Building construction basics: Key terms firefighters need to know
A firefighter’s guide to building loads, forces and hazards
Regardless of construction type or materials used, all buildings are under stress. The weight of the building’s structural elements as well as the equipment, furnishings and people inside all contribute to a series of forces trying to equalize gravity and architectural design faults, especially when it’s burning.
Whether by building type, occupancy or in combination, specific load and force specifications related to stress are described and defined in NFPA 5000: Building Construction and Safety Code. The load parameters are translated into hazard levels as they relate to materials failure and the risk of building collapse, both important elements in a firefighter’s survival strategy.
An experienced firefighter applies these principles to the buildings and assemblies in their response area prior to any incident. Your job is to understand the importance of these established structural loads, their forces and the associated hazards as described in the structures you are looking at and learning about.
NFPA 5000 and occupancy guidelines
NFPA 5000: Building Construction and Safety Code describes loads and the forces on them, and they are defined and given value as they relate to design requirements and safety.
Load combinations are calculated using various equations that can accurately define the load requirements for a wide variety of structure features, including heliports, cell block walls and grab bars in hospitals.
As a firefighter, you need see the relationship between assembly and strength. This will allow you to be able to anticipate the consequences of non-regulatory construction.
All building types and their occupancy classifications – as described in NFPA 5000, NFPA 101: Life Safety Code and regional regulatory codes – are defined by requirements. These requirements are categorized by hazard levels ranging from I through IV:
- Level I – Occupancies and structures that represent a low hazard to human life in the event of failure. These include agricultural facilities, storage buildings and certain temporary structures.
- Level II – Structures considered ordinary by construction standards and light hazard by occupancy and contents. Level status changes easily with content, use and remodeling alterations. Structures in this category include residential, commercial and industrial buildings not included in any other category.
- Level III – Buildings that pose a substantial hazard to human life in the event of failure; this includes prisons, power-generating stations, large educational buildings as well as any occupant load greater than 5,000.
- Level IV – Buildings and essential structures such as fire and police stations, emergency shelters and aviation towers. All buildings important to national defense and critical power facilities such as those related to water and electrical emergencies.
Structures are designed to carry loads and, as such, are under extreme pressure or stress to keep their associated forces neutralized. Loads can be described in three general categories: dead loads, impact loads and live loads.
- Dead loads: These are static forces that are relatively constant and can be under tension or compression. Examples include the construction elements themselves, such as walls and roofs, or additional fixed items, such as air-handling units and boiler and heating assemblies. It is important to understand that dead loads contain many items not ordinarily thought of as structural, such as plumbing stacks and risers, electrical boxes, heating, ventilating and air conditioning systems. Even fire sprinkler arrangements can be thought of as part of a building’s dead load. A firefighter needs to understand dead loads as the largest amount of a building’s structural strength and subsequent weight and the lethal consequences of their failure during a fire.
- Impact loads: These are the loads associated with moving heavy machines, vehicles or stored merchandize inside a structure. The random movement of elevators can be described as an impact load. Such movement during a fire can collapse a building without any structural indicators, such as a softening roof or spalling concrete.
- Live loads: Live loads include dynamic loads, such as people, furniture and weather. These can become critical in times of emergency or disaster, especially as they increase in scope and scale. Evacuation of panicking civilians may become a primary initial tactic, and high winds can render master streams useless, putting interior crews at increased risk.
Forces: Power and direction
Load forces are difficult to define and predict. A firefighter can think of force in three ways:
- Concentrated force: There are concentrated loads that are single forces acting over small areas, such as a column or a beam resting on another beam. An active fire at one of these point locations can initiate a complete ceiling collapse.
- Linear force: There are line loads that act along a line such that the force of the load travels along that line. The weight of a bearing wall on a floor is a good example of a line load force. Fire hidden in the floor joists under surface coverings of carpet, wood or tile and adjacent to a bearing wall can result in partial or full floor collapse.
- Distributed force: This involves a load carried over a large area, such as a windstorm on siding or snow accumulating on a flat roof. Adding to the load of a building by subjecting it to high winds or an additional six inches of snow or rain during a fire can change the parameters of collapse dramatically.
Regardless of the type, loads are transmitted throughout a structure as stress, and stresses are imposed on structural materials by force. The stresses on construction materials are defined as compression, tension and shear:
- Compression: A direct load on a column or wall. Firefighters should watch for where a room or building is weakest, such as a glass wall with no partitions or a bearing wall separating the kitchen from the dining room.
- Tension: A load that pulls on another structural member, like the forces between turnbuckles. These loads can be seen in curved roof assemblies and in pre-stress concrete construction where complete walls have been placed in a connecting pattern relying on little or no additional structural framing.
- Shear: Shear energy is the stress on a horizontal bolt connecting two vertical metal roof sections. While fire can compromise such assemblies, weight and shifting forces can also contribute to a shearing failure, providing no direct indicator for a rescue or suppression crew.
How it all comes together: On-scene structural considerations
Every load and subsequent force varies in terms of how it is degraded during a fire. How much of a load is involved, over how much space and distance, the material used and how it was assembled all contribute to its failure rate. How old and what alterations were made during a building’s life can contribute greatly to its potential for structural compromise.
For firefighters on scene, understanding the loads and forces of the materials and construction processes associated with a building on fire is critical to suppression efforts and crew survival. Determining exactly what is on fire, whether contents or the actual structure, becomes a tactical determinate in addition to how much and how fast it is burning. An open-air pavilion with little or no furniture will burn at a different rate and intensity than a school with desks, bookshelves and paper supplies in every room.
Appreciating how a building is designed, what materials are used in its construction as well as its occupancy and use allows a firefighter to predict entry points, fire spread, search and rescue timing, evacuation corridors and ventilation routes for a final suppression outcome.
Day or night and in good weather and bad, for increased fireground effectiveness and safety, firefighters must respect the real value of understanding the physics of loads and forces associated with unstable materials and construction failings when compromised by exterior power, especially fire.
Learn more about concepts of force, load and stress in references from NFPA and IFSTA, and even books covering physics 101.
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