Performance-Based Design of Fire-Exposed Reinforced Concrete Elements using an Equivalent Standard Fire

Fire events represent one of the most severe loading scenarios for reinforced concrete (RC) buildings. In lieu of intensive computational analysis, designers need simplified methods to assess RC members exposed to natural fires. This thesis focuses on the development of a time equivalent (te) to replace a natural fire with an equivalent standard fire, allowing for the implementation of existing simplified analysis methods. The proposed te is based on the average internal temperature profile (AITP) that develops in a section during fire. Two AITP te are proposed to accurately or conservatively approximate the AITP of natural fire exposed sections. A size adjustment factor (φsize) is also proposed to account for the influence of section dimensions. Suitability of the AITP te in the performance-based fire design of RC beams and columns was examined based on the relationships of moment-curvature, axial load & axial strain, and moment capacity & axial load capacity

Equivalent standard fire duration to evaluate internal temperatures in natural fire exposed RC beams

With the recent shift towards performance-based fire design, practical methods to account for natural fire loading when designing concrete structures are needed. Available design methods and analysis approaches are based on standard fire curves. To apply these methods, a natural fire event can be converted to a standard fire with a specific duration (time equivalent). However, existing time equivalents often ignore the influence of internal temperature gradients on the section behaviour, which is unacceptable for concrete structures. This paper introduces a time equivalent method suitable for reinforced concrete (RC) beams exposed to natural fire. The method is based on the actual temperature gradient within a concrete section. To simplify analysis of RC beams exposed to fire, an average internal temperature profile (AITP) can be utilized, which records the average temperature variation along the height of a section. Two equations are provided such that a standard fire duration can be determined to accurately or conservatively represent the AITP of a beam section exposed to natural fire. Characteristics of the natural fire, as well as the influence of section dimensions are accounted for. The developed AITP time equivalent method is found to be superior to the existing methods and accurate in approximating the moment-curvature response for RC beam sections

Performance-based design of RC beams using an equivalent standard fire

The design of buildings for fire events is essential to ensure occupant safety. Supplementary to simple prescriptive methods, performance-based fire design can be applied to achieve a greater level of safety and flexibility in design. To make performance-based fire design more accessible, a time equivalent method can be used to approximate a given natural fire event using a single standard fire with a specific duration. Doing so, allows for natural fire events to be linked to the wealth of existing data from the standard fire scenario. In this paper, the use of an existing time equivalent method is reviewed and assessed for application in the performance-based design of reinforced concrete (RC) beams. The assessment is established by computationally developing the moment-curvature response of RC beam sections during fire exposure. The sectional response due to natural fire and time equivalent fire are compared. It is shown that the examined time equivalent method is able to predict the sectional response with suitable accuracy for performance-based design purposes. The research is the first to provide a comprehensive evaluation of the moment-curvature diagram of RC beams using time-equivalent standard fire scenarios that model realistic fire scenarios

Influence of Natural Fire Development on Concrete Compressive Strength

With increasing acceptance of performance-based design principles in the field of fire safety, it is imperative to accurately define the behaviour of materials during fire exposure. Real-world fire events, otherwise referred to as natural fires, are defined by four characteristics: heating rate, maximum temperature, exposure duration, and cooling rate. Each of these four characteristics influences concrete’s behaviour in a different manner. In this paper, the available experimental work for concrete, tested at elevated temperatures, is examined to identify the influence of the four natural fire characteristics on concrete compressive strength. This review focuses on normal strength concrete tests only, omitting parameters such as unique additives and confinement. The intent is to provide a fundamental understanding of normal strength concrete. The findings show that maximum temperature and cooling rates have a significant influence on concrete strength. Exposure duration has a moderate impact, particularly at shorter durations. Variable rates of heating have minimal influence on strength. Detailed conclusions are provided along with review limitations, practical considerations for designers, and future research needs

Performance-Based Design of RC Columns using an Equivalent Standard Fire

The extreme variability of natural compartment fires poses a significant challenge in the process of performance-based fire design. To reduce this variability, the severity of a natural fire can be related to that of a standard fire, known as a time equivalent (te). In this paper, the applicability of a time equivalent, previously derived based on the average internal temperature profile (AITP) that develops within reinforced concrete (RC) beams exposed to fire from three sides, is examined for RC columns exposed to fire from four sides. A parametric study is presented to examine the suitability of the existing AITP te in representing the internal temperatures of RC columns. The accuracy of the AITP te in approximating column performance, is judged based on the moment-curvature, axial load-axial strain, and bending moment-axial force relationships during fire exposure. Comparison with existing methods is provided to further demonstrate the superior suitability of the AITP te in representing natural fire severity for RC columns