A Weatherproofing Risk Matrix for Multi-Storey Buildings

Transcription

1 ICBEST 2014 June 09-12, 2014, Aachen, Germany A Weatherproofing Matrix for Multi-Storey Buildings Dianne Johnson 1, Karl Jones 2, Peter Lalas 3 and John Sutherland 2 1 Capital Improvements Ltd, Wellington, New Zealand 2 Mott MacDonald NZ Ltd, Auckland, New Zealand 3 Janus Facades Pty Ltd, Sydney, Australia Corresponding author: Peter Lalas, peter@janusfacades.com Abstract The New Zealand Building Code Acceptable (Deemed to Satisfy) Solutions for External Moisture (weathertightness) includes a Matrix which gives the designer an indication of a number of the influencing factors that lead to success, or failure, of cladding systems for residential buildings up to 3 storeys. Weathertightness failure is not limited to this category of buildings. Many multi-storey buildings in New Zealand have weatherproofing problems because facade components such as windows, balcony doors and cladding have been installed without due consideration to the increased exposure, imposed building movements and risk of these components in multi-storey buildings. This paper is a first attempt to document the performance parameters and a risk matrix for buildings up to 15 storeys. The intent is to identify key success/failure parameters for the buildings and formulate a matrix that allows key decision makers to understand weathertightness risks associated with various cladding related decisions/approaches. The matrix is not intended as a compliance tool, rather it should be an informative, easy to use, facilitator of informed decision making, that elucidates risks that may be otherwise overlooked. 1 New Zealand Building Industry Context The New Zealand (NZ) Building industry is a relatively immature one when it comes to designing and constructing tall buildings. There are a limited selection of frontline construction companies who have an established track record and/or capability to plan and construct medium to large commercial and apartment buildings. NZ has never been able to build up a cadre of experienced site managers and construction staff due to successive financial crises which have regularly decimated/ hollowed out the industry by sending trades people into premature retirement or to alternative geographic locations to continue work in the industry. There is an even shallower pool of options with respect to subcontractors who are capable of taking on all aspects of cladding design and installation. There are some cladding and curtain wall subcontractors who are developing skills. The general approach is to extrapolate small building technologies and in the recent past it would not have been unusual to have tall buildings proposed that are no more than a vertical arrangement of fifteen floors of basic residential windows and cladding. Unfortunately this does still happen and was a significant contributor of the leaking building syndrome striking multi-storey apartments. The NZ building industry has a tendency to understate risk and overcompensate with unsustainable enthusiasm. This paper is timely in its attempt to balance enthusiasm with knowledge and awareness and put rational and comparative tools in the hands of the designer. 2 New Zealand Building Code Residential Matrix The external moisture (weathertightness) section of the New Zealand Building code has an acceptable solution (deemed to satisfy) document that contains a risk matrix. This acceptable solution E2/AS1

2 and the matrix within it addresses buildings within the scope of NZS 3604:2011 Timber Framed Buildings, 1 Buildings or 2 Single Household or Commercial under 10m. The risk matrix can be found on the Web by searching for E2 Matrix, New Zealand. The matrix was largely developed as a tool to help lead to prioritise cavity based cladding solutions in New Zealand at a time when there was some resistance to this within the building industry. In terms of intent the document [1] itself states This guidance document is aimed at designers, builders and building consent officials to help in assessing the weathertightness risk of low rise, timber-framed buildings using the risk matrix in Acceptable Solution E2/AS1 ( E2/AS1 ) for Clause E2 External Moisture of the New Zealand Building Code. The acceptable solution presents reasonable practice details that have a good track record of success and assists those with limited knowledge in resolving compliant solutions cost effectively and avoid the need to demonstrate code compliance. Standardisation of detailing leads to some familiarity in industry. The E2/AS1 Matrix usefully provides explanation and examples to make it easier to Assess the building envelope design and environmental factors which effect Weathertightness Decide when a cavity solution must be provided Calculate a risk score for the building Use this risk score to identify wall claddings/systems that can be expected to function adequately within the risk parameters Adjust the risk score to improve the likelihood of a successful building façade. The E2 / AS1 Matrix is easily understood and can be used by building developers, architects, builders and owners to manage the design/construction risks associated with their envelope. 2.1 The Matrix 15 Our matrix introduced by this paper is referred as the Matrix 15. The purpose of the Matrix 15 is to help in assessing the weathertightness risk of medium rise buildings (residential and commercial) and the relevance of this analysis for the selection of appropriate wall cladding systems. It is not intended to be a compliance tool. Rather it should be an informative, easy to use, facilitator of informed decision making that elucidates risks otherwise overlooked. 3 E2/AS1 Matrix Factors In considering the development of a new risk matrix it is potentially useful to compare to the E2/AS1 Matrix guidance document The six risk factors in the E2/AS1 Matrix and their scores ranges are: Wind zone (scoring range of 0-2) Number of storeys (scoring range of 0-4) Roof/wall junctions (scoring range of 0-5) Eaves width (scoring range of 0-5) Envelope complexity (scoring range of 0-6) Deck design (scoring range of 0-6) Of these factors, wind zones or more accurately in our case wind pressures, envelope complexity and deck design are relevant to the Matrix 15. The number of storeys is not considered relevant given that a taller building will typically experience higher wind pressures which is already covered under a separate heading. Eaves are not typically included in multi-storey buildings and would only benefit the top two or so stories, if at all and so are not considered relevant.

3 4 Success Factors in Multi-storey Building Cladding In this section we seek to identify and discuss the factors/parameters that are critical determining factors in the success or otherwise of building envelope installations. We present a table under each category that is a first attempt at identifying a scoring system and risk category under which various options might be placed. Quantification of risk scores and interpretation of risk categories is discussed in section Façade Design Wind Pressures Design Wind pressures should be determined in accordance with the wind effects Standard, AS/NZS [2] by an experienced Façade or Wind Engineer or by carrying out a wind tunnel test. Façade wind pressures should be given in terms of typical and corner pressures, including all local pressure factors and both Serviceability Limit State (SLS) and Ultimate Limit State (ULS). SLS pressures are used to check member deflections for ergonomic comfort considerations and ULS pressures are used to check structural adequacy. Table 1. ULS Design Wind Pressures (corner pressures) Design Wind Pressure Low To 1.5kPa kPa 3-4 High Greater than 2.5kPa >5 4.2 Geometric Complexity The Matrix has an item of Roof-Wall Junctions which relates to façade system complexity. It is considered that increased complexity brings increased risk. Table 2. Façade Geometric Complexity Low High Description Simple rectangular structure. Less than 2 internal corners. Complex plan, with most floors similar. Recesses within the façade. Balconies over habitable spaces. High Geometric complexity with varying floor plans. Non vertical façade. Multiple recesses >5 4.3 Types of Facade Systems The choice of façade system or façade type can contribute to the envelope risk. Different systems such as a unitised curtainwall or precast concrete walls have different inherent weathertightness properties. One building can have several different façade systems. Two different types of curtainwall on one building may constitute two different systems. A façade system may include details such as windows, doors and balcony and/or structural penetrations. These are not considered separate systems, but their interfaces must also be addressed. In Table 3 we consider generic descriptors related to the critical features of the most commonly used façade systems in New Zealand and Australia, we then provide in Table 4 some typical examples with a risk score for each.

4 Only systems which are pressure-equalising and drained are considered, as face-sealed systems will typically not be allowed in New Zealand by local government authorities. It is also assumed that the chosen systems have been performance tested to a recognised façade (or window) testing Standard and that they have a proven Performance Rating (performance wind pressure, air leakage and water penetration test pressure). Table 3. Types of Façade Systems. Low System Characteristics Prefabricated systems that utilise accurate fabrication techniques. Engineering responsibilities easily defined and design completed to local standards. Building movements accommodated by mechanisms that are comprehended by simple rational analysis. Weathering systems do not rely on exposed sealants, no flashing tapes or foam tapes. Limited use of site applied sealant in well protected positions. Overall responsibility for design, fabrication and installation by a contractor/subcontractor experienced with the system. More than 10 years successful history of use of similar systems on similar buildings. Systems with some off site fabrication and some on site fabrication. Engineering of system requires coordination by multiple parties and design or testing completed to less familiar standards that are not cited in New Zealand or Australian building codes. Building movements accommodated by mechanisms that are comprehended by more complicated rational analysis. Weathering systems incorporate some exposed sealants in protected locations and some site applied sealants to stable substrates. No flashing tapes or site applied foam tapes. Overall responsibility for design, fabrication and installation by no more than two contractors/subcontractors experienced with the system years successful history of use of similar systems on similar buildings

5 Systems with mainly site fabrication. Engineering of system by multiple parties and design or testing completed to unfamiliar standards or to methods not codified. Building movements accommodated by mechanisms that are comprehended by complicated rational analysis or unable to be rationally assessed. High Weathering systems incorporate exposed sealants and site applied sealants to stable substrates. Flashing tapes and site applied foam tapes. Overall responsibility for design, fabrication and installation by more than two contractors/subcontractors experienced with the system. Less than 5 years successful history of use of similar systems on similar buildings. Table 4. Example Scoring for Types of Facade Systems System Comments Panelised (or Unitised) curtainwall Stick System curtainwall Precast Concrete With punch windows Aluminium framed and glass visions, spandrels that are any panel material (glass, metal granite & etc.); factory assembled. Only pressure-equalising and drained stick curtainwall systems are acceptable. This generally excludes pressure-plate systems. Stick systems are assembled on site and so have a greater reliance on site workmanship. Stick systems also have a limited movement joint capability compared with panelised curtainwall systems Pre-cast concrete panels are a single skin (the thickness of the concrete panels) system, as detailed NP:PCH-2009 Precast Concrete Handbook (available from Standards Australia infostore.saiglobal.com) [3]. The horizontal and vertical joints between concrete panels must be pressure-equalised and drained. Interfaces, especially with windows, doors and balconies must also be specifically designed to be pressure-equalised and drained. Pre-cast concrete panels are usually designed by one company and installed by another with windows installed by a different party. Each without connection and separately employed by the Main Contractor. If the Main-Contractor (the builder) does not adequately co-ordinate this work and ensure that adequate installation drawings are provided and engineering is coordinated, then the indicated score of 2 should be increased to (5)

6 Cladding built in-situ with RAB, Rainscreen & cavity battens This is cladding of any durable sheet material (e.g. compressed fibre cement (CFC) sheet, solid aluminium zinc-alume or Colorbond steel panels or aluminium composite panels used as a rainscreen. The airseal is made of an air barrier of metal or CFC sheet. The cavity is formed by installing battens/rails onto the air barrier and fixing the rainscreen to the battens. The system is usually fixed to a timber or galvanised steel infill frame and has integral windows and doors and often balcony penetrations Number of Different Systems Different systems includes different façade systems, but also doors and windows, canopies and decks. Consideration should be given to increasing scores to account for penetrations to the façade, such as at decks & balconies, piping and electrical cables & etc. these should be considered as an interface to be properly considered and designed. There is a tendency in the industry to apply a number of different façade systems to one building to provide architectural diversity. Each façade system is designed and installed by one or two different subcontractors. This is often done without consideration of increased complexity of detailing at interfaces and resolving practicalities of coordination. Often main contractors do not proactively manage and co-ordinate this issue. Quantity surveyors do not accurately cost the complexity of the various systems. They typically consider the costs of systems only without considering interface costs accurately which leads to false representation of cost savings available, often without understanding of risks involved. The risk related to number of systems in one façade is considered an important factor as more systems result in more interfaces and more different interfaces and the risks quickly escalate as shown in table 5. The number of systems as a measure is considered more appropriate than number of façade subcontractors as a single subcontractor with multiple systems will still have a number of additional interfaces to deal with which increases risk. The consideration of what constitutes a system is important given the number of rainscreen cladding systems that are installed over air barriers by others often with a timber frame by someone else. Therefore we consider that for systems that are installed by multiple contractors, the score should be at least doubled. As presented in the examples of table 6, precast with punch windows are low risk (given the qualifications indicated above), therefore 1-2 systems (the precast and the windows) is considered low risk. Once another system is added into the envelope or combining a rainscreen system (by one façade sub-contractor over an RAB (by another sub-contractor) then the installations typically become more complicated and risky, on that basis the doubling of systems installed by multiple subcontractors seems fair given that takes a score to 3 and out of the low risk category. The transition to high risk category is the next consideration, in our experience two different systems over air barrier by others and a window system tends towards a high risk so perhaps a 5 or more systems score is appropriate. Table 5. Number of Systems. Number of Systems Low High >5 >5

7 Table 6 presents some examples of how this scoring might work: Table 6. Number of Systems Scoring Examples. Systems Description calculation precast concrete with punch 1+1=2 Low windows 1 curtainwall with composite 1+(1x2)+1=4 aluminium rainsceen over RAB and windows 1 curtainwall and composite aluminium rainscreen over RAB and terracotta tiles over RAB with windows and precast with punch windows 1+(1x2)+(1x2) =8 High 4.5 Façade Contractor Experience In the New Zealand context there are not many medium to large commercial buildings and a limited number of experienced sub-contractors. There is a model of procurement for aluminium framed systems with a Prime die holder who develops and markets systems and a network of franchisee businesses who undertake fabrication. This is more suitable to domestic construction but unfortunately is often applied to multi-storey commercial where the inherent separation of system designers from system documenters, and from system fabricators and installers creates risks of gaps in communication of requirements and understanding of system limitations. Table 7. Façade Contractor Experience Low Description Good experience >10 Years Has successfully completed many similar systems on similar buildings 1-2 Little experience Some staff have experience on similar buildings for different 3-4 companies High No experience of similar building >5 4.6 Building Movements A building façade must be able to tolerate all reasonably expected building movements without compromising weathertightness integrity. For example interstorey-drift due to Seismic effects, edge beam deflection due to self-weight and imposed loads as well as thermal effects and column shortening. For curtain walls, these effects typically require a horizontal joint of 25mm or so every floor and a 10mm joint at each mullion. Other systems such as pre-cast concrete require less as the concrete material expansion due to temperature changes is less. A typical value for inter-story drift is 5 to 6mm for Serviceability and 20mm for the Ultimate condition. However, recently structures of some buildings have been designed to be lightweight and so much more flexible. Some buildings have been quoted with a serviceability inter-storey drift of

8 40mm and ultimate of 100mm. Conventional façade systems may not be able to tolerate such serviceability movements and remain serviceable. Façade designers must determine the movement requirements for its façade system and in particular obtain the values of the building structure s movements from the building structure engineer. Table 8. Building movements Low High Description building seismic serviceability movements <6mm horizontal vertical differential serviceability <10mm building seismic serviceability movements 6-15mm horizontal vertical differential serviceability 10-15mm building seismic serviceability movements 15-40mm horizontal vertical differential serviceability 15-35mm >5 4.7 Performance Testing All façade systems should be performance tested so that their performance rating can be determined. A building with a number of systems should have both systems tested including their interface. There is a tendency in New Zealand and Australia to have façade systems tested separately. The result of this is that façade systems are installed with the interfaces which have an unknown performance rating. Many façade systems, such as cladding are not covered by a local standard and so are often installed untested and with an unknown performance rating. The accepted façade testing Standard in New Zealand and Australia is AS/NZS 4284[4]. This applies pressure and suction to the external face of the façade specimen and includes cyclic variation of the water penetration test pressures. The 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. Table 9. Performance Testing Low Description all façade systems for the building have been performance tested in one specimen, including all interfaces to project specific conditions. façade systems for the building have been performance tested separately, without project interfaces being tested High façade systems for the building have not been performance tested >5 4.8 Other considerations For this paper we have not considered the following: complications of accepting a foreign fabricator who manufacture overseas, where inspection of testing and fabrication is difficult and expensive the ability to maintain the façade, we consider this does not affect the initial performance of the façade, but may affect its longevity

9 5 Informing Decision Making As summarised in table 10 we have indicated a simple scoring value for each of the chosen parameters. A lower score indicates a lower risk. The three risk categories indicate the following: Low; this is a minimum risk indication which gives the best chance of successful performance in service ; the risk of inadequate performance is higher than the writers recommend; the designer should consider re-design of the higher-scoring parameters to reduce the risk to the low category. Alternatively, be aware of the risks and manage then with good design and installation quality control. High; the risk of inadequate performance is too high and the writers recommend a major review to reduce the risk, back towards the low category. The reader may wish to add importance values to each of the parameters or possibly increase the scores of the parameters which are perceived to be more important. This possibly complicates the matrix to an unreasonable extent. Table 10 The Matrix 15 Scoring Summary Parameter Low High Design Wind Pressure >5 Façade Complexity >5 Façade Type >5 Number of Systems >5 Façade Contractor Experience >5 Building Movements >5 Performance Testing >5 Total >35 6 Conclusion This paper has been written as a first attempt to improve the awareness of participants in the building industry of the difficulties in designing and building the facades of medium-rise concrete and steel structure buildings compared with timber-structure houses up to 3 stories. It provides a point score so that different façade options can be compared and the reviewer can determine which option gives the lesser risk of failure. The failure risk can also be reduced by addressing the options with an identified high risk. The user of the Matrix 15 is free to vary the point scores as they deem appropriate for their project. We expect that application of the Matrix 15 will improve the understanding in the building industry of the parameters to be considered in façade design and improve building industry outcomes. References [1] Ministry of Business Innovation & Employment. (2014). Acceptable Solutions and Verification Methods For New Zealand Building Code Clause E2 External Moisture. Third edition amendment 6, New Zealand [2] Standards New Zealand., Standards Australia. (2002). AS-NZS1170 Structural Design Actions., New Zealand, Australia.

10 [3] National Precast Concrete Association Australia :PCH-2009 Precast Concrete Handbook [4] Standards New Zealand., Standards Australia. (2008). AS-NZS4284 Testing of Building Facades., New Zealand, Australia.