Take the Surprise Out of BUILDING COLLAPSE
FIRE FIGHTING BASICS
Everyone has seen news coverage of fire buildings collapsing “suddenly,” trapping fire fighters. But was the collapse as sudden as the media would have us believe? There are many indications of weakening and possible collapse of the building.
If these collapse indicators are routinely recognized and channeled to the officer in charge of the operation, injuries to personnel from collapse can be eliminated or at least lessened.
Collapse! One of the most dreaded words in the fire service.
Most of us have witnessed firsthand or have seen pictures of a collapse scene. And for fire fighters around the world, the picture doesn’t need much description—somber fire fighters clawing and risking their own safety to locate and remove brother fire fighters trapped under tons of rubble that just moments before was the scene of a fire fighting episode. It can happen in many ways…
- A supermarket roof collapses, hurling fire fighters to the floor below or dropping them to their deaths, trapped in the confines of a blazing cockloft.
- A commercial occupancy, converted many times, experiences a large cellar fire. Its overburdened first floor collapses, carrying fire fighters to their deaths.
- In a small suburban hamlet the trussed roof of a house of worship loses its strength and tumbles, trapping two fire fighters operating in its nave.
- A fire in a supermarket cellar has been punishing fire fighters for a considerable time. The first floor lets go, trapping 13. One is killed. The recovery of the other 12 a miracle.
- At a “routine” fire fighting operation, a tower ladder team is conducting an outside attack on a heavily involved, vacant frame structure. The building wall lets go, shearing off the tower basket from the boom. The basket and its two fire fighters fall 30 feet to the street below.
At the tragedies listed above, and at practically all others to my knowledge, sufficient indications were present to warn that not only was the collapse a distinct possibility, but was very probable in most cases.
Collapse indicators are many and varied. Some indicators stand by themselves as a major signal of collapse. Others may be minor but contribute to the overall assessment picture of the building’s stability.
Among the size-up factors that must be considered when determining the possibility of collapse are building construction, occupancy of the building, the length of time the fire has burned, size of the fire, location of the fire, presence of bulges and cracks, the condition and behavior of smoke, sounds, distortion in windows and doors, large-caliber stream use, water runoff, storage, exposed steel construction members and cast iron supports, floor sag, sliding plaster and dusty atmospheres, roof formation, ground support, and the age of the building.
Building construction can strongly affect the risk of collapse, plus how it will fall, where voids will be found and where people will be trapped.
Construction (for studying collapse potential) can be grouped into two major classifications. The first, unframed, refers to structures whose walls support the floors and roof assemblies. Failure of any wall will cause extensive collapse with large amounts of crushing debris. Rescue and search will be a long, difficult and dangerous operation. Voids will be formed by structural members, machinery and furnishings.
The second broad classification of construction is framed structures, in which walls do not support floors and roofs. These buildings are erected as a steel and/or concrete skeleton. Horizontal platforms are not supported by outside walls. Rather, the walls are hung on the skeleton for their support. These buildings will withstand a tremendous amount of fire stress without much danger of extensive collapse potential. For example, an explosion ripped through a high-rise framed office building in New York City. The walls on almost all floors were blown out to the street below. When fire fighters arrived, the building was still standing and fire fighters could safely search all floors for injured. Under fire conditions, collapse in these structures will be relatively local and not nearly as extensive as is the case with the unframed structures.
In order of resistance to collapse, construction classification is further broken down into four categories:
- Fire-resistive structures. They contribute very little fuel to the fire, are usually framed construction, withstand the heat of fires well, and collapse is rare or local.
- Heavy timber type construction (mill construction), featuring heavy masonry and brick walls, heavy dimensional timber used for columns and beams, heavy thick wooden floors, collapse only after many hours of fire fighting, and lack of concealed spaces.
- Brick and joist type construction has brick or masonry walls supporting floors and roofs. Overloading is the most common cause of collapse during a fire in this type.
- Wood frame construction has balloon or brace frame type. Collapse in these structures is rare during interior fire fighting. When the structures eventually do collapse, the fire is of such magnitude that the interior attack has been abandoned. One must be aware that if collapse of these structures is not total, a second collapse can be expected.
- Noncombustible construction (the most potential to collapse under fire conditions of all other construction) is mostly one story with masonry walls that have little fire load and often with corrugated steel decking on the roof.
The new lightweight 2 X 3-inch or 2 X 4-inch parallel chord truss construction is making wood frame buildings and brick and joist buildings even more liable to sudden collapse. These parallel chord trusses are constructed of gusset plates or “gang nails,” small squares of sheet metal pressed to create rows of triangular points. These gusset plates are then pressed into the joints of the chords and webbing holding the truss together.
Under fire conditions, the gang nails lose hold and the truss fails rapidly.
Noncombustible constructions use roll bar trusses placed for roof support at spans of up to 6 feet or more on center. The truss’ top chord under compression is laid on the enclosure walls to support the roof. The bottom chord under tension is tied into the top chord by a series of triangles. In a serious fire situation, failure of the roll bar truss is assured within 5 to 10 minutes after it has been exposed. Because the construction of a noncombustible structure is light, a major concern is the safety of the ventilation team on the roof.
Once on the fire scene, the commanding officer must act on any collapse signs that manifest themselves either visually from the exterior or via communication with the interior fire fighting forces. The officer must call for any help needed in determining doubtful structural stability (building department personnel, structural engineers, etc.). Any reported conditions must be noted immediately and information recorded in some type of fireground log. If the information received is sketchy, have it investigated by someone whose opinion is valued. In any event, begin record keeping. Note the location of the forces assigned, position of logistics, signs of collapse, etc. Seemingly insignificant data may give support to strategic decisions that may have to be made rapidly in the near future.
—photos by Bob Pressler.
Additional collapse indicators: Are there exposed iron or steel beams within the structure? Are there parapets (free-standing walls extending above the roof level)? Parapets have no sheer stress strength, and if they fall (as in a taxpayer-type construction) they could bring down the entire facade with it. Is truss construction evident? Truss construction in a structure may be determined by the classical hump in the roof; by large, open, unsupported areas (as found in bowling alleys, supermarkets, skating and dance halls, bingo parlors, etc.); or from pre-fire planning.
Building occupancy offers another clue about collapse. Due to economic conditions, failing businesses, or new proprietors, the building originally designed as a soda store could become an appliance warehouse. These occupancy storage changes may have increased the load bearing on the structural members of the building, a load the building and the structural members were never designed to withstand.
Increased demands by the commercial world for ventilation and air conditioning also cause occupancy load changes. The roof team can discover large heating, ventilation and air-conditioning (HVAC) systems in place on a roof that was only designed to support an added snow or rain load. Other add-on loads that may be found at the roof level are large signs and marquees with their tremendous sheer pull on parapet or free-standing walls. In high-security areas, the roof team may discover heavy steel plating on the roof or floors.
How long has the fire been burning? Remember, once exposed steel reaches 1100°, it expands 9 1/4 inches for every 100 feet of length. The expanded material must go somewhere. The beam will either twist and drop its load of roof beams or floor joists and the horizontal structural members they support, or the beam will push a free-standing parapet or a wall out of plumb and to the street.
The size of the fire is another indicator. If two or more floors are heavily involved, the command post should put a priority on searching for additional signs of collapse.
The location of the fire is another signal. If the fire has extended to a cockloft where truss construction is used, there is a very real danger of collapse. If a truss roof or a cockloft of truss construction is so in need of venting that a hole must be cut, we are so near to a collapse situation in this instance that to continue to operate on the roof would spell disaster. A heavy, difficult fire condition not easily controllable in a cellar location should give reason for collapse planning.
Bulges in the wall should be observed from the building line. Walls out of plumb have very little strength left; wood frame walls out of plumb may and do fail suddenly and in one piece, dropping all its interior contents pancake fashion.
Cracks either are seen or indicated by smoke or water seeping through the exterior wall. This indicates an unnatural, unplanned opening in a wall from inside to outside. A crack is not necessarily a panic situation but, if in doubt, mark the extent of the crack and have it observed periodically. A traveling crack is a major indicator of building movement and eventual collapse.
Heavy smoke indicates abnormal pressure on the interior structural members. Only 1-psi lateral pressure is necessary to cause failure of a 12-inch brick wall. Smoke coming through the bricks could easily mean an early sign of a crack, failing mortar or a sagging wall. In any event, it is a definite sign of an opening existing from outside to inside.
Sounds inside or outside the building must be reported immediately. Creaks and groans are cause for caution. During an immediate defensive (exterior) attack, fire fighters may hear noisy indications of furnishings falling unseen from level to level. Yet, collapse of the structure’s interior may not give a loud unmistakable noise. For example, fire fighters momentarily attacking a vacant five-story building from the exterior to gain headway heard a soft whoosh. Access to the first floor hall showed the roof bulkhead enclosure at the first-floor stairway. Sounds may also be very evident from the inside only.
Distortion is another exterior indicator that the building is not of the same strength as when it was constructed. Watch for windows that are sagging, door jambs that are of a different shape than the door, and doors that suddenly move untouched from one position to another.
The effects of a large-caliber stream (800+ gpm) could cause problems leading to structural collapse. For every minute the stream is in use, 3 1/2 tons are added to the live load of that floor. Time is another factor to consider if the large-caliber streams have been operating for some time with no visible effect on the fire. This would indicate that the fire is of greater magnitude than what was originally sized up. There already may have been some undetected interior collapse. If the water on the walls turns to steam, the building’s shell is too hot, steel must be at its failing temperature and the mortar must be crumbling.
Know where the water runoff is. Is the water running out of the building, or adding to the live load that the burned structural members are forced to support? How is the water leaving the building? If it is coming through unnatural openings (cracks) the load and pressure inside must be great and the structure is weakening.
Stock and storage practices are of great importance to collapse size-up. If the interior live load is not commensurate with the original designed use of the building (a clothing store stacking appliances in large quantities), this must be known immediately and transmitted to the commanding officer. Remember, overload of structural members is the most common cause of brick and joist collapse. High piling will cause fire to gain headway due to lack of penetration of the extinguishing streams. An absorbent type of stock such as mattresses, rags, paper or cardboard rolls will greatly increase the live load in the building since this stock retains the water from the fire fighting streams.
Interior construction features should be noted and communicated. Interior exposed trusses and unprotected steel in the major fire area are an important factor, since at 1100° steel expands and at 1300 to 2000° steel fails.
Sagging floors should also be transmitted. Water-laden floors are a danger sign in themselves; however, if the water is pooled in the center of a large area (floor or roof), the collapse danger is multiplied.
Command responsibility must react to stability questions of the fire structure In one or more of the following ways:
- Record the information at the moment.
- If there is doubt as to the relative importance the report will play in the decision of strategy change, have the information checked out by a more competent evaluator. However, if the report received from any source on the fireground is of a serious nature, action must be taken immediately and reevaluated later.
- Revise strategy immediately if necessary.
- Evacuate the forces from the interior.
- Plan for an exterior attack.
- Remove forces, apparatus and equipment from the collapse zone.
An examination of supporting wall beams should be made to determine the status of the beam-end support. If new wood shows on the floor beams where they enter the wall for support, this is an indication that the building or its parts are moving.
The coefficient of expansion for all structural materials under fire conditions is uniquely different. In other words, all parts of the building will expand and contract at different rates during a fire. This causes bonds to break and one type of material used for support to move away from another when subjected to the stress of fire.
Unusual conditions on the roof must be communicated. Look for sagging from a heavy unnatural load such as air conditioning, pumps or signs, or any covered opening (closed skylights, scuttles, bulkheads, etc.). Steel plating used for security purposes to cover roof and/or floors in commercial type occupancies definitely increases the fire load. In most cases, this is the sole reason for the sudden collapse of these buildings under fire conditions. The classical hump design of the dreaded bow-string truss construction may only be detected by the roof team. In any event, all these vital pieces of information must be made known immediately to the officer in charge.
Also give serious consideration to ground support of the structure and marginal, beach or landfilled areas. The additional fire fighting load of water, manpower and equipment may cause the load or its support or ground area to shift.
The age of the structure will also come into play in the size-up. Wood is at its strongest when it possesses the natural moisture it once owned. As wood ages and dries out, driving the natural moisture out, it becomes weaker. Absorbing, shedding and reabsorbing water and moisture from rain and dampness also causes the wood to decay and rot. Now, not only are the structural members weaker, but they have the ability to absorb water from hose streams, adding weight to the already weakened and fire-stressed building.
All these collapse signs will never exist at once. They are all listed here together so that they may be more easily recognizable and understood by all the fireground forces. Collapse indicators must be familiar to all to be easily recognized by all.
At all serious structural fire incidents, both fire fighters and supervisors have a tremendous responsibility. Once human life is accounted for by search, rescue and removal procedures, the stability of the structure must be our next emphasis.
All information gathered by fire forces on the condition of the building must be related to the command post. The commanding officer must not fail to act on this data, because it is only through these small, varied pieces of information that a bigger size-up picture can be put together. And once the size-up picture becomes clearer, the overall attack strategy can be reformulated, reevaluated and, if necessary, radically changed. Reports from the line forces should also be accompanied by some type of value judgment or remedy from the sources reporting the indicator of structural weakness:
- “Heavy water accumulations on the first floor. No other weakening indicators. We are attempting to open flooring to drain the excess load.”
- “Roof is very spongy and loaded with heavy appliances. Recommend vacating the roof area and below.”
- “Roll bar trusses are holding up the roof. Recommend abandoning roof operations.”
- “Chief, we are in exposure no. 2. Plaster is sliding off the walls. There is no apparent reason for it, and it is accompanied by noisy rumblings. The building appears to be moving.” This would be a mayday message—a message of imminent collapse.
The command post must not passively wait for needed information from its line units. Officers in charge should actively seek it out, especially when any doubt exists about the stability of the structure.
When reports questioning the building’s integrity are received, the chief must act. At the very least, he must record all information. If any doubt exists as to the validity of a report, he must have it investigated. The officer in charge may have to change his strategy if necessary (defensive instead of offensive), or pull out his forces immediately and set up an exterior attack and secure the collapse zone.
All these actions must be done immediately, for in a minute it may indeed be too late.
Once the collapse is determined, a defensive attack employed and the collapse zone defined, our major problems will come from the fire forces themselves. Overzealous, meaningful activities will have to be controlled. Equipment must be repositioned and manpower not essential to the operation must be released—if not for other fire department duties, for their own safety.