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Calculating Fire Exposure
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The design of structural fire resistance under fire conditions requires three major steps: (1) determination of the thermal boundary conditions to a structure resulting from a fire; (2) determination of the heat transfer to the structure, or portion thereof; and (3) determination of the structural response of the structure. Calculating fire exposures to Structures is Step 1 in the process.

SFPE S.01 Standard on Calculating Fire Exposures to Structures

SFPE Standard S.01 provides methodologies to predict thermal boundary conditions for fully developed fires to a structure over time. Information developed using this standard will provide input to thermal response and structural response calculations undertaken as part of an engineered structural fire resistance design.

This standard addresses fully developed fire exposures, which include exposures arising from a fully developed fire within an enclosure and exposures from a localized fire involving a concentrated fuel load that is not affected by an enclosure. The methodologies presented in this standard do not take into account the effects of fire suppression or smoke management systems. The purpose of this standard is to provide methods to calculate the thermal boundary conditions to a structure resulting from a fully developed fire.

EVALUATION OF ENCLOSURE TEMPERATURE EMPIRICAL MODELS

The methods presented in SFPE S.01 for computing a time-temperature profile in an enclosure are shown in SFPE Report Evaluation of Enclosure Temperature Empirical Models (2010, 9 November) This report outlines 23 different methods or method variations. All methods considered had been documented in published material and did not involve the use of computer simulations. The methods included simplistic approaches such as a constant temperature exposure, correlations of particular data sets, generalized parametric approaches, and correlations of computer generated data.

The selection process involved assessing the performance of all 23 methods against a database containing about 130 fully developed single-compartment fire tests. The database was compiled largely from four decades of published enclosure fire test results. Most of the tests were conducted using full-scale compartments that represented a wide range of parameters that would have some influence on the time-temperature development. The parameters included the absolute enclosure dimensions, the absolute opening dimensions, the number and location of the openings, the type of boundary materials, the ventilation factor, and the type of fuel burning.

Structural Fire Protection’s Shifting Paradigm

An article in the 2017, Quarter 2 issue of Fire Protection Engineering magazine by LaMalva discuss the role that calculating fire exposures plays in the structural fire engineering process. Applied structural fire protection serves as a secondary safety measure to sprinklers in the rare case that a fire becomes uncontrolled. However, prescriptive codes do not require the analysis of structural systems under fire exposure, they only prescribe structural insulation coverage of the nominal structural system design. It is expected that buildings will be able to provide safe evacuation of occupants and shelter for those in refuge areas. This is dependent on the assumption that the building’s structural system will remain stable during an uncontrolled fire; but the building codes are not written in such a way that does not consider the size of the building. This one size fits all approach may be too lenient for tall buildings and too stringent for lower-risk buildings.

Currently there are two options for structural building design. Standard fire resistance design is the traditional option where designers select qualified assemblies from available listings to meet prescribed levels of fire resistance. Structural fire engineering is an alternate approach that evaluates the demands of structural systems under fire exposure and allows the system to be improved by adjusting specific elements.

In 2016, ASCE/SEI 7, the parent standard for structural engineering building codes in the US was updated to include a chapter on fire resistance. This chapter now gives the designer the option to adopt a structural engineering approach as constituted in appendix E. Appendix E details the evaluations and considerations that must be made using this approach. Until this code was updated, designers decided on their own what constituted a satisfactory structural fire engineering design. It is anticipated that this code will significantly reduce the inconsistency in the industry.

By allowing the engineer to influence more design variables, he or she can design for aesthetics, functionality, and/or costs without compromising fire safety. Additionally, it is envisioned that this code will legitimize the practice of structural fire engineering and ease stakeholder’s reluctance to adopt this approach for building projects. In the near future a new ASCE/SEI Guideline: Structural Fire Engineering will be coming out and there are plans to incentivize the adoption of structural fire engineering

Probabilistic Study of the Resistance of a Simply-Supported Reinforced Concrete Slab According to Eurocode Parametric Fire

An article by Heidari in Fire Technology journal presents a method to identify the most important parameters that need to be considered in a fire safety engineering design. It allows engineers to exclude variations in some parameters of the Monte Carlo analysis, reducing the overall number of runs needed to obtain an answer. Using the one-at-a-time (OAT) method a range of input variables were investigated to determine which ones had the highest impact on the final results. It also highlighted the range of possible fire scenarios for which the design element was structurally safe. The Monte Carlo analysis was then run on the reference case. A Monte Carlo analysis performs numerical experiments using a large number of randomly generated sample sets from the inputs according to their probability distributions.

This methodology was applied to a simply supported reinforced concrete slab in an open plan office building. The parameters and assumptions were made in accordance with the Eurocode. A number of time-temperature curves were created by varying ventilation, nature, distribution, and quantity of fuel. The scenario that produced the highest temperatures was chosen to be the reference case for further analysis. The OAT method was applied to the reference case for variables including the characteristic fire load density, fire fighting measures index, axis distance of reinforcement, opening factor, and concrete density. The OAT sensitivity analysis identified the key input parameters which had the greatest effect on the maximum rebar temperature for the purpose of the Monte Carlo simulation. Monte Carlo simulation was run 1500 times, a number at which the mean value of the outputs converged. Then considering the results of the sensitivity analysis, three lower impact variables were removed allowing the simulation to be run only 750 times with a lower probability of failure.

The study shows that the OAT sensitivity analysis provides an insight into the range of fire parameters for which the design is structurally safe. It also found that the unavailability of fire protection measure leads to an increased probability of failure. The results of this study demonstrate that sensitivity and probabilistic analysis can provide a comprehensive understanding of the factors affecting the structural fire resistance and inform further fire development and detailed structural analysis.

Traveling Fire Methodology

An article by Rein in the 2017, Quarter 2 issue of Fire Protection Engineering magazine describes how the traveling fire methodology can be used to calculate fire exposures for fires in large compartments. Modern architectural preferences for large open compartments are forcing engineers to design outside of their known boundaries. This has created a demand for more research and the development of new methodologies for large compartments. Experiments and recent incidents have shown that fires in large compartments tend to travel, burn for long periods of time, and have highly non-uniform temperature distributions. Researchers are working to advance the design concept of Traveling Fires Methodology (TFM), a framework that incorporates the actual behavior of fires in large open compartments.

TFM considers design fires to be composed of a near-field (flame) region and a far-field (smoke) region, each with different characteristics. This allows for a more accurate representation of large compartment fires. Recent studies have shown that structural members are more likely to reach higher peak temperatures in traveling fires, while in other cases fire duration is more important than fire type. These results among others, show that a single worst-case fire scenario cannot be established. To ensure a safe design a number of fire types including traveling fires must be considered.

The TFM research has identified that column section sizes and their change on different levels of the frame are important in defining the weakest floors in a building. Also, research has shown that long uniform fires result in the highest average frame utilization likely because they result in the highest temperatures and therefore the greatest stress in members. Research is also being done on fires that travel vertically and the results have shown that failure time is greatly dependent on the fire time and number of floors involved. TFM is still a new concept that will continue to be improved, but it has already proved to be very useful in developing new knowledge about large compartment fires.

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