The building façade is one of the main contributing elements to the spread of fire between floors. Fire spread through the exterior façade can be deadly and lead to significant property damage and business interruptions. Fire on the exterior of the building is becoming increasingly challenging for firefighting operations, building code enforcement professionals, and building safety experts due to advancements in cladding materials that often incorporate combustible materials, including plastics.
In the past decade, there has been an increase in fires that propagate rapidly through the exterior of buildings due to the use of substandard exterior cladding. This has led to deaths, injuries, property damage, and business continuity issues. The most unfortunate example is the deadly fire incident at Grenfell Tower in the UK in 2017, which resulted in the deaths of more than 70 people and injured many more. The fire spread through the exterior Aluminum Composite Material (ACM) cladding system, engulfing the entire building. Another example is the Address Hotel fire in Dubai in 2015, where the fire spread through the exterior ACM cladding system and destroyed the entire façade. Additionally, a fire broke out in the TAM building in Baku, Azerbaijan in 2015, starting at the ground level and spreading through combustible exterior materials, resulting in the deaths of 15 people.
The main question is why fire spreads so quickly through the exterior façade of a building. The rapid spread is due to the use of combustible materials and a phenomenon known as the “chimney effect” or stack effect. In this effect, air movement in the vertical cavity behind the exterior cladding is driven by differences in pressure and temperature, causing hot air to rise rapidly.
Construction Components of Exterior Cladding with Metal Composite Materials (MCM)
Metal Composite Materials (MCM) are widely used in exterior cladding systems due to their aesthetic appeal, durability, and insulation properties. These materials consist of multiple layers that provide both functional and decorative benefits. The construction components of MCM cladding systems include the following key elements:
- Core Material: The core is the central layer sandwiched between the outer metal skins. It can be made from various materials, including polyethylene, mineral-filled cores, or fire-retardant materials. The choice of core material significantly impacts the panel’s fire resistance, structural integrity, and overall performance.
- Metal Skins: The outer layers of MCM panels are typically composed of aluminium, but other metals such as zinc, copper, or stainless steel can also be used. These metal skins provide the primary weather barrier and give the cladding its visual characteristics. They are coated with protective finishes, such as PVDF (polyvinylidene fluoride), to enhance durability, colour retention, and resistance to environmental factors.
- Adhesive Layers: Adhesive layers bond the core material to the metal skins, ensuring the structural stability of the panel. High-quality adhesives are crucial for maintaining the panel’s integrity under various environmental conditions, such as temperature fluctuations and moisture exposure.
- Protective Coatings: The metal skins are often coated with protective layers to improve resistance to corrosion, UV radiation, and other environmental elements. These coatings can also include self-cleaning properties, enhancing the appearance of the cladding.
- Subframe and Support System: The subframe, often made of aluminium or galvanised steel, provides the structural support for the MCM panels. It includes brackets, rails, and fixings that attach the panels to the building’s façade. The design of the subframe ensures proper alignment, ventilation, and load distribution.
- Sealants and Gaskets: Sealants and gaskets are used to seal joints and seams between panels, preventing water infiltration and ensuring a weather-tight installation. They also accommodate thermal expansion and contraction, maintaining the integrity of the cladding system over time.
- Insulation and Vapor Barriers: Insulation materials can be incorporated behind the MCM panels to enhance thermal performance. Vapour barriers are also used to prevent moisture penetration into the building’s structure, reducing the risk of condensation and mold growth.
International Building Code Requirements for the MCM
Building codes and regulations around the world have evaluated the use of MCM and other combustible exterior finishing materials, such as plastic veneers, exterior insulation finishing systems (EIFS), and insulation. While there might be different approaches to regulating the use of these materials from one country to the other or in different model codes, the Intranational Code Council (ICC) Codes and standards are one of the leading references globally.
The International Building Code (IBC), which is one of the ICC codes, has specific fire performance requirements for Metal Composite Materials (MCM) used in building construction. These requirements are designed to ensure that MCM does not contribute to the spread of fire. Following are the main areas where the IBC focuses:
Compliance with NFPA 285:
MCM used in exterior walls must comply with the fire propagation performance criteria of NFPA 285, which is known as the “Full-Scale Test”. These standards tests the ability of wall assemblies to resist fire spread when exposed to a fire involving combustible materials. Passing the NFPA 285 test is crucial, especially for high-rise buildings or another chimney effect as discussed earlier can be present.
Surface Burning Characteristics:
The IBC requires MCM to meet specific surface burning characteristics when tested by ASTM E84 “Test Methods for Surface Burning Characteristics of Building Materials”, or as know “Tunnel test”. This test measures the flame spread index and a smoke-developed index of material surfaces. According to IBC, the MCM must achieve a flame spread index of 25 and a smoke development index of 450, to ensure the material surface will not support fire spread.
Thermal Barrier:
The MCM must be separated from the interior of the building by a 12 mm (1/2 inch) gypsum Board or by the equivalent material to ensure it won’t ignite within the building or catch fire due to high temperature from within the building. A thermal Barrier might not be required if the MCM passes the NFPA-286 “test or is used on architectural trims and balconies.
Height and Area Limitations:
The IBC requirements for MCM are also based on the height of the building, the percentage of the face covered by the MCM, the combustibility class of the MCM, the separation of the MCM panels, provision of sprinkler systems and the separation of the exterior faced from other buildings and properties. More strict requirements are applied to taller buildings or those with a higher percentage of wall coverage.
In conclusion, architects should pay more attention to the design and specification of exterior wall cladding. The use of MCM shall be limited to a height of 22 m only and shall pass both tests NFPA -285 and ASTEM-E-84. The cavity behind the MCM system shall be also considered carefully, in terms of the combustibility of the insulation, opening protection, and ignition sources. In addition, the areas covered by the MCM shall be limited and separated effectively from each other to eliminate the possibility of vertical spread.
