Estimating Structural Stability

Use and misuse of the 20-minute rule.

NFPA Journal®, January/February 2009

An incident commander (IC) facing a structure fire deals with myriad factors when deciding which strategy and tactics to employ. When creating a plan of action and making offensive and defensive decisions, life-safety factors are paramount, and there are few factors more important to this decision process than structural stability. While the signs of imminent structural collapse are at times obvious, more often the stability of the structure is not completely known. In addition, structural conditions tend to get progressively worse until the fire is brought under control, so the IC must make sure to evaluate the structure’s stability continually.

Some ICs use an old axiom known as the 20-minute rule to help them estimate structural stability. While there may be other definitions, we define the 20-minute rule as: “When a heavy volume of fire is burning out of control on two or more floors for 20 minutes or longer in a building of ordinary construction, anticipate a structural collapse.” Despite its long use by the fire service, this “rule” remains simply a rule of thumb and does not take into consideration numerous factors that contribute to collapse.

Let’s quickly dissect the 20-minute rule to assess its validity and to ascertain its limitations.

Fire intensity, burn time, content loads, construction methods, and building materials all affect structural stability, and the 20-minute rule addresses all of these factors, with one exception: the content load, a major factor when determining structural stability. In general, ICs use the 20-minute rule based on what they consider a “normal” content load, an assumption that can be dangerous. Each content load is unique, and a large or fast-burning fuel load can accelerate collapse.

Is 20 minutes an exact time? Absolutely not! In most cases, determining when a fire started or even when it progressed to an advanced stage is impossible, so the 20-minute clock should start when the fire reaches an advanced stage. This advanced stage of fire intensity is commonly and loosely defined as a “heavy volume of fire,” but what exactly does that mean? What one IC perceives as a heavy volume of fire may be only a moderate volume to another. And the IC’s ability to evaluate fire intensity may be compromised. A large volume of fire inside a building may not be visible from the exterior or even on the interior if the fire is in a concealed space.

There is also reason to question the “two or more floors” component of the rule. Many one-story buildings collapse, and multistory buildings have collapsed even when fire occurred only on one floor. Yet, it is clear that, when a fire extends to more than one floor in a multistory building, the building has a greater risk of collapse.

The 20-minute rule is based on ordinary (Type III) construction, but a frame (Type V) building may be completely consumed in less than 20 minutes, and unprotected steel or wood trusses have been known to fail in much less than 20 minutes. Conversely, a fire-resistive (Type I) building would be expected to withstand a fire for a much longer period.

The 20-minute rule addresses fire intensity, burn time, construction methods, and to some extent, building materials. Thus, it provides a framework that can be helpful in determining structural stability as long as the IC understands the limitations and modifies the 20-minute time element accordingly.

An experienced IC knows the 20-minute rule means more than the risk of structural collapse. The probability of saving lives decreases over time as the fire increases in intensity. Different structures, ventilation profiles, fuel configurations, fuel loads, and other factors affect the actual time from ignition to flashover and collapse. An IC should keep that probability in mind while viewing structural collapse as a major consideration when developing an incident action plan based on a risk-versus-benefit analysis. They should also remember that the acceptable level of risk decreases as the building progresses toward probable collapse.