Coupled CFD-FEM Simulation of Steel Box Bridge Exposed to Fire

Increasing fire-induced bridge failures are demanding more precise behavior prediction for the bridges subjected to fires. However, current numerical methods are limited to temperature curves prescribed for building structures, which can misestimate the fire impact significantly. This paper developed a framework coupling the computational dynamics (CFD) method and finite element method (FEM) to predict the performance of fire-exposed bridges. The fire combustion was simulated in CFD software, Fire Dynamic Simulator, to calculate the thermal boundary required by the thermomechanical simulation. Then, the adiabatic surface temperatures and heat transfer coefficient were applied to the FEM model of the entire bridge girder. A sequential coupled thermomechanical FEM simulation was then carried out to evaluate the performance of the fire-exposed bridge, thermally and structurally. The methodology was then validated through a real fire experiment on a steel beam. The fire performance of a simply supported steel box bridge was simulated using the proposed coupled CFD-FEM methodology. Numerical results show that the presented method was able to replicate the inhomogeneous thermomechanical response of box bridges exposed to real fires. The girder failed due to the buckling of a central diaphragm after the ignition of the investigated tanker fire in no more than 10 min. The framework presented in this study is programmatic and friendly to researchers and can be applied for the estimation of bridges in different fire conditions.

Experimental study on fire resistance of a full-scale composite floor assembly in a two-story steel framed building

PurposeThe purpose of this paper is to report the first of four planned fire experiments on the 9.1 × 6.1 m steel composite floor assembly as part of the two-story steel framed building constructed at the National Fire Research Laboratory.Design/methodology/approachThe fire experiment was aimed to quantify the fire resistance and behavior of full-scale steel–concrete composite floor systems commonly built in the USA. The test floor assembly, designed and constructed for the 2-h fire resistance rating, was tested to failure under a natural gas fueled compartment fire and simultaneously applied mechanical loads.FindingsAlthough the protected steel beams and girders achieved matching or superior performance compared to the prescribed limits of temperatures and displacements used in standard fire testing, the composite slab developed a central breach approximately at a half of the specified rating period. A minimum area of the shrinkage reinforcement (60 mm2/m) currently permitted in the US construction practice may be insufficient to maintain structural integrity of a full-scale composite floor system under the 2-h standard fire exposure.Originality/valueThis work was the first-of-kind fire experiment conducted in the USA to study the full system-level structural performance of a composite floor system subjected to compartment fire using natural gas as fuel to mimic a standard fire environment.

Fire Experiment Inside a Very Large and Open-Plan Compartment: x-ONE

The traditional design fires commonly considered in structural fire engineering, like the standard fire and Eurocode parametric fires, were developed several decades ago based on experimental compartments smaller than 100 m2 in floor area. These experiments led to the inherent assumption of flashover in design fires and that the temperatures and burning conditions are uniform in the whole of the compartment, regardless of its size. However, modern office buildings often have much larger open-plan floor areas (e.g. the Shard in London has a floor area of 1600 m2) where non-uniform fire conditions are likely to occur. This paper presents observations from a large-scale fire experiment x-ONE conducted inside a concrete farm building in Poland. The objective of x-ONE was to capture experimentally a natural fire inside a large and open plan compartment. With an open-plan floor area of 380 m2, x-ONE is the largest compartment fire experiment carried out to date. The fire was ignited at one end of the compartment and allowed to spread across a continuous wood crib (fuel load ~ 370 MJ/m2). A travelling fire with clear leading and trailing edges was observed spreading along 29 m of the compartment length. The flame spread rate was not constant but accelerated with time from 3 mm/s to 167 mm/s resulting in a gradually changing fire size. The fire travelled across the compartment and burned out at the far end 25 min after ignition. Flashover was not observed. The thermocouples and cameras installed along the fire path show clear near-field and far-field regions, indicating highly non-uniform spatial temperatures and burning within the compartment. The fire dynamics observed during this experiment are completely different to the fire dynamics reported in small scale compartments in previous literature and to the assumptions made in traditional design fires for structural design. This highlights the need for further research and experiments in large compartments to understand the fire dynamics and continue improving the safe design of modern buildings.

Turbulent Momentum Flux Behavior above a Fire Front in an Open-Canopied Forest

Atmospheric turbulent circulations in the vicinity of wildland fire fronts play an important role in the transfer of momentum into and out of combustion zones, which in turn can potentially affect the behavior and spread of wildland fires. The vertical turbulent transfer of momentum is accomplished via individual sweep, ejection, outward interaction, and inward interaction events, collectively known as sweep-ejection dynamics. This study examined the sweep-ejection dynamics that occurred before, during, and after the passage of a surface fire front during a prescribed fire experiment conducted in an open-canopied forest in the New Jersey Pine Barrens. High-frequency (10 Hz), tower-based, sonic anemometer measurements of horizontal and vertical wind velocity components in the vicinity of the fire front were used to assess the relative frequencies of occurrence of the different types of momentum-flux events, their contributions to the overall momentum fluxes, and their periodicity patterns. The observational results suggest that the presence of surface fire fronts in open-canopied forests can substantially change the sweep-ejection dynamics that typically occur when fires are not present. In particular, sweep events resulting in the downward transport of high horizontal momentum air from above were found to be more prominent during fire-front-passage periods.

Fire Detection Factor of Polyurethane Foam Fire Experiment in UL 268

This study focused on fire detection factors by confirming the adaptation level of fire detection due to black smoke generated in the case of polyurethane foam fire, and it compared and analyzed the change in combustion products. An experiment was conducted to derive fire detection factors using the UL 268 polyurethane foam real fire test standard. Combustion products were measured using a particulate matter sensor (PMS), combustion gas analyzer (CGA), and gas analyzer (GA). As a result of the experiment, it was confirmed that the adaptability of the existing fire detector deteriorated when black smoke was generated. The fire detection factor was derived using the measured values of combustion products. In particular, among the measured factors, the most adaptable combustion products were found to be PM 2.5, PM 10, NO, and CO. They were considered to improve reliability when used as fire detection factors, as compared to existing fire detectors.

Development and validation of simple-shield thermocouple in fire environments

A novel simple-shield thermocouple that is capable of radiation correction in fire environments and that has a simpler structure and greater convenience than conventional aspirated thermocouples was proposed. The measurement errors of bare-bead, double-shield aspirated, and simple-shield thermocouples were then compared in a simulated fire environment. In addition, a fire experiment using wood cribs was performed in order to verify the measurement performance of the proposed simple-shield thermocouple in a real fire environment using a one-third-scale ISO 9705 room. The simple-shield thermocouple produced fairly accurate temperatures that fell within 5% of the actual gas temperature in the simulated fire environment. In addition, variations in the surface emissivity and the installation angle of the simple-shield thermocouple in the real fire environment further reduced the measurement error. With a radiant heat flux of 20 kW/m2, it was confirmed that the bare-bead thermocouple had a relative measurement error of up to 80% compared to the aspirated thermocouple, while the proposed simple-shield thermocouple was capable of measuring the temperature within a relative error of around 15% compared to the aspirated thermocouple.

Modern Regulatory Requirements for Gas Fire Extinguishing to Improve the Safety of Facilities

To implement the technical regulations of the Eurasian Economic Union «On the requirements for fire safety and fire extinguishing equipment» (TR EAEU 043/2017), modern regulatory requirements were developed based on the advanced domestic and foreign research in order to increase the efficiency of using automatic gas fire extinguishing installations. The domestic methodology of experimental determination of the gas extinguishing concentration in the fire chamber has a number of advantages in comparison with the foreign analogues. It is established that it is necessary to take into account the conditions of fire experience for the selection of equipment for a fire extinguishing installation with FK-5-1-12 in the process of its design, in order to ensure more complete evaporation of the liquid phase of FK-5-1-12 after leaving the nozzles. An assumption was substantiated that allows setting the normative concentration of fire extinguishing gases at the level of heptane without conducting a fire experiment to extinguish fires of flammable liquids with a flash point of more than 61 оC. In this case, the concentration value can be taken 15 % less than according to the requirements of ISO 12854: 2009. Experimental studies on extinguishing smoldering materials with gases were conducted. It was established that nitrogen, CO2, freons 114B2 and 227ea at a concentration for extinguishing heptane eliminate not only flame combustion, but also smoldering if the free combustion time does not exceed 3 min. With free combustion for more than 6 minutes, reliable suppression of smoldering is achieved when the oxygen content in the volume of the fire chamber is up to 2.5 % vol. The results of the experiments were taken into account in the draft norms for the design of fire extinguishing installations. Additional requirements were developed for gas fire extinguishing containers and selector valves, which will make it possible to exclude the use of low-quality or obsolete products as part of gas fire extinguishing installations in the territory of the Eurasian Economic Union. The developed modern requirements are included in the drafts of new regulatory documents, which were publicly discussed, and are at the approval stage.

Effects of fire frequency on savanna butterfly diversity and composition: A preliminary study

Fire plays a major role in many biomes, is widely used as a management tool and is likely to be affected by climate change. For effective conservation management, it is essential to understand how fire regimes affect different taxa, yet responses of invertebrates are particularly poorly documented. We tested how different fire frequencies influence savanna butterfly diversity and composition by using a long-term savanna fire experiment initiated in 1954 in the Kruger National Park (South Africa). We compared butterfly abundance, species richness and community composition across three fire frequencies: high (burnt annually), medium (burnt triennially) and low (burnt twice in 60 years). Plots with high fire frequency hosted higher abundance than medium- or low-frequency plots. Fire frequencies did not affect species richness, but they led to distinct communities of butterflies. Our findings suggest that, in view of the three fire frequencies tested, a spatial diversity of fire frequencies may increase butterfly diversity at the landscape level in wet savannas. Managers may need to promote a greater diversity of fire frequencies by increasing fire frequency in some areas to provide habitat for species requiring high fire frequency, and by decreasing fire frequency in a large proportion of the landscape to provide fire refuges. This study provides new insights for butterfly conservation in savannas and highlights several knowledge gaps, which further studies should address for insect responses to be given adequate consideration in fire management strategies.Conservation implications: A spatial diversity of fire frequencies may increase butterfly diversity. Managers may need to promote a greater diversity of fire frequencies by increasing fire frequency in some areas to provide habitat for species requiring high fire frequency, and by decreasing fire frequency in other areas to provide fire refuges.