Supporting References: Recruit Student Handout
"Building Construction for the Fire Service", Third Edition by Frank Brannigan
"Collapse of Burning Buildings" by Vincent Dunn
I. F. S. T. A.'s "Building Construction and the Fire Service"
· Understand basic terminology related to building construction in general.
· Understand the different loads imposed on buildings.
· Understand the different stresses developed in structural components.
· Recognize building classification systems.
· Recognize common structural components.
· Understand basic failure patterns of different types of construction.
There are five (5) factors that need to be taken into consideration when evaluating (sizing up) a building as it applies to how it will withstand fire and fire extension, and what will be required for firefighting crews to extinguish the fire.
By knowing type of construction, firefighting crews can evaluate potential routes of fire extension throughout the structure, estimate the time that extension will take, how long it will take to get fire attack teams in position ahead of the fire, weak points which may fail under fire exposure and lead to collapse allowing them to establish danger zones, and strong areas they may use in defending the uninvolved portions of the structure.
The size of the building will allow firefighters to determine the extent of involvement, an approximation of the potential fuel and occupant loads, and the amount of firefighting resources that need to be applied to try to bring this incident under control, and to eventual extinguishment. The physical dimensions of the building, and knowledge of the location of the fire within the building will allow incoming companies to estimate the length of hose lines which will be needed to begin fire attack.
The age of construction will help gauge whether it is conventional or lightweight construction. We can obviously operate in a building of conventional construction for a longer period of time than we can in a building of lightweight construction. Lightweight tends to fail much more quickly. Additionally, knowing the approximate age of the building will help us evaluate features specific to the type of construction we are dealing with. For example, pre-’33 masonry construction is certainly more of a hazard to us than masonry buildings built after the codes were upgraded following the earthquake of 1933. The life of a building may be 75 to 100 years or more. Over that time, erosion of mortar in brick walls, corrosion of exposed metals, rotting and, or insect infestation of wooden structural members all contribute to degradation of the structural integrity.
How do we determine the age of the building? Start with the overall appearance. What kind of siding does it have? Due to the cost of construction, labor intensive sidings like board and batten, and clapboard are no longer built, so the presence of these types of siding would indicate an older structure. Due to the cost of maintenance, wood siding is used less frequently today. Vinyl sidings are becoming more prevalent in new construction. Vinyl siding is also used in remodeling buildings, so its presence may be deceptive. No one (1) indicator should be relied on solely when estimating the age of a building. Look at the rafter tails, if visible, two-by-four (2 x 4) rafter tails indicate either a very old building, or lightweight construction. Look at the condition of the roof. Older structures may have a noticeable sag to the roof due to the age, the added weight of several layers of roofing and the fact that codes were less stringent regarding size and spacing of roof rafters. How many layers of roofing are present? Older structures may have several layers of different roofing materials. Most roof systems were expected to last fifteen (15) to twenty (20) years, so a building with three distinct roof layers is probably at least thirty (30) years old, depending on how old the top layer of roofing is. These are just some examples of ways of visually estimating the age of a building.
Renovations are very important. When a building undergoes renovation, you should take notice because frequently a building was built for a specific use, for a specific type of floor loading. A new owner will come in and renovate the building, remove some structural elements because they’re in the way of what he/she wishes to do. Removing those structural elements weakens the building, even though they may be loading it to the same point as the original design intended. Or they may go in and load it even more heavily than it was originally designed for. In either case, the original safety factor designed into the structure has been reduced or eliminated. There are a number of fire-collapse related incidences that can be tracked back directly to renovations in which structural elements were altered and did not take into account the structural loading that was then subsequently placed into the building. Remodeling a building makes determination of age and construction more difficult, and requires even more diligence during inspections, to get an accurate handle on the true type of construction present.
The type of occupancy allows a determination of what kind and number of firefighting resources to bring to bear. If its 3:00 in the afternoon, and it’s a residential complex, chances are all of the occupants are awake, alert and able to take effective action to remove themselves from risk. If its 3:00 in the morning, it’s a different matter. Most people will be unaware of the situation that may be developing around them.
When structures are loaded, three different types of stress are created, these stresses occur separately or in combination. The stresses are:
1. Compression stress crushes material together
1. Tensile stress pulls material apart
1. 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.
Loads are applied in one or a combination of three (3) ways axial, eccentric or torsional.
1. Axial loads are transmitted along the central longitudinal axis of the structural element.
B. Eccentric loads
1. Eccentric loads are transmitted along an axis parallel to the longitudinal axis, but off-center, which reduces the maximum load the structural element can carry, compared to an axial load, and induces lateral (buckling) instability.
C. Torsional loads create a twisting stress on structural members
1. 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 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.
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 diagramed. 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.
We need to be able to differentiate between “conventional” construction and “lightweight” construction. In conventional construction, you have solid structural elements -- 2x6, 2x8, 2x10 or larger rafters, floor joists, as examples. The distinction is important for the simple reason that solid structural elements will result in a longer burn time, flame exposure time before yielding to the stresses they are undergoing and failing, resulting in either a localized or more extensive building collapse.
In the case of lightweight types of construction, this is not the case. Engineered materials such as metal gusset plate trusses, plywood “I” beams, open web bar joist trusses, unprotected structural steel elements, will yield under direct flame exposure or fire exposure much more quickly because in the old days of conventional construction, if the architect wasn’t quite sure if a structural element would carry the anticipated load, they would simply move to the next larger size. If they weren’t quite sure a 2x6 would cut it, they would put in a 2x8 or a 2x10 just for the additional safety factor. Nowadays, computer aided design systems engineer structural elements literally down to the exact safety factor that the building codes require for the anticipated loading the structure will be expected to support. As a result, if the computer tells them that a plywood “I” beam will suffice where they used a 2x8 before, they’ll use that plywood “I” beam and it will fail much more quickly than a conventional 2x8 would. This is due to the plywood “I” beam having less mass than the conventional beam/rafter had, and elements of less mass that carry the same load are less fire resistant. Engineered components usually have a higher surface area-to-mass ratio, resulting in heat conduction into the structural component occurring more quickly than it would in a solid element.
In order to determine if you are dealing with lightweight construction it is necessary to examine the structural elements used in the floor and roof systems. If you find any extensive use of 2 X 4 or 2 X 3 materials, or unprotected structural steel, or any engineered components which use these lightweight elements, it should be considered lightweight construction. For the purposes of evaluating older wood frame structures, this would include those buildings with 2 X 4 roof rafters, even though they were conventionally built, as they will fail in the same rapid time frame that contemporary lightweight construction styles will fail in.
Walls may be either bearing, in which they support their own weight as well as other parts of the structure, or non-bearing, in which they support their weight only. Masonry or stone veneers are examples of non-bearing walls. Buildings may be designed as bearing wall buildings, in which the walls are intended to support the floor and roof systems, or as framed structures, in which the walls, floor and roof systems are supported by a structural framework of columns, girders and beams. Framed buildings usually tend to be large, steel reinforced concrete or steel framed structures. Bearing walled buildings will have at least two (2) bearing walls, which are usually the longest walls of the building (this keeps the length of structural elements in the floor and roof systems to a minimum because the span between these walls is shorter than the that between the shorter walls). Buildings with flat, arched or gable roofs have only two (2) bearing walls, with the other two (2) being non-bearing. This distinction is as important as the two (2) bearing walls will be more stable than the non-bearing walls due to the superimposed weight of the floor and roof systems. Any wall in buildings with hip roofs is a bearing wall if it supports any portion of the roof, which is indicated by the presence of rafter tails at the top of the wall. Wood frame and masonry types of structures are almost always bearing wall buildings. Steel reinforced concrete buildings may be either bearing wall (tilt-up) or framed buildings. And, steel frame buildings are almost universally “framed” structures.
The most common roof styles are flat, gable, hip, arched, sawtooth and bridge-trussed. Wood frame buildings normally have flat, gable or hip roofs. Masonry construction may have any of these roof types. Steel reinforced concrete and steel frame normally are flat-roofed. Once the structural components are in place, they are sheathed. In older structures, the sheathing may be space sheathed (there is a gap between the edges of the boards) usually with 1 X 4 or 1 X 6, or close sheathed (in which there is no gap between the edges of the boards). “One-by...” roof sheathing was replaced by plywood, due to the lower cost of installation. And, plywood is now being replaced in new construction with oriented strand board (O. S. B.) sheathing. Once sheathed, the roof is covered with roofing paper, and the final roofing, which may take a variety of forms.
The bearing walls are the primary support in buildings with flat roofs. In the case of peaked roofs (Gable and Hip) the bearing walls and the ridge pole are strong areas, with hip rafters being a secondary level of support. The bearing walls are the primary strong areas in arched, sawtooth and bridge-truss roofs. There may also be secondary strong areas (interior beams/girders) in the case of the sawtooth type roof.
Related to the Fire Service
Axial Load A load bearing on the axis of a structural element in such a way that the stress imposed by the load is carried by center of mass of the structural element. Structural elements are capable of carrying the maximum load only when that load is applied axially.
Beam A structural member subjected to loads perpendicular in its length.
Bearing Wall A wall which supports all, or a portion, of a superimposed load such as a floor or roof.
Brick Veneer A single thickness of brick wall facing placed over frame construction or masonry other than brick.
Chord Main members of trusses as distinguished from diagonals.
Column A structural member which transmits compressive force along a straight path in the direction of the member.
Compression A crushing force which tends to push the mass of material together.
Dead Load Weight of the building and any equipment permanently attached or built in.
Drywall A system of interior wall finish using sheets of gypsum or button board and taped joints.
Duct A channel, usually for ventilating, heating or air conditioning.
Load A load bearing in line with a structural element from one end to the other, but
which does not pass through the center of mass of the element axially. The resultant stress causes the structural element to bend along its length (leading to lateral or buckling instability). A load which can be carried safely if axially loaded may cause catastrophic failure of the structural element if it shifts to an eccentric load.
Fascia A flat vertical board located at the outer face of a cornice or eave. A decorative feature built on the front and occasionally sides of a structure to hide unsightly roof features and increase the architectural appeal of the building. See also Cornice.
Footing That part of the building which rests on the bearing soil and is wider than the foundation wall.
Header A brick laid at right angles to the length of the wall in masonry construction. In wood frame construction, the beam spanning over a door or window opening.
Hip The junction of two sloping roof surfaces forming an exterior angle.
Impact Load A.K.A. shock load: A load delivered in short time which may cause structural collapse.
Joist A horizontal beam used to support a floor or ceiling.
Lamination Several layers of lumber making up a laminated beam.
Lateral Load A force applied to the side of a structural member.
Lath Narrow, rough strips of wood, or wire mesh, used to support plaster or stucco.
Live Load Any load other than a dead load.
Monolithic Consisting of one piece of stone or stone like material such as concrete. In monolithic frames, the frame is strong enough to withstand the loss of one structural element without causing failure of the entire structure. The resultant load is transferred to the other structural elements around the one that failed.
Neutral Plane That area of a beam which carries neither compressive nor tensile loads, but simply serves to keep the top and bottom parts of the beam, where these loads are localized, separated by the same distance over the length of the beam.
Non-bearing Wall A wall which bears no load other than that of its own weight.
Pier A supporting section of wall between two openings. Also a short masonry column.
Plate (Frame construction) The top or bottom horizontal structural member of a frame wall or partition e.g., top or sole plate.
Treating The process of impregnating wood with mineral salts under heat and pressure, which reduce susceptibility to absorption of moisture, thereby reducing the risk of wet-rotting of the wood. Pressure treated lumber is required in making the sole plate (sill) of wood framed structures. The mineral salts used in pressure treating are suspected human carcinogens, hence S. C. B. A. should be worn to avoid inhaling the products of combustion from pressure treated lumber which is burning.
Rafter A beam that supports a roof.
Shear A force tending to cause molecules of a material to slide past one another where they are in contact in the same plane.
Sheathing The covering applied to the framing of a building to which siding or roofing is applied. May be space sheathed, in which boards are not laid edge to edge, or solid sheathed in which they are. Older styles of sheathing used 1 X 4 or 1 X 6 boards, either straight or diagonal sheathed. In newer construction, the boards have been replaced by plywood or oriented strand board.
Sill Frame construction: The bottom rough structural member which rests on the foundation. Synonymous with Sole Plate. See “Plate” also.
Soffit The underside of the fascia of a building; also, false spaces above cabinets, etc.
Static Load A load applied slowly which remains constant.
Stucco A material made of cement, sand and plaster and applied as siding.
Stud Vertical structural uprights which make up the walls and partitions in a frame building.
Surface Area-to-Mass Ratio The relationship between the surface area and the mass of structural members.
Tension A stress in a structural member which tends to stretch a structural member or pull it apart.
Torsional Load A load imposed on a structural element in such a manner that it causes the structural element to twist or spiral in response to the load.
Veneered Wall A wythe of decorative stone or masonry attached to the bearing wall but not carrying any load but its own weight.