BY JAY D. MICHAEL
Understanding Structural Loads and Loading
Members of the fire service are constantly encouraged to study and understand building construction. Initial fire training building construction classes range from several hours to a little over a day, sometimes with a site visit. The types or classifications from Type I to Type V are reviewed, and then a quiz or exam is given. At the end of your initial training, a few questions on the exam pertain to building construction. Over the years, you review the basic material, take more classes, and move up the ranks. Soon, you are telling the younger firefighters to “know building construction and structural loads” Are you just repeating what you are hearing, or do you understand how buildings are constructed?
|(1) This wood column is in compression; the horizontal beams or girders transmit the load imposed on the beam or girder to the column. (Photos by Paul Dansbach.)|
The late Francis Brannigan used to implore us: “Know your enemy; the building is your enemy.”1 Many times, he reminded us that the structure is merely a gravity-resistance system. Firefighting strategy needs to change once the fire transitions from the burning contents to a burning structure.
|(2) The bottom chord of this heavy timber wood truss is in tension. As a load is imposed on the top chord of the truss, the top chord will be in compression and the bottom chord will be in tension.|
We must understand loads and the forces present on the various building components and structural members. If we can get a basic understanding of the normal loading and forces on building components and structural members, we would be better prepared to evaluate the safety of our operation, whether it involved fire, collapse, or wind damage—or any other time our response takes us into or around structures.
RELATED FIREFIGHTER TRAINING
FORCES, STRUCTURAL LOADS, AND STRESS/STRAIN
Internal and external forces act on structural components. An external force is commonly referred to as a structural load; an internal force is a stress. Another way to look at it is action and reaction. The load is the action; the stress is the reaction. For every action, there is an equal and opposite reaction. The load applied to a column would place the column in compression; conversely, a load hanging from a rod would place the rod in tension. Strain is the deformation of a structural member because of stress within the member. Stress is usually measured in terms of pounds per square inch or pounds per square foot. Strain is measured in the percentage of elongation that happens when the material is tensioned.
|(3) This exterior wood-frame wall is a bearing wall. The wall supports the floors as evidenced by the floor joists arranged at a 90° angle to the wall. The walls of the lower floors will also support the floor above and potentially the roof of the structure.|
Compression is a crushing force. In compression tests, the material is compressed until it crushes or crumbles. Tension is a pulling force that is trying to make the building material longer. In tension tests, enough force is applied to pull the material apart. Shear attempts to make the building materials or structural components slide past one another. In shear tests, force is applied to pull the materials apart. Use of a concrete anchor is an example of shear resistance. When the anchor is placed in a hole in the concrete, a bolt or screw is inserted and tightened. The tightening causes the anchor to swell. Shear resistance keeps the anchor in the concrete. The common nail is another type of building fastener that resists the forces of shear. A gang-nail plate used on a lightweight wooden truss somewhat resists shear.
|(4) The wood floor joists (beams) are supported by the steel girder by bearing on the top flange of the girder.|
Structural members that are under a load must change shape. If the member does not change shape, no structural load is present. The force applied causes this change in shape. In most structures, the eye will not detect the change in shape when the structural member is carrying a normal load.
The basic structural members are columns, girders, beams, floor decking, and roof decking. Walls are a part of structural members. They are sometimes structural load bearing, although most of the time the rules of columns apply to walls just as girder/beam rules apply to floors and roofs.
A column is a structural member that is under compression and transfers its load along a straight path in the direction of the column. Columns are normally thought to be vertical, but they may be horizontal or diagonal. The John Hancock building in Chicago, Illinois, has wind bracing as part of its visible structure. This wind bracing provides rigidity to the structure when wind loads are applied. These braces must resist both compression and tension forces in the structure caused by the wind. Bracing is often designed and placed diagonally between columns; sometimes it is placed horizontally between columns.
Often, braces are referred to as rakers or struts—for example, temporary bracing used in tilt wall construction when the wall panels have been raised vertically but before the wall panels are tied into the roof structure. Not all horizontal members are columns; a structural load must pass through the member in compression. Often, nonvertical columns are referred to by other names such as raker or strut. Sometimes construction crews will refer to a “bent,” which is a row of columns in a line. The bay is the open floor space between any two bents.
There are three basic types of columns; they are differentiated by the manner in which they generally fail. These basic names are piers, intermediate columns, and long slender columns. (1) Not much thought went into the names of these columns, but they fail differently. The long slender column fails by bending or twisting, referred to as “buckling.” The pier, or squat column, fails by crushing. The intermediate column may fail in either manner.
The length of the column determines its load-carrying capacity. The shorter a column, the more it can carry. The structural loads carrying capacity of a column decreases as the length is increased. A column will lose strength by the square of the change in length. A 16-foot-high column will carry one-fourth the load of an eight-foot-high column. Once a column starts to fail, there is very little reserve strength to resist the collapse—in other words, once a column starts to fail, you are probably not going to stop it. Bracing securely placed at the midpoint of a column will effectively increase the column’s load-carrying ability, making two columns of equal length and strength. Even with this bracing, a column will fail rapidly when it reaches the critical yield load.
A column is cylindrical and hollow, allowing the load to be placed as far away from the center of the cylinder as possible. Building with round cylinders is difficult. Attaching beams to round columns can lead to design problems with aesthetics in the final finishing. Popular column shapes are round, box, and probably the most popular “H.” These shapes will lend themselves well to the cylindrical column theory. The “H” and box designs will allow a circle to be drawn encompassing the top and bottom of each leg of the “H,” or the four corners of the box, while looking down on the end of the column. It is easier to attach girders, beams, walls, and other building components to the box and “H” shapes than it is to round building components.
Understanding Structural Loads: Beams/Girders
A beam/girder trasmits forces in a direction perpendicular to such forces to points of “reaction” (points of support, typically columns). A girder is a beam that supports other beams. The loading delivered to a girder is the same as the load delivered to a beam. As the beam receives the load, the force is transmitted perpendicularly, or at a right angle, to the supporting members of the beam. Loading of a beam will cause the beam to bend downward. The bending action causes the top of the beam to be in compression and the bottom under tension.
There are many types of beams: simple, continuous, fixed, overhanging, and cantilevered are but a few. Beams receive a load, turn the force laterally, and stress the beams’ supporting members. I mentioned above that some columns may be horizontal; the same is true for beams: A beam may be horizontal or vertical. A roof rafter, for instance, is a beam that sits in an orientation that prevents you from thinking of it as a beam.
The depth of a beam determines its load-carrying capacity. The beam’s load-carrying capacity increases by the square of its depth. Look at a truss: The greater the distance between the top chord and the bottom chord, the more the truss can carry, or the farther it can span. When looking at a beam, notice its depth and the distance from the top to the bottom. You will then be able to determine the load-carrying capacity relative to the surrounding beams. When looking at the beam, you will probably notice girders in some areas. They will most likely be larger than the beams sitting on them.
Beams may also be suspended instead of sitting on columns. The rules still apply: The force is turned laterally and then delivered to its supporting members. In cases of suspended beams, the beam’s load places an attached supporting member in tension. The supporting member may be a chain, a cable, or a steel rod held by something else. In the arrangement of suspended beams, the top portion of the beam will be under compression and the bottom portion will be under tension, which is the same as for any other beam.
A beam supported on each end with a load placed midspan will deflect. This deflection results in the top portion of the beam’s being placed in compression and the bottom portion of the beam’s being placed in tension. There will be a small portion of material in the center of the beam that has no stress applied; this is known as the “neutral plane.” This neutral plane serves only to keep the compressed and tensioned portions of the beam separated an equal distance for the length of the beam. Less material is needed in this neutral plane. That’s why many trusses are of open-web design. Beam loading refers to the distribution of the load on the beam. We in the fire service refer to this as a “concentrated” or “distributed” load. The more the load is distributed, the better. Concentrated loads may lead to local collapse.
Understanding Structural Loads: Walls, Floors, and Roofs
Walls, like columns, transmit loads by compressive force to the floor below, another wall, or earth through the foundation wall. The wall unit will react to force like a long slender column. A wall may also be required to act like a beam, resisting flexing force such as a wind load.
Walls have two main classifications: load bearing and nonload bearing. A nonload-bearing wall must support only its own weight and the weight contained in the wall. A strip mall most generally has masonry and steel load-bearing walls. The interior wall separating the various stores is nonload bearing. An interior wall that may be removed without supporting the structure above is a nonload-bearing wall. A partition wall in an office and the brick veneer on the outside of a residential structure are additional examples of nonload-bearing walls. A load-bearing wall will support a part of the structure above the wall, which may be another wall, floors, or the roof. A load-bearing wall will be more stable than a nonload-bearing wall because it can be attached top and bottom. This weight will create a more stable environment for the wall. Bearing weight will cause a deflection and determines that a load is present; this load will help to stabilize the wall.2
Many new structures, some of which are multistory, are built with tilt slab concrete walls. The tilt slab concrete wall may be a bearing wall or a nonbearing wall. Nonbearing tilt slab wall panels have perimeter steel columns and edge beams to support the vertical floor and roof loads.3 Pay attention to new construction in your territory. The roof assembly (and floors of multistory buildings of this type of construction) of both load-bearing tilt slab construction and nonload-bearing tilt slab construction is tied to the walls. If the roof is being lost in fire conditions, firefighters must be very concerned of wall collapse.
The tilt slab concrete walls will fail in a 90º angle collapse. This collapse may be outward or inward; either way, it will be dangerous to firefighters in the collapse zone. A tilt-slab building under construction is very dangerous. The walls are held in place temporarily with a false work of bracing called “tormentors.” These tormentors may be lightweight aluminum poles or wooden planks staked to the ground; they surround the wall and are angled into the slab. Sudden loading, such as in a wind storm, may overload the tormentors. Unstable ground at the construction site may also cause the tormentor to loosen, increasing the demand on the remaining tormentors. (1)
The roof system is of vital importance to the fire service. The roof assembly is where we will find the “dominate vertical channel”4 for ventilation. This is where the fire will naturally travel and where we must ventilate to reduce fire spread and make interior conditions more tenable. Roof assemblies vary from just preventing the weather from entering the structure to providing stability for the exterior walls.
Most new lightweight construction methods require that the roof be a very important structural component. Older construction doesn’t necessarily require the roof to be an integral portion of the structure; for example, a parking garage doesn’t even have a roof. The roof assembly holds the walls together in most lightweight construction methods; supports snow and rain load; resists wind stress; and supports concentrated loads such as the HVAC unit, signs, and many other nondesigned loads in altered buildings.
Roof assemblies may be very simple, such as wood rafters, to the more complex truss systems supporting large-span roofs. Design loads for roof structures are not as great as for floor loads. A noncombustible structure may have a combustible roof covering or roof structure.
With the current trend toward “going green” or environmentally friendly buildings and renovations, the use of natural light is becoming more common. New structures have more openings for the passage of light. They may be traditional skylights or thinner panels. Renovated structures are using these same construction techniques and building features, possibly in roofs that were not designed for energy-efficiency techniques and the natural lighting we have today. Mixing modern building techniques with older buildings can lead to the structure’s not performing the way it was designed or the way we may think it will. During the initial size-up during fire conditions, building renovations may not be noticed. Your quick observations may lead you to believe the building has never been renovated. Renovation of older buildings will continue, as it should. Remember it may not react in the way you would expect the original construction method to react. The initial size-up rarely, if ever, allows for observation of the roof assembly. Older building materials replaced with newer lightweight construction methods will not react the same way.
Floor boards transmit loads to joists (beams). The amount of the load transmitted depends on the distance the load point is away from the joist. Some floors are designed to support heavy loads, and some may be designed to support only the load presented in a residential structure. Think about the residential structure with a designed floor load of 40 pounds per square foot that has been converted to a lawyer’s office, which could be equated with a library with book stacks where the design load is 150 pounds per square foot. (1) Check to determine if proper reinforcement has been installed to support the additional loading.
It is important to have a good working relationship with your local building code officials. These officials have the experience and knowledge base to determine the load capacity of structural components. It is just as important for fire crews to get out into their assigned areas and look at the building methods. With a good working relationship, code enforcement and fire crews can work together to keep the community safe.
How the load is centered on the building components is important, too. A torsion load is a twisting load. Some vehicles have torsion bar suspension; the vehicle is attached to one end of a spring steel bar (the torsion bar) while the wheels are attached to the other end. Bumps in the road are absorbed by the twisting action of the torsion bar. Although a building torsion load is not designed to be a spring, the load and stresses are the same. Buildings that have structural component failure often place undesigned torsion loads on remaining structural members. These undesigned stresses may lead to further collapse.
|(5) The steel fire escape imposes an eccentric load on the wood-frame wall, as the load of the fire escape does not pass through the center of the wood-frame wall.|
A roof system containing an open web parallel cord lightweight steel truss is designed to transfer the roof load horizontally to the supporting columns. All the roof trusses work in concert with each other. Should one or more of these trusses fail, the remaining members, next to the failed ones, will have a torsion load placed on them. This torsion load is undesigned. The truss is able to deal with the designed load by transferring the load through tension and compression to the supporting column. The torsion load placed on it may cause failure, as the truss cannot deal with all of the additional forces.
Axial load is one that passes through the centerline of the structural member; all portions of the structural member are equally stressed.
|(6) This photo details the use of a metal gusset plate connector on a lightweight wood truss. Notice the limited thickness of the connector and the lack of quality control, which allowed 25 percent of the connecting surface of the gusset plate to connect to nothing.|
An eccentric load is one that is concentrated to one side of the supporting wall or column. The load is straight but stresses only one side of the column. An example of an eccentric load would be the sidewall of a multistory balloon frame construction building. The floor attachment is on a ribbon board that is attached to the side of the sidewall. This ribbon board may be mortised into the stud or just nailed to the stud. The floor load will place the wall stud in compression on the load side and in tension on the opposite side. An exterior fire escape is another example of an imposed eccentric load.
|(7) As the metal gusset plate is exposed to fire conditions, the teeth of the gusset plate conduct the heat into the wood fibers, and decomposition of the wood fiber begins. The gusset plate may also warp and begin to peel off the lightweight wood truss.|
How the load is transmitted to the ground is of vital importance. Brannigan often requested that we “undress the building,” meaning that we look at the supporting structure. He used the term “gravity resistance system” to relate to the supporting columns, beams, floors, and roofs. (1) Building materials are tested for their resistance to compression, tension, and shear. While looking at this gravity resistance system, look at the components, and trace the load to the ground. This will help you determine how structures are built. Understanding fire behavior and heat transfer will also assist in determining how the structure will react.
REACTION OF STRUCTURAL COMPONENTS TO HEAT
Heat affects building components in various ways. Some components burn and add fuel to the fire; others absorb heat for a time and conduct the heat through the structural component to another location. Understanding the compression, tension, and shear stresses the structural components have will assist you in determining the reactions that may occur when the structural components are exposed to the heat from a fire.
(8) This building is an example of hybrid construction. The lower floors of the structure are constructed with steel and masonry materials; the upper stories are lightweight wood-frame construction. As construction progresses, we will be able to determine if the steel columns and beams will be protected with a fireproofing material to give the steel a fire-resistance rating.
Items under tension are resisting the forces trying to make them longer. Heat elongates metal objects; thus, the reaction to heating is manifested by exactly the same force the tensioned object is resisting. This is thermal expansion; when heated, the atoms that make up the material increase movement. We learned in basic training that a 100-foot-long steel beam may elongate by nearly 10 inches at 1,000°F (1), sometimes causing wall failure. We now see this same reaction to heat in lightweight steel structures—not with the wall pushing out but by the lightweight materials’ becoming curved or “S” shaped perpendicular to the load or showing some other deformity. The strength is not in the material to push the wall out but with the elongation. Something has to give; thus, the deformity.
For the most part, tensioned members are smaller than compressive members; thus, the mass is not present to absorb the heat transfer. The structural component will elongate or deform. Simply playing a hose stream on the structural components and cooling them below the reaction temperature will return steel components to near their original strength. Keep in mind that the component will not return to its original shape. The structural steel may not now be shaped to support the structure, but it will be strong.
Much has been written about the metal gusset plates or gang nails in wooden truss construction. The wooden truss industry suggests the metal reflects heat away from the wood, which doesn’t really matter, since the wood is so thin it cannot absorb much heat. The gang-nail plate is also very thin, unable to absorb much heat. The nail plates on the bottom chord are in tension. When heated, they will deform and become unattached from the wood. This, again, is an example of a structural member under tension reacting to heat.
Look at wooden trusses involved in fire. Most times, the gang-nail plates on the compression chords are intact and present, although deformed, while the gang-nail plates on the tensioned chords are missing. Members under tension may be the first to fail under fire conditions. Construction members in shear stress may have various reactions to heat. In the case of the aforementioned concrete anchor, if the bolt is inserted into a lead anchor, heat will cause the lead to weaken and the anchor to pull out. Given a steel anchor in concrete, the steel may swell and become tighter as the concrete absorbs heat and transfers it to the metal anchor. Of course, once the steel reaches a certain threshold, it will itself become weaker. Nails, which resist pulling out because of shear, vary in shear strength. A long and skinny nail, especially one that is cement coated, will conduct less heat to char wood than a common nail. Shear holds screws in place, offering greater holding power and less damage than nails to the core materials.
Many variables exist when determining what will happen to building materials when they are exposed to fire. We can talk in general terms of buildings, or we can speak in specifics of building or structural components. Much research has been posted on the Internet. When viewing material, look at when and how the experiment was conducted and determine if the results can be replicated. Manufacturers’ testing sometimes augments data favorable to the manufacturer. When reviewing material, look to see how the test was performed and to what standard.5 When testing organic building material, the moisture content is a variable—new wood will not be as dry as the wood in an old house. Very dry wood is not as strong as wood that contains moisture, such as when it is just out of the kiln.6 For example, southern yellow pine in the nominal dimension of two inches is kiln dried to 12- to 15-percent moisture content. “Green wood,” or fresh cut lumber, is considered to be of 23 percent moisture content. Kiln-dried wood has a range of six to 15 percent, depending on the type of wood and the desired usage.7 Kiln-dried wood is also considered stronger than wood that is stacked and allowed to dry naturally.8
The behavior of hybrid construction or construction that uses combinations of building materials (concrete, steel, and wood) is very difficult to predict in a fire situation. Many buildings today are built using “performance” code requirements instead of traditional prescriptive code requirements (where structural member dimensions are specified), meaning that the structure must support a given load, using whatever building material/structural design the designer chooses. Many buildings are now built using mathematical formulas and lighter-weight materials. (5) This math vs. mass (the heat-absorption ability) will gain even more popularity as time passes. Technology for energy conservation and for reducing costs (using recycled materials and newly developed building materials) will challenge the fire service in the future.
What we should be thinking about when walking in a building are the way the structural building components are loaded and how the stress will react under fire conditions. Let this “awareness of your surroundings” surface in your mind. Try to make it a habit to be aware of what is around you, how structures are built, and how the load gets to the ground. Additionally, think in terms of how heat will affect the gravity-resistance system.
1. Brannigan, Francis L.. Building Construction for the Fire Service, 3rd Edition, fifth printing. The National Fire Protection Association, 1995.
2. Dunn, Vincent. Collapse of Burning Buildings: A Guide to Fireground Safety. Fire Engineering, 1988.
3. Remmetter, Walters, and Steinbicker, “Multi-Story Tilt-Up Buildings,” Structure Magazine, July 2008, 22-25.
4. Coleman, John.Incident Command for the Street-Smart Fire Officer. (Tulsa, Okla: Fire Engineering, 1997).
5. Dansbach, Paul, “Building Construction for Firefighters,” FDIC 2002 class notes.
8. Firefighters Handbook, Delmar, 2000.
JAY D. MICHAEL, a 30-year veteran of emergency services, has served 24 years with the Elkhart (IN) Fire Department, where he is a lieutenant. He is an Indiana fire officer III and an instructor II/III. He is an adjunct instructor at the Elkhart Fire Department Training Academy and at the Michiana School of Fire and Emergency Services at the University of Notre Dame. He is a member of the Indiana Fire Instructors Association and the International Society of Fire Service Instructors. His areas of expertise include incident management, building construction, and fire behavior.
Originally ran in Volume 163, Issue 6.