Curtain walls have been shaping the visual identity of modern buildings since the late 19th century, especially for sky-high multi-occupancy structures. However, design and safety standards have evolved – and so have the passive fire protection measures employed to safeguard the lives of those using these buildings.
To protect these properties effectively in a fire, it is vital to ensure that compartmentation between the floor slab and the façade—an area referred to as the spandrel zone— is robust. However, there are many different passive fire protection products on the market that have been tested to different standards or combinations thereof. It is important that designers, specifiers, and fire engineers appreciate the variances in the scope of these tests for a more holistic understanding of the firestopping measures installed within the spandrel zone and to ensure the highest levels of fire safety are achieved.
GOING BEYOND PRODUCT
To prevent fire spread through the movement joint, there must be a continuation of fire resistance of the compartment floor right up to the façade using perimeter firestopping measures. Many codes and regulations around the world make provision for this, demanding the use of slab-edge fire seals that have had their fire resistance certified to different product test standards. However, the test paths to compliance offered can vary hugely in scope, and the types of curtain walling systems they can apply to.
For example, in the UK, the legacy path is to use BS 476-20: 1987 – Fire tests on building materials and structures. Method for determination of the fire resistance of elements of construction (general principles). This is a small-scale, static test that requires a specimen to be installed between two concrete lintels within a furnace and subjected to defined heating and pressure conditions. Whilst BS 476-20 can provide a basic understanding of a product’s standalone performance, the test is designed to be generalised and therefore cannot give a clear indication of a perimeter fire seal’s ability to resist fire in its intended application. EN 1366-4 – Fire resistance tests for service installations – Part 4: Linear joint seals is a test method similar to that of BS 476-20. However, in 2021 it was amended to exclude curtain walling perimeter seals from its scope.
As anyone who works in fire safety knows, it is often not the performance of individual products that determine how fire will spread, but the combination of all the materials, elements and how they have been assembled. Whilst perimeter firestops are highly effective at inhibiting fire and smoke spread, compartmentation is reliant on them being able to maintain their fit between the floor slab and façade. This is why many regions look toward standards that offer a much more holistic perspective of the perimeter firestop’s performance in a spandrel construction rather than its performance as a standalone product.
This is particularly important when we consider that most curtain wall systems comprise non-fire-rated aluminium framing and non-fire-resistant vision glazing. These are sometimes referred to as ‘Type A’ systems. With a melting point of approximately 660°C, aluminium loses its strength around 350°C. This can result in the framing structure distorting and gaps forming at the wall/slab edge intersection, impacting the perimeter fire barrier. Not only can this open multiple paths for fire, smoke, heat, and gasses to spread upwards internally, but the increased ventilation can also fuel the fire, enabling it to grow in size and ferocity.
To reduce this risk, a high-density stone wool fireboard that shields the spandrel panel and the mullions within the spandrel zone can be used alongside the perimeter firestop. This can help to prolong the stability of the assembly, allowing the firestop to fully perform its compartmentation function whilst reducing the chance of the fire breaking through the spandrel panel itself. This approach is common practice in jurisdictions such as the United Arab Emirates and Australia and can be assessed by both European (EN) and American (ASTM) test standards.
EN STANDARDS
EN 1364-4: 2014 – Fire resistance tests for non-load bearing elements (Curtain walling – part configuration) is widely referenced in building codes and regulations in many countries across Europe— including the UK— Middle East and Asia Pacific. It is also the recommended test standard for curtain wall perimeter barriers in the Association for Specialist Fire Protection’s (ASFP) Advisory.
Note 7, with CWCT having announced that it proposes updating Technical Note 98 to recommend the test too.
As a ‘part configuration’ test, it looks at the spandrel assembly of a curtain wall system, comprising the mullions, transoms, and infill and spandrel panels. This makes it far more representative of a perimeter firestop in its end-use application. It also allows for multidirectional movement which would occur in the event of a fire as different building elements heat up and deform.
It tests individual parts of the assembly such as the perimeter seal or the anchoring attaching the curtain walling to the floor slab, or systems with fire resistance requirements only to the spandrel zone. There are specific configurations of the test that can be chosen depending on the primary element that needs to be assessed. In very simplified terms, Configuration 5 is used to assess the ability of the perimeter fire seal to contain the fire inside the compartment, under the stress of temperatures and pressures that lead to in-test deflection of the façade and floor slab. Meanwhile, Configuration 2 tests the upstand and downstand with the perimeter seal and anchoring, or a combination of and is often used by façade manufacturers as it incorporates flaming on both the inside and outside of the açade; this configuration would be used by curtain wall system manufacturers for specific data on the façade.
Additionally, to validate the durability of the perimeter firestop, EN 1364-4 also includes provision for long-term façade movement by referring to the European Technical Approval Guidelines 026-3, which is now superseded by European Assessment Document (EAD) 350141-00- 1106. This requires that the perimeter seal is subjected to a minimum of 500 cycles between the minimum and maximum joint width, to simulate in-service wind sway, seismic activity and thermal load at a rate designated by the test applicant 24 hours before fire exposure.
It is important to note that this test and its configurations focuses on the performance of materials and elements just within the spandrel. EN 1364-3: 2014 – Fire resistance tests for non-load bearing elements (Curtain walling – full configuration, complete assembly) on the other hand offers a full configuration test to evaluate the performance of the complete curtain wall assembly and includes any transoms and mullions, infill panels and the spandrel zone. However, it is only applicable to ‘Type B’ fire-rated systems, which are rare.
ASTM STANDARDS
Another common test standard for perimeter edge fire seals used in USA, UAE, and parts of Asia Pacific (using the international code council’s (ICC) International Building Code (IBC) that takes into consideration the performance of the spandrel) is ASTM E2307, Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using Intermediate- Scale, Multi-story Test Apparatus.
This measures “the performance of the perimeter fire barrier and its ability to maintain a seal to prevent fire spread during the deflection and deformation of the exterior wall assembly and floor assembly during the fire test.” It uses intermediate scale multi-storey apparatus to better replicate a fully developed curtain wall fire. The perimeter joint and perimeter firestop are exposed to fire from both inside the test room and outside to simulate the room where the fire started, as the fire plume exits the room of fire origin through a window opening and attempts to leapfrog to the next floor. Like EN 1364- 4, it also includes provisions for pre-cycling, or ‘cold’ movement before the test begins, covering thermal load, wind sway, seismic movement, and all these combined.
Alongside this perimeter firestop test, there is also now a relatively new standard that looks more specifically at assessing the spandrel assembly’s resistance to fire spread and the leapfrog risk. Unlike the aforementioned test standards, ASTM E2874 Standard Test Method for Determining the Fire Test Response Characteristics of a Building Spandrel-Panel Assembly Due to External Spread of Fire Using Intermediate-Scale, Multi-story Test Apparatus has been designed to emulate a fire scenario at post-flashover stage inside a compartment. It analyses the ability of the spandrel assembly to prevent fire from spreading over the façade exterior to a room’s interior on the floor above the compartment of origin. The test specimen includes the exterior wall spandrel panel assembly, fasteners, structural supports, and any glazed openings. When applied in conjunction with ASTM E2307, this can provide a uniquely rounded view of how both the perimeter firestop and spandrel insulation can achieve complete compartmentation, preventing fire from spreading floor to floor both internally through the curtain wall system and externally up its surface.
INDIAN STANDARDS
The Indian standard, IS 18190: 2023 – Fire Resistance of Perimeter Fire Barrier Joint System released in June 2023 by the Bureau of Indian Standards (BIS) references ASTM E2307, as well as UL 2079 – Tests for Fire Resistance of Building Joint Systems specifically for assessing air leakage of the fire test assembly, the resulting L-rating is an important factor when considering the ability of a passive fire protection measure to provide an effective smoke seal. Additionally, IS 18190: 2023 mandates adherence to the ISO 834-1 time-temperature curve for evaluating the perimeter fire barrier joint system, which is the same time-temperature curve followed by EN 1363-1 and subsequently used within testing to EN 1364-4.
APPLYING BEST PRACTICE
Curtain wall systems present several unique challenges when it comes to ensuring passive fire safety. Choosing curtain wall perimeter firestops that have had their performance verified with testing that is reflective of their end use in the ‘as-built’ condition, such as the EN or ASTM standards, can not only enable compliance in many jurisdictions but also encourage best practice and, where possible, help to go beyond regulatory requirements to deliver enhanced spandrel protection, contributing to safer curtain walled buildings around the world.
