Protected Steel Members Research Articles

Probabilistic risk assessment of the effects of fire service intervention on structural fire resistance design of unsprinklered buildings

This paper presents a probabilistic analysis method to explicitly include the influences of fire service intervention (FSI) in structural reliability analysis of unsprinklered buildings exposed to fire. The method considers different scenarios of fire behaviour following FSI, based on probabilistic analysis of the current UK fire data to estimate different components of intervention time, size of the fire on arrival of the fire service and extent of damage of the fire at the end of intervention. These statistical data are then used to generate random fire temperature-time curves to calculate the probability of structural failure of intumescent coating protected steel members using the Monte Carlo Simulation method. To aid deterministic design, a simplified method is developed to derive fire load modification factors (δn,FSI) to account for the effects of FSI so that when the fire load modification factor is applied, the deterministic design method gives the same probability of structural failure as that from explicit probabilistic calculation. An extensive parametric study covering a wide range of practical design factors has been carried out and an Analysis of Variance (ANOVA) is performed on the results to examine whether any design factor has significant effect on the fire load modification factor. The results show that fire growth rate and compartment floor area are the most influential. The effects of fire load, steel critical temperature and fire protection thickness have moderate effects. Based on the results of this study, it is suggested that the design fire load should not be reduced if the fire floor area does not exceed 250 m2, or the fire growth rate is ultrafast. If the fire growth rate is fast, the design fire load should not be reduced for fire floor areas less than 1000 m2. For other situations, a fire load modification factor of δn,FSI = 0.78, as in the current Eurocode, may be applied.

Reliability analysis of intumescent coating protected steel members under the standard fire condition

This paper presents a methodology and the results of a study to characterize variabilities in temperature dependent thermal conductivity, as defined in EN 13381-8, and dry film thickness (DFT) for one type of intumescent coating under the standard fire condition, and their influences on reliability of intumescent coating protected steel members. The variabilities in temperature dependent thermal conductivity and DFT of the coating were found to be best described by lognormal and normal probability distributions respectively. Monte Carlo simulations were carried out to determine associated probabilities of structural failure based on comparisons of steel limiting temperatures. The EN 13381-8 variable thermal conductivity method was found to give temperature dependent thermal conductivity of the intumescent coating that leads to a probability of failure of 0.1, corresponding to a reliability index of 1.28. This level of reliability may not be sufficient (i.e. not able to achieve the required level of safety), depending on the conditional probabilities of fire occurrence and flashover. To achieve lower probabilities of failure (and thus higher reliability), this paper suggests using a safety factor of greater than 1.0 to multiply the variable thermal conductivity obtained using EN 13381-8. For example, to achieve target conditional probabilities of failure of 0.001 and 0.01, the safety factors are 1.7 and 1.4 respectively.

Tests on intumescent paints for fire protection of existing steel structures

Intumescent coatings (i.c.) are often used on steel structures to reduce the heating during a fire when the structural, aesthetic, and architectural value of the structural members should be preserved. Indeed, i.c. form a thin protective layer on the surface. When the i.c. are exposed to fire or excessive heat, volume expansion and density reduction take place. Therefore, the protective layer absorbs heat and protects the structural member from damage or excessive deformation.To perform rigorous and realistic analyses on protected steel members, thermal properties of all materials, including i.c., should be known. Nevertheless, the thermal properties of these systems are not available, especially for existing buildings.This paper shows experimental test on steel members protected with i.c., taken from an existing structure, which is thirty years old. I.c. thickness measurement and adhesion tests were performed in situ, whereas furnace tests were carried out at the Laboratory of the Italian National Fire Services.The tested steel members have various section factors and behave two fire curves (Standard Fire and Smouldering Fire) during the tests in furnace. The furnace tests were performed on steel members with original i.c. (reference) and steel members protected by applying a new i.c. after removing the existing paint (restored) or simply by applying it on the existing paint (repainted). The results show that the existing i.c. is not very efficient, whereas the restored and repainted specimens showed similar performance. A law for the thermal conductivity of the i.c. was obtained from the experimental results, according to an European code, and used in a finite element numerical model.

A simplified approach for predicting temperatures in fire exposed steel members

For evaluating fire resistance of steel structural members, temperatures in the cross section of the member are required. In this paper, a simplified approach for evaluating cross sectional temperature in contour protected steel members exposed to fire is presented. The approach is derived utilizing simplifying assumptions to the general heat transfer equation for standard fire and is then extended for application under design fire scenarios. The proposed approach is applicable for both protected and unprotected steel sections. The validity of the approach is established by comparing predicted temperatures with those obtained from finite element analysis generated via ANSYS. In addition, predictions from the proposed method are also compared with the temperatures predicted by “best-fit” method. The comparisons to test data, finite element results and best-fit method indicate that the proposed simplified method gives good predictions of fire induced thermal gradient and temperature history in steel sections under any fire exposure. The simplicity of the proposed method makes it attractive for use in design situations.

Assess the Fire Resistance of Intumescent Coatings by Equivalent Constant Thermal Resistance

Intumescent coatings are now the dominant passive fire protection materials used for steel construction. Intumescent coatings will react at high temperatures and the thermal properties of intumescent coatings can not be measured directly by the current standard test methods which are originally designed for the traditional inert fireproofing materials. This paper proposed a simple procedure to assess the fire resistance of intumescent coatings by using the concept of equivalent constant thermal resistance. The procedure is based on the approximate formula for predicting the limiting temperatures of protected steel members subjected to the standard fire. Test data from investigations on both small-scale samples and full-scale steel members are used to calculate the equivalent constant thermal resistance. Using the equivalent constant thermal resistance of intumescent coatings, the calculated steel temperatures agree well with the test data in the range of the limiting temperatures from 400°C to 600°C. The procedure needs no complex computation and is recommended for practical usage. The equivalent constant thermal resistance could be used to quantify the insulation capacity of intumescent coatings.

Rise in structural steel temperatures during ISO 9705 room fires

An experimental programme was undertaken to study the temperature rise of protected and unprotected structural steel during a fire within a small enclosure (an ISO 9705 room). The fuel (wood crib) was placed at two locations (front and back) within the ISO room. Each location had two fire scenarios present: the first fire scenario was for recording the temperatures of protected steel members within the enclosure, and the second fire scenario was to measure the temperatures of the directly exposed members. Six steel columns and two steel beams were strategically placed, and their temperatures were measured. Other data recorded were gas temperatures and heat release rates (HRRs). Thermocouples were kept in identical locations during the tests with protected and unprotected steel members to facilitate direct comparison. Despite the natural variability in fire development in identical situations, data up to ≈20 min were found suitable for direct comparison between protected and unprotected steel members. Comparison of these results with Fire Dynamics Simulator (FDS) version 5.3.1 modelling (with prescribed HRRs) results is presented to show the usefulness of the data collected.

Energy based time equivalent approach for evaluating fire resistance of reinforced concrete beams

The current prescriptive approaches for evaluating fire resistance of reinforced concrete beams under standard fire exposure have a number of drawbacks and do not provide realistic performance assessment. As an alternative, time equivalent concept can relate the severity of design fire exposure to that of standard fire exposure. However, the current empirical formulae for evaluating such time equivalency are mainly derived for protected steel members and may not be applicable for reinforced concrete members. This paper presents an energy-based time equivalency method for evaluating the fire resistance of reinforced concrete beams under design fire scenarios. The proposed method is based on the principle of equivalent energy, and estimates fire resistance based on the equivalency between standard and design fire exposures. The validity of the method is established by comparing the predictions from the proposed approach with those from existing methods (equivalent area method and empirical formulae) and with nonlinear finite element analysis. The applicability of the proposed approach to design situations is illustrated through a numerical example. It is shown that the proposed energy based method is capable of predicting equivalent fire resistance under design fire scenarios with an accuracy that is sufficient for practical purposes.

Predicting protected steel member fire endurance using spread-sheet programs

The simple method of computing the time-temperature response of protected steel members that has been recommended by the European Convention for Constructional Steelwork (ECCS) is presented in this paper. The method is a one-dimensional heat flow approach that explicitly accounts for the thickness and thermal properties of the insulation as well as the area and perimeter of the steel section.

Nonconventional fully developed polyethylene and wood compartment fires

Seventeen experimental fires with polyethylene and wood fuel were carried out in a full-scale compartment. The combustion air was supplied by a fan and the special feature of the compartment was the use of a slit which enabled the vertical position of the neutral plane to be well defined for outside observation. Large vertical and horizontal gradients in the gas temperature were found. In the polyethylene fires the highest mean compartment temperature measured was 1200°C. In the fully developed period all fires were ventilation controlled. Simulations of the experiments were carried out by means of a computer program. A general design model for polyethylene fires is proposed. This model is compared with a similar model for wood fires, and the models are applied to structural design. For wood structures and unprotected or lightly protected steel members the polyethylene fire is the more severe.