Burnout Resistance Research Articles

Numerical study on the behaviour of reinforced concrete columns exposed to fire including the cooling phase

It has been proven that reinforced concrete (RC) structures subjected to fire are likely to collapse during cooling phase. However, many insights into this phenomenon have not been covered in both research and building codes/standards. This paper presents a quantitative study on the behaviour of RC columns subjected to natural fire including cooling phase, particularly the fire resistance R and burnout resistance DHP of columns. The study adopted Eurocode parametric fire model to simulate the natural fire curve and finite element models in SAFIR to simulate the during-and-post-fire responses of RC columns with various cross-sections. A parametric study on the effects of various factors (concrete cover, load ratio, and reinforcement ratio) on the DHP-R relationship is also presented.

Burnout resistance of concrete‐filled steel tubular (CFST) columns under realistic fire conditions

AbstractIn this paper, we investigate the structural behaviour of concrete‐filled steel tubular (CFST) columns under various fires. Our contribution is based on two fire scenarios in modelled FDS with subsequent 3D FE numerical analysis (Abaqus), where material properties are defined for heating and cooling using user subroutines. Our goal is to examine the burnout resistance of CFST columns under different realistic fire scenarios involving varying number of burning cars.The loss of compressive strength of concrete due to high temperature is irreversible. Hence, the burnout resistance is affected by the field of the highest temperatures reached inside the column. Moreover, the pre‐existing load affects the deformation of the column during heating and influences the post‐fire behaviour. The analysis is performed in several steps: first, results from the CFD models are extracted and appropriately mapped, then the heat transfer analysis between fire and solid is performed, and the resulting temperature history of the CFST column is calculated. Subsequently, a mechanical analysis is performed, where (1) the column is loaded, (2) the analysis of the column's structural fire behaviour is carried out, (3) if the column sur‐vives burnout, then in the last step, it is loaded until failure.

Numerical analysis of the effects of fire with cooling phase on reinforced concrete members

Fire exposed structures may collapse during or after the fire decay phase, with risks for building occupants and firefighters; yet, understanding of the effects of the fire decay phase on structural loadbearing capacity remains limited. This paper describes a numerical investigation on the behavior of reinforced concrete columns, beams, and walls under natural fires including cooling down phases. Finite element models are benchmarked against experiments capturing the behavior during heating. The models are then used to simulate the structural response of the concrete members under fires with various cooling rates and load ratios. The analyses capture the irreversibility of material properties through tracing of the temperature history in the structure. The results show that temperatures and deformations continue increasing after the end of the fire heating phase. As a result, concrete columns, beams, and walls may fail during the cooling phase. Faster cooling rates reduce the likelihood of failure in cooling. For beams, failure can be inferred from the maximum reinforcement temperature reached throughout the fire, but for columns and walls a thermal–mechanical analysis of the member throughout the fire history is needed. A relationship is proposed to evaluate the burnout resistance from the fire resistance rating and cooling rate. The presented numerical method allows assessing the structural stability throughout a fire event, an important requirement for designing a fire resilient built environment.

Experimental investigation of structural failure during the cooling phase of a fire: Concrete columns

Structures may collapse during the cooling phase of a fire, yet standard furnace tests only measure the response under heating. There lacks experimental test protocols and design methods to assess resistance until burnout. This paper describes a new experimental approach for burnout resistance evaluation, reports experimental data on loaded reinforced concrete columns in furnace tests with cooling down phases, and presents numerical models of the tests. The test results show that columns designed for a standard fire resistance of 60 min exhibited a fire resistance of 83 min in the furnace but failed during the cooling phase when the burners were shut off after 72 min while the load was maintained. Two other specimens survived exposure to heating of 45 and 55 min, respectively, and their residual capacity was measured. Finite element analyses show agreement with the tests, showing applicability of numerical methods for evaluating burnout resistance of concrete columns. These findings demonstrate experimentally that delayed thermal-mechanical effects can jeopardize structural stability in real fires, and provide a framework to measure these effects. Moving beyond fire resistance to quantify the response until burnout will support designs for safety of occupants and firefighters throughout the fire and promote repairability and resilience.

The cooling phase, a key factor in the post-fire performance of RC columns

The focus of recent literature is primarily on the heating phase of concrete columns in relation to possible delayed failure, in combination with a linear descending cooling phase. The research is only recently started, and this is mainly based on a parametric temperature-time curve, including a linear cooling regime as the first kind of approximation suitable for design purposes. Gernay (2019) [1] proposed a straightforward linear relationship between the burn-out resistance of a column subjected to a parametric fire and the ISO fire rating time, valid for heavily loaded columns. However, it is known that the linear cooling curve in the case of the parametric fire does not follow Newton's fundamental law of cooling in a transient situation and, and therefore, is non-natural. Exploring the importance of the cooling regime delivers the need to investigate more profoundly other slopes of, or shapes than, linear cooling, which might be closer to reality. Also, the importance of the load level should carefully be investigated. Indeed, in real buildings, not every column is heavily loaded to obtain some uniformity in dimensions. A simple column is chosen and heated on four, three, or two sides to investigate the effects of the cooling regime and the load level. A combination of thermal loads coming from the parametric fire, the more natural cooling BFD curve, a heavily forced, very slow cooling regime was created. For the material model, a proposed strength loss of 10% (EN 1994-1-2, 2005) is verified and compared against a 20% loss observed in several other experiments out of literature. Explicit formulation of creep will be used to account for the time-dependent loss in strength. Research relevance can be found in the assessment of the residual capacity of concrete structures in post-fire circumstances. This capacity can be expressed in terms of load-bearing capacity or post-fire fire rating; both are important to assess the reliability of a structure for further use after it has been subjected to a fire. It is shown that a difference in the cooling regime, load level, number of exposed sides can cause failure and do influence the post-fire bearing capacity.

Fire resistance and burnout resistance of timber columns

For any construction material, failure during or after the decay phase of a fire is possible, notably due to delayed temperature increase and material properties degradation. As mass timber construction is increasingly proposed for large buildings, there is a need to better understand the susceptibility of timber members to fire decay phases. This paper investigates the resistance to full burnout of timber columns. First, a dataset of glulam columns tested under standard fire is analyzed using finite element modeling, showing conservative agreement. Then, the numerical model is used to analyze the columns under ‘standardized’ natural fires comprising a cooling phase. Adopting a systematic procedure based on the minimum Duration of Heating Phase (DHP) that leads to failure, the burnout resistance of the columns is found to range from 20 to 50% of their standard fire resistance. These results indicate significant susceptibility to failure during the cooling phase, which is due to the combined effect of delayed heating and loss of mechanical properties at relatively low temperatures for timber. Based on heat transfer and structural mechanics, it is thus shown that guaranteeing resistance to full burnout with timber structures presents important challenges, even when setting aside the question of combustibility and auto-extinction.

High Single-Event Burnout Resistance 4H-SiC Junction Barrier Schottky Diode

This paper presents a single-event burnout (SEB) resistance method for 4H-SiC Junction Barrier Schottky Diode (JBS) under high bias voltage and linear energy transfer (LET) conditions. The method is validated via two-dimensional numerical simulations. The analysis and comparison of conventional 4H-SiC JBS and 4H-SiC Multi-Buffer Layer JBS (MBL-JBS) diodes verify the resistance tolerance of the latter device to SEB. Silvaco TCAD simulation results prove that the 4H-SiC MBL-JBS modulates the drift region's electric field distribution and disperses the high-peak electric field at the N-/N+ junction. Moreover, it reduces the impact generation rate at the Schottky, PN and N-/N+ junctions to achieve SEB tolerance. The device's SEB performance is optimized by adjusting the 4H-SiC MBL-JBS structure parameters, and the SEB threshold voltage is significantly improved compared with the traditional structure.

Burnout resistance of concrete slabs: Probabilistic assessment and global resistance factor calibration

Traditional prescriptive requirements for structural fire design do not explicitly ensure the load bearing capacity of the structure up to and including burnout. Therefore, a deterministic design method is presented, which enables to perform a quick check of the burnout bending resistance of a concrete slab exposed to a natural fire, and allows for the evaluation of (delayed) bending failure that can occur during or after the cooling phase, that would otherwise remain undetected. Subsequently, the influence of the inherent uncertainties associated with material properties and geometry on the design until full burnout is investigated. Reference fragility curves are presented and subsequently a global resistance factor (GRF) safety format for the fire design considering complete burnout scenarios is proposed. Application of the determined GRF allows for an explicit reliability-based design for fire exposed concrete slabs, without requiring the application of full-probabilistic methods. The concept is applied to simply supported reinforced concrete slabs and the calculated values are applicable to any compartment within the Eurocode parametric fire framework, through application of a scaling approach. The methods proposed in this contribution are intended as a stepping stone towards the development of GRF-based safety formats for more complex geometries and advanced numerical tools.

Burnout resistance assessment of air tuyere in blast furnace

To assess the burn resistance air tuyere in blast furnace using the finite element method (axisymmetric problem), DEFORM-2D software was used. Ceramics with a slurry coating applied to it was used as thermal insulation for protection against air tuyere burnout. The resistance to air tuyere burnout with the selected thermal insulation on its outer surface is estimated by calculating the stress intensity.

Fire resistance and burnout resistance of reinforced concrete columns

This paper quantifies the resistance to complete burnout of reinforced concrete columns under fire exposure. It adopts a new metrics to complement the fire resistance rating that captures the specific effects of the cooling phase and characterizes explicitly the ability of a structural member to survive burnout. The analysis is performed on a dataset of 74 standard fire resistance tests on columns. Finite element modeling is applied first to reproduce the standard tests. The obtained ratio between the computed and experimental fire resistance has an average value of 0.95 with a standard deviation of 0.29. Then, finite element modeling is used to analyze the 74 columns under ‘standardized’ natural fire comprising a cooling phase. The effects of cooling are carefully incorporated in the materials laws including thorough consideration of irreversibility of properties and explicit modeling of transient creep. An iterative computational procedure is applied to subject each column to increasing durations of fire exposure until finding the shortest standardized fire that cannot be survived until burnout, from which the burnout resistance metrics is defined. The duration of this shortest fire increases approximately linearly with the fire resistance of the columns, but with a factor smaller than one, revealing an increased propensity to delayed failure in columns with longer fire resistance. Finally, a simple equation is formulated to estimate the burnout resistance from the fire resistance. Such equation can support design for complete burnout which has benefits for safety of fire brigades and first responders, as well as for property protection and resilience of the built environment.

EMOTIONAL BURNOUT AS A MODERATOR OF TV NEWS IMPACT ON YOUNG ADULT AUDIENCE: EEG STUDY


 
 
 The paper considers changes of the EEG parameters while viewing the emotionally accented TV messages depending the emotional burnout formation. We found that the character and degree of EEG changes during the perception of emotional stimuli significantly depend on the characteristics of a particular stimulus and the current level of emotional state of the viewers (emotional burnout, fatigue). The subjects with the phase of resistance of the emotional burnout syndrome at the stage of formation (“Burnout RP under development”) differ significantly from the groups with the unformed and formed Burnout Resistance Phase (“Burnout RP unformed”, “Burnout RP formed”) in the dynamics of changes of spectral power of the EEG subbands during the exposure to the emotional stimuli. The development of burnout is characterized by an increase in State Anxiety and fatigue, the desensitization to the emotionally accented information, an augmentation in social alienation and, eventually, manifests in changes in the brain response to emotional stimuli, which consist in the inhibition of high-frequency EEG components.
 
 

Doctor as a guard of mothers and babies' health in Kazakhstan and Germany: two psychological models of burnout resistance

Doctor as a guard of mothers and babies' health in Kazakhstan and Germany: two psychological models of burnout resistance

Ideality and noise figure characteristics of r.f. sputtered millimetre GaAs diodes

Refractory Schottky barriers have been incorporated in millimetre GaAs mixer diodes to improve r.f. performance and burnout resistance. It is shown that low power r.f. sputter deposition of TiW and Ti-Mo refractories results in degradation of noise temperature ratio and to a first approximation does not affect d.c. parameters, especially the ideality factor.

X-Band High-Burnout Silicon-Schottky Mixer Diodes

The development of a highly reliable thermal-compression bonded X-band silicon-Schottky barrier diode is described. A highburnout diode capable of handling peak RF power of 10-14 watts ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau</tex> = 1 µs, 103 Hz) with a noise figure of 6.5-7.0 dB (SSB) was suceessfully demonstrated. These diodes are more rugged than whisker type point contact and Schottky-barrier devices and exhibit two to three times higher burnout resistance to RF pulses than Schottkybarrier diodes manufactured today. For this reason, they are very suitable for high-power military radar systems and especially useful for medium-power systems where limiters are not used to protect the mixer diodes.

Self-passivated GaAs/W mixer diode

GaAs/tungsten Schottky barrier mixer diodes have been fabricated in a self-passivated, grown guard layer configuration. These devices have exhibited 0.64 eV barrier voltages together with the ability to withstand nanosecond RF pulses in excess of 8 ergs with no resulting deterioration in noise figure performance. Such high burnout resistance properties are attributed to the absence of any significant metallurgical interaction between the GaAs and tungsten at temperatures as high as 600°C. The somewhat high noise figures exhibited by these devices are attributed to interface states, possibly arising from strain.

High-spatial-resolution scanning Auger spectroscopy applied to analysis of X-band diode burnout

Low-noise-figure Schottky-barrier mixer diodes for use at X-band and above have small active areas and are thus susceptible to burnout or degradation from high rf power levels. As part of a program to improve burnout resistance of mixer diodes for a Navy system, metal diffusion couples were studied and diodes were fabricated, burned out, and analyzed. The microspot Auger analysis technique in the Auger-spot mode provided the required submicron identification of the interdiffusion phenomena in the failed regions of the mixer diodes. For cw-failed diodes, interdiffusion between the various diffusion couples found in both Pt–Ti–Mo–Au and Ti–Mo–Au diodes has been identified as the primary failure mode. In the pulsed case the lack of interdiffusion as made evident in the analysis of edge-burned-out sites indicates the existence of an alternate failure mode.

Properties of a Light-Modified-Breakdown Detector in GaAs

The properties of a bulk-effect millimeter-wave detector made from high-purity epitaxial GaAs are discussed. The devices are operated at 4.2 K in a new light-modified-breakdown (LMB) mode with dc bias above avalanche breakdown. When operated in the LMB mode as a direct video detector, circuit-limited response time (10-90% pulse) less than 20 nsec is observed, which is an order of magnitude faster than the response of bulk GaAs and InSb mixers. NEP values of less than 10(-10) W (D* on the order of 10(10)) in the video detector mode have been measured over the 10-70 GHz frequency range. The dependence of detector performance on operating parameters and frequency is given and compared with the predictions of hot-electron theory developed for InSb detectors. Its performance is also compared with that of microwave-biased InSb, thermal, pyroelectric, and photoionized-impurity GaAs detectors and with point contact, Schottky barrier, and MOM (tunneling) diodes and is shown to provide significant advantages of either burnout resistance, risetime, or noise over competitors in the millimeter spectral region.

Design and fabrication of high burnt-out Schottky barrier crystal video diodes

A design procedure for crystal video diodes which maximizes burn-out resistance and dynamic range for a given detection sensitivity is outlined. The design variables are the semiconductor, the metal, the substrate and epitaxial doping, the epitaxial thickness and the diode diameter. A key result is that, to first order, the diode sensitivity is independent of the metal. Diode design is only weakly dependent on the epitaxial doping but strongly dependent on the semiconductor, the diode are and the epitaxial thickness. The design and fabrication of a 10 W S-band diode is discussed. Flip mounting is required for efficient diode cooling, and a special 3-mesa tripod structure is employed for mechanical stability. The tangential sensitivity and burn-out power of these diodes is respectively −55 to −57 dbm and greater than 10 W.

Microwave backward diodes in InAs

The many advantages of backward diodes for microwave mixing and detecting—zero bias, low local oscillator power, low 1/ƒ noise, and environmental insensitivity—are significantly offset by the high capacitance associated with a narrow tunneling junction. This either severely limits high-frequency operation or requires small-area junctions which are fragile and susceptible to burnout at low power levels. This limitation can be reduced considerably by selection of a semiconductor with small band-gap and low carrier effective mass, with consequent high tunneling probability and reduced capacitance per unit area. Thus, a device of given size permits higher frequency operation, or equivalent electrical performance with larger area, leading to improved ruggedness and resistance to burnout. Computer-aided studies of the characteristics of tunneling junctions have confirmed the initial estimate that, among elemental and III–V semiconductors, InAs offers improved detector performance. Such devices have been fabricated, and microwave characterization of InAs backward diodes has been carried out. Measurements at 9 GHz on relatively large-area diodes show tangential sensitivity comparable to commercial (small-area) germanium backward diodes, with improved ruggedness and burnout resistance. Extension of this work to mixed compound semiconductors is suggested as a means of circumventing ambient temperature limitations of InAs.

Low-noise GaAs Schottky-barrier beam-lead mixer diodes

Gallium arsenide Schottky barrier beam lead mixer diodes with a noise figure of 5.5 dB at X-band and a high burnout resistance are described. Limits of the performances are related to noise temperature ratio and these are discussed in connection with fabrication process and low-frequency noise.