Structure separation experiments :
Authors: Maranghides Alexander , Nazare Shonali , Hedayati Faraz , Gorham Daniel , Link Eric , Hoehler Matthew , Bundy Matthew , Monroy Xareni , Morrison Murray , Mell William (Ruddy) , Bova Anthony , McNamara Derek , Milac Thomas , Hawks Steven , Bigelow Frank , Raymer Bob , Frievalt Frank , Walton William 
Published: September 2022

This report describes the experiments conducted during the first phase of a multi-phase project designed to assess structure-to-structure fire spread for structures in the Wildland-Urban Interface (WUI). The experiments focused on quantifying thermal exposures from auxiliary structures, e.g., sheds. The effects of shed sizes, construction materials, fuel loading, and separation distance on the ignition of primary structures (residential dwellings) with no wind have been studied and reported. Preliminary structure separation experiments using natural gas burners and an instrumented target structure were conducted to optimize instrumentation and experimental design for shed burn experiments. Full-scale fire experiments were conducted in which various sizes and types of sheds were used to generate typical radiative and convective heat exposures. Experiments were conducted indoors at the National Institute of Standards and Technology (NIST) to quantify exposures from small source structures (sheds) in terms of mass loss rate (MLR) and heat release rate (HRR). HRR was measured using oxygen consumption calorimetry. Large and very large sheds were tested outdoors at the Insurance Institute for Business & Home Safety (IBHS) test facility. The mass loss method was employed to estimate the HRR for the outdoor shed burns. In addition, indoor shed burn experiments with a target structure were conducted to assess target performance against exposures from varying sheds (construction, size, fuel loading) and separation distances. All experiments were performed without an artificially generated wind field. The preliminary experiments helped troubleshoot and provided insights into the airflow bias effects within the indoor test facility. The indoor shed burn experiments demonstrated that the mass loss measurements can be used to infer HRR measurements. The peak HRR (PHRR) was a function of construction materials, shed size, and fuel loading. Heat fluxes were measured and the effects of the burning shed on the target structure were documented under no-wind conditions as a function of separation distance, construction materials, shed size, fuel loading, and shed openings. The mass loss method was used to infer HRR for from very large auxiliary structures tested outdoors at the IBHS test facility. Generally, for a given shed size and fuel loading, the PHRR, which is related to the fire hazard, depended on the shed construction material. Combustible shed source structures generated a greater PHRR than non-combustible sheds, almost by a factor of 4. The exposures quantified in this study demonstrate that any auxiliary structures made from combustible materials can pose a significant fire hazard when placed next to the primary structure. For non-combustible source structures, the shed orientation with respect to the primary structure and the size of the door opening are critical factors. Sheds and auxiliary structures exceeding 120 ft2 floor area are subject to placement requirements with respect to residential structure(s) on the same lot through Chapter 7A of the California Building Code. The location and spacing of smaller sheds (e.g., under 120 ft2) may or may not be regulated and current codes often allow these smaller sheds to be as close as 5 ft from a residence or property line. The findings from this study can provide technical input for national, state, and local building codes to alleviate fire spread from auxiliary structures within WUI communities.