Structure Survivability in the WUI

• Structure survivability is the result of the relationship between the structure construction and local intensity and duration of exposures (fire and embers).

• High fire exposures can readily cause direct ignition of exposed combustibles.

• Communities should assess both components (exposure and hardening) in the process of structure and community hardening.

Primary factors influencing the effectiveness of structure hardening/mitigation actions:

1. Defensive actions by first responders save structures: NIST WUI fire reconstructions have determined that most (> 90 %) damaged structures were defended and saved by specific first responder actions. However, the number of exposed structures can quickly outnumber statewide resources.
   
   – Defensible space contributes to making structures defensible by first responders.
   – Additional actions may also reduce potential exposures, such as removal of fuels before the fire arrives.
   – The reduction in exposure further increases the chance of survival.
    
2. Partial structure hardening impacts structure survivability: The impact of partial hardening on structure survivability is not directly proportional to the fraction of hardening actions implemented. As an example, doing half of the ember hardening specified in Table A "Structure and attached combustible hardening against ignition from embers" does not automatically translate to a 50 % increase in ignition resistance and structure survivability.
   
   – In low-exposure scenarios, partial hardening may provide sufficient ignition resistance. However, structure survivability cannot be guaranteed.
   – Effectiveness of partial hardening is inversely proportional to the exposure level and availability of first responders to conduct suppression and other defensive actions to mitigate the vulnerabilities remaining from the partial mitigation.
   – The impact of a partially hardened structure on the community is inversely related to SSD; partially hardened structures need to be further apart.
    

In low-density communities, a structure ignition will increase local ember exposures; observations have shown these ember exposures to be significant in the first 300 ft downwind. Locally there will also be increased exposures from fire (radiation and/or convection). These exposures may impact parcel-level combustible features; however, there should not be any direct fire exposures to adjacent properties due to larger SSD. The impact to the adjacent properties and community will be indirect in terms of fire, and possibly low for ember exposures (because of the large SSDs). Therefore, in low-density communities with large SSDs the impact of structure ignition on the surrounding structures is low.

In moderate-density communities, a structure ignition can ignite an adjacent property under a “favorable” wind direction and other local conditions. The embers generated from a burning structure will generate significantly higher exposures to the downwind structures relative to the low-density case due to separation distance. Compared to low density, the relative impact of a structure ignition to the surrounding structures can be seen as medium.

However, high-density communities are in a very different exposure category. Here the ignition of a structure will almost invariably result in the ignition of one or more adjacent properties and will likely result in the loss of a significant fraction of the community, as evidenced by several large loss WUI fires in the U.S. This is not only because fire spread occurs very easily between tightly spaced structures, but also because it is very difficult to contain a fully involved structure fire, even when only moderate winds of 10 mi/h to 15 mi/h are present. Fire spread is difficult to stop as it is very challenging to remove/block the heat between residential buildings when they are constructed 6 ft to 10 ft apart. In high-density construction, a single ignition can have a disproportionate impact on the overall community losses.