Editor's note: Part 1 of this report was published in Firehouse® in February 1996. This article continues summarizing a small portion of the 79-page Chapter 2, "Principles of Construction," of the 667-page third edition of Building Construction For The Fire Service, published by NFPA. Copies may...
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(Facts about structures are printed in regular type. Firefighting implications are printed in italics. Page references are to Building Construction For The Fire Service, third edition.)
A column delivers a compressive load along a straight path in the direction of the member. We think of columns as vertical but any member which delivers a compressive load, whatever the direction, is subject to the "laws of columns," which state that columns lose strength by the square of the change in length. For example, a 24-foot column can carry only one-fourth of the load carried by a 12-foot column of the same material and dimensions (12 squared is 144, 24 squared is 576, 576 divided by 144 equals 1/4).
Photo by Thomas R. Webster/Pensacola Beach Fire-Rescue
Flames break through the roof of a Pensacola Beach, FL, restaurant in April 1995. The building had been the city's first firehouse until 1965, when it was converted to a hamburger stand and later remodeled as a restaurant. Firefighters must be aware of concealed hazards in remodeled structures.
Take note of high steel scaffolding. The load is successfully carried on columns of hollow tubes about two inches in diameter because the structure is braced about every eight feet by connections (an important topic to be discussed later). The scaffolding columns are really a set of eight-foot columns one atop the other. Consider using a power saw to slice down along the column and cut all the connections. The columns would buckle and the scaffold would fail.
A theater collapsed when a connection attaching the balcony to the column failed. The connection was not only transferring the load of the balcony to the column but cutting the column into shorter lengths. Even though the loss of the balcony load lightened the load on the column, the increase in length made it impossible for the column to carry the remaining lesser load.
This was a problem in the Oklahoma City bombing when columns weakened by the loss of bracing floors had to be rebraced with steel trusses, some of which were manhandled into place by firefighters.
Note the construction of wood studs. Often, a piece of the studding is cut and placed from stud to stud about mid-height. This is sometimes erroneously called firestopping. In fact, its purpose is to "cut" an eight-foot stud into two four-foot studs one atop the other, thus increasing the load-carrying capacity of the stud wall and stiffening it.
When we discuss trusses, we will learn that the top chord (top member) of a truss is under compression and therefore subject to the laws of columns. Any member under compression, no matter if vertical, horizontal or diagonal responds to the laws of columns.
We must repeat a paragraph from part 1 (February 1996, page 103), which reads, "The shape of a member under compressive load is very important. The further the material of the member is placed from the center the stronger the member is. A hollow steel column can carry a far greater load than the same amount of steel rolled into a solid rod. Structural members under a compressive load tend to buckle. Placing the material away from the center reduces or eliminates the buckling tendency."
All things being equal, a hollow round steel column is probably the best use of available column material. However, other considerations such as use of floor space and ease of making connections are also important.
Steel columns are often H-shaped. Note that a circle could be drawn through the four points of the H. On the other hand, the strength of a beam lies in its depth. Steel beams are therefore shaped like an I.