An approach to integrated fire safety design of mass timber apartment buildings

Abstract

Based on previous studies, the Dutch Building Regulation (BBL) is inadequately equipped to address fire safety in timber-structured buildings, particularly when the timber is exposed and in the context of tall buildings. Fire compartments containing large volumes of structural timber suffer from additional fire safety risks compared to traditional buildings without a combustible structure. These extra risks include: a longer fire duration, unpredictable fire scenario, potential second flash-over, failure of timber members due to heat wave-penetration, larger protruding flames from openings and increased smoke production (Brandon et al., 2022). This master thesis focuses on studying fire safety of CLT (Cross Laminated Timber) compartments with properties typical for apartment buildings. The master thesis contains a literature study, divided into material level, structural component level, compartment level and building level. This literature study can be helpful for people who want to understand the fundamentals of fire safety in contemporary CLT timber buildings and want to understand the further master thesis with more ease.
In this master thesis we used three fire behaviour models to analyse compartment fires previously tested in full scale by Brandon et al. (2021a). The outcomes of simulations done with these models were compared to full-scale compartment fire test data (for a compartment of 48 square meters) to get an understanding of the accuracy, and strengths and weaknesses of the models. The study suggests that the ZHM and Brandon models are able to simulate the average char depth in the ceiling. For compartments that have percentage of exposed mass timber and an opening factor equal to that in test 1 of Brandon et al. (2021a), the outcomes of the Brandon and the ZHM models give conservative results for the the maximum char depth in the ceiling. There is a discrepancy between the simulation results and the maximum char depth in the walls when amount of exposed timber and opening factor were increased beyond the values used in test 1 of Brandon et al. (2021a). Suggestions for correction factors based on linear interpolation are given to arrive at a better approximation for the maximum char depth. It is advised to use CFD and/or compartment fire tests for any compartments that differ from tests 1-5 in Brandon et al. (2021a). Targeted design measures were listed to protect the critical areas where maximum char depth is expected.
In order to get a better understanding of the fire behavior models for CLT compartments, I simulated 24 different realistic compartments, with differing sizes, shapes, amounts of exposed timber and opening factors. The parameters were chosen in such a way, that the variations encompass a range of typical CLT apartment buildings. I performed this analysis using three different fire behaviour models. The models calculate the charring depth into the CLT cross-section. We then added the zero strength layer (timber without any structural resistance due to the heat-wave) to the modelled charring depth, resulting in the effective char depth. The larger the effective char depth, the more of the cross-section’s initial resistance is lost. Over the three models, the largest correlation was found to be between opening factor and effective depth. The second largest correlation was found to be between amount of exposed timber and effective depth. Using the results of the models, without any correction for maximum char depth, none of the simulated compartments suffer failure of the ceiling panel according to calculation.
Between the three models, the zone model (Brandon et al., 2021b), is best capable of simulating protection for a finite number of minutes, as the exact number and thickness of the protection can be input in the model.