(Facts about structures are printed in regular type. Firefighting implications are printed in italics. Page references are to Building Construction For The Fire Service, third edition.)
A column delivers a compressive load along a straight path in the direction of the member. We think of columns as vertical but any member which delivers a compressive load, whatever the direction, is subject to the "laws of columns," which state that columns lose strength by the square of the change in length. For example, a 24-foot column can carry only one-fourth of the load carried by a 12-foot column of the same material and dimensions (12 squared is 144, 24 squared is 576, 576 divided by 144 equals 1/4).
Photo by Thomas R. Webster/Pensacola Beach Fire-Rescue
Flames break through the roof of a Pensacola Beach, FL, restaurant in April 1995. The building had been the city's first firehouse until 1965, when it was converted to a hamburger stand and later remodeled as a restaurant. Firefighters must be aware of concealed hazards in remodeled structures.
Take note of high steel scaffolding. The load is successfully carried on columns of hollow tubes about two inches in diameter because the structure is braced about every eight feet by connections (an important topic to be discussed later). The scaffolding columns are really a set of eight-foot columns one atop the other. Consider using a power saw to slice down along the column and cut all the connections. The columns would buckle and the scaffold would fail.
A theater collapsed when a connection attaching the balcony to the column failed. The connection was not only transferring the load of the balcony to the column but cutting the column into shorter lengths. Even though the loss of the balcony load lightened the load on the column, the increase in length made it impossible for the column to carry the remaining lesser load.
This was a problem in the Oklahoma City bombing when columns weakened by the loss of bracing floors had to be rebraced with steel trusses, some of which were manhandled into place by firefighters.
Note the construction of wood studs. Often, a piece of the studding is cut and placed from stud to stud about mid-height. This is sometimes erroneously called firestopping. In fact, its purpose is to "cut" an eight-foot stud into two four-foot studs one atop the other, thus increasing the load-carrying capacity of the stud wall and stiffening it.
When we discuss trusses, we will learn that the top chord (top member) of a truss is under compression and therefore subject to the laws of columns. Any member under compression, no matter if vertical, horizontal or diagonal responds to the laws of columns.
We must repeat a paragraph from part 1 (February 1996, page 103), which reads, "The shape of a member under compressive load is very important. The further the material of the member is placed from the center the stronger the member is. A hollow steel column can carry a far greater load than the same amount of steel rolled into a solid rod. Structural members under a compressive load tend to buckle. Placing the material away from the center reduces or eliminates the buckling tendency."
All things being equal, a hollow round steel column is probably the best use of available column material. However, other considerations such as use of floor space and ease of making connections are also important.
Steel columns are often H-shaped. Note that a circle could be drawn through the four points of the H. On the other hand, the strength of a beam lies in its depth. Steel beams are therefore shaped like an I.
It is an error which betrays ignorance to speak of I beam columns. For any one of several reasons, a steel column may be shaped like an I beam but it is not a beam, it is a column.
Excavation shoring used by contractors must resist the compressive load of the walls trying to cave in. In small excavations, it often consists of adjustable hollow round steel tubes. In a cave-in accident, the improvised wood shores should be of the next best shape, which is square. A four-by-four-foot cross-section piece is the same amount of wood as a two-by-eight but the shape of the two-by-eight is wrong, and it would bend and buckle.
Note the bracing of a free-standing block wall. Traditionally, it is made of wooden planks. A high wind hitting the wall sends a compressive load down the plank, which can buckle and snap. In addition, if the ground is sandy, rain will liquefy it and lower its resistance to the shear load of the stake being pushed backwards. In wind and rain conditions, avoid free-standing masonry walls under construction.
Deep excavations are usually braced by vertical soldier beams, which in turn are connected by horizontal walers, braced by diagonal rakers. Other arrangements may be found. The loads are twice what would be permitted in permanent construction. All this unprotected steel is in the combustible environment of plywood, fuel and plastics. Steel fails faster when it is overloaded. A serious fire could cause the bracing to fail. Buildings around the excavation would fall into the excavation.
It is vital that pre-plans provide for cooling the steel. If the steel cannot be reached by hose streams, very rigid controls should be placed on the construction fire load. It would be well if the extreme hazard is presented to the builder and the building department in writing. "Gee, nobody ever told us THAT!!!" may make the fire department look incompetent after the disaster. There is no disaster experience to point to but all the elements are present (page 278).
Beams may sag and give warning but column failure is often sudden and catastrophic. Any sign of column failure should lead to evacuation and competent assessment of the risk of collapse.
The failure of unprotected steel basement columns supporting a concrete floor caused the flaming death of four Pennsylvania firefighters who responded on mutual aid. No one on the scene was familiar with the hazardous nature of the building.
Do not forget that the stud walls of wooden buildings are really a series of wooden columns. When the wall is involved in fire, collapse is imminent. Never get into the collapse zone to cover the exposure of a burning wooden building to the adjacent building. In recent years, at least three firefighters have died in this manner. Use long-range appliances. Pre-plan for safe exposure coverage.
A wall can be thought of as a column extended along a line. A wall may be carrying part of the load of the structure or it may simply enclose a space.
A bearing wall carries part of the load of the building and its contents. Walls which do not carry part of the load are non-bearing walls. The necessity for thicker walls on lower floors in taller buildings limited the height of bearing wall buildings. A newer technique, reinforced masonry, permits high-rises of masonry wall bearing construction; the walls are the same thickness throughout (page 340).
The load on the bearing wall helps to stabilize the wall, thus bearing walls are more stable than non-bearing walls. On the other hand, the collapse of a bearing wall is more serious to the structure than that of a non-bearing wall.
The collapse of any wall is serious to firefighters on it, caught under it or hit by missiles from it.
As we walk down Main Street looking at the ordinary brick (or masonry) buildings, in general the side walls are the bearing walls, the front and rear are generally non-bearing. They may look and actually may be constructed exactly alike. Study any building under construction or demolition to determine which walls are which.
In reinforced masonry (page 340) buildings, which require multiple bearing walls, the corridor walls may be of reinforced masonry, rather than the usual gypsum board on studs. Masonry walls cannot be breached to bypass a multi-lock door, as can be done with gypsum walls. Know your buildings.
Veneer walls. Veneer walls of brick, stone or imitation stone are used to improve the appearance of the basic building, which most often is wood but may be steel, cast concrete or concrete block. Some buildings may have brick masonry bearing walls and brick veneer over wood, non-bearing walls. Some have a masonry first floor and brick veneer upper floors.
A veneered building should be described according to its basic construction. The brick veneer building is not another type of masonry construction. The building is of wood, steel or concrete construction, brick veneered. The veneer wall cannot stand up by itself. It depends totally on the basic structure. If the basic structure fails, the brick veneer comes down. A New York City fire officer died when an old wooden building about to be overhauled, and seen to be bowed, collapsed and dumped the veneered imitation stone on him.
Party walls. Party walls are built half on each lot and are common to both buildings. Often, joists were set into sockets common to both buildings.
Be aware of the code in your area and the extent to which party walls were permitted. Fire extension through openings is subtle. Smoke may be believed to be coming from the basic fire. Fire gets a grip on hidden spaces. When fire becomes evident, the voids of the exposure building may be heavily involved. Ceiling voids in exposures must be opened and examined.
In some cases where masonry fire walls were required between townhouse units, the building department permitted the main girders supporting the first floor of adjacent units to be supported in a common socket in the "fire wall." When the basement is finished off, this fire path is hidden.
Panel/curtain walls. Many buildings have no bearing walls. They are supported on a steel (or concrete) frame (or skeleton). Panel walls are defined as covering only one story and curtain walls as covering more than one story but the terms are often used interchangeably. They may be of almost any suitable and approved building material. Brick, stone, glass, steel concrete and assemblies of imitation concrete are all used.
Earlier framed buildings had masonry panel walls built floor by floor on concrete floors, providing an excellent inherent firestop. The perimeter firestopping of many more recent buildings ranges from poor to non-existent. If the building is unsprinklered, there is a good possibility of fire spreading from floor to floor in the perimeter space between floor and wall panel. Sprinklers on the floor above would flood the floor and deter this extension. In an unsprinklered building, we can improvise sprinklers with open hose butts laid on the floor and accomplish the same effect to remedy a fundamental design defect.
Fire walls. Fire walls should be able to stop fire with little or no assistance from firefighting forces. All penetrations of the fire wall should be equal to the fire wall in fire resistance. Openings should be protected with properly rated fire doors. Fire walls in steel-framed buildings are probably unstable if there is fire on both sides of the wall (page 309).
A high percentage of fire doors do not operate properly during fires. In some fire departments, checking that fire doors operate properly is a routine ladder company responsibility. In any case, this responsibility should be pre-assigned and not left to chance.
Fire doors, both horizontal and overhead rolling, are triggered by fusible links, sometimes located high up in the overhead. A burst of heat might trip the fusible link. It may be impossible to raise an overhead fire door and it can be very difficult to open a sliding fire door. In finished buildings fire doors may be concealed in pockets and tripped by an inconspicuous link in the ceiling. Find and note these on pre-plans.
Firefighters should never advance through a fire door without blocking the door, to prevent being trapped behind it. The blockage should be removed when all firefighters have returned through the doorway.
Shear walls. Steel or concrete framed buildings are vulnerable to wind, which can overturn the building. Interior or exterior cross bracing is often used to resist this force. One method of countering this lateral thrust is by the use of shear walls of reinforced concrete. These are placed so as to resist the greatest thrust which is usually on the widest dimension of the building.
Such walls are expensive. For economy, they are often incorporated into the required enclosure walls around vertical shafts, such as those for stairs or elevators.
If you are breaching a shaft wall and find reinforced concrete, go around the corner. You will probably find easy-to-breach, concrete blocks or even gypsum board on studs.
Be aware of the construction of shaft walls. Gypsum board on studs can meet fire resistance standards but may be blown out by hose streams, leaving a deadly open shaft. Partition walls. Partition walls simply subdivide an area. In newer or remodeled buildings, they rarely extend into the void space above the suspended ceiling. Even if they do, they will likely be opened for wiring or duct work.
Smoke and heat will extend over the entire floor above the ceiling. Heated non-combustible tiles or burning combustible tiles have fallen and started additional fires.
Demising walls. This is a code term indicating the walls around a tenant occupancy.
Hollow walls. Some masonry walls are built with a space between the front and back wythes (a single vertical thickness of masonry). This increases stability, provides a space to drain off water penetrating the outer wythe and lately is filled with insulation, either rigid or foamed. Utilities may be located in the void.
Unexplained smoke may be due to insulation in the void ignited by an electrical fault. Use caution opening such a wall; flashover is conceivable.
The basic principle of the arch (page 74) is that all elements must be in compression. The arch does not deliver its load straight down as does a beam but with an outward thrust. This thrust must be resisted by a mass of masonry or by tying the ends of the arch together with steel rods. In older fire-resistive buildings, sometimes the steel ties holding floor arches together are visible overhead.
Arches are not only segmental (round). They can be flat or hinged (pointed). The Gothic Arch, often used in churches, is an example of a hinged arch. Many churches are built with laminated wood three-hinged arches or rigid frames. One-story industrial buildings, bowling alleys and skating rinks are often built of unprotected steel rigid frames.
Many masonry buildings show fine-looking arches on the exterior, while the interior wythes are carried on wooden beams. If the wood burns away, collapse will result. Examine old masonry buildings for this dangerous practice (page 159).
Steel rigid frames start to fail at the thinnest cross section. Pre-plans should emphasize the necessity of cooling the steel even before attacking the fire in the contents. If the rigid frame building (steel, concrete or wood) has no basement, just a concrete floor laid on the ground, the steel ties will be encased in the concrete. However, if there is a basement, the rods are in the basement ceiling, unprotected from fire. Heat could cause the rods to elongate and thus possibly cause the rigid frame to fail, causing collapse. This hidden steel might cause the same sort of tragedy as occurred in a Montreal church. In that case, unprotected steel columns abutting the crawl space were heated by a fire in the crawl space, collapsed and brought down the masonry wall. Two firefighters died (page 314; also see "On The Job Montreal: Death Knell Tolls For Firefighters At Church Blaze" by Ian Stronach, Firehouse®, November 1987, page 42).
At times, two materials are combined to take advantage of the best characteristics of each. Concrete, an inexpensive material, is strong in compression but weak in tension. Steel is strong either way but is more costly. By providing steel at the locations where tensile stresses develop, a composite material, unfortunately called reinforced concrete, is developed.
All elements of a composite material must react together. If the materials separate, the composite no longer exists and the two materials separately will be unable to carry the load.
Composite Structural Elements
Two different materials may be combined in a structural element. Steel and reinforced concrete are combined in composite floors. In some cases, studs are welded to steel beams and imbedded in the concrete of the floor to produce a diaphragm floor, which stiffens the structure. Composite concrete-steel floors can be constructed with bar-joist trusses. The top chord of the truss is set below the tops of the truss web, allowing triangles of steel to project upwards. These are imbedded in the concrete.
A flitch plate girder is made by sandwiching a piece of steel between two wooden beams. A sheet of plywood may also be used as the "meat" of this "sandwich."
A brick-and-block composite wall (page 51), in which cheaper concrete block substitutes for brick where it will not be seen, should not be confused with a brick veneered concrete block wall, in which the brick and block are not structurally united. In older construction, hollow tile is found instead of concrete block.
Any sign that composite materials or structural elements, particularly reinforced concrete, are separating is cause for serious concern, and withdrawal should be considered until a competent engineer can survey the situation. Because they carry unusually heavy loads, flitch plate girders should be noted on pre-plans.The loss of the wood would permit the beam to fail.
Composite construction is sometimes used to describe buildings in which two different materials carry structural loads.
Ascertain the construction of the top floor of a concrete building. One had a wooden penthouse. A steel structure with combustible ceiling tiles was erected on the roof of a Montgomery, AL, high-rise to house a restaurant. The exits were inadequate; 25 died.
A later article will include a most important topic: "Collapse Is Not The Only Hazard." In the meantime, please refer to a pertinent article by John Norman, Firehouse® contributing editor and captain of FDNY Rescue Company 1, in his January 1996 Fireground Tactics column, "Unusual Fire Behavior Equals Firefighters Trapped" (page 16).
(TO BE CONTINUED)
Correction: In my February 1996 article, I erroneously named Thomas Gallagher as the FDNY firefighter killed when fire burst out of a ceiling. Firefighter Peter McLaughlin is the man who was killed.
Francis L. Brannigan, a Firehouse® contributing editor, was a fireground commander from 1942 to 1949. Since 1966, he has concentrated on the hazards of buildings to firefighters.