Structural Loading in Firefighting Operations

Structural Loading When structures are loaded, three different types of stress are created, these stresses occur separately or in combination. The stresses are: A. Compression Compression stress crushes material together. B. Tension Tensile stress pulls material apart. C. Shear Shear stress causes material to fracture and slide across the fracture in opposite directions. A wooden beam, supported on each end with a load placed mid-span, will deflect. This will result in the wood on the top of the beam compression stressed, and wood on the bottom of the beam tensile stressed. There will be a small amount of wood along the center of the beam from end to end that, in fact, is not experiencing any compressive or tensile stresses. This area is known as the neutral plane. Since the wood in this area is not carrying any of the load on the beam, but simply serves to keep the top (compressive stressed) part of the beam, and the bottom (tensile stressed) part of the beam separated the same distance along the entire length of the beam, less wood is needed along this neutral plane area. This is why trusses are more efficient at carrying loads than are solid structural elements. They have a top chord carrying the compression stress, a bottom chord carrying the tensile stress, and a web to keep the top and bottom chords separated the same distance along the entire length of the truss. Types of Loads Loads are applied in one or a combination of three (3) ways axial, eccentric, or torsional. A. Axial Loads Axial loads are transmitted along the central longitudinal axis of the structural element. B. Eccentric Loads Eccentric loads are transmitted along an axis parallel to the longitudinal axis but off-center. C. Torsional Loads Torsional loads create a twisting stress on structural members. Steel beams, superheated in a fire, will expand by elongating. If both ends are restrained securely enough that this elongation cannot take place, the beam will expand by twisting, introducing torsional loads that it and its supports were not designed to withstand. This may result in localized, and possibly even total collapse. As a general rule, the longer and thinner a column is, the more susceptible it is to buckling as its load is increased. A shorter, wider column is less susceptible to buckling and will fail by crushing when overloaded. And, torsional loads may be applied laterally to the central longitudinal axis, causing the structural element to twist. Columns can carry the greatest load axially. If this load should suddenly shift from an axial to an eccentric or torsional load, it can cause failure of the column. The load a column can carry is reduced by a factor of four (4) if a column’s length is doubled and all other factors remain the same (Euler’s Law). An example of this can be seen when a fire has burned away most of the second floor attachment to a column in a two (2) story building. Essentially the column’s length has been doubled. Hence, the maximum load it can safely carry is now one-quarter what it was when originally built, with the second floor in place, which provides mid-point bracing of the column. Loads can also be classified as dead or live. Dead loads are the entire structure and everything that is permanently attached to it, such as flooring, columns, beams, roof air conditioners, marquees, etc. Some of these features can also represent live loads, which are any loads developed by things introduced into the building that are not permanently attached to it, furnishings, machinery, people etc. Any source of vibration would be a live load, hence, an air conditioner when not operating would be a dead load, but once it begins operating then becomes a live load also. Live loads are introduced via a number of routes during a firefight. The water we deliver into the building, our own weight as we move through or on top of the structure, ground and aerial ladders placed against the walls, etc., all affect its structural strength. If a firefighter weighs 250 pounds when fully suited up and carrying equipment, he/she presents a 250-pound live, but static load if he/she remains completely still. The instant he/she begins moving the building experiences at least twice the stress in the form of an impact load, and if he/she were to jump onto a roof, for example, the building would experience up to 1000 pounds of stress. This impact loading (which more accurately for our purposes should be called shock loading) of an already compromised structure may result in its giving way. This is why firefighters should never step onto a roof without first “sounding” it. FOUNDATIONS Foundations may be slab or raised.  The California bungalow style of single family residences are mostly on raised foundations.  Why should we care?  Because they also usually had floor furnaces, which due to lack of maintenance, age and mechanical failure, start fires, and completing overhaul of these appliances sometimes requires fire personnel to enter the crawlspace under the building. Most older residential structures were built with raised foundations, but due to the difficulty of making these structures earthquake resistant, most structures (residential and otherwise) are now built on slab foundations. COLLAPSE PATTERNS There are a number of things that a firefighter should be aware of when they arrive on scene.  They should consider the type of construction, the way that type of construction will fail in fire exposure, evaluate the type of collapse that kind of construction may experience, and establish appropriate collapse zones. Depending on the type of construction, they will experience a number of different types of collapses. In the case of walls, they may see what is called a 90-degree wall collapse in which the wall fails and literally rotates outward from its foundation 90 degrees onto the ground in one piece.  And at that point, a 30-foot tall wall would create a 30-foot long collapse zone that we should stay out of. This may result in fire attack teams having to take up flanking positions adjacent to the corners of the building in order to stay clear of the collapse zones.  This severely limits the effectiveness of fire attack, and usually signals a defensive firefight. Another type of collapse that we will commonly see is what’s called a “curtain fall” wall collapse.  Essentially, you’ll have a type of construction where they’ll have masonry or stone or some other type of veneer material applied on the outside of the actual structural element.  Under certain conditions, this veneer will peel off the wall and end up falling straight down, similar to the way a curtain drops, hence the name.  At this point, the collapse zone is close to the base of the wall.  This type of collapse is also frequently seen in unreinforced masonry (U. R. M.), as the degraded mortar usually doesn’t cause the bricks to adhere together in a monolithic manner, and there is no other reinforcement present in the wall that will keep what essentially amounts to a stack of loose bricks together as they collapse. We may also see what’s called an “inward/outward” wall collapse where as the wall begins to collapse, it breaks somewhere in the middle and into one or more pieces, and one piece kicks out, and the reaction of it kicking out causes a lower piece to kick inward, such as is diagrammed.   These cracks will frequently occur at points where the wall already possesses some weakness, such as where the floor joists or roof rafters enter it, or at door or window levels.  These small breaks in the continuity of the wall are enough to create a weakness that will cause that spot to fail earlier than the rest of the wall will.  Both the “curtain fall” collapse discussed earlier and this inward/outward type of collapse are commonly found in unreinforced masonry.   Information adapted from Brannigan, Frank. Building Construction for the Fire Service, Third Edition; Dunn, Vincent. Collapse of Burning Buildings; and IFSTA's Building Construction and the Fire Service. 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