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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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 gravity, time, trauma, and disease can hasten the failure of the human organism, gravity, time, trauma, and fire can hasten the failure of a building.
A competent surgeon must possess a solid knowledge and understanding of human anatomy and physiology. Likewise, a competent fire officer must understand the anatomy and physiology of the square-foot fireground: building construction. Just as human anatomy and physiology provide the foundation of medicine, building construction provides the foundation for developing fireground strategy and for making fireground decisions.
The consequence of being the third planet from the sun, there are two kinds of gravity loads: dead load and live load. Of the two, dead load is the easiest for engineers and architects to deal with. Dead load is determinate, which means it can be calculated precisely. For example, a two-by-four-inch wall stud will always weigh what it weighed when it was installed in the wall; that is, once installed in the building, the dead load of the stud won't change. It's the same with doors, columns, walls, floors, windows, carpeting, roofing materials, etc.
Because it is indeterminate (estimated), the second gravity load, live load, is the most challenging for engineers and architects to contend with, so they overcompensate for it. This overcompensation is called a "factor of safety." Examples of live load are furniture, people, snow, rain and, in the case of a high-rise building, wind. Bottom line: If it is not a determinate, precisely calculated dead load, it is a best-guess indeterminate live load. An example of the strategic significance of the two gravity loads is the floor of a typical residential occupancy.
Gravity Load Exercise
Imagine that you are in a room in your condominium. You look around and admire your stuff that is supported by the floor. Can you guess how much this stuff, the live load, weighs? Let's suppose:
Your weight - 100 pounds (don't worry, you are not anorexic, but this makes the math easier)
Two chairs - 30 pounds
Futon - 100 pounds
Dresser - 100 pounds
Book shelf and books - 65 pounds
Desk - 80 pounds
The initial live load is 475 pounds.
Now, visualize you are sharing the feng shui of your room by hosting an intimate gathering attended by 10 party animals:
10 party animals - 1,000 pounds (again, 100 pounds per party animal makes the math easier)
The initial live load - 475 pounds (from above)
That makes the party live load 1,475 pounds.
The floor of your room measures 12 feet wide by 15 feet long, thus the floor area is 180 square feet. This means that your floor consists of 180 one-foot-by-one-foot squares. If you could evenly spread the 1,475-pound party live load across the floor so that each one-foot-by-one-foot square carries the same load, each square foot of your floor would be loaded with about eight pounds. This theoretical eight-pound-per-square-foot (psf) situation is called a uniformly distributed load. Building codes require a design live load of 40 psf for a residential floor - five times what you just calculated.
Engineers and architects can only estimate what live loads may be placed on this same floor by another occupant in the future. So, your floor is designed to support 7,200 pounds (40 pounds times 180 square feet) of live load - that is 3.6 tons of live load! That sounds like a lot, but we haven't factored the heaviest load of all, the floor itself.
Included in the dead load are the floor structure at 100 psf, attractive floor tiles at five psf and non-load-bearing partition walls at 20 psf. Note: Exterior walls do not rest on the floor; the studs (columns) support their load. Add this dead load to the design live load and you have the total load per square foot of the floor during your intimate gathering: