When it comes to the protection of adjacent buildings, commonly used terms such as Exposure Protection and Spatial Separation should be familiar to most. The need to protect adjacent buildings arises from the possibility of spread of a fire from the building of origin to a neighbouring building/structure due to heat exposure from the fire by radiation and/or convection or from flying brands from the fire. Building codes across the world have established design criteria aimed at protecting the combustibles within, and the exterior of, a building exposed to a fire. Local construction codes must be reviewed to identify the applicable spatial separation and exposure protection requirements. The purpose of this blog is not to review the building code provisions, but to provide an overview of the contributing factors of potential fire spread between adjacent buildings and the various generally acceptable methods to protect such fire spread.
An important term related to understanding the spread of fire from one building to another is Exposure Severity. To understand the term Exposure Severity, defined as the intensity of an exposing fire, we must first look at the primary contributor to fire spread between adjacent buildings, namely the Radiant Heat Transfer. Radiation exposure to an adjacent building from a fire in a nearby building can be a result of the radiant heat transfer from an interior fire. The radiant heat of an interior fire is emitted through windows and other exterior wall openings. Radiant heat transfer can also result from flames issuing through windows of the burning building and flames issuing from the burning façade of the building. Exposure Severity is me
ant to be the measure of the radiation level developed per unit window area by the exposing fire (I0), calculated using the below formula.
By inputting the lowest pilot ignition temperature among most cellulosic products to the above equation, a maximum tolerable level of radiation at the façade of an exposed building has been established as 12.5 kW/m2 [1]. The duration of burning is considered vital as the longer the fire burns, the fire spread increases the radiator size and eventually the radiation levels become high enough to require unattainable separation distances between the buildings to limit the effects of radiation. The duration to manual intervention by the local fire department also plays a part in determining the required separation distances. The National Building Code of Canada requires that the separation distances for a non-sprinklered building be doubled where the time for the first fire department vehicle to arrive on site exceeds 10 minutes in 10% or more of all fire department calls to the building.
Most of the minimum separation distances identified in codes and standards originate from an evaluation of the building construction and contents based on their resistance to ignition from the radiation exposure of an adjacent burning building. The effect of convective heat transfer was not considered in determining the separation distances where the source of fire hazard is through openings in the façade of the burning building since pilot ignition at greater distances can occur from radiation than those at which flame impingement and convective heat transfer can cause an ignition.
In addition, it is worthwhile to discuss the formation of a thermal pump through an opening (typically a window) on the wall of a burning compartment. This results from the buoyancy differences between the hot combustion products and the ambient air outside. This difference provides a positive means of furnishing fresh air to the combustion and discharges flames and combustible products through the opening. This contributes to the exposure severity and, as it is understood, any type of fresh air supply to the room and ventilation would contribute to the exposure severity. Several other factors that affect the exposure severity are discussed in the NFPA Fire Protection Handbook [2], Chapter 3.
Based on the above discussions, common means to protect adjacent properties from fire spread include; providing adequate separation distances, limiting combustible construction of the building where considerable combustible contents are contained within the building, limiting openings in the façade of the building where sufficient separation distance cannot be maintained, providing sprinkler protection, etc. Further details on the various means of protection of adjacent buildings from fire are provided in NFPA 80A [1] and applicable building codes.
PLC Fire Safety Engineering (PLC) has utilized domestic and international codes over the years in helping our clients identify the required exposure protection and formulating alternative compliances in achieving the intended objectives of the codes and standards where a direct compliance to the requirement is not feasible. PLC has methods that allow us to quickly determine the required spatial separation distances for various types of construction. Please contact us should you require help with exposure protection for your building.
[1] – NFPA 80A-2010, Protection of Buildings from Exterior Fire Exposures.
[2] – NFPA Fire Protection Handbook, Twentieth Edition.
