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Both the student and the practitioner in bridge-designing will do well to recognize and bear constantly in mind certain first principles of design; and to enable them to do so, the author offers the following, which he considers will cover the essential fundamental principles that should govern the designing of all structural metal-work. Most of these will be repeated in the "General Specifications" given in Chapter XIV. under the heading "General Principles in Designing all Structures," for the reason that the said specifications would be incomplete without them.
The reason for this special chapter being devoted exclusively to these general principles is that the subject is of the utmost importance, and needs much more elaboration than could properly be given it in specifications. On this account the statement of each principle herein will be followed by remarks of an explanatory nature giving its raison d'être or application. It is to be noticed that the numbering does not agree with that of the "General Principles" in Chapter XIV.
The attention of the reader is called to the fact that this chapter is by far the most important one in the book, in that it contains in a concentrated form the most important conclusions drawn from the author's entire experience in his chosen specialty. The principles given have been established mainly by observation of the mistakes of others, and in a few cases, it must be confessed, by those of his own.
Few designers care to make public their errors for fear of the result being to their disadvantage; nevertheless far more is learned from the mistakes of construction than is learned in any other way.
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