By John W. Mittendorf
Fireground time comprises numerous factors, including the time a fire has been burning, the time of detection and reporting of the fire, dispatch time, and time of response. In Part 3, we focused on the approximate time it will take you to make an “initial impact on a fire”. Notice that definition did not address the time it will take you to extinguish fire once you begin to apply water. Until a fire is extinguished, it will continue to weaken a structure. Remember these three factors that work against you during suppression operations:
- Fire weakens a building.
- Gravity wants a building.
- Water weighs 8.35 lbs per gallon.
So, fireground time (once you arrive on-scene) is actually a combination of the time it will take you to apply the first drop of water, and the additional time it will take for extinguishment.
Another factor when considering fireground time is how much time the building will give you before it collapses. This “basic principle” has the ability to determine your mode of operation – offensive or defensive. If you are in an offensive mode, it will help you decide how long you should remain inside the building. With this in mind, you should develop a rough fireground baseline help you evaluate how much time you have before a structural collapse can occur when fire is exposing (and weakening) structural members. This is the baseline the author has used over the years:
- If fire is exposing lightweightt structural members for more than 5 minutes, rethink an interior attack.
- If fire is exposing conventional structural members for more than 15-20 minutes, rethink an interior attack.
This is a rough guideline, but it has proven fairly accurate in numerous structural incidents. Lightweight construction in residential occupancies is generally the same (i.e., 2×4 truss construction), despite where you live. Most lightweight construction will fail in less than 7 minutes when exposed to fire, and some types will fail in significantly less time, depending on the combination of dead and live loads that are imposed on the construction. However, conventional construction can vary from older “rough cut” 2X4’s to modern 2X4’s, 2X6s, all the way to post and beam type construction. It is imperative that you be familiar with the particular definition of conventional construction in your specific area and be familiar with its collapse potential. As an example, most residential conventional construction in Southern California comprises 2X6 members, and sometimes larger. When exposed to fire, you can expect this type of construction to last 15-20 minutes before collapse. If you take the time to develop your rough fireground baseline, favor the low side of the time scale so it allows you some leeway. Remember the famous fireground safety rule, “Falling debris always has the right of way.”
John W. Mittendorf joined the Los Angeles City (CA) Fire Department (LAFD) in 1963, rising to the rank of captain II, task force commander. In 1981, he was promoted to battalion chief and in the year following became the commander of the In-Service Training Section. In 1993, he retired from LAFD after 30 years of service. Mittendorf has been a member of the National Fire Protection Research Foundation on Engineered Lightweight Construction Technical Advisory Committee. He has provided training programs for the National Fire Academy in Emmitsburg, Maryland; the University of California at Los Angeles; and the British Fire Academy at Morton-in-Marsh, England. He is a member of the editorial advisory board of Fire Engineering and author of the books Truck Company Operations (Fire Engineering, 1998) and Facing the Promotional Interview (Fire Engineering, 2003).