Question: I would like to ask a question of someone who has a record of actual interior structural firefighting. I am sort of tired of getting the same trite answers to questions. My concern is this: we have so many firefighters burned today. Why is flashover such a common event?
Answer: Well I dont know if I fit your expertise criteria but I would like to give your question a shot! See, from my perspective, we did not have that many flashover events. As a matter of fact, in comparison to the amount of interior structure firefight operations we had (7 to 15 a day) we had none. At least we werent putting firefighters in burn centers every day.
There are a few reasons for this problem. The most blatantly important I will leave for last. Energy conscious America had tightened up the buildings. The pressure of interior fire no longer is able to easily cause failure of some enclosure as a window, wall, door. Thicker walls filled with insulation material and triple-pane windows are contributing to the buildup of heat to the flashover point. Note: Should the materials hold in the fire conditions from breathing we may really have the backdrafts that our buffs believe we have on the fireground. Security conscious America has wrapped our commercial buildings like vaults! The rears of these occupancies are covered with concrete block and road-construction-grade steel plates where there once were windows and doors.
Remember, rear ventilation of a commercial occupancy is one of the critical tasks that must be accomplished. We have faster burning fire and HOTTER fire today. One pound of building stuff: chairs, drapes and rugs, flooring, etc., when put in a box and burned, would give off 8,500 BTUs and no more over time. Now, when we put todays space-age replacements for the material that constructed those furnishings of the past in a box, one pound gives off 17,000 BTUs in a much shorter time.
Building construction trends
We have open trussing on every floor of a building. Fires that penetrated ceilings and cocklofts only ate 16 inches on center, full dimension wood bays. Now it has play with 1,500 to 3,000 square feet of match sticks. We get to the scene earlier than ever before because of America becoming alarmed automatically. Smoke detectors give early notification and phone alarms follow. This is truly a great thing, but you have to remember you are at the ENVELOPE earlier and usually just before flashover. Flashover condition would have to be vented in the old days just to get in there! Now our protection levels and positive-pressure breathing devices allow us to probe the fire compartment without the ventilation needed to cool the area.
Finally, it is the murderous effects of not enough people showing up at structural fires to perform the critical tasks necessary to prevent flashover from occurring (in most instances). You cannot trickle in the tasks of a building on fire. You cannot stage crews until some vested person decides what tactic is next on his or her waterproof checklist. You cannot afford to have teams of three or four firefighters performing the same single extinguishment support tactic. You cannot have hose lines move fast enough above the first floor with only two firefighters on the entire line! So there you are! But that is only my opinion!
Question: In your view, is the risk of backdraft explosions increasing because modern-day houses are better insulated? Or is it possible that the risk is lessened because of the increased use of fire retardants? These questions are based on the assumption of a relatively small, self-contained fire, with low ventilation.
Answer: To give some sort of a reply to this question can either be short and simple OR long and complicated. I will give you both.
Short and simple: Yes!
Complicated and long: In my view (your word) the terms backdraft, smoke explosion, flashover, and rollover are getting all confused with one another. First, backdraft and smoke explosion are two different phenomena. One is virtually impossible to find on the fireground, and the other is becoming more common. The problem is that the texts of the past used the terms as if they were synonymous. They consistently referred to backdraft/smoke explosion. In my experience, nothing could be further from the truth; OR you need to make up another term for the latter.
Backdraft is a condition that exists within an enclosure (building, for the rest of this letter) and is caused by a fire condition within that building when the system remains closed. The heat builds beyond the fire point or flash point or ignition temperature of any fuel. Pressure continues to increase, and the only thing that decreases is air.
Now, the only part of the fire triangle (tetrahedron) that is missing is air. The textbooks here miss the boat by saying that backdraft can be prevented by vertical ventilation! Nonsense! If the conditions are present for backdraft, it will go in all its glory of rapid energy release. Period! As firefighters, we can only choose where it goes! If the condition gets its air from the bottom of the enclosure the door, the air rushes in or is sucked into the process and the release of combustion is rapid and expansive and blows through the front door and moves other building structure to possible collapse. However, if we choose to open the top of the envelope (building) where the pressure is so great that the air cannot be sucked into the process, the process must exit to the air! Now we have the same energy go off as flame through the roof and no structural damage by concussion or detonation. You did not prevent backdraft; you just professionally chose where it should release itself relatively harmlessly.
With all that said, it is very difficult to have a true backdraft condition. The building parts usually fail before that can occur. (More on this later, if you want.) However, smoke explosion is another matter!
Now, among other things, energy conservation plays a dominant role in this phenomenon becoming more frequent on the fire ground. Buildings do not vent themselves through flimsy panes of glass or thin layers of horizontal surface above the fire condition. The pressure extends the byproducts of incomplete combustion (which are flammable and explosive) into remote areas of the enclosure. These areas include adjacent horizontal spaces that would not ordinarily have been extended to. They also include areas above the fire, such as cocklofts or attic spaces, and into voids of today, such as truss lofts between each floor of multi-story buildings. The gaseous product is flammable and explosive but does not have sufficient heat, as does backdraft. If this condition receives heightened heat energy from an opening below it or from flame being pushed horizontally into it, it certainly will ignite with more force than even flashover. Therefore, in our vernacular, if we pull the ceiling in the fire room before opening a hole above the space first, we will have an explosive force that creates a pressurized fireball that envelopes the room area and blows down the weaker ceiling membrane. If the nozzle is pushing fire horizontally as it extinguishes the immediate flame, that tick of energy will ignite the explosive gathering of combustible gases that found its way into other horizontal spaces.
The firefighting premise here is that this phenomenon may be reversed by prompt and proper ventilation techniques. What is proper? you ask. OPENING UP OF A STRUCTURE IN A SYSTEMATIC MANNER TO RELEASE THE PRODUCTS OF COMBUSTION AND REPLACE THEM WITH COOL FRESH AIR. Sound familiar? Notice I did not say Blow into the building envelope with forced air!
Now to the heart of your question. Our energy-conscious world has created a tighter envelope for us to ventilate. Walls are thicker and filled with insulation in both cold and warm climates. (I live in Florida now, and my walls are 8 inches thick.) Windows have double- and triple-pane glazing. This alone is a ventilation nightmare for both interior- and exterior-venting firefighters. The breaking of glass no longer is the evidence of effective ventilation of that space. Often one or two of the glass panes are broken, and for some reason the second or third remains intact. This is the reason that this explosive event has been called cold smoke explosion; it is so remote from the actual fire location as to not normally be planned for. The key here is that smoke explosion, flashover and rollover need a couple of things to GO: air, for sure, but also some additional heat energy in the form of a flame, flash, or spark to ignite. Backdraft needs nothing but air!
Question: We currently carry 600-700 feet of 3-inch hose and 1,100 feet of 4-inch in our hosebed. We are buying two new trucks with rear discharge. Our 3-inch is approximately 25 years old. I would like to switch to 2½-inch, approximately 400 feet with 200 feet preconnected and a nozzle connected to it. We would also be increasing our 4-inch to 1,400 feet and load it so we could lay a dual line if needed. We use 1¾-inch for our attack lines. We have adquate water supply with hydrants and short supply lays. Some want to keep the 3-inch for an attack line and also buy 2-inch attack line. We do have a bidder for our 3-inch hose. Any suggestions? We are a typical bedroom community 70,000 in the Midwest.
Answer: You send a lot of information, but I am missing some that could be vital for my opinion to make sense. I will attempt to discuss my points (remember, they are mine only) with some assumptions.
Write more on this to me please?
Question: Opening a door to a fire occupancy: Do we stand to the side of the door at the building wall or behind (in front of) the door? There are two theories here in this department. One says that we should use the door for protection from the extending fire condition and the other says that the wall will keep us protected if the fire BLOWS out of the opening, burning awaiting firefighters. What is your opinion?
Answer: I think this dilemma is
set in motion by card-table drill periods. Let me get
something off my chest that should be said by every
professional fireperson often in his career. You study or
drill for two different operations in most fire
departments if you are interested in performing the job
very well AND passing promotion tests.
It seems that the question has no basis for experience at the locked door to the fire occupancy? Sorry!
If you are a police officer, you cannot make much noise in front of the flimsy panel door and must be successful at the first noise and then stand aside from the probable bullets.
But firefighters tend to make a lot of noise as they use differing procedures and steps to force doors behind which is the cause for the alarm.
You cannot stand aside and be successful at pulling cylinders and using the key tool, OR using conventional forcible entry OR even the new hydraulic force tool used on steel buck doors.
The main thing is to be successful and not get hurt by the escaping conditions inside the occupancy before water starts and you have a chance to begin search procedures.
Learn to set up control procedures for your door before you apply the final pressure to the lock assembly. A simple piece of rope looped at the doorknob will do if a firefighter stands on the working end that hit the floor. There are other makeups that resemble the effectiveness of the rope, but you get the idea.
You should have a pretty good idea of the fire condition behind the door before you put the final force operation to work.
Size up for forcing doors:
No. 4 is the part you are asking about. A second thought here.
Most of the time, the out-of-control door into the fire apartment causes the condition to escape to the public hall. The forcible-entry / first-hoseline teams are not the injury problem. It is the escape of superheated atmosphere to the ceiling and the stairs to the upper floors on which may be either an orderly escape by civilians or very courageous acts being performed by your team members. In either case, your open door throws havoc and injury into the operations.
Set up the locking devices for failure, secure the door for control, and force the door.
Question: In Building Construction Related to the Fire Service (top of page 53), steel and reinforced concrete are being compared. The book claims that steel expands in temperature at the same rate as concrete. I have been instructed that steel expands and/or warps at approximately 1,000 degrees Fahrenheit. At what temperature does concrete fail? And, does it change in temperature at the very same rate as steel, given that the variables such as thickness are equal?
Answer: First I don't know what book you are reading. I have a couple or more and am not familiar with the title you reference in your question. Basically I use Brannigan and Dunn. Could you forward the name of the author to me?
In the meantime I would like to discuss your question in relation to the study material I have digested over the years (most of which is still on a massive bookshelf in this room) AND on my experience with both of the materials you mention.
First, steel and concrete have similarities: both are elastic materials, both (as do all materials on earth) have a coefficient of expansion but certainly not the same!
Second, you say, in the hypothetical example you use, that all variables are constant. That is impossible. Concrete will never have the conduction rating or rate that steel does. That alone accounts for the more rapid reaction to heat over time that steel has and why it is so dangerous in a fire condition.
Next, if steel is heating to a dangerous failure point, hitting it with water will stop that process and in most cases return it to a safer state and have the expansion reduce! That is how elastic it is. Concrete heated to that temperature (almost impossible to be a threat because the time required would have caused us to vacate the occupancy or enclosure) and hit with water will cause failure and shattering of major portions of the mass.
Now, further, concrete is used as an insulator for steel to give it time to extend the ability to absorb and conduct the heat necessary for it to fail. Next, steel wants to expand; it will move once it is all heated to the temperature necessary 800 to 1,000 degrees Fahrenheit.
Steel will move make no mistake about it. If the walls supporting it are weak, it will push the walls out 12 inches for every 100 feet of steel. If the walls and buttresses are strong, the steel will find the 12 inches by twisting itself and dropping its load of support rafters for floor or roof assemblies.
Next, if that steel continues to heat (as seen with covered pier fires and large mill buildings), it will lose all strength at 1,800 degrees and fail much as if a Happy Birthday ribbon were used to hold up its original load.
Concrete danger is really that there may be water trapped within it during the pour-and-drying process. This water expands at 212 degrees and has a small steam explosion that blows pieces of concrete off the mass. This is mostly called Spalling. The danger of collapse of concrete is during the pouring and curing process. Fires in buildings under construction are the concern here.
Exposed steel support members: reinforcing bars and tension assemblies are exposed steel and will fail around 800 degrees.
I don't know what else to tell you. I would get a copy of Brannigan's Building Construction for the Fire Service. I tell everyone in this business that if you buy just one book in your career, it should be Brannigan's; it will save your life. Read the chapters on steel and the one on concrete construction; it is more than enough information to make life-saving choices on the fire ground.