It is possible for one to correctly lay out or plan a trussed roof without being able to determine the stresses in the various parts, but to do so a knowledge of the mechanical principles involved is absolutely essential, unless one can find a similar case to follow, and even then he might copy a poor example.
Every architect, draughtsman and master builder should thoroughly understand the way in which the various members of the common types of trusses are made to support the weight of the roof, or other loads, why the braces run this way or that, and which members are in compression and which in tension. This much is. necessary to enable one to make the preliminary drawings of the roof in an intelligent manner. If the general design is correctly laid out, a structural engineer may be engaged to determine the stresses and compute the size of the members, detail the joints, etc., while the general drawings are being finished by the architect, although, of course, it is desirable for the architect to be able to do all of this work himself.
In this chapter, the author has endeavored to explain the way in which the stresses act in the more common types of wooden trusses, and to give examples of nearly all the various forms used in building.
According to Professor Lanza the term "truss" may be applied to any framed structure intended to support a load.
In order, however, to distinguish a truss from a mere framework the author prefers the following definition:
"A truss is a triangular, polygonal or curved framework supported only at the ends (or in the case of a cantilever truss at the centre), and so designed that it cannot suffer distortion without crushing or pulling apart one of the pieces of which it is composed."
A true truss does not depend upon the rigidity of its joints for its stability, and imposes only a vertical pressure on the walls.
"A joint" of a truss is the intersection of two or more members of a truss. If a member is built up lengthways with two or more pieces of material, the place where the pieces join would ordinarily be spoken of as a joint, but such joints are not truss joints. In this work they will generally be designated as splices, in distinction from the joints proper.
"A member" of a truss is any straight or curved piece which connects two adjacent joints of the truss. Members are also often called pieces.
"Ties" are those members of a truss which are in tension only.
They may be either of wood, iron or steel, and of any cross section.
"Struts" are those members of a truss which are in compression only. They may also be of either wood, iron or steel, and of any cross section, capable of resisting flexture.
"Tie-beams" are ties that are also subjected to a transverse strain. The main horizontal tie of wooden trusses is often called the "tie-beam," even when it has no transverse strain, but the term is not then strictly correct.
"Strut beams" are struts that are also subject to a transverse strain.
"Chords." In horizontal and bridge trusses, the top member and the main horizontal tie, are often called "chords," the upper one being the top chord and the lower one the bottom chord. (In queen rod trusses the top chord is sometimes termed the "straining beam.")
In the King rod or Queen rod trusses, the main slanting struts are often called "rafters" or "principal rafters," because they are usually parallel, or nearly so, with the rafters of the roof.
"Purlins" are horizontal beams, sometimes trussed, extending from truss to truss to support the rafters or ceiling joists.
"Stress." The term "stress" denotes an internal resistance which balances an exterior force, or if we imagine a piece of material, subject to an external force, cut in two at any point, the force with which one part of the piece acts upon the other at this section is called the "stress." In connection with trusses, the term i also very commonly used to denote the force which any given member is required to resist. In this country stress is commonly measured in pounds or tons.
"Unit Stress" is the stress on a unit of area, generally the square inch.
The "stress per square inch" is equal to the total stress divided by the number of square inches in the section on which it acts. Thus if a strut 6 inches square, is subject to a compressive stress of 18,000 lbs., the unit stress is 18,000, divided by 36 or 500 lbs. _
"Strain." When a solid body is subjected to a stress of any kind, an alteration is produced in the volume or shape of the body, and this alteration is called the "strain." Strain is, therefore, the result of a stress or stresses. For safe stresses the strain produced in a strut or tie, is very minute; in the case of a beam, the strain is the elongation of the fibres on one side and the shortening of the fibres on the opposite side, due to the bending of the beam, the "pull" on the bottom fibres being commonly termed, the "fibre stress."