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Seismic Focus Part 2: What are the categories of earthquake design, and how do they affect design?




Our recent blog covered why Australia has seismic requirements and why our seismic standard has been getting more attention lately.

We have also covered some of the NCC’s essential requirements on earthquake compliance, in particular, the need to comply with AS 1170.4-2007: Structural Design Procedures Part 4: Earthquake Works in Australia.

In this blog, we’ll review how these requirements work on interior wall, ceiling and frame linings, and their associated fixtures.

What are the earthquake design categories and how do they affect design?

Australia has three Earthquake Design Classes (EDC), covering low-level, low-risk commercial buildings up to the most severe multi-level and multi-personal buildings.

The relevant EDC is identified with reference to Table 2.1 of AS1170.4-2007.

Briefly, the implications of these three categories are summarized below:

EDCI: Seismic design class I requires an engineering analysis of the minor lateral load applied to each level. This means that the building structure must be able to withstand the lateral force of an earthquake. (See Section 5 of AS1170.4)

EDCII: Seismic Design Class II requires static engineering analysis. This means that the building must be able to handle the force of the earthquake in all directions. (See Section 6 of AS1170.4)

EDCIII Earthquake Design Category III requires complete engineering design with dynamic analysis. This is required for the highest risk levels and tallest structures. (See Section 7 of AS1170.4)

This third category involves calculating the effect of earthquake force in all directions, including inertia and vibration.

At the end of the day, to stop motion framing or refraction in the event of an earthquake, seismic restraints or props are specified by engineers.

What is the Australian Standard for Seismic Strength?

The Australian Standard for Structural Seismic Design is AS 1170.4-2007.

This includes the requirements for seismic reinforcement and earthquake resistance for all components.

The National Building Code (NCC) requires buildings to comply with AS1170.4 with reference to Section 8 of the Non-Structural Components Standard.

Seismic reinforcement has long been in building code, but was rarely thought to be necessary in Australia for its implementation. As a result of a lack of enforcement, builders and contractors often overlooked the supporting requirements of the seismic standard.

In line with more interest in the seismic standard, a new standard for suspended ceilings was released in June 2020, Suspended Ceilings – Design and Installation AS/NZS2785: 2020.

ANZ Sylvia Park Project – Seismic Pillar Detail

What do architects need to know about AS1170.4?

A common misconception is that only the basic structural framework should be taken into account for earthquake works, but this is not the case. Non-structural components and fasteners for earthquake forces shall be designed as required by AS1170.4. This applies to all categories of earthquake design.

AS1170.4 Clause 8.1.4 provides a comprehensive list of non-structural components which require consideration of seismic loads as follows:

Walls that are not part of a seismic force resistance system Annexes, including parapets, gables, porches, awnings, awnings, chimneys, roof components (tiles, sheet metal), containers and miscellaneous components Connectors (fasteners) for wall attachments, curtain walls, non-load-bearing exterior walls, partitions, floors, Ceilings (see also fixed and floating ceilings below) Architectural equipment, including storage racks and library shelves over 2.0 meters high.

The Australian Standard also states that many mechanical and electrical components and their fasteners commonly found in tall buildings also require consideration of their ability to accommodate seismic loads. for example:

Lighting fittings Ducts, cables and pipes for distribution of fire suppression systems and sprinklers. For a comprehensive list of all components, which require consideration of earthquake loads, refer to the Australian Standard.

What this means is that lightweight steel frames such as the partition and suspended ceiling frame require a new structural assessment for each project.

A structural engineer is required to ascertain if the zoning and roof systems on the project require seismic framing and to ensure that the framing system is safe in the remote possibility of a seismic event.

The structural engineer does this by running required seismic load calculations according to the relevant earthquake design category.

What is Fix and Float?

Part of the design of the seismic restriction begins in the architectural phase, in consultation with the structural and installers engineers to allow for appropriate spacing between the structural and non-structural elements of the building. This reduces or prevents collision or damage in the event of an earthquake.

For roofs, a common seismic design is called fix-and-float, in which the roof frame is fixed to only one of the four walls, while the other three are floating. Supawood’s recommended 15mm circumference fitting can still be used, but the frame is not sealed to the other three walls. Usually, the most stable wall is chosen as the installation wall. This allows the roof to move in the event of an earthquake without rupturing.

Note that walls are usually installed anyway, so the main effect of seismic engineering is changes to the roofs, fixtures, and services running through them.

What do builders and contractors need to know about AS1170.4?

Building contractors have the ultimate responsibility to ensure that the buildings they construct comply with the requirements of AS1170.4.

Builders need to ensure that all non-structural elements such as the architectural, mechanical and electrical components referenced above are properly checked for earthquake loads in their design and even installation.

It may be best to ask the structural engineer to provide effective floor acceleration and inter-floor drift allowances to manufacturers of architectural, electrical and mechanical components to ensure that the systems selected have the performance characteristics required to accommodate expected earthquake actions.

In summarizing…

All commercial buildings in Australia need to allow earthquakes to occur, no matter where they are located. Depending on the use of the building, engineering calculations are made to determine whether the building structure can withstand the force of an earthquake. Non-structural components also need to allow earthquakes the way they are repaired and the way they are located near structural, non-structural, or other structural components. The structural engineer will evaluate the building in accordance with AS1170 Part 4. Although there are common or general seismic designs for roof and wall bracing, etc., this is required for every project. Builders have the ultimate responsibility to construct a building in accordance with AS1170.4 earthquake standard. However, architects need to ensure that their designs and selected materials can be adopted accordingly.

For your peace of mind, most Supawood systems are available with seismic engineering calculations at no additional charge.

Photos: ANZ Sylvia Park, Auckland NZ




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