Editor's note: This article finishes 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, by Francis L. Brannigan. Part 1 was published in Firehouse® in February 1996, part 2 in July...
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Editor's note: This article finishes 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, by Francis L. Brannigan. Part 1 was published in Firehouse® in February 1996, part 2 in July 1996.
(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.)
TRANSMISSION OF LOADS
"All loads must be delivered to the earth." This absolute maxim is often ignored by the proponents of one building material or another. The wood industry often shows a picture of a deep laminated wood beam only charred by fire. A twisted steel beam lies across it. The caption doesn't mention that many, many laminated wood beams are supported on unprotected steel columns.
It is important to understand how loads are transmitted from the point of application to the ground. Consider an ordinary brick and wood-joisted building with interior columns:
- A load is placed on a wood floor.
- The floor boards deliver the load to the joists.
- The joists deliver the load to a masonry wall at one end and to a girder (a beam which supports other beams) at the other.
- One end of the girder rests on a masonry wall. The other end rests on a column.
- The amount of the load transmitted to each support point depends upon the distance of the load point from each of the supports, the nearer points receiving the greater load.
- The structural engineer must calculate the distribution of the loads. The building does it automatically, according to the laws of physics. This example makes use of a simple masonry and wood-joisted building. Regardless of the size of the building or the construction materials, the principles are the same. Any weight on the roof of a giant high-rise building is transmitted to the ground through the structure of the building.
All loads must be transmitted continuously to the ground from the point at which they are applied. Any failure of continuity will lead to partial or total collapse.
Illustration by Christopher J. Brannigan
The building load supported on a too-short cast iron column built up by cast iron blocks was seen in Denver. Such a connection, like a child's building blocks, has no resistance to a lateral thrust.
There is a tendency to make light of partial collapse and consider it unimportant. A partial collapse, like any collapse, is very important to at least two groups those under it and those on top of it. The fatal collapse of the walkways in the Kansas City Hyatt Regency Hotel (page 60), which claimed more than a hundred lives, was "only a partial collapse."
Roofs are very important to the fire service. In some cases, the roof is necessary only to keep the rain out; in other cases, the roof is vital to the stability of the building. Roofs will be discussed in more detail as we discuss each type of building but a few vital points are given here.
In so-called tilt-slab concrete buildings, the roof is vital to the stability of the structure, and roof damage can cause wall collapse (page 373).
The recent, very necessary special attention to truss roofs should not cause firefighters to ignore the hazard of the sawn joist roof (page 181). Combustible metal deck roofs, when burning, are unsafe to be on, and firefighters cannot possibly or safely open the huge vent holes which would be required (pages 302-309; also see "Follow-Up Report" by Dave E. Williams on page 48 of this issue). Trusses are fully discussed in Chapter 12, pages 517-563. The subject will be summarized in a future issue. Single-ply (or membrane) roofing is a relatively recent problem.
To ventilate a ballasted roof, remove the ballast from an area several feet larger in all directions than the desired hole. Cut the membrane with a knife or scissors, not with an axe or power saw. Peel back the membrane and open the roof. If the membrane is glued to the roof, peeling is difficult.