Avoiding risky behaviour

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By , and - , Build 145

A suggested risk matrix for steel and concrete buildings up to 15 storeys could inform decision making in façade component choice and installation for some buildings that fall outside the scope of E2/AS1.

SINCE RISK MATRIX tables were introduced to New Zealand Building Code Acceptable Solution E2/AS1 in 2005, the way designers assess and manage weathertightness risk in residential construction has improved. However, this matrix only applies to residential buildings up to 3 storeys within the scope of NZS 3604:2011 Timber-framed buildings.

Now, a recent paper has suggested a similar risk assessment and awareness method for medium-rise steel and concrete buildings up to 15 storeys.

Called Risk Matrix 15, it is not intended as a compliance tool but as a way to raise awareness of the difficulties and issues in designing and building the façades of medium-rise concrete and steel structure buildings above 3 storeys.

The authors intended Risk Matrix 15 to enable informed decision making, clarifying weathertightness risks that may be overlooked.

From E2/AS1 to Matrix 15

The E2/AS1 risk matrix has been accepted as an easily understood tool for managing and designing construction risks associated with the building envelope of houses.

In contemplating the new risk matrix, comparisons were made to the 2013 E2/AS1 risk matrix guidance document. The six risk factors in the E2/AS1 risk matrix and their scores ranges are:

● wind zone 0–2

● number of storeys 0–4

● roof/wall junctions 0–5

● eaves width 0–5

● envelope complexity 0–6

● deck design 0–6.

When developing Risk Matrix 15, the wind zones or wind pressures, envelope complexity and deck design were considered relevant or transferable. The number of storeys and eaves were considered not relevant.

Critical success factors identified

The risk categories considered critical for determining factors in the success or otherwise of building envelope installations were:

● façade design wind pressure

● geometric complexity

● type of façade systems

● number of different systems

● façade contractor experience

● building movements

● performance testing.

Tables were developed for each category proposing a scoring system and risk category that the various options might be placed under.

Façade design wind pressures

Design wind pressures should be determined in accordance with the wind loads standard by a façade or wind engineer or by a wind tunnel test.

They should be given in terms of typical and corner pressures, including local pressure factors and provided as both a serviceability limit state (SLS) and an ultimate limit state (ULS). SLS pressures are used to check member deflections, and ULS pressures are used to check structural adequacy.

ULS design wind pressures up to 1.5 kPa are low risk, with risk increasing with pressure. Greater than 2.5 kPa is suggested as the threshold for a high risk.

Geometric complexity

Increased geometric complexity brings increased risk in any building. Geometric complexity can be introduced by features like multiple recesses and sloping façade systems.

Types of facade systems

Several different façade systems or façade types are often applied to one building for architectural diversity. The choice of these can contribute to the envelope risk, as different systems, such as a unitised curtain wall or precast concrete walls, have different inherent weathertightness properties. Even two different types of curtain wall on one building may constitute two different systems from a weathertightness risk perspective.

More systems add risk

The risk generally increases as more systems are designed and installed by different subcontractors. Often when adding systems to a project or concept, little consideration is given to the increased complexity of detailing at interfaces.

Often main contractors do not manage and coordinate this issue. Quantity surveyors do not accurately cost the complexity of the various systems, primarily considering the costs of systems without accurate interface costs. This can lead to false representation of cost savings without an understanding of the risks.

There may be rainscreen cladding systems installed over air barriers that have been installed by others, often with a timber frame installed by yet someone else. With systems that are installed by multiple contractors, the authors suggested that the risk score should be at least doubled.

Looking at the numbers

The risks quickly escalate as the number of systems (and different interfaces) in one façade increases. Using 1–2 systems was considered in the low risk category, 3–5 systems a medium risk and 5 or more systems a high risk with a risk score of 5 or more.

Façade contractor experience

There are not many medium to large commercial buildings and a limited number of experienced façade subcontractors in New Zealand. Using a contractor with no experience on similar buildings would be considered a high risk.

A New Zealand procurement model used for many aluminiumframed systems has a ‘prime die holder’ who develops and markets systems and a network of franchisee businesses who undertake fabrication. This is considered more suitable to domestic construction but, in our market, is often applied to multi-storey commercial where the inherent separation of system designers from system documenters from system fabricators and installers creates risks of gaps in communication of requirements and understanding of system limitations.

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Building movements

A building façade must be able to tolerate all reasonably expected building movements without compromising weathertightness integrity.

Movements include inter-storey drift due to seismic effects, edge beam deflection due to self-weight and imposed loads as well as thermal effects and column shortening.

Recently, structures of some buildings are being designed to be lightweight and more flexible.

Some buildings have quoted a serviceability inter-storey drift of 40 mm and ultimate of 100 mm, putting them into the high risk category. Conventional façade systems may not be able to tolerate such serviceability movements and remain serviceable.

Façade designers must determine the movement requirements for their façade system and obtain the values of the building structure’s movements from the building structure engineer.

Performance testing

Preferably, all façade systems for the building should be performance tested in one specimen, including all interfaces, to project-specific conditions to determine their performance rating.

Many façade systems are not covered by a local standard and so are often installed untested and with an unknown performance rating, placing them in the high risk category.

The widely accepted façade-testing standard is AS/NZS 4284:2008 Testing of building facades. This applies pressure and suction to the external face of the façade specimen and includes cyclic variation of the water penetration test pressures.

A performance test should include a structural test at serviceability pressures, air leakage test, water penetration test, serviceability seismic racking, repeat water penetration test, ultimate structural test and ultimate seismic racking.

Scoring summary

The risk assessments enable comparison of different façade options. Risk categories are summarised as follows:

● Low – a minimum risk indication that gives the best chance of successful service performance.

● Medium – the risk of inadequate performance is higher than recommended. The designer should consider redesign of the higher-scoring parameters to reduce the risk to the low category. Alternatively, be aware of the risks and manage them with good design and installation quality control.

● High – the risk of inadequate performance is high, and a major review is recommended to reduce the risk where possible.

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For more

This article is an edited version of a paper first presented by the authors at ICBEST 2014 in Aachen, Germany, June 2014. 

The full paper with tables for each risk parameter can be downloaded here.

Download the PDF

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