Irving Schnider, author of Façades – a Practical Guide to Curtain Walls and Cladding, Architectural Glass and Natural Stone, outlines the key lessons that can be learnt – and reiterated – from the UK's Grenfell Tower fire.
Schnider is a building construction technologist and freelance cladding advisor.
The recent Grenfell Tower fire tragedy in London attracted considerable international attention, and, regardless of a project’s location, lessons of significant importance should be learned from it to avoid repetition. Unfortunately, there is a history of flammable cladding materials being applied on numerous high-rise towers in the UAE as well as other Gulf states over recent years, and several major fires attributable to such materials have subsequently resulted in the introduction of stringent regulations. The latest fire to erupt on the Torch Tower in Dubai Marina – for the second time in two years – is of significant concern, and sheds light on the critical role of the new UAE Fire & Life Safety Code of Practice’s regulations. Additionally, it is also crucial to address the vulnerability of existing buildings clad with materials that do not meet or satisfy the safety requirements detailed in the latest edition of the UAE Fire and Life Safety Code of Practice.
This is a dilemma confronting municipalities and local authorities, because of the likelihood that fires will continue to erupt until these façades have been remedied to conform, which is what is currently under review throughout the UK. This has resulted in the occupants of several buildings being evacuated, because the properties are considered hazardous. The ownership of buildings in the UAE is likely to preclude such a task being feasible; thus, such buildings may be uninsurable, which would compound the problem.
Cognisance must be taken of the circumstances leading up to the Grenfell Tower fire. Expert reviews should examine why unsuitable materials were incorporated in the structure, and whether specifications, detail design, and workmanship could have been contributory factors. A study of these factors may also support high-rise construction in the UAE.
Next page: Façade cladding
In any review of building façades, it is important to understand that, whilst they may appear to be similar in appearance when completed, the external cladding composition may fundamentally differ. In principle, the cladding is likely to comprise a variety of components – including metal, glass, stone or other appropriate enclosing material – that are mechanically fixed to the exterior of a building. This can be to form either a weather-tight or a non-weather-tight envelope, the latter being termed a ‘rain-screen’.
Typical external façades are either curtain walls, which are a form of non-load bearing cladding or building envelope system, and most commonly comprise a relatively light-weight metal carrier framework that includes opaque, translucent, and/or transparent infill panels. While the term embraces many different construction methods and materials, the predominant similarity is that the frameworks are hung from the building structure, and are located outside the perimeter of the floor slabs. External façades may also be window walls, a vertically – and sometimes horizontally – discontinuous glazed façade fixed to the structural frame between floor slabs, or between an upstand wall and soffit or downstand beam. Cladding to the spandrel substrate is in-situ, and fixed separately to the windows, which was the case with Grenfell Tower.
Next page: Fire resistance
Typically, only in very exceptional cases are external façades fire-rated, and ideally, each of the components assembled in a cladding installation should be fire-resistant. Individual components cannot be fire-rated in isolation; only a composite system into which they are incorporated – that has been appropriately tested and certified by a recognised authority – should be used. Fire-resistance tests are used to measure fire behaviour, as well as the combined performance of all components used in construction when exposed to a fully developed fire.
Noting that a predominantly glass curtain wall is unlikely to be fire-rated – unless this is specifically required, which is mainly due to the type of occupancy or building proximity to adjacent structures – there is a need for a horizontal perimeter fire-resistant joint or barrier system. Its primary purpose is to seal the linear gap between the edge of the floor slab and the external façade to provide resistance during the early stages of a fire whilst the curtain wall is still intact, specifically to contain hot smoke and flame – both vertically and laterally.
This is not applicable to a window wall, because there is no interior perimeter gap to be sealed. However, a mechanically fixed external cladding system, such as that applied on the Grenfell Tower, creates a void between the structure and the cladding, which then acts as a chimney – unless horizontal barriers are fitted optimally at each floor level.
Next page: Fire barriers
A fire-resistant barrier system – or a fire-stop – may be defined as a material or combination of materials used to retain the integrity of fire-rated construction by maintaining an effective barrier against the spread of flame, smoke, and hot gases through penetrations in fire-rated wall and floor assemblies. Only tested and certified systems should be used between the edge of floor slabs and curtain walls. If no test certification is available for the proposed system, then an individual test, representative of the project conditions, needs to be performed for its acceptability and certification, unless an engineering judgement can be derived from similar tested and certified system designs.
The perimeter joint fire barrier is classified by its fire-resistance rating – this is the duration for which a passive fire protection system can withstand a fire-resistance test, which is typically expressed in units of time, such as 30 minutes, 60 minutes, and 90 minutes, or 0.5 hours, one hour, and 1.5 hours, ranging up to four hours.
Fire protection systems require the incorporation of fire-retardant materials that are capable of resisting burning and withstanding heat without deformation or disintegration for such set periods of time. For example, perimeter fire-stop systems shall meet the requirements of ASTM E 2307 when tested and certified, or listed assemblies shall provide fire-resistance ratings no less than that of the construction in which the joint occurs.
Many compounds of smoke are highly toxic and/or irritating, and one of the more dangerous gases is carbon monoxide, because it is relatively undetectable by the senses. Therefore, smoke inhalation can quickly lead to incapacitation and loss of consciousness, potentially culminating in serious injury and death. It is largely to prevent such an occurrence that minimum time periods are applied to fire-rated compartments in buildings, which allows sufficient opportunity for the evacuation or rescue of people.
Next page: Fire barriers – material components
Material components and combinations of different fire-stop systems include:
- Intumescent sealants: These act in tandem with other restricting barriers – such as galvanised steel flashings fixed to the underside of the floor slab – and expand to seal apertures when exposed to heat, and carbonise to form an insulating barrier against the spread of smoke;
- Man-made mineral fibre (MMMF) or rock-wool insulation: Because a single sheet of steel is insufficient to retard propagation of heat and its destructive effects by itself, MMMF, generically referred to as rock-wool, insulation is a favoured slab edge barrier, with its effectiveness increasing when used with proprietary flowable self-levelling sealants to inhibit smoke transfer; and,
- Fire-resistive intumescent latex-based dispersion sealant: This applied by either spray or brush to the top surface of compressed mineral fibre insulation, and extends to adjacent surfaces of the floor slab and the inner face of the external façade treatment to maintain in-service integrity.
In many instances, the configuration and composition of perimeter barrier systems is influenced by the fire-resistance rating required between floors and to the exterior. Different international standards and codes define typical system arrangements to be used. The system rating should, therefore, meet with the specified code’s requirements.
As a result of flame passage out of broken glazing in the floor below, fires can be expected to impinge upon the bottom of the perimeter joint system, as well as the exterior of the curtain wall adjacent to its junction to the floor edge. Typically, the perimeter joint system is primarily a smoke barrier rather than a fire-stop, which is a common cause of confusion. A conventional fire-stop system is one that prevents flames from entering the floor above by virtue of the composition and depth of its construction; this is achieved by the incorporation of suitably insulated panels at least 1m deep, comprising a continuous galvanised steel sheet, or a lining of any one of cementitious fibre, calcium silicate, and gypsum board, as the inner face to which the horizontal perimeter joint system connects integrally.
To be fully effective, a perimeter joint system must include protection from intense heat and flame of the aluminium mullions comprising the framing of external curtain walls.
When certain materials are heated, they undergo thermal decomposition and release flammable gases – the basic ingredients of smoke. Oxygen-deprived fires contain a significant concentration of compounds that are flammable, due to the fact that materials have not had the ability to become fully oxidised by the burning process. Thus, smoke coming into contact with a new source of oxygen has the potential to be ignited, either by another open flame, or spontaneously – by its own high temperature.
It is essential that thermal insulation is not only non-combustible, but also does not give off smoke or toxic gases, such as with polyisocyarunate (PIR) foam – which as was used in the Grenfell Tower – when subjected to intense heat. MMMF – or rock-wool – is a suitable insulating material that is commonly used, and this is manufactured in varying densities, thicknesses, and sizes to suit design and application requirements.
Model building codes frequently require the provision of active fire protection systems, such as sprinklers, as well as passive barriers to minimise physical damages to property and injury to persons resulting from the effects of fire. However, for acceptability, passive fire-protection of buildings is dependent on incorporation of materials and systems that have been tested and assessed to determine and, thereon, certify their resistance-to-fire characteristics.
Many authorities require materials, devices, and assemblies for fire-stopping systems to be tested by an accredited testing laboratory, after which the test results are published by an accredited quality assurance agency to permit that the items bear a Listing Label, and be certified accordingly. All products forming part of the system shall bear a design listing and approval label to conform to the construction type and fire-rating requirements of each item.
This is especially relevant when the external façade cladding incorporates metal composite panels (MCP), the core of which, sandwich construction, often comprises low-density polyethylene (LPDE), which is a thermoplastic polymer. It is a commonly applied cladding material that is normally incorporated in spandrel panel construction. As a result of polyethylene being flammable and the contributory cause to serious fires, regulations for public, commercial, and residential buildings prohibit the use of composite metal sheet material for exterior façade applications, unless it comprises a modified inner core of fire-resistant material that meets the EN standard 13501-1 Fire Classification of Construction Products and Building Elements.
Next page: Fire testing
Product tests include surface spread of flame to ASTM E 84 and BS 476: Part 7 standards – with items being rated from Class 1 to Class 4, the former being the best result achievable – and combustibility (fire propagation) to ASTM E 119 and BS 476: Part 6 standards. The latter set are the traditionally accepted ‘reaction-to-fire’ standards that are used to assess the performance of external surfaces. However, their intensity may not be sufficient to simulate the effects of a fire in reality.
BS 8414 Parts 1 & 2 are large-scale fire performance test methods for non-load bearing exterior wall and cladding assemblies, in which the test methodology enables the overall fire performance of the system and its relevant components to be assessed as closely as possible to end-use fire exposure conditions. These conditions are typically developed to represent an external fire source or a fully developed (post flashover) fire – rather than a furnace or gas burner – into a room, venting through openings, such as a window aperture, which exposes the cladding to the effects of external flames.
Next page: Design
A lack of understanding about how a fire propagates is perceived to have contributed to the failure of Grenfell Tower’s design to curtail the spread of flames. The cavity between the thermal insulation – attached to building substrate – and the façade cladding panels acted as a chimney, which may be applicable to similar fires experienced in the Gulf. Simply installed horizontal sheet steel flashings at each floor level between the insulation and the cladding would have prevented the formation of an uncontrolled vortex – created by rising heat – that fuelled the rapidly expanding fire.
That there were no sprinklers in the Grenfell Tower prevented the dousing of flames that entered the building when the glass shattered, creating an ignition condition when oxygen was re-introduced to the fire upon disintegration of the glass in the external wall. Combustion would have rapidly restarted, resulting in an explosive effect, as the newly introduced cooler gases quickly heated and expanded. As a matter of course, high-rise buildings should be equipped with fire sprinklers to actively control any fire that may occur within the interior, before it has a chance to spread.
Next page: Financial considerations
It is understood from some UK media reports that the retrofit works executed on Grenfell Tower were subject to cost savings, either because of inadequate budget or by the contractor proposing and having accepted materials that could be considered as inferior quality products. These are currently subjects of speculation, but have similarities to numerous construction markets, where it is standard practice to drive down prices for specialist works below what is feasibly economical to execute. Something must give, and that is usually the quality of final product to the detriment of the integrity of the building, which is evident for all different types of external cladding in buildings throughout the Middle East.
Contributing to the delivery of poor quality installation is inadequate or lack of supervision, which may be both on the contractor’s behalf or the project engineering team’s, because no budget allowable was made available. This provides the opportunity to cut corners, substitute materials, and cover up errors.
Next page: Workmanship
Typically, much of façade installation work is outsourced, which removes some of the specialist façade contractor’s direct control. In times of intense construction activity, when available resources are scarce, semi-skilled operatives are engaged to do the work of skilled artisans. This, coupled with inadequate supervision, is a recipe for cladding failures.
In the words of one contractor guilty of serious misdemeanour: “I know we were wrong as it was sheer cheating and carelessness by my sub-contractor. Everyone closed their eyes – my supervisors, and even the quality assurance and quality control inspectors of the main contractors and the consultants.”
The contractor said it “has been the practice of workmen to save time and get more money for the same amount of time, as payments are made on the basis of production”, which is an indicting admission, indicative of a prevailing attitude in the industry.
There is increasing evidence of external façade cladding failures on buildings constructed within the last decade or so. Warranties have expired and building ownership has transferred, so recourse to those initially responsible for a remedy does not exist. It is understandable that new owners may be reluctant to expend what is necessary for rectification. Depending on the type of cladding, there is a commensurate increase of risk to either the occupants or the public beneath – the latter potentially putting them in physical danger.