The Forth Bridge/Experiments on Wind Stresses
Experiments on Wind Stresses.
The scanty information existing on the very important subject of the action of wind pressure on the surfaces of structures, whether flat or curved, induced Mr. Baker to make a series of experiments, upon the results of which it would be possible to form some definite conclusions. These experiments are so interesting, and the appliances by which they were obtained were so ingenious, that no apology is needed for their repetition here. Realising the difficulty of working with models in actual wind, which is never, so to speak, of the same intensity or direction for two consecutive moments, and labouring under the disadvantage of not having an instrument which would reliably indicate the actual pressure at any time, Mr. Baker simply reversed the order of things by making the wind stationary and the apparatus movable. The latter then consisted of a light wooden rod, suspended in the middle, so as to balance correctly, by a string from the ceiling. At one end of the rod was attached a cardboard model of the surface the resistance of which was to be tested, be it a portion of a round tube, a flattened strut, a piece of top member, or of the internal viaduct, or even of a whole cantilever. On the opposite end of the rod was placed a sheet of cardboard facing the same way as the model, so arranged that by means of another and adjustable sheet, which could slide in and out of the first, the surface at that end could be increased or decreased at the will of the operator.
The mode of working this contrivance is for a person to pull it from its perpendicular position towards himself, and then gently release it, being careful to allow both ends to go together. If this is properly done, it is evident that the rod will in swinging retain a position parallel to its original position, supposing that the model at one end and the cardboard frame at the other end are balanced as to weight, and that the two surfaces exposed to the air pressure coming against it in swinging are exactly alike. Should one area be greater than the other, the model or the cardboard sheet, whichever it may be, will be lagging behind, and twist the string. By now increasing or diminishing the area of the cardboard sheet and repeating the experiment over and over again, a point will be reached when the whole mass will swing without twisting being produced. The area of the cardboard will then represent the exact area of the model which is affected by wind pressure.
The experiments carried on in various ways by different people and at different times are generally in agreement with each other, and not very different from those arrived at by scientists with most complicated apparatus, and by laborious and painstaking processes.
The points on which reliable information was more particularly wanted were in respect of surfaces more or less sheltered by those immediately in front of them. In the case of any box lattice girder, for instance, assuming that the wind was blowing fully square at it, it might also be assumed that the side nearest the wind would cover the side of the girder lying behind; but if the wind blows at an angle to the girder, it is certain that the second surface receives its proportion of full pressure of the wind; and in cases where two lattice box girders are close together—as, for instance, in the top member—all four surfaces will receive a proportionate amount of wind pressure.
It may easily be understood that the distance from each other of these surfaces has a great deal to do with the amount of wind stress they receive, and it was with a view of obtaining some useful data with regard to this question that Mr. Baker's experiments were carried out.
Information exists about flat surfaces and curved surfaces, and also about cubes—that is, two or more sides of any rectangular box or girder upon which the wind acts in a more or less diagonal direction. In all these Mr. Baker's experiments agreed with those of other observers, and obtained with different apparatus; but in the case of sheltered surfaces the results were somewhat different. On the whole, however, Mr. Baker satisfied himself that in no case was the area affected by the wind in any girder which had two or more surfaces exposed more than 1.8 times the area of the surface directly fronting the wind. As the calculations have been made for twice this area, the stresses which the structure will receive from this cause will be in all cases less than those provided for.
Mr. Baker also tested models of girders built of metal, both in air and in water, and although some slight differences with the former result were found, yet on the whole they fully confirmed the general conclusions arrived at.
The observations now made on the completed structure will no doubt help to throw further light on this subject of great importance to engineers, since in large structures the wind stresses are of considerably greater moment than the train loads, and should therefore, for economical considerations, be reduced to the narrowest limits compatible with absolute safety.