Investigating effect of wind speeds on structural firebrand generation in laboratory scale experiments

Highlights

• Firebrands generated from structures source of rapid flame spread in large outdoor fires.

• Projected area of firebrands proportional to firebrand mass for experiments over various scales.

• The influence of wind speeds on firebrand production revealed in this study.

Abstract

Firebrands generated from structures are known to be a source of rapid flame spread within communities in large outdoor fires, such as wildland-urban (WUI) fires, and urban fires. It is important to better understand firebrand generation mechanism to prevent structure ignitions by firebrands. Though the wind plays an important role during the large outdoor fires, little known is the influence of wind speeds on firebrand production. To this end, a series of experiments were performed using mock-ups of full-scale wall assemblies exposed to wind. The objective of this study was to examine if experiments with mock-ups of full-scale wall assemblies may provide insight into firebrand generation from structures. Specifically, generated firebrands were collected and compared with those collected from full-scale components and a full-scale structure. The relationship between projected area and mass of firebrands were compared with previous experimental data. It was found that the projected area of firebrands was proportional to the firebrand mass in this study, which is the same as those from experimental studies performed for full-scale components and a full-scale structure. The slope of the relationship of the projected area and the mass of firebrands was the same under the same wind speed and was affected by the applied wind speed within this experimental range.

Introduction

Large outdoor fires, such as urban fires, wildland-urban interface (WUI) fires and wildland fires/wildfires are an urgent problem across the world. WUI fires are frequently seen in the news, however oftentimes called wildfires mistakenly. WUI fires, which happen where communities and wildland vegetation coexist, have resulted in loss of life and property damage. In Europe, both Spain and Portugal have suffered large WUI fires, as well as California (USA) in 2017. Japan, whose cities are densely populated, has more of an urban fire problem. These may or may not be produced after the occurrence of strong earthquakes. The recent 2016 Itoigawa City fire that occurred in Niigata, Japan, is an example where no earthquake was present, but a large-scale urban fire developed.

One of common features in both urban fires and WUI fires are structure ignitions [1]. While both urban fires and WUI fires are complex, it is possible to develop scientifically-based mitigation strategies by attempting to understand how structures are ignited in these fires. Post-fire disaster investigations have pointed to the significance of firebrands, or embers, as a leading driver for both fire spread and structure ignition [2], [3].

Firebrand research may be divided into three important research areas, firebrand generation, firebrand transport, and ignitions by firebrands. Past research on firebrands have focused on firebrand transport, with little research conducted on firebrand generation and ignitions by firebrands [4]. Recent development of the firebrand generators and studies using those firebrand generators have advanced understanding of vulnerabilities of structures to firebrands significantly [5]. Nonetheless, far less progress has been made on understanding firebrand generation from structures. Understanding the characteristics of firebrands generated from structures will advance the understanding of firebrand transport and structure ignitions by firebrands.

Firebrand generation from structures was investigated both experimentally and during post-fire disaster investigations. In previous research on experimental firebrand generation, several actual structures were burned and firebrands were collected by polyurethane sheets [6], [7], along with other data. The size of firebrands was determined by measuring holes on those sheets and it was reported that the size of 85% of firebrands captured by those sheets were less than 0.23 cm². In another study, structure burns in wind tunnel facilities were performed and firebrands were collected by pans filled with/without water. Eighty-three percent of firebrands collected by pans filled with water were between 0.25 cm² and 1 cm² [8]. Firebrands collected after a three-story wooden school burn experiment reported the lengths of most of firebrands were between 1 cm and 3 cm. In post-fire investigation of an urban fire, firebrands were collected and also analyzed [9]. The projected areas of most of firebrands were smaller than 10 cm² with the mass of each firebrand less than 1 g. [9]. Past experiments have focused on investigating firebrand generation by burning entire structures. While it is realistic, conducting many full-scale experiments of structure firebrand production is costly and not easy to undertake. As a result, the authors have embarked on an experimental program to determine if smaller-scale experiments may produce useful insight into the firebrand production process.

First, an attempt to understand firebrand generation from an actual residential structure was made. During firefighting training with an actual residential structure, firebrands were collected with pans with water [10]. Second, an experimental burn under similar wind speed was performed with a simple structure made with Oriented Strand Board (OSB) and wood studs [11]. Firebrands collected from the simple structure had similar mass and size classes to those collected from the actual residential structure. Thirdly, a repeatable experimental method with full-scale wall assemblies were developed and showed possibility to predict characteristics of firebrands from those experiments [12]. In addition, effect of sidings on wall assembly was investigated using the same methods [13].

The objective of this study is to investigate if experiments using mock-ups of full-scale wall assemblies may provide insight into firebrand generation from structures. Firebrands generated in the current work were collected and compared with those collected from previous experiments that made use of full-scale walls and a full-scale structure, constructed of common building materials [11], [12]. The wind effect on firebrand production using a newly developed experimental method using mock-ups of full-scale wall assemblies was investigated.