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The misconceptions about performance-based design

Prescriptive building codes are useful to ensure that minimum standards are satisfied.
Prescriptive building codes are useful to ensure that minimum standards are satisfied.

Some people think performance-based design solutions are too confusing, expensive or a sneaky way to cut corners. But according to Sid Thoo, performance-based solutions offer a holistic approach that can see buildings meet optimum standards, rather than just minimum ones.

Have you ever wondered who wrote the very first building regulations? The Code of Hammurabi is thought to be the oldest known example of a written building code, dating back more than 3500 years. While it covers a number of different areas of law, one section explicitly outlines the responsibilities of a builder, and some of the consequences should there be a construction failure:

  • performance-based design

    The Code of Hammurabi on clay tablets at the Louvre

    If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death

  • If it kills the son of the owner, the son of that builder shall be put to death
  • If a builder builds a house for someone, even though he has not yet completed it; if then the walls seem toppling, the builder must make the walls solid from his own means

Thankfully, building codes have come a long way since the Code of Hammurabi. While they are constantly evolving, major changes in building regulation will often be enacted in response to catastrophic events, loss of life or significant property damage.

For example, in 1666 the Great Fire of London destroyed over 13,000 buildings, and was able to spread quickly due to the close proximity of mostly timber buildings. In response, the Rebuilding of London Act included requirements for fire resistant construction and separation.

performance-based design

The Great Fire of London, depicted by an unknown painter

One of the more significant innovations in building regulation has been the development and implementation of the performance-based code. The United Kingdom was one of the first countries to adopt a performance-based approach, with the Australian Building Codes Board (ABCB) implementing a performance-based code in the 1996 edition of the Building Code of Australia (BCA), which now forms part of the National Construction Code (NCC). There are now over a dozen countries around the world that have some form of performance-based solutions, including the United States, Canada, China, New Zealand, Singapore, Norway and the Netherlands.

And yet even though they have been around for almost half a century, there still seems to be some misunderstanding about what performance-based solutions are and how they can be used. Some common misconceptions include that they are too confusing, make buildings more expensive or allow people to cut corners during design and construction. In actuality, performance-based solutions allow a more holistic and integrated approach to building design and construction, can result in significant cost savings, and can also ensure that buildings meet an optimum standard rather than just minimum one.

For example, consider the ceiling height in a room or space. In a Class 1 residential building, Part 3.8.2 of NCC 2016 Volume Two advises:

Heights of rooms and other spaces must be not less than:

(a) in a habitable room excluding a kitchen – 2.4m; and
(b) in a kitchen – 2.1m; and
(c) in a corridor, passageway or the like – 2.1m; and
(d) in a bathroom, shower room, laundry, sanitary compartment, airlock, pantry, storeroom, garage, car parking area or the like – 2.1m;

and so forth, and also includes a useful diagram on the following page. It explicitly states what the minimum ceiling height in a room or space must be, depending on its intended function or purpose. This is known as a prescriptive or “deemed-to-satisfy” approach, and has long been used in building codes and regulations around the world. In simple terms “build something exactly as described, and it will comply”.

However, there are a variety of reasons why the height of a room might vary from these minimum dimensions, or may not be built exactly as described in the regulations. A ceiling might be curved or have some other irregular geometry, or a portion of the ceiling in a habitable room might be lower than 2.4m due to a structural or mechanical engineering requirement. While a well-written Deemed-to-Satisfy solution can cover the majority of scenarios, it’s still possible that a situation may occur in a building where a prescriptive approach can’t be easily applied.

Here is where a performance-based approach towards building codes can be more flexible. Let’s have a look at the performance provisions for room heights in the Part 2.4 of NCC 2016 Volume Two:

P2.4.2 Room heights: A room or space must be of a height that does not unduly interfere with its intended function.

But wait a minute! It doesn’t state the minimum height for a room or space, or give any dimensions. How can that be a building regulation? This is the unique nature of performance-based design; rather that dictating specific measurements or dimensions, the height of the room or space could be almost any dimension, as long as it is suitable for purpose.

For example, take a living room with a bulkhead at 2.1m along one wall (maybe to conceal an airconditioning duct and vertical outlet), but where the rest of the ceiling is at 2.7m. Technically, under the Deemed-to-Satisfy requirements of the NCC, the lower ceiling height doesn’t comply.

performance-based design

Technically, the room height near the window wouldn’t meet the Deemed-to-Satisfy NCC requirements. However, the NCC Performance Provisions make the design of this seating nook possible

However, the location and size of the bulkhead is unlikely to have an adverse effect on use of the space – it may even be a deliberate design strategy to create a cosy nook or window seat. This is made possible when you take a performance-based approach, rather than having to write numerous prescriptive clauses to cover every possible design scenario. We can also use the Deemed-to-Satisfy requirements as a guide as to what dimensions would be considered an acceptable performance solution.

Room and ceiling heights are a relatively simple example of how a performance-based code works. They can be particularly effective in more complex buildings and projects, where a “one size fits all” approach may not be appropriate. For example, what if you had to renovate an early 20th century office building to meet current fire safety standards, without compromising the heritage-listed architecture? This was the challenge faced by the design and consultant team working on 39 Hunter Streetin Sydney. They were able to use a performance-based approach to demonstrate building compliance, while also preserving the building’s open staircase and atrium. You can read more about their solution in the ABCB’s case study.

Providing equitable access for people with disabilities is another important consideration in the design of a building. It was particularly challenging for SJ Bishop Architect when designing a five storey mixed-use apartment building on a long, narrow site in Wolloomooloo NSW. Under the Deemed-to-Satisfy provisions of the NCC, disability access required a lift with sufficient width for a wheelchair user to be able to turn around inside the lift. However this would have compromised the size of rooms adjacent to the lift shaft. With the assistance of an access consultant, Accessibility Solutions, and using a combination of evidence of suitability, comparison with the Deemed-to-Satisfy provisions and expert judgement assessment, a narrower “through” lift with doors on opposite sides was specified that satisfied the NCC performance requirements for disability access. You can read more about this solution in the ABCB’s case study.

Improving energy efficiency and productivity in buildings is another important design consideration, what with the built environment accounting for around 25 per cent of Australia’s total carbon emissions. There are a plethora of applications, calculators and algorithms available, which all have different methodologies for calculating and estimating energy use in buildings. This often means they cannot be used to compare results between tools. Rather than only writing prescriptive codes that dictate which tools can or can’t be used (which would also have to be updated every time a new version is released), the NCC includes provision for a “reference building” performance solution. This allows a Deemed-to-Satisfy building to be modelled and compared to a proposed building design in the same analysis tool for a more objective comparison of a building’s energy use.

Now it’s also important to note that performance-based building codes complement – rather than compete with – the Deemed-to-Satisfy requirements, and their effectiveness is very much dependent on how the performance requirements (or expectations) are defined. If too rigidly or prescriptively defined, there would be little difference to the Deemed-to-Satisfy requirements. But if the performance requirements are not defined clearly enough, then they can be open to misinterpretation or compromise the intended performance outcome. In this regard, the ABCB is supporting practitioners by developing NCC supporting material to increase capacity and competent use of performance-based design.

Performance-based solutions are a progressive tool that help to ensure our buildings are safe, accessible, structurally sound and energy efficient. Additionally, they allow building design professionals to propose the most appropriate or optimum solutions for achieving building compliance, not just the ones that meet the minimum Deemed-to-Satisfy requirements.

By providing a flexible framework that can accommodate innovative building compliance solutions, performance-based solutions can result in better performing building solutions, which can also be more cost-effective to implement.

Sid Thoo is an architect, consultant and educator. He is a member of the Australian Building Code Board’s Subject Matter Expert (SME) Network, a volunteer pilot network that comprises a range of industry experts to assist practitioners in the application of performance-based design.

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