Computational Analysis of Cross-Laminated Timber (CLT) Under Elevated Temperatures

Technical Summary: Computational Analysis of Cross-Laminated Timber (CLT) Under Elevated Temperatures

Problem Statement
Cross-Laminated Timber (CLT) is experiencing exponential growth in construction due to its sustainability, dimensional stability, and prefabrication advantages. However, the lack of comprehensive understanding regarding its thermomechanical behavior under fire conditions hinders its widespread application. While wood naturally forms a protective char layer that insulates the inner core, the performance of CLT is complex due to its orthogonally layered structure, adhesive properties, and the use of metallic connectors (screws, nails) which can act as thermal bridges. When exposed to high temperatures, metallic connections lose resistance, potentially compromising the entire structural system. Current design standards, such as Eurocode 5, provide calculation methods for carbonization but require further validation regarding the interaction between varying layer thicknesses, heat transfer, and connection integrity.

Methodology
This study employed computational simulation using Ansys software to analyze the thermomechanical behavior of CLT floor-to-floor panels under fire exposure. The research utilized the Finite Element Method (FEM) with the following parameters:

• Specimens: Three panel configurations (P1, P2, P3) with 3, 5, and 7 layers, respectively. Total thicknesses were 10 cm, 16 cm, and 22 cm. The panels were modeled using Pinus spp. (Strength Class C24) with polyurethane-based adhesives.
• Geometry and Loading: Panels measured 40x60 cm. A uniformly distributed load (accounting for self-weight and accidental loads) was applied, varying slightly by panel thickness (6.19 to 6.86 kN/m²).
• Connectors: HBS ø 10TX80 screws were used, spaced at 10 cm intervals, with bonded contact assumed between the steel and wood to simulate mechanical interaction and heat transfer.
• Fire Exposure: Simulations followed the ISO 834-1 standard temperature-time curve. Panels were exposed to temperatures ranging from 200°C to 1200°C for a duration of 60 minutes (3600 seconds).
• Analysis Stages: A two-stage sequential analysis was performed: first, a transient thermal analysis to determine temperature distribution; second, a mechanical analysis incorporating the resulting temperature field to evaluate displacements and stresses.
• Monitoring: Nine nodal points were defined across the panel thickness (external, internal, and central) to track temperature gradients and heat penetration.

Key Results

• Thermal Penetration and Carbonization: As temperature increased, heat transfer to the interior of the cross-section was observed. The 3-layer panel (P1) exhibited significant heat penetration, with internal temperatures approaching the carbonization threshold (approx. 300°C) at the 45-minute mark. In contrast, the 5-layer (P2) and 7-layer (P3) panels showed limited thermal evolution; the charred outer layers effectively insulated the inner core, keeping internal temperatures significantly lower (e.g., P3 internal points remained below 70°C even at 1200°C external exposure).
• Connector Temperatures: The metallic screws acted as thermal bridges, but the number of surrounding layers significantly influenced their temperature. In P1, screw temperatures reached a maximum of 162.3°C. In P2 and P3, maximum screw temperatures were 126.3°C and 175.9°C respectively (noting P3's highest value occurred at the tip closest to the fire, though the text indicates P1 generally had higher internal interference). Crucially, in all cases, screw temperatures remained below the 200°C threshold often cited as the limit for maintaining connector properties, though the study notes that indirect thermal effects can still influence mechanical capacity.
• Displacements: Under fire-induced loading, all panels exhibited linear displacement growth over time. P1 (3 layers) demonstrated the highest deflection, displacing 6.3% more than P2 and 8.2% more than P3. The maximum vertical displacement for P1 was 6.15x10⁻¹ mm, while P3 showed the least movement (5.33x10⁻¹ mm).
• Screw Movement: Horizontal and vertical displacements of the screws were minimal due to the fixed support conditions. Maximum horizontal variations were on the order of 10⁻² mm. Vertical displacements showed a "pull-out" effect for outer screws and inward movement for inner screws due to bending stresses, with P1 again showing the largest deflections.

Significance and Conclusions
The study concludes that increasing the thickness and number of layers in CLT panels significantly improves thermomechanical performance under fire conditions. Thicker configurations (5 and 7 layers) provide superior insulation, delaying heat transfer to the core and protecting metallic connections from reaching critical temperatures that would compromise structural integrity.

The research validates that while CLT panels undergo section reduction due to carbonization, the multi-layered structure maintains structural stability better than thinner configurations. The findings suggest that thicker CLT elements reduce the degradation of wood stiffness and strength during fire exposure and delay the onset of failure in connection zones. However, the authors maintain that the thermal response remains highly dependent on the intensity and duration of the fire exposure. The study reinforces the utility of computational simulation in predicting these behaviors, offering data that aligns with experimental observations regarding the insulating properties of charred layers and the protective role of increased panel thickness.