Although trusses have been used for supporting the roofs of large buildings for centuries and there are few modern buildings designed for public or semi-public purposes which do not require one or more trusses to support some part of the building, yet there is a great deal of ignorance on the part of draughtsmen and builders and also otherwise well-informed architects as to the manner in which the stresses act and the correct way of arranging the various members in different types of trusses to sustain the loads. In fact, the mechanical principles of any but the simplest form of truss and the manner of determining the stresses seems to be difficult of comprehension by persons who have not had the advantages of instruction at a technical school and it has been the experience of the author that it is somewhat difficult to make the subject perfectly plain without oral explanation.
In this work, therefore, a special effort has been made to make the explanations as clear as is possible in print, even at the risk of appearing rather elemental and common place, and the author hopes that he has so far succeeded that any person who will diligently follow the explanations and go to the trouble of drawing out the diagrams may become fairly proficient in designing ordinary trussed roofs.
The proper designing of a trussed roof requires not only a knowledge of the theory of trusses and the strength of materials but also a familiarity with the various types of trusses that are employed for the support of roofs and their adaptability to different forms of roofs; a practical knowledge of the economical spacing of the trusses and of roof construction in general and how to meet any special forms of construction in the most economical manner, or in short how to lay out the entire framework so as to meet the requirements of strength, the special requirements of. the building and a wise economy. In the preparation of this book, therefore, the author has endeavored, first, to give practically all of the types of trusses that are used in buildings, with sufficient explanation as to their advantages and limitations as to enable the reader to select the type of truss best suited to his purpose; second, to show how various kinds and shapes of roofs may be best supported; third, to give the method of computing loads and determining stresses; and, finally, how to proportion the members and joints to the stresses.
To do this in the best manner it seemed advisable to divide the book into chapters and to describe certain kinds of roofs, such as domed roofs, church roofs, armory roofs, etc., in separate chapters. The method of paragraphing employed in Parts I. and II. of this series has also been retained, because of its convenience for cross-references, and also for convenience when used as a text-book.
One of the most valuable aids to the architect is a knowledge of what has been done in different lines of building construction, and for this reason a great many examples of existing trusses and of trussed roofs are illustrated. These examples have been selected as guides to the architect and draughtsman as to the shape of the truss, section of the members, detailing of the joints and the manner of bracing laterally and to the posts or walls. It is seldom that a truss can safely be copied outright, but after the stresses have been computed, such illustrations as have been given will be found of much assistance in deciding on the manner of building the truss, proportioning the joints and bracing the roof. This is particularly true in the matter of steel trusses.
In conclusion the author would advise those readers who are studying the subject for the first time and who wish to thoroughly understand the correct manner of designing a roof truss to first study carefully Chapters I., II. and III., which in a general way explain the mechanical principles of different kinds of trusses and then to study with great care Chapters VII -VIII. The only way in which one can learn to draw a stress diagram is by taking paper and pencil and drawing the diagram to a scale, line by line, in accordance with the instructions given. It can never be learned by simply reading the book. After a few of the examples given have been worked out, the student should apply the method to similar trusses with different proportions, when the general principle will become apparent and once this is understood it can readily be adapted to almost any kind of truss.
By persistent effort, any person of average intelligence should be able to master the principles of graphic statics, as applied to ordinary types of trusses, and once mastered the stresses can be very easily and quickly determined. Without being able to determine the stresses, however, it is impossible to economically proportion a truss to its load and span with any degree of accuracy, or with complete confidence in its safety.