Part 1 - Why a Competent Fire Officer Must Understand Building Construction Building construction is the anatomy and physiology of the structural fireground. Just as the human body must resist the assault of gravity and time, so must a building resist the assault of gravity and time. Just as...
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Dead load - 125 psf
Design live load - 40 psf
That makes the total load 165 psf.
Multiply the total load by the design live load and you will have the total design load, or "code load," for the entire floor of your room:
165 psf x 180 sq. ft. = 29,700 pounds
A ton is 2,000 pounds, so the total design load for your floor during the intimate gathering is nearly 15 tons (29,000 psf Ã· 2,000 pounds)!
I'm sure that some of you are thinking: "OK, that was interesting, but what's the strategic significance of this exercise?" Consider this: Suppose your condo is on fire. By the time the fire department arrives, your condo is fully aflame. Since your condo is 100% involved, fire officers determine that the operation will be transitional (defensive to offensive) coordinated with aggressive evacuation and stabilization of uninvolved occupancies.
Prior to the offensive transition, two teams directed two 250-gpm hose streams into your condo for seven minutes. By flowing 500 gpm for seven minutes, a total of 3,500 gallons of water was directed into your condo. Each gallon of water flowed weighs around 8.34 pounds, so a total of 29,190 pounds of water was directed in your condo (8.34 times 3,500). In case you don't grasp the significance, this is nearly 15 tons of water (live load) flowed in just seven minutes.
- How does this undesigned live load compare with the design load of your room?
- What does this undesigned live load do to the safety factor engineered into the floor?
- What about more gpm for more minutes into a building in your community? (Command Caveat: Following a defensive operation, somebody needs to do the math before commencing offensive overhaul.)
Throughout history, building project developers (the people who pay for construction projects) have sought to reduce two things: dead load and time. Dead load and time represent the two greatest costs of a construction project. Dead load is expensive; that's why the steel frame of a contemporary high-rise building is no longer protected with concrete. Concrete is heavy (dead load) and requires a lot of labor hours to encase (form and pour) every high-rise column, girder, purlin and joist with concrete.
Instead, the steel frame (the structural hierarchy) of a contemporary high-rise is protected by sprayed-on fire resistive material - it's lightweight, fast and thus much less expensive. It is also the least effective method of protecting structural steel. (The best protection is gypsum sheetrock.)A contemporary high-rise structure is a lightweight building and an older high-rise, such as the Empire State Building, is a conventional (or legacy, if you prefer) structure. Lightweight construction methods and structural components are a developer's dream: they reduce dead load and reduce the time it takes to assemble all the components of the structural hierarchy.
The term "lightweight" is not limited to trusses and glued together I-joists, lightweight means reduced dead load. Just as there are contemporary lightweight trusses, there are conventional legacy trusses. Examples of conventional trusses are the timber arch truss and the timber parallel chord truss. The metal plate connected truss, comprised of two-by-four-inch web and chord members, is an example of a lightweight truss.
Stress and Strain
Just as the human body adjusts to the stress and strain of gravity, time and the environment, so too must a building adjust to the stress and strain of gravity, time and the environment. Strain is the physical evidence of stress. For example, consider a section of rope that measures 10 feet. After the rope is pulled it lengthens to 10 feet, three inches; pulling the rope caused the rope to lengthen three inches. If the change in shape - the strain - is lengthening, the stress has to be tension. The relationship is symbiotic: if there is strain, there is stress; if there is no strain, there is no stress; if there is no stress, there is no strain.