The last decades have seen a surge in the construction of tall buildings all over the world. Due to their, often, innovative and complex layouts, tall buildings can pose unique challenges to architects and engineers. Previous tall building failures raised significant concerns on the applicability of prescriptive fire design for these structures. The use of structural fire engineering can enhance the safety of a tall building under fire by strengthening any vulnerable areas in the structure and at the same time reduce the costs of fire protection by removing it when unnecessary. Commercial finite element and specialist structural fire engineering software have their advantages and disadvantages. In this thesis, the object-oriented and open-source finite element software OpenSees is presented along with its development with structural fire capabilities by the author and other researchers at the University of Edinburgh. Specifically, new pattern, element, section and material classes have been introduced. All the developed code follows the object-oriented paradigm and is consistent with the ethos of the existing framework. Verification and validation studies of the developed code are also presented. Several procedures including that for dynamic analysis of structures in fire for the collapse assessment of structures are discussed. The development of OpenSees with structural fire capabilities allows the collaboration of engineers across geographical boundaries and disciplines using a community tool. In this work, the behaviour of tall buildings under different fire scenarios has been modelled using the developed OpenSees code. Firstly, the collapse mechanisms of generic tall buildings are investigated, namely the strong and weak floor mechanisms are demonstrated, and criteria are established on when each of these mechanisms occurs. The parametric study performed demonstrated that the weak floor collapse is less probable for generic composite buildings however this type of failure can become easier to appear as the number of floors in fire increase. The effect of vertically travelling fires on these mechanisms is also examined. The results of the study show that slower travelling rates delay or avoid the global failure of a tall building compared to quicker travelling rates allowing for the time required for steel members to regain their strength during cooling to ambient temperature. However, it was seen that higher tensile membrane forces were observed in the floors as the travelling rates increased which could result in possible connection failure. Most of the research and design codes, such as Eurocode, typically assume a uniform thermal environment across the floor area of a structure when defining the design fire. However, in reality fires are more likely to travel in large enclosures, hence there is a need to understand how tall buildings behave under more realistic fire conditions such as travelling fires. A methodology for defining the thermal environment of large enclosures using travelling fires has been recently developed at the University of Edinburgh. Taking into account OpenSees' programmable architecture and its recent inclusion with heat transfer capabilities by other researchers, there was a collaborative effort in order to understand the thermal and structural response of a generic composite tall building under horizontally travelling fires. The findings of the study showed that larger travelling fire sizes produce quicker heating to the steel beams while smaller fire sizes produce higher peak temperatures in the concrete slab. The structural analysis also demonstrated that travelling fires produced higher midspan deflections in comparison to Eurocode parametric fires and higher plastic deformations which is an indication of higher damage. Further work focused on looking at the behaviour of tall buildings under the combined scenario of horizontally and vertically travelling fires. The results of the study showed that the travelling fires produce lower maximum compressive and tensile membrane forces in the composite floor compared to the Eurocode parametric fires for the building examined and thus in a multi-floor scenario the columns are pulling-in less after large deflections develop in the floor. More specifically, the short-hot fire produced the most demanding response. This suggests that in long floors where uniform heating is really impossible, the time of failure predicted by parametric fires in a multi-floor scenario can be more onerous. The outcomes of this work can aid designers when considering the structural fire response of tall buildings in a performance based design context. It was demonstrated that multi-floor fires could be a threat for tall buildings, and thus this possibility should be considered in design. The use of more realistic fire definition for large enclosures, such as travelling fires, should also be considered. The travelling fire methodology can provide an enhanced level of confidence for the safety of a building since it can represent a range of similar fires to those that may occur in a real fire scenario.

The current design methods used to determine fire progression and temperature-time development in fire compartments today are being questioned to not give accurate results in large and complex enclosures (larger than 500 m2). The established design methods proposed by Eurocode and used by fire safety engineers today are primarily the standard temperature-time curve and the parametric temperature-time curves. The parametric temperature-time curves are based on the heat and mass balance equations and both methods assume homogenous temperatures and uniform burning. These assumptions are being questioned for use in large enclosures such as open-plan compartments and compartments with multiple floors connected which are typically modern and common building types in today’s society. Today there are no established design methods developed to determine fire progression in large enclosures, but the Improved Travelling Fire Method (iTFM) and the New MT model II are new, alternative design methods which are prospects to become established engineering tools in the future. The iTFM is developed at the University of Edinburgh for travelling fires in large size compartments and the New MT model II is developed by RISE, Research Institutes of Sweden, for large tunnel fires. These two new design methods have been investigated and compared to established methods in a case study. Also localised fires from Eurocode with proposed interpretations by Ulf Wickström has been investigated and compared to the standard temperature-time curve and the parametric temperature-time curves. The new interpretation suggests that the given heat flux boundary conditions in Eurocode are interpreted as adiabatic surface temperatures based on given emissivities and convection heat transfer coefficients according to Eurocode. Through a case study the different methods were compared throughout reference buildings with constant material properties and fire loads, but with varying floor area and height. The result focused on if the new methods have more bearing on reality than the standard fire curve and the parametric temperature-time curves methods when determining fire progression and temperature-time development. Desired benefits with the new methods are to better predict and describe fire development in large enclosures. The referenceIIIbuildings were considered as occupancy class 2 (Vk2) and Br2 buildings with a load bearing fire resistance capacity demand of 30 minutes. This report is an early stage in the process of developing new fire models to improve the fire designing process when working with large compartments. The aim with the new, alternative methods and localised fires with proposed interpretation is to enable them to become engineering tools used by fire safety engineers in the future to create a more efficient and adapted design process. The results differ significantly depending on used method and reference building. The maximum temperatures conducted by the iTFM are in general higher than the standard fire curve and the parametric temperature-time curves. When applying the method to the reference building with high ceiling height and low spread rate the resulting temperatures were lower than the standard fire curve. The fire progression of the New MT model II is highly dependent on opening factor and time until temperature increase starts. In comparison to the parametric fire curves with the same opening factors the New MT model II resulted in considerably faster temperature development and higher temperatures. Localised fires with the new proposed interpretations resulted in adiabatic surface temperatures which were compared to the standard temperature-time curve after 30 minutes of fire and the maximum temperature of the parametric temperature-time curves. The comparison resulted in slightly lower temperatures for the localised fires with the new proposed interpretations compared to the standard temperature-time curve and similar temperatures compared to the parametric temperature-time curves in the case study. The results of the iTFM and the New MT model II differs significantly depending on physical parameters used in the calculation processes. The models are customizable and vary depending on fire scenarios and compartments and could possibly be future alternative methods when designing for fires in large compartments. Further studies and development together with real fire tests would provide the models with better accuracy and continuity. Localised fires with proposed new interpretations are a future prospect to become a future standard method for determination of maximum temperature of member surfaces in fire safety design.

Many studies of the fire induced thermal and structural behaviour in large compartments, carried out over the past two decades, show a great deal of non-uniformity, unlike the homogeneous compartment temperature assumption in the current fire safety engineering practice. Furthermore, some large compartment fires may burn locally and they tend to move across entire floor plates over a period of time as the fuel is consumed. This kind of fire scenario is beginning to be idealized as 'travelling fires' in the context of performance‐based structural and fire safety engineering. However, the previous research of travelling fires still relies on highly simplified travelling fire models (i.e. Clifton's model and Rein's model); and no equivalent numerical tools can perform such simulations, which involves analysis of realistic fire, heat transfer and thermo-mechanical response in one single software package with an automatic coupled manner. Both of these hinder the advance of the research on performance‐based structural fire engineering. The author develops an extended travelling fire method (ETFM) framework and an integrated comprehensive tool with high computational expediency in this research, to address the above‐mentioned issues. The experiments conducted for characterizing travelling fires over the past two decades are reviewed, in conjunction with the current available travelling fire models. It is found that no performed travelling fire experiment records both the structural response and the mass loss rate of the fuel (to estimate the fire heat release rate) in a single test, which further implies closer collaboration between the structural and the fire engineers' teams are needed, especially for the travelling fire research topic. In addition, an overview of the development of OpenSees software framework for modelling structures in fire is presented, addressing its theoretical background, fundamental assumptions, and inherent limitations. After a decade of development, OpenSees has modules including fire, heat transfer, and thermo‐mechanical analysis. Meanwhile, it is one of the few structural fire modelling software which is open source and free to the entire community, allowing interested researchers to use and contribute with no expense. An OpenSees‐based integrated tool called SIFBuilder is developed by the author and co‐workers, which can perform fire modelling, heat transfer analysis, and thermo-mechanical analysis in one single software with an automatic coupled manner. This manner would facilitate structural engineers to apply fire loading on their design structures like other mechanical loading types (e.g. seismic loading, gravity loading, etc.), without transferring the fire and heat transfer modelling results to each structural element manually and further assemble them to the entire structure. This feature would largely free the structural engineers' efforts to focus on the structural response for performance-based design under different fire scenarios, without investigating the modelling details of fire and heat transfer analysis. Moreover, the efficiency due to this automatic coupled manner would become more superior, for modelling larger structures under more realistic fire scenarios (e.g. travelling fires). This advantage has been confirmed by the studies carried out in this research, including 29 travelling fire scenarios containing total number of 696 heat transfer analysis for the structural members, which were undertaken at very modest computational costs. In addition, a set of benchmark problems for verification and validation of OpenSees/SIFBuilder are investigated, which demonstrates good agreement against analytical solutions, ABAQUS, SAFIR, and the experimental data. These benchmark problems can also be used for interested researchers to verify their own numerical or analytical models for other purposes, and can be also used as an induction guide of OpenSees/SIFBuilder. Significantly, an extended travelling fire method (ETFM) framework is put forward in this research, which can predict the fire severity considering a travelling fire concept with an upper bound. This framework considers the energy and mass conservation, rather than simply forcing other independent models to 'travel' in the compartment (i.e. modified parametric fire curves in Clifton's model, 800°C‐1200°C temperature block and the Alpert's ceiling jet in Rein's model). It is developed based on combining Hasemi's localized fire model for the fire plume, and a simple smoke layer calculation by utilising the FIRM zone model for the areas of the compartment away from the fire. Different from mainly investigating the thermal impact due to various ratios of the fire size to the compartment size (e.g. 5%, 10%, 25%, 75%, etc.), as in Rein's model, this research investigates the travelling fire thermal impact through explicit representation of the various fire spread rates and fuel load densities, which are the key input parameters in the ETFM framework. To represent the far field thermal exposures, two zone models (i.e. ASET zone model & FIRM zone model) and the ETFM framework are implemented in SIFBuilder, in order to provide the community a 'vehicle' to try, test, and further improve this ETFM framework, and also the SIFBuilder itself. It is found that for 'slow' travelling fires (i.e. low fire spread rates), the near‐field fire plume brings more dominant thermal impact compared with the impact from far‐field smoke. In contrast, for 'fast' travelling fires (i.e. high fire spread rates), the far‐field smoke brings more dominant thermal impact. Furthermore, the through depth thermal gradients due to different travelling fire scenarios were explored, especially with regards to the 'thermal gradient reversal' due to the near‐field fire plume approaching and leaving the design structural member. This 'thermal gradient reversal' would fundamentally reverse the thermally‐induced bending moment from hogging to sagging. The modelling results suggest that the peak thermal gradient due to near‐field approaching is more sensitive to the fuel load density than fire spread rate, where larger peak values are captured with lower fuel load densities. Moreover, the reverse peak thermal gradient due to near‐field leaving is also sensitive to the fuel load density rather than the fire spread rate, but this reverse peak value is inversely proportional to the fuel load densities. Finally, the key assumptions of the ETFM framework are rationalised and its limitations are emphasized. Design instructions with relevant information which can be readily used by the structural fire engineers for the ETFM framework are also included. Hence more optimised and robust structural design under such fire threat can be generated and guaranteed, where we believe these efforts will advance the performance‐based structural and fire safety engineering.