How dwell time affects the determination of occupant density in retail occupancies using footfall data

Abstract

During the fire engineering design process, occupancy numbers are typically determined through the application of occupant load factors recommended by guidance documents or required by fire safety codes. It is often unclear how these factors have been derived or what upper bound of likely percentile of real-world values they represent. In retail occupancies the number of people using a space across a given time, referred to as footfall, is often recorded to set ground rents, etc. and the length of time that people remain within that space is often called the dwell time. This paper proposes a computational method to determine maximum occupant densities in retail spaces as part of a fire engineering design by combining measured footfall data with dwell time distributions. This method has then been applied to a case study of a large UK mall, using recently recorded footfall data. Simulations suggest that the maximum occupant density in common circulation spaces could be around 2.7 m2 per person and 5.2 m2 per person within the associated retail units whereas current design occupant load factors give 3.5 m2 per person for circulation spaces and 2–10 m2 per person for retail areas.