Why is façade and building movement an important consideration when specifying curtain wall perimeter seals?
Buildings are constantly moving, vertically and laterally, impacting on the interaction between the façade and the building structure. Curtain wall façade systems must be designed, constructed and tested in ways that ensure they can withstand the different forces that make them deflect and move. This consideration of movement must also extend to the slab edge firestopping measures because they are subject to distinct boundary, restraint and fixity conditions, movement stresses and fire exposure conditions — as we explore below.
What types of loads influence façade and building movement?
Buildings must be able to manage any movement caused by various predictable and unpredictable forces leading to loading from:
Wind
Rain
Snow
Seismic sway
Temperature
Occupancy
Settlement
Other loads
The likelihood and severity of these loads should be modelled and analysed in the early design phases. However, whilst design and engineering measures can be taken to reduce movement, it is impossible to eliminate it. Therefore, it is vital that glazed façades and the components from which they are constructed, can tolerate these forces — this includes passive fire protection.
Curtain wall passive fire protection at the perimeter joint
Curtain wall buildings are commonly built with a gap between the edge of the floor slab and the façade to allow for both vertical and lateral deflection. This is often referred to as the movement gap, perimeter joint or expansion joint. The designed gap can vary considerably in void size. For example, it can be wider for buildings in locations with high movement risk (e.g. frequent seismic activity) or include more dynamic, complex design elements (e.g. curved elevations).
Whilst vital for allowing free movement, this gap - if left unprotected - can provide a path for flames, smoke, and heat to be drawn upwards due to the chimney effect, thus enabling fire to spread floor to floor. To prevent this, perimeter seals are installed in the gap of the perimeter joint at each floor level, continuing the fire resistance of the compartment floor right up to the internal surface of the curtain wall to provide compartmentation.
However, under the stress of the loads listed above, the width of this gap can be in a constant state of flux.
To accommodate this movement, it is commonly recommended that curtain wall perimeter fire seals provide a 7.5% or greater movement capability at the movement joint, allowing both compression and flex. This is a very real requirement, with deflections sometimes being considerably larger in comparison with the void dimension being sealed. To do this, they need to be installed under compression. The importance of this is clearly explained by the Association for Specialist Fire Protection (ASFP) in its Red Book- Firestopping: Linear Joint Seals, Penetration Seals & Cavity Barrier (4th Edition):
“The effectiveness of the fire stop will depend on the ability of the curtain walling / cladding system to maintain the compression fit for the duration of the required fire resistance period. Unless the system is installed pre-compressed and can move to maintain compression, premature failure of the fire-stopping may occur.”
If slab edge firestopping products cannot adequately recover from the repeated compression cycles, then gaps could form between the curtain wall and the floor slab, allowing the vertical spread of fire and smoke.
This means that when specifying perimeter edge seals, it is important to ensure that their testing not only assesses their ability to achieve the required fire resistance, but that they can withstand constant inward and outward movement for the lifetime of the building to maintain a tight seal throughout their in-service life.
Firestop movement cycling before fire testing
The importance of durability and resilience in accommodating movement is reflected in the provision for movement in different standards. For example, the European product standard for curtain walling, EN 13830: 2015, stipulates that fire resistance testing for perimeter fire seals should be carried out in accordance with EN 1364-4: 2014 – Fire resistance tests for non-load bearing elements (Curtain walling – part configuration), and durability and cycling in accordance with ETAG 026 (now superseded by EAD 350141-00-1106). This requires that the perimeter seal is subjected to at least 500 cycles between the minimum and maximum joint width, to simulate wind sway, seismic activity and thermal load at a rate designated by the test applicant.
After this cycling, the test construction is left to stabilise for 24 hours and cannot be altered before fire testing. The test assembly incorporating the pre-cycled perimeter seal is then exposed to furnace testing of over 1000°C at 20Pa positive pressure as per the EN 1364-4:2014 test standard. This process is fundamental to obtaining a CE Mark.
The American ASTM E2307-20 test standard also includes provisions for pre-cycling, or ‘cold’ movement before the test begins, covering thermal load, wind sway, seismic movement, and all these combined.
Movement from in-test fire loads
In-test deflection of the façade and floor slab, incurred as a result of being subjected to over 1049°C during the EN 1364-4: 2014 test procedure, simulates the heat generated in building fires that can often cause the slab to deflect. Firestops should be capable of maintaining their seal against the slab edge when placed under the forces caused by vertical movement.
This dynamic and flexural property requirement is not replicated in any other test making EN 1364-4: 2014 the most onerous in terms of examining perimeter seal performance.
Perimeter seal design
Traditional passive fire protection products typically use non-combustible stone wool insulation with a horizontal fibre orientation. This orientation is resistant to lateral compression. Continual exposure to movement and compression can result in the breakdown of the fibre structure. In such cases they essentially become crushed, resulting in a gap forming between the product and the façade, potentially causing the firestopping to fail in the event of fire.
To resolve this, Siderise engineered Lamella, a special formulation stone wool insulation with vertically oriented fibres. The fibres are laterally compressed under quality-controlled factory conditions to remove any gaps between the adjoining cut sections and assist with uniformity of product density. This ‘pre-compression’ is held in place by heat-applying a foil facing to the cut face of the fibres. When exposed to fire, this foil facing is sacrificed, releasing the factory-induced compression and allowing the Lamella core to seal against the substrate — even under the movement caused by the fire load.
Lamella systems are further compressed by 10-20% when installed.
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