What percentage of involvement constitutes a Defensive posture for a Heavy Timber Arch?

• A. 10%
• B. 25%
• C. 50%
• D. 75%

Answer: (B)

Defensive posture is chosen when interior fire control is no longer safe and operations must be conducted from the outside. For a Heavy Timber Arch, the point at which you switch to defensive strategy is when about 25% of the structure is involved. At this level, the combination of heavy timber members and the arched design makes the risk of rapid structural failure and collapse significant, so interior attack becomes too dangerous. You shift to exterior water application, exposure protection, and preserving unburned areas while watching for signs of redistribution or collapse. If involvement is only around 10%, interior operations may still be feasible; very high involvement, like 50% or 75%, indicates extreme danger and collapse potential, but the standard guideline used here identifies 25% as the tipping point to a defensive posture.