A critical review by Nikola Perković, Chiara Bedon, Jure Barbalić, and Vlatka Rajčić

In the evolving world of façade engineering, the conversation around fire safety is growing more urgent — and complex. As building envelopes become more intricate, more layered, and more experimental, their behavior in fire conditions reveals a gap between architectural ambition and regulatory comprehension.

This is the challenge addressed by Nikola Perković, Chiara Bedon, Jure Barbalić, and Vlatka Rajčić, authors of the in-depth study “Study of Fire Resilience Challenges to Promote the Structural Use of Load‐Bearing Composite Timber‐Glass Walls: Experimental and Numerical Analysis.”

The research explores the behavior of innovative timber-glass composite systems under fire exposure, offering rigorous testing, detailed simulations, and comprehensive recommendations for the future of hybrid structural façades.

Rethinking Fire Performance in Transparent Structures

As architecture shifts toward more transparent, lightweight, and sustainable façades, the use of materials such as cross-laminated timber (CLT) and laminated glass (LG) has become increasingly attractive. These materials support carbon-neutral design and can enable elegant, high-performance buildings. However, their use as load-bearing structural elements—particularly in composite configurations—raises complex questions about fire performance.

Unlike traditional reinforced concrete or steel walls, timber-glass composites behave dynamically under heat: they degrade unevenly, transfer stresses unpredictably, and respond to fire in ways that challenge current modeling standards.

The very advantages that make timber and glass sustainable and expressive — renewability, transparency, low weight — become potential vulnerabilities in fire conditions.

The study's central objective was to address this critical challenge: Can we rely on composite timber-glass walls not only to support vertical loads but also to resist the progressive damage induced by real fire scenarios?

To answer this, the research team undertook both experimental full-scale fire tests and nonlinear numerical simulations, focusing on how these systems perform structurally and thermally under standardized and realistic fire conditions.

It is one of the first efforts to fuse laboratory experimentation with calibrated digital modeling for this specific hybrid configuration.

The authors also stress the context of use: These composite systems are particularly relevant for schools, mid-rise housing, pavilions, and civic buildings where visual transparency, natural materials, and environmental credentials are as important as structural reliability.

The Experimental Setup: Full-Scale Under Fire

The core of the investigation involved a full-scale load-bearing wall prototype constructed with CLT and laminated safety glass, bonded together using structural adhesives without mechanical anchors. The experimental configuration featured:

• CLT panel as the primary vertical load-bearing component
• Annealed laminated glass (LG) bonded continuously to timber
• Epoxy adhesive layer, without fire retardants, mimicking realistic construction practice

The furnace test, based on EN 1365-1 (load-bearing walls) and EN 1363-1 (general fire testing), subjected the composite wall to controlled ISO 834 standard temperature curves, reaching temperatures of over 800°C within the first 20 minutes.

Key Monitoring Parameters:

• Internal temperatures in timber, glass, and adhesive joints
• Vertical and lateral displacements
• Cracking, delamination, adhesive failure initiation
• Charring depth over time (tracked through embedded sensors and post-test sectioning)

The setup included precise displacement sensors and thermocouples, enabling the team to capture the complex interplay of degradation phenomena and their impact on global stability.

Video recordings and thermal imaging complemented sensor data to provide a full-spectrum understanding of failure sequences.

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Failure Observations:

1. Adhesive Detachment: At temperatures exceeding 120°C, the adhesive began softening, leading to a loss of composite interaction between timber and glass. This bond deterioration was not uniform — it initiated near the hottest zones and propagated non-linearly.
2. Glass Delamination and Breakage: Laminated glass, while resisting initial temperature rise, experienced interlayer weakening and subsequent pane cracking. Delamination reduced stiffness and caused asymmetric force redistribution, amplifying strain on the CLT panel.
3. Charring and Section Reduction: Timber charring followed predictable patterns, reducing the effective section thickness while increasing nonlinear strain paths in the panel. Char depths of 35–40 mm were observed at failure.
4. Combined Failure Mode: Rather than a sudden collapse, failure emerged through a sequence of interrelated degradations: adhesive yielding, glass cracking, timber charring, and finally buckling due to eccentric loads.

Despite these degradations, the wall sustained its load-bearing function for approximately 60 minutes, marginally exceeding standard REI-60 benchmarks.

The structural system showed inherent resilience due to its redundancy — a hallmark of robust hybrid design.

Finite Element Simulations: A Deeper Computational Layer

The numerical modeling employed the ABAQUS and SAFIR platforms to replicate the test scenario with calibrated material behavior.

The digital twin included:

• Thermal conduction modeling through multi-layered façade profiles
• Time-dependent stiffness degradation of adhesives
• Delamination modeling at the glass interlayers
• Progressive charring modeled with Eurocode 5 equations

_{Model Highlights:}

• Bond failure was simulated through cohesive zone models for the epoxy, capturing softening and eventual rupture.
• Glass was modeled with temperature-sensitive modulus reduction, reflecting loss of rigidity above 300°C.
• Timber was divided into active and charred layers, using temperature-activated element deletion to simulate loss of section.

_Validation:

Simulations aligned closely with physical outcomes:

• Predicted failure time was within 3 minutes of the experimental collapse
• Strain paths and deformation patterns matched test records
• Stress fields highlighted the non-intuitive load redistribution after glass detachment

_{Parametric Variants:}

Researchers explored:

• Improved adhesives with fire-resistant performance (e.g., silicone-based or intumescent-modified)
• Use of triple-laminated glass with asymmetric plies to delay cracking
• Increased CLT thickness for charring redundancy

These changes resulted in simulated performance improvements of up to 30 minutes, suggesting that composite façade walls could reach REI-90 or REI-120 ratings with appropriate detailing.

Furthermore, a sensitivity analysis was performed to evaluate how construction tolerances, such as bonding uniformity and surface flatness, influence thermal degradation and deformation modes — underscoring the role of craftsmanship in fire resilience.

Architectural and Regulatory Implications

The study doesn't just deepen understanding—it shifts the conversation about what’s possible.

_{Design-Level Impacts:}

• Reconsideration of adhesive choice: The bondline is the first vulnerability and must be engineered for high-temperature exposure.
• Hybrid redundancy design: Systems must assume progressive failure modes and be designed for residual strength.
• Facade-integrated safety design: Rather than relying solely on internal suppression systems, the façade must act as a protective skin.
• Detailing matters: The precision of adhesive application, edge protection for glass, and spacing of timber joints all contribute to performance.

_{Code-Level Implications:}

• New testing standards may be needed for bonded hybrid wall systems beyond BS 8414 and NFPA 285.
• Updates to Eurocode 5 and Eurocode 9 are recommended to include glass-timber-adhesive assemblies.
• Adoption of performance-based fire engineering must become routine for complex façade geometries.

These recommendations could influence future international guidelines and push forward innovations in digital permitting and parametric code compliance tools.

A Blueprint for Resilient Transparency

The combined expertise of the authors — rooted in structural engineering, façade science, and fire mechanics — is evident in the rigor of the study. Their work provides an indispensable resource for:

• Structural engineers designing with CLT or LG
• Architects pushing the boundaries of transparency and lightness
• Fire safety consultants tasked with mitigating envelope-driven risk
• Façade consultants navigating the intersection of aesthetics, performance, and regulation

This isn’t just a research paper — it’s a blueprint. It offers a vision where material innovation and fire resilience evolve in tandem, rather than in opposition. In a world where façades increasingly define buildings, this work urges us to treat them not just as skins — but as active systems that protect, perform, and endure.

The authors conclude with a call for interdisciplinary dialogue: future fire-safe façades will not come from materials alone, but from collaboration between engineers, architects, chemists, and regulators.

Citation:
_{Perković, N., Bedon, C., Barbalić, J., & Rajčić, V. Study of Fire Resilience Challenges to Promote the Structural Use of Load‐Bearing Composite Timber‐Glass Walls: Experimental and Numerical Analysis. 2020.}

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