Spotlight Academia: Mechanical behavior of rainscreen walls subjected to fire load

Mentors: Angelo Lucchini (Full Professor – SSD CEAR-08/A – Architectural Engineering), Enrico S. Mazzucchelli (Associate Professor – SSD CEAR-08/A – Architectural Engineering), Giacomo Scrinzi (Research Fellow – SSD CEAR-08/A – Architectural Engineering) 

Ventilated facades have repeatedly come under the spotlight for their behaviour in case of fire, and especially for the rapid spread of flames in the cavity in presence of materials with an inadequate reaction to fire. Certainly, the fire at Grenfell Tower in London in 2017 is among the most striking cases, causing over seventy casualties and dozens injured. It was no isolated case as in late decades more and more fires have affected building façades. The public outcry and the increasing frequency of fire events have therefore highlighted the need for an urgent re-evaluation of specific regulations in many European countries, mainly focused on: 

• limiting the spread through the façade of fire originated inside or outside the building;
• avoiding parts of the façade falling during the fire, to allow people to safely escape and rescue teams to intervene. 

While fire spreading can be contained using materials with adequate reaction to fire, façade’s structure mechanical resistance during such incidents is often neglected. Regulations clearly define reaction to fire requirements for different envelope components, but an EN harmonized experimental method to evaluate fire resistance of ventilated facades is still not available. 

The aim of the thesis was therefore to investigate the mechanical behaviour of a typical ventilated façade system in case of fire to evaluate the influence of temperature on the material’s mechanical properties along with the effects of thermal expansions on the stability of the façade. Five among the most common configurations were analysed (see Fig. 1). 

As a first configuration, a ventilated facade with porcelain stoneware cladding, fixed to an extruded aluminium alloy T-shaped profile by stainless steel retaining clips, was analysed. The structure presents at the top end a bolted upright-bracket connection which acts as a fixed point (hinge) and further supports connecting the vertical profile to brackets with slotted holes (rollers) to allow for ordinary thermal expansion.  

The second configuration is similar to the previous one except for the sliding capability of lower supports to accommodate longitudinal thermal expansion which is realized by fixing bolts heads into a continuous groove in the extruded profile.  

Further configurations involve a stone cladding (granite) supported by aluminium alloy brackets fixed onto C-profiled mullions respectively made of aluminium alloy, carbon steel and stainless steel. Tiles are held in place by brackets vertical blades which accommodate in the kerf cut on the tiles’ horizontal edges. Thermal expansions are permitted by bolted connections through slotted holes on the uprights. 

International regulations propose different methods to evaluate temperature’s evolution in external members subjected to fire, such as the Eurocode 1 external fire curve which considers a uniform temperature on the facade plane with no regard for the spatial distribution and the thermal gradient as a function of the flame’s height, which appeared to be limiting for this work’s purposes. 

Another method to determine the behaviour of external structures exposed to fire is proposed in Eurocode 1, considering the convective and radiative exchange with hot gases and flames; unlike the external curve, this approach provides temperature as a function of the height, but it does not provide indications regarding the evolution in the time domain. To overcome these issues, experimental data were derived from the results of real tests performed in accordance with the NFPA 285 method which involves the use of thermocouples on the external and internal side of the cladding in order to gather temperature curves as a function of both space and time. These curves were used to perform a dynamic thermal analysis (see Fig.2) and evaluate the temperature’s evolution in the thirty minutes assumed as the reference time interval, considering convective and radiative exchanges between the different components, flames and hot gases and the temperature-dependent behaviour of thermal properties, such as specific heat and conductivity.  Briefly, for configurations no. 1 and no. 2 the temperature at the base of the upright reached up to 680°C, which is extremely critical for an aluminium profile. For configurations no. 3, 4 and 5, given the thermal inertia of the heavy cladding, the temperature in the uprights oscillates between 100°C at the top and 400°C at the base.   

The temperatures presented above are not interesting as such but rather for their effects, since an increase in temperature for metal structures involves two main effects: the reduction of mechanical properties in terms of yield stress and elastic modulus and a significant dimensional variation.  

Aluminium alloys are particularly susceptible to these phenomena, as its structural resistance is immediately affected by external temperature and completely cancelled over 550° C.  

Instead, the behaviour of carbon steel (stainless steel performance is comparable) is markedly better, as up to 400°C the yield stress is unchanged and the elastic modulus reduction is slower compared to aluminium, while the linear thermal expansion coefficient is about 50% lower than this of aluminium. 

Uprights for ventilated facades are usually designed to work in tension, being constrained at the top with a hinge while the lower brackets behave like rollers, as described above.  

Due to the high temperatures reached in this scenario, thermal expansions exceed the design tolerances in a few minutes, altering the starting static scheme and leading to a hyperstatic structure, moving from a hinge-roller to a hinge-hinge scheme. Hence, vertical profiles are no longer in tension but begin to work in compression, which is detrimental for profiles characterized by high slenderness which makes them particularly susceptible to buckling phenomena.  

For configuration no. 1, after only 6 minutes of exposure to fire, thermal expansion exceeds the length of the slotted holes altering the structure’s static scheme, and the profile reaches a buckling condition after 9 minutes. The upright thermal expansion causes the lower slabs to slide downwards (see Fig.3), reaching after 15 minutes the tile retaining clip length resulting in an incipient collapse condition. Tile retaining clips, although directly exposed to flames, showed an excellent mechanical behaviour as they are generally made of stainless steel. After 22 minutes of exposure to the fire, the profile presents significant deformations, and it’s permanently compromised. 

In configuration no. 2, although the upright’s groove is supposed to accommodate thermal expansions without altering the static scheme, the over-friction generated between bolts and the inside of the groove results in an axial profile compression and in buckling effects after 9 minutes. While a larger thermal elongation capability helps in delaying buckling effects, it also results in relevant deformations that make the upper tiles slide off and eventually fall after 21 minutes. 

Referring to configurations no. 3, in which tiles are supported by brackets accommodated in kerf cuts, the system’s behaviour is different. Deformations alter the static scheme in a few minutes, leading to flexural buckling and resulting in a critical profile’s deformation which tends to dislocate the cladding elements and induces significant stress particularly around the kerf cut, which is clearly the weakest point (see Fig. 4). The high local stress eventually leads to the breakage of tiles at the constraint points and to their fall (see Fig. 5). 

For configurations no. 4 and 5, alike the latter but in presence of different upright’s alloys, the outcome is basically the same but delayed since the steel thermal expansion coefficient is about 30 to 50% lower than aluminium. The profile is subjected to axial compression only after 15 minutes and the failure is not reached due to the decrease in the resistance of steel, but rather the increase of stress induced by thermal expansion.  

In conclusion, it can be stated that: 

• Aluminium substructures are extremely sensitive to the action of fire, both due to the quick loss of mechanical properties and the significant thermal expansions;
• Steel presents a better mechanical behaviour but is also affected by buckling phenomena induced by thermal expansion. The behaviour of stainless steel is generally comparable to that of carbon steel;
• Cladding panels are the weak element of the system, and their retainers play a crucial role.
• The following should certainly be considered as the main design strategies to be further investigated:
• Providing a substructure with mullions and transoms which offers a greater number of cladding panels retainers and allows to subdivide the whole cladding and the substructure along the height;
• The use of safety mesh glued onto the back of the cladding panels to conserve their integrity in case of cracking;
• Consider the use of additional safety retainment systems to avoid detached panels to fall (e.g. steel cables). 

New regulations certainly open to encouraging scenarios, as they’re limiting the use of combustible materials on façades and aiming for passive protection strategies. Reaction to fire of materials represents a fundamental aspect but it’s not the only one. Structural resistance in case of fire is also worthy of greater attention from the legislator.  

At the moment some proposals and reports, although preliminary to the development of a European regulation on the behaviour of façades in case of fire, expressly focus on the issue of debris possibly detaching and falling to the ground, defining limitations in terms of weight and of compromised surface, with the primary intent of protecting human life.