The material in the following chapters has appeared, in the main, as a series of articles in Western Engineering, In being arranged for publication in the present form, this has been revised and enlarged. The matter contained therein is the result of some eleven years experience in timber-framing, during which time I have been intimately connected with the design and the superintendence of construction of nearly two hundred million board feet of timber, most of this being represented by the structural features of two expositions.

In this work, I have found that the published record of timber construction is meagre. Especially is this statement true of details of design, and strength of timber joints. I have searched through all available engineering literature for the results of tests of timber joints and fastenings, and have been disappointed in finding so few recorded. To supplement these few tests, I have made additional ones on various types of timber joints.

I have, in my own work, always tried to design the particular structure, so that it would be effective and efficient in action, and at the same time be simple and direct for the carpenter to frame. These two conditions are not always possible to obtain; their correlation, however, is always to be sought. With this end in view, I have, whenever possible, followed my designs into the field, observed the framing and erection of the structure, its behavior under load, and the effect of fime and the elements.

The results of this experience and study are presented in the following pages. As explained in the introductory chapter, this volume is in no sense a text-book, and does not cover equally all phases of timber-construction. Its many shortcomings are realized, but it is hoped that the contents may be of some benefit to those who may have occasion to design or construct timber-framing.

With some of the theories advanced, and conclusions drawn, there may be differences of opinion. I am frank to state that certain of these conclusions may have to be modified in the light of future tests. It is always unwise to attempt to extend the results of tests too far. However, until such further tests are made, it is imperative that working-values be established for present use. The best that can be done under those circumstances is to use the most reasonable theory that can be found, utilizing the available tests as a guide. This method is certainly better than a blind guess, or a rule-of-thumb method. As an illustration of the condition just mentioned, the present method or methods of designing bolted joints may be cited.

While timber as a structural material has been largely supplanted by steel and concrete, especially in permanent work, there are still many occasions where it may be employed advantageously in bridges and buildings, and other structures of a somewhat permanent nature. A knowledge of the properties of timber, its capabilities, and its limitations for use in construction, is therefore an essential part of the education of a civil engineer.

The old-school bridge engineer was a past master in the art of timber framing. Many of his structures, it is true, were framed more by experience and judgment than by considerations of theory and of computed stresses, yet the number of timber railroad bridges still giving service testifies to the soundness of his design. The results of his experience have been handed down to his successors and are represented today in the accepted standards of the railway engineer's office. Outside of this class of engineers, however, it may be truthfully said that neither is the art of timber framing generally understood, nor is the value of such knowledge appreciated.

For the design of wooden buildings of exceptional size or of unusual proportions, a structural engineer is now generally retained; otherwise, the plans for framing are prepared in the architect's office. In the latter event the work is usually given to an architectural draftsman possessing little experience in actual construction, and only a superficial and therefore frequently dangerous knowledge of structural mechanics. This practice results from the commonly accepted ideas that timber designing consists in computing the sizes of beams and girders, or in solving the stresses in a roof truss, and that, given the required sizes of the principal structural members of a frame, the carpenter is fully capable of designing the joints. This conception of the scope of timber designing is erroneous. There is no timber structure of an appreciable size which will not justify a careful and intelligent study of the framing details, not alone on the ground of safety, but also from the consideration of economy. Important details should not be left to the judgment of the contractor or carpenter. With all respect for the ability of the experienced carpenter, there is at times nothing so impractical as a so-called practical man. I have seen instance after instance where it would seem that the carpenter had gone out of his way to frame a joint in the weakest possible manner.

Obviously, the method of finding the stresses in a structure is the same whether the material be timber or steel or concrete, and timber joints are as susceptible to analysis for strength as are details in any other material. The cause of the weak details so often seen in timber trusses has been largely the failure on the part of the designer to realize that the joints needed attention. As a test for the display of ingenuity and as a problem to develop one's knowledge of practical and efficient construction, the design of an ordinary mill building in timber and the superintendence of its framing and erection has few equals.

For an intelligent design in timber, a knowledge of sawmill and timber-yard methods is essential. The difficulties of actual framing and erection must also be anticipated and provided for; the designer must imagine himself in the carpenter's place and realize, for example, what cuts will be most difficult to make and what holes will be hard to bore ; in other words he must foresee in what details careless work is most likely to occur. The possibility of the timber being green and the consequent shrinkage must be recognized, and if such shrinkage is detrimental to the strength of the structure, means must be provided for tightening the joints after the shrinkage has taken place. As possible incipient causes of failure by shear, the checks due to seasoning must not be neglected. In short, all the limitations of the material must be fully realized.

In the case of structures of steel, the majority of the details can be made in accordance with the standards of present-day practice, fully treated in the text-books and the handbooks of the steel companies and bridge shops. In the realm of reinforced concrete design, certain standards for detailing are being formed rapidly. For timber, however, there are no such standards, except those for bridge and trestle-work generally followed by the railroad engineer. Or, it may be said that in timber construction, many details called standard can be justified by no consideration of efficiency. Even the standards used by the old-school bridge engineers cannot be employed indiscriminately. Certain of these, while entirely suitable for the woods obtainable in the Eastern
States, have been transferred to the West and applied without modification to timbers with entirely different properties from those for which the details were designed. The most notable example of this practice is the use of the standard cast or malleable-iron washer with wood as soft as Douglas fir.

For both steel and concrete design and construction there are many good text-books and standard specifications, but for timber framing, such as heavy building and bridgework there are only a few text-books and, to my knowledge, no standard specifications. Among the few books dealing with this subject, Jacoby's Elements of Heavy Framing' and Howe's 'Simple Roof Trusses' are notable for their excellence, and their contents should be mastered by anyone interested in the design of timber structures. It is with the view of supplementing these and other existing works, by bringing into correlation the drafting-room design and the requirements of the field, rather than covering the whole subject of structural design in timber that the present treatise has been undertaken.

A general knowledge of structural design on the part of the reader has been assumed and no attempt has been made to cover the whole field of timber framing, but by discussing the advantages and disadvantages of different typical details, I have tried to point out the structural limitations of the material and the difficulties which arise during construction. Only by a thorough understanding of the many elements that enter into the design of joints and details in timber framing is it possible to make the finished structure safe, efficient, and economical.

A set of general specifications for timber-work is given in the concluding CHAPTER. These specifications are intended primarily for buildings, but with certain obvious modifications are applicable to any timber structure. Since the greatest forests occur in the West, these specifications apply particularly to Douglas fir, but with different unit stresses they may be used for any other timber. The properties of Douglas fir are not far different from those of long leaf yellow pine, so that the specifications may be used with but slight changes for structures built of the latter timber. With these specifications, I hope to establish to some extent certain workable standards for timber framing in general, and for building construction in particular.