lateral stability needs in mixed-use developments.
• Structural Retrofits: In existing structures with unique challenges, a combination of systems may be necessary to enhance lateral stability.
• Specialized Structures: Buildings with irregular shapes or load distributions may benefit from a combination of lateral stability systems to ensure optimal performance.

Figure 6: Bracing & moment resisting frames are often combined with internal shear walls

Benefits

• Allows for tailored solutions to complex design scenarios.
• Enhances overall structural performance.

Disadvantages

• May increase design complexity and construction costs due to more different types of systems needing to be designed and procured.

Design Procedure

• Step 1: Conduct a detailed structural analysis to determine the lateral load
• Step 2: If the above systems are individually insufficient to satisfy the load then consider combining them together to achieve sufficient capacity.
• Step 3: Ensure compatibility and proper integration of individual systems.

Example: Combined Systems for Lateral Stability in a Residential Building

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.

Conclusion

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.