Analys av lokal brand under oskyddade fackverkstakstolar i stål : En riskanalys över invändig släckinsats vid bränder i matvarubutiker lokaliserade i hallbyggnader

Sammanfattning

This report examines the structural fire safety in grocery stores located in hall-type buildings with unprotected steel structures. The fire department conducts annual firefighting operations that involve smoke diving in these structures, and therefore there is a demand for guidelines on when interior interventions should be avoided. The purpose of this report is to analyze how a fire that starts in a shelf unit under a truss roof made of unprotected steel affects the stability of the roof structure. The goal is to determine whether critical temperatures in the roof structure can be reached as a result of a localized fire before flashover occurs, making it impossible for rescue personnel to remain in the building. To determine this, the critical temperatures of roof structures in typical hall buildings must first be calculated. These are presented according to the slenderness ratio of the steel profile that the truss is made of and are found to range from about 545-612 ºC. Next, a localized fire in a typical shelf unit is modeled according to methods developed in previous thesis works at Luleå University of Technology. To derive the fire load and fire effect curves for shelf units of varying heights, a method developed by Ludvigsson and Rydstedt in 2014 is used. Then, to calculate the temperature profiles that develop in the fire plume, the simplified factor method developed by Troedson and Östlund in 2016 is used. The temperatures in the plume are then converted to steel temperatures, and the relationship between the height of the shelf unit, the height of the steel from the ground, and the steel temperature reached is presented graphically. The time at which critical steel temperatures are reached, if they are reached, is then compared to the time of flashover. The time of flashover is calculated using a two-zone model in the software CFAST, where the height of the shelves and the ceiling height are varied. The result shows that flashover in a typical medium-sized grocery store with a floor area of 2400m² is expected to occur after between 20 and 25 minutes. In extreme cases, where unusually tall shelves and a low ceiling height are present, flashover may occur earlier. The results also show that in cases where critical steel temperatures are reached due to the localized fire, this happens before flashover occurs, but the distance between the shelf and the steel must be small. If the shelf unit is less than 2,25 meters high and the height to the steel in the roof structure is greater than 4 meters, the steel will not reach critical temperatures before flashover occurs. If the steel height is 5 meters, the shelf can be as high as 2,75 meters, and if the steel height is 6 meters, the shelf must be over 3 meters for critical temperatures in the steel to be reached before flashover occurs. When flashover occurs, rescue personnel will not enter the building, and those already inside will have evacuated due to the heat development and the clear risk, as flames will emerge from openings in the building. Since a typical store shelf is only 2 meters high and the height to the steel truss is rarely less than 4 meters, it can be concluded that the risk of death due to structural failure in a medium-sized grocery store is very low. The interior firefighters working inside to extinguish the fire will experience intense heat from the fire and will need to retreat before critical temperatures in the roof structure are reached. Therefore, it is the working environment for the interior firefighters that sets the limit for interior interventions, not the risk of structural failure.