The importance of non-structural elements in earthquakes

Good performance of non-structural elements during earthquakes can save lives and costs, and relies on well-managed processes during the design, procurement and construction project phases. In the past, buildings designed after the adoption of AS1170.4 in 1993 are believed to have adequate structural resistance to seismic loads. However, between 1993 and 2019, it was common for non-structural elements to have little-to-no earthquake resistance elements in their design.

So, what are non-structural elements?

What are non-structural elements?

Non-structural elements are those elements within a building that are not considered to be part of either the primary or secondary structural systems. Examples of non-structural elements include components such as mechanical and electrical plant, ducting, pipework, cable trays, suspended ceilings, light non-load bearing partitions, and cladding systems such as brick veneer. The illustration below shows a snapshot about all the non-structural elements in the building, which actually covers majority of the building.

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Let's breakdown all the components in a building.

I would simplify this into 2 major departments, the structural components and the Operational and Function Components (OFCs).

Structural components are the basic elements which are designed to carry and transfer all loads to the ground without total or partial collapse of the building. Seismic damage of structural components causes casualties, building disruption, and repair costs.  

Non-structural components (the Operational and Functional Components, OFCs) are those elements housed or attached to the slab and walls of a building which are not part of the main load-bearing structural system, but also be subjected to large seismic forces. While non-structural components may not play a momentous role in the overall design of the building, they have a substantial impact on how the building will perform during a seismic event, and most importantly, provide a safe path for people to escape or the emergency services to enter to save people. OFCs are divided into three categories of sub-components: architectural (external and internal), building services (mechanical, plumbing, electrical, and telecommunications), and building contents (common and specialized). OFCs can be classified into three categories in accordance with the nature of their seismic response sensitivity:

1. Inter-story-drift-sensitive components,
2. Floor-acceleration-sensitive components, and
3. Both Inter-story-drift and floor-acceleration-sensitive components.

Remembering the event of February 2011, Milton How, Director of VAICO said “I have lived through two disastrous earthquakes of over 6 magnitudes on Richter scale in a 5 month period and, the fact that these two violent earthquakes happened so close to each other, it weakened the building and caused a disaster on a big scale”. Milton further added “he was very close to the Canterbury Television Building event and spent 24 hours on site along with the first responders. There was a gas leakage in one lane; Telco’s were down, so there was no communication. The whole experience was overwhelming”. This made him realise that they must take steps to minimise the damage these extreme weather events can cause, which in turn led to the innovation of the V.LOCK system.

This image below, summarised by Sensequake explains the breakdown of multiple elements in the building.

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Why do we care about non-structural elements?

Non-structural elements comprise up to 70% of a building’s capital value. It’s not surprising then that the damage to these elements in an earthquake has resulted in many buildings being declared economic losses, even when the structure itself was not badly damaged. Damage to non-structural elements also presents a significant risk to building occupants and affects whether the building can be reoccupied following an earthquake. Recent research suggests there is a need to focus on reducing the damage to non-structural elements.  

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A research conducted by MBIE of New Zealand summarises the costs for code compliant seismic restraints to non-structural elements in 6 different buildings, car parking building, office building, hospital, comunity library and service centre, fire station and primary school. Depending on the types of building and the function it serves, the costs can vary quite significantly. For example, a hospital would have over 75% components that are non-structural elements; where as car park building would have 40% non-structural elements.

Earthquake Provisions for Class 1 buildings

For Class 1 buildings, provisions for seismic design are contained in Appendix A of AS 1170.4. Generally, Class 1 buildings designed and detailed for lateral wind forces in accordance with the relevant material standard (e.g. AS 1684 or AS 3700) are considered able to also resist earthquake forces. However, unreinforced masonry elements in particular, including gable ends, chimneys and parapets, must be designed to resist an additional horizontal racking force, as specified in Appendix A.

Earthquake Provisions for Class 2-9 buildings

NCC Volume One contains a Performance Requirement (BP1.1) that requires Class 2 to 9 buildings to perform adequately under all reasonably expected design actions, including earthquake actions. The corresponding Deemed-to-Satisfy (DTS) Provision for earthquake actions is B1.2, which references AS 1170.4 Structural design actions – Earthquake actions in Australia.

Section 3 of AS 1170.4 applies a hazard design factor to all regions of Australia. This requires earthquake loads to be considered in the design of any building irrespective of where it is located.

Compliance with AS 1170.4

For Class 2-9 buildings, Section 8 of AS 1170.4 provides information about certain non-structural building parts and components that would need to be designed to resist horizontal and vertical earthquake forces.

These parts and components include:

• Walls that are not part of the seismic force resisting system.
• Appendages including parapets, gables, verandahs, chimneys and the like.
• Partitions.
• Ceilings.
• Mechanical and electrical components including smoke control systems, fire suppression systems, boilers, escalators, transformers and the like.

Therefore, in order to comply with AS 1170.4, the design of the above parts and components must be carried out for earthquake actions by one of the methods provided in Section 8 (i.e. using established principles of structural dynamics, or using the general or simplified methods expressed in Clauses 8.2 or 8.3 of Section 8).

Designing and detailing for earthquake actions

Careful detailing is required for non-structural elements so they can resist earthquake actions. Critical details, such as connections, restraints and, where required, flexible elements and/or separations should be identified and documented.

There are generally two ways to detail non-structural elements and their connections:

1. Using details or proprietary systems verified as being compliant with the Building Code. These can be prescribed in Acceptable Solutions, standards or in literature developed by technical groups.
2. Using details which have been subject to specific engineering design. Details that result from a specific design should generally be the responsibility of an engineer with competency in structural engineering in the application of earthquake actions, such as a Chartered Professional Engineer (CPEng).

For owners/designers

Ensure the connections of all non-structural elements comply with the National Construction Code. Pay particular attention to ceiling systems. It is important that these elements are properly restrained against earthquake shaking to prevent them from collapsing on people, cutting off escape routes, or failing to support critical ceiling-mounted emergency evacuation components.

For manufacturers/importers of proprietary non-structural/building services components

Ensure proprietary non-structural/building services components, whether manufactured in New Zealand or overseas, are capable of meeting New Zealand seismic performance criteria (by either calculation or testing).

Buildings with low–damage systems

Take care with buildings that incorporate low-damage primary systems that have the potential for large displacements in earthquakes. These displacements will need to be accommodated by non-structural elements. Systems that reduce actions such as base isolation of a building are an effective way of protecting the structure and its contents.

Fit-outs

It is important that architects and other parties collaborate and seek specialist engineering advice for seismic restraints and detailing associated with building fit-outs, particularly when a structural engineer is not directly involved with the fit-out project. There are too many projects out there where the non-structural elements have never been designed, and the risks would fall back on the project owner and its associated stakeholders.

Coordinate design

Effective earthquake performance of non-structural building elements and systems depends on good communication between all stakeholders. Early engagement of key stakeholders in the design process facilitates good design and can help avoid problems later. Stakeholders may include the:

• design coordinator, for example, an architect, engineer or project manager
• specialist non-structural seismic engineers
• structural engineers
• building services engineers
• fire protection engineer
• suspended ceiling manufacturers/suppliers
• building consent authority (BCA).

Early coordination of non-structural systems helps avoid conflicting specifications and work plans. In addition, it will minimise procurement issues by providing a better basis for costing and tendering the work and making the process of issuing a Building Consent and Code Compliance Certificate easier.

Regulation 126

ArchiEng specialist engineers provides independent peer review on various projects. In recent years, there has been an increase on requests for Regulation 126 checks on non-structural elements, such as partition walls, seismic ceilings, seismic restraints for MEP services, staircases etc. This was partly driven by building certifiers and stakeholders who wish to do the right thing. Click here to find out on who can issue Regulation 126 Certiticate in Victoria.

Conclusion

For owners and architects, be aware that owners are responsible for their buildings. Engage the right building’s structural design engineer, or consider engaging specialist engineer with similar expertise, for the seismic design of non-structural elements, to prepare construction details for the work and to coordinate the work of the relevant disciplines.

For engineers/designers, include a section on non-structural elements in the Design Features Report as well as providing relevant information in the specification and on the drawings.

For builders, always check that your project has a non-structural element section involved, that has been adequately specified, designed and documented.

For building surveyors, check that the design documentation adequately covers non-structural elements. The details for non-structural elements may be part of the main contract or be the subject of a separate contract depending on the engagement circumstances. If in doubt, always engage a specialist engineer to conduct an independent peer review with Regulation 126 signed off for a complete peace of mind.

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