on both theory and practice by a committee of bridge specialists chosen from its members.

The advent of the reinforced concrete bridge may prove to be the inauguration of a better state of affairs in highway bridge construction and the means of correcting the crying evils which have existed for so long. Counties that are tired of replacing worn-out and rusted-out "tin bridges" are beginning to call for reinforced concrete structures, because these require very little annual expenditure for maintenance and repairs, and, as far as is known at present, when properly designed and built, they will last practically forever. But the same prostitution of design and the same criminality in building that for decades have been the curse of the metal bridge business are becoming the bane of reinforced concrete construction. It requires engineering skill of a higher order and greater practical experience to plan bridges of reinforced concrete than it does to design steel structures, and the former need much more rigid inspection of materials and workmanship than the latter. The reasons therefor are as follows:

First. The building of reinforced concrete bridges is a new art that is only beginning to be systematized.

Second. Concrete bridges are an eminently proper type of structure for some locations, but for others they are absolutely unfit; and when used in the wrong places they are liable to involve trouble and disaster.

Third. It is just as easy to skin the life out of the reinforcing bars of a concrete bridge as it has been in the past to cut down the sections of steel bridges below the danger limit; in fact, it is far easier, for, when the deed is once done, all proof of it is hidden permanently until after disaster has overtaken the structure.

Fourth. The prevention of the use of improper cement for the concrete throughout the entire construction is a very difficult matter, and a barrel or two of inert cement worked into a critical place might result in the destruction of the bridge. When one span of a concrete arch bridge collapses the others are more than likely to follow suit, the whole structure from abutment to abutment falling down like a house of cards, because the piers are generally incapable of resisting the unbalanced thrust from the dead load of a single span. To make them capable of so doing would involve an expenditure of money that is not warranted, for the dead load thrust of any span should be resisted by that from the adjoining spans, except at the ends of the bridge, where it is taken care of by the massive abutments.

Fifth. The safety of a reinforced concrete bridge is primarily dependent upon a proper proportion of ingredients in the concrete and a thorough mixing of them, and therefore it is at the mercy of the workmen and subject to the vigilance and care of the foremen and the inspectors. The practicing of that all too common and most reprehensible trick of saving cement in order to reduce the cost of construction would involve far more

serious consequences in a reinforced concrete arch bridge than in the piers for a steel structure.

If county commissioners will have the good sense to consult competent bridge engineers before deciding to build reinforced concrete bridges, will retain them to make the plans and specifications and to supervise the construction, and will pay them upon a sufficiently liberal basis to permit of their hiring all the good inspectors that the work needs, they will succeed in effecting a great improvement in highway bridge building. But, alas! this is too much to expect from ordinary county commissioners, who are too often chosen from the ignorant classes for political and other improper reasons; hence it is to be feared that the highwaymen, the scalpers, and the unfit designers will continue to get in their nefarious work, and that reinforced concrete structures will prove no more reliable or durable than the notorious "tin bridges."

Since the preceding was written the author has received a letter from his friend, J. C. Ralston, Esq., C. E., formerly City Engineer of Spokane, Wash., from which, with the writer's permission, the following extract is made. It confirms very effectively the preceding anticipation of future disaster to reinforced concrete bridges designed by incompetent or improperly interested parties. Speaking of a certain highway bridge builder, Mr. Ralston says as follows:

"He is the man who designed and once put forward seriously an arch made of an intrados ring of concrete about four (4) inches thick and an extrados ring of the same thickness, the two rings being separated about twelve (12) or sixteen (16) inches, and this interior filled with a well-rammed, nice, juicy clay. This, of course, furnished an ample play-ground for the neutral axis and the lines of pressure to play hide-andseek, besides offering special plastic inducements for these frisky functions to stay at home. In fact, I surmise that such a design, in the opinion of the designer, circumscribed their sphere of action within the middle third by barriers of actual concrete. Thus we reach the superlative-the very acme of perfect design, when by such simple mechanical means we confine all such ill-bred functions to an argillaceous field of innocuous desuetude. Need we congratulate ourselves on being members of a profession in which its great leaders weave in such epoch making fashion the dulcet lines of theory and practice into an incomparable fabric of royal perfection?"

But, seriously speaking once more, the reinforced concrete bridge, which certainly has come to stay, is eventually going to prove the cure for the ills of highway bridge building, and the medicine that will effect it is the motor truck. That type of traffic-vehicle has proved itself to be economic, and it has rapidly become heavier, until now its loads rival those of the famous road-roller-that bugbear of highway bridge builders. Furthermore, it must be remembered that the road-rollers traverse bridges so slowly that their impact is assumed to be zero, while the motor trucks. pass over at speed, necessitating the usual highway impact allowance; hence in designing the floor systems it will nearly always be found that the motor truck is the ruling factor. The ordinary county bridge of steel trusses with wooden floor is so lacking in strength, rigidity, and mass as

to be incapable of carrying heavy motor trucks with perfect safety; and as these vehicles have no restricted area of operation, and as their use in the country districts is rapidly increasing, it appears likely that light wooden trestles and tin bridges will soon throughout North America have to be relegated to oblivion.

CHAPTER LXVII

BRIDGE FAILURES AND THEIR LESSONS

THE Scope of this chapter does not permit of an enumeration of all the railway bridge failures that have occurred since structural designing was placed on a rational basis by Squire Whipple; nor has the author available the necessary statistics for making such a compilation. However, it is desirable that the reader should have an appreciation of the influence that past failures have exerted in advancing the standard of bridge designing and construction and in hastening the adoption of the bridge specialist's recommendations. To the newer generation of engineers, it might seem that the present excellence of bridge design and construction has been attained without much effort. Such, however, is not the case; for the present standard has been reached by a costly weedingout process-the defects being brought to light by failures of structures or of parts thereof. It has cost a great many lives and dollars to attain the present standard of excellence. The mental inertia of those in authority which had to be overcome was enormous. Improvement has been brought about through the persistent efforts of the consulting bridge engineer by raising the requirements in his specifications so as to keep pace with the acquisition of new facts, and through his insistence that the said specifications be adhered to.

There is always something to be learned from a failure; but too often failures are smoothed over and minimized and given insufficient publicity, so that their lessons are not duly observed nor appreciated. That there have been numerous failures in times past one can readily see by glancing through the back numbers of the engineering periodicals. For instance, the Engineering News, Vol. 23, page 373, gives the following table of railway bridge failures covering the period of years from 1879 to 1889, inclusive.

This table shows a total of 286 failures in eleven years, or an average of twenty-six per annum. Forty-three of these failures were of iron bridges, an average of nearly four per annum. The number of lives lost and persons injured in the eleven-year period is not given, but for the year 1889 there were reported nineteen deaths and sixty-four persons injured in twenty-two wrecks of bridges. In 1889, the last year of the period, there were some 24,450 iron spans and 15,250 wooden spans in service; and of these, five iron spans and seventeen wooden ones failed. Four of these iron spans which succumbed were wrecked by derailed cars and one by a defective pier. Of the wooden-span failures, four structures were burned, three were wrecked by freshets, six were knocked down by derailed cars, and three succumbed from inherent weakness.

In many cases impact due to derailment of cars produced the failure. Lack of precaution at the ends of the structures in the way of guard rails, re-railing frogs, and collision posts was a contributing cause to many of these accidents. Hence it is reasonable to conclude that some of them might have been prevented and the effects of others minimized, and this remark applies to the cases of the burned wooden bridges and those washed out by freshets. In 1895 there were thirty-seven failures of railroad bridges, causing a loss of fifty-seven lives, besides injuring eighty-six persons. Fourteen of these structures were knocked down, five were "square falls," six were destroyed by fire, and five were carried out by freshets. Seven of these thirty-seven failures were of iron or steel bridges, six of which were knocked down and one wrecked by a freshet. Six electric line bridges also failed that year, resulting in forty-six persons killed and nine injured. Further details concerning these failures will be found in the Engineering News, Vol. 37, page 93. It will be observed that the year of 1896 shows an increase in failures over that of 1889, which, perhaps, is to be expected, as the number of bridges had increased considerably. On the other hand, improvements had been made in design and construction, and safety devices had been developed, so that if the railroad companies had availed themselves of these things to a larger extent, this number of failures would have been reduced. However, as fourteen spans were wrecked by derailment, six were burned, five failed because of inherent weakness in some part, and five were washed out by freshets, it seems that the lessons of the earlier failures had not been heeded. Moreover, very little publicity was given by the technical press to these accidents at the times of their oc