Fire dynamics in multi-scale timber compartments

Abstract

The modern building industry is continually seeking materials and products that are less pollutant, stronger, more aesthetically appealing and easier to construct with. As a response to these drivers, Engineered Wood Products (EWP) have entered the construction market as an evolution of very well-known conventional products such as plywood. However, contrary to plywood, EWP consist of much thicker layers and contemporary glues. New manufacturing technologies enable the use of EWP on an entirely different scale. Today, wooden high-rise buildings, bridges, and other macro-structures can effectively be designed.Despite the multiple benefits of timber construction, new fire safety hazards have sparked with the arrival of EWP. These hazards correspond to increased fuel load density and the potential for structural collapse since timber structures are combustible. These changes challenge the fire safety strategy for timber buildings.The purpose of any fire safety strategy is to ensure a safe evacuation and operation of a building during and after a fire event. Most fire strategies rely on compartmentalisation (physical boundaries that restrict fire spread to other parts of the building) and on a robust structure that maintains its integrity after all the fuel has burned out. Compartment fire dynamics and external flaming are key to understanding how compartmentalisation can be achieved in timber buildings. To ensure compartmentalisation and structural integrity, it is fundamental that the timber walls and floors stop burning, i.e. self-extinguish. Charring materials such as timber have the ability to self-extinguish without external intervention under certain circumstances.This research aims at investigating the effect of exposed timber on the compartment fire dynamics and the external flaming with respect to the current fire dynamics’ theory for buildings with non-combustible structures. This thesis also aims to study the mechanisms that lead to self-extinguishment of timber in compartments with a geometry representative of residential construction. For this purpose, three experimental campaigns using Cross Laminated Timber (CLT) were conducted. The first consisted of medium-scale compartment fires with varying configurations of exposed CLT with a kerosene pool fire as the fire source. The use of kerosene as the fuel facilitated the characterisation of the fire dynamics. The second consisted of medium-scale compartment fires with two exposed CLT walls and different densities of wood cribs as the fire source. The use of wood cribs as the fuel allowed for a progressive and long decay phase to be achieved. The third consisted of a large-scale demonstration test to enable a scaling analysis and validation of the results from the medium-scale tests.The most novel finding from this research is that the burning of the internal CLT surfaces induces additional momentum-driven flows, thus altering the energy balance in the compartment. As a result, it was found that the commonly-used compartment fire framework developed by Thomas et al., which relates temperatures and fire regimes to the compartment geometry only, is not applicable. The location of the exposed timber surfaces is shown to be relevant for the fire behaviour; an exposed CLT ceiling always presents a lower burning rate than any other vertical wall.As in any under-ventilated fire, the excess of pyrolysis gases produced by the CLT walls is found to be unable to combust inside the compartment due to a lack of oxygen. These gases are forced to flow outside, producing a much larger external flame than those obtained in fires with non-combustible boundaries. This thesis proposes new models to predict the projection and thermal exposure of external flaming as a function of exposed CLT inside the compartment.Finally, design criteria are proposed to achieve self-extinguishment of timber in the context of compartment fires. These criteria are a function of the amount of exposed timber and its location within the compartment.To conclude, this thesis provides an insight into a new framework for compartment fires with exposed timber walls. This framework serves as input to engineering new fire safety strategies for this type of structures. The outcomes from this research indicate that by adding additional exposed timber surfaces the fire dynamics within the compartment experience a change in regime, the external flaming is more severe and self-extinguishment may or may not occur in time depending on the ratio of exposed timber and its configuration.