Mass Timber and Seismic Risk: Evaluating CLT Performance in Chile

Title, Authors, and Publication

The paper, titled "Collapse Fragility of a 5-Story CLT Structure Under Chilean Subduction Earthquake Records," was authored by Alberto Aravena, Alan Hellman, Joaquín Miranda, Franco Silva, and Diego Valdivieso. It was presented at the World Conference on Timber Engineering (WCTE) 2023 in Norway.

Objective and Background

Cross-Laminated Timber (CLT) is increasingly used for mid-rise and high-rise construction as a sustainable alternative to concrete and steel. However, in high-seismic regions like Chile, there is limited research on the seismic performance of CLT structures. This study assesses the collapse probability of a government-subsidized 5-story CLT building under Chilean subduction earthquake records, applying the Performance-Based Earthquake Engineering (PBEE) framework.

Given Chile’s high seismicity, it is crucial to evaluate whether CLT buildings can meet seismic safety standards and how their structural connections influence their collapse fragility. The study employs Probabilistic Seismic Hazard Analysis (PSHA), Nonlinear Time History Analysis (NLTHA), and Multiple Stripe Analysis (MSA) to quantify the collapse risk of CLT buildings.

Introduction

Chile, located on the Nazca-South American subduction zone, frequently experiences megathrust earthquakes. Although CLT construction is growing in Latin America, it lacks formal seismic design codes tailored for this material. Existing studies from North America and Europe highlight the nonlinear behavior of CLT-to-CLT connections, which play a key role in energy dissipation during earthquakes.

This research aims to:

1. Quantify the collapse probability of a 5-story CLT building under Chilean earthquake conditions.
2. Compare numerical modeling approaches (2D vs. 3D finite element analysis) in predicting structural response.
3. Assess the impact of seismic drift limits and design constraints on CLT performance.

Methodology

1. Building Model and Design Assumptions The building follows government-subsidized housing criteria in Chile. It has five stories (2.6 m per floor), CLT walls, and floors made of 3-ply CLT (120 mm thickness). Structural stability relies on monolithic CLT walls, hold-downs, and shear keys as seismic force-resisting elements.
2. Numerical Modeling Approaches 2D planar model: Represents a simplified elevation. 3D finite element model: Captures the full structural system with wall-to-wall and hold-down connections. Models were developed in OpenSeesPy, using shell elements for CLT panels and nonlinear springs for shear keys and hold-downs.
3. Seismic Hazard Characterization Probabilistic Seismic Hazard Analysis (PSHA) determined site-specific ground motion parameters. Ground motion records were selected based on the Conditional Mean Spectrum (CMS), ensuring consistency with Chilean seismic conditions.
4. Collapse Fragility Analysis Multiple Stripe Analysis (MSA) estimated collapse probabilities at different seismic intensities. Collapse was defined based on exceedance of inter-story drift limits (0.2%) and loss of lateral stability.

Key Findings

1. CLT Buildings Have a Low Probability of Collapse Under Chilean Earthquakes The 3D model predicts a collapse probability of <0.1%, suggesting that well-designed CLT buildings can withstand major earthquakes. The 2D model underestimates collapse resistance, highlighting the importance of full 3D modeling for CLT seismic assessment.
2. Seismic Drift Limits Are Critical in CLT Performance Chilean design code limits inter-story drift to 0.2%, ensuring minimal earthquake-induced deformation. The study confirms that adhering to this drift limit significantly enhances seismic resilience.
3. Base Connections Are the Most Vulnerable Elements Shear keys at the base experience the highest forces and deformations, suggesting that reinforcement at these points is critical. The use of tension rods or alternative base connections could improve seismic performance.
4. 2D vs. 3D Models Show Different Collapse Predictions The 2D model collapses at lower intensity measures (IMs) than the 3D model, meaning simplified 2D models do not accurately capture CLT system effects. The 3D model more realistically distributes seismic forces across all structural components.

Conclusion

This study demonstrates that properly designed CLT buildings meet Chilean seismic safety standards, with a collapse probability of less than 0.1%.

• Seismic drift limits play a critical role in preventing collapse.
• Numerical models should use full 3D representations to avoid underestimating structural capacity.
• Seismic performance of CLT hinges on well-designed base connections, as shear keys are the weakest link.

The findings support the expansion of mass timber construction in Chile, but further refinements are needed to optimize design codes for CLT seismic resilience.

Future Work and Applications

1. Developing Seismic Fragility Curves for CLT Components Establishing damage thresholds for CLT walls, connections, and non-structural elements.
2. Evaluating Loss Estimation in CLT Buildings Assessing repair costs and downtime following earthquakes.
3. Investigating Hybrid CLT Systems for Enhanced Seismic Performance Combining CLT with steel or reinforced concrete to improve energy dissipation.
4. Improving CLT Base Connection Designs Testing alternative foundation detailing to reduce stress concentrations at shear keys.
5. Adapting CLT Seismic Design Guidelines for South America Establishing CLT-specific seismic codes for high-risk seismic regions.

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