Mr. Thorpe.

Mr. Wilson.

Mr. W. H. THORPE suggested that it was reasonable to presume, with respect to big structures, that the effect of a gust lasting 1 second only would be much less than if it persisted for, say, 30 seconds, though the pressure developed might be the same. Inertia of mass had to be overcome, and the work of stress development had to be done, both of which took time. He thought it manifest that no more work could be absorbed by the structure than was given up by the wind in the time during which it acted. Sir Benjamin Baker's inference that maximum pressures did not obtain over large areas, because the force required to blow in particular windows was greater than would overturn the building in which they were situated, might have been less well-founded than his deductions usually were, for the wind that would shatter a glass sheet of perhaps 3 lbs. per square foot might be quite harmless on a structure having walls fifty to one hundred times as heavy. The position, however, was not disturbing, for accepting the Author's conclusions, it would appear that, whereas 30 years ago it was not unusual to assume the probability of high wind-pressures, and to discount these for large areas, it would now appear reasonable to consider pressures of say one-half as much, with little or no reduction for large areas, the ultimate effect on the provision of strength in the structure being much the same in the case of large structures, and favourable in the case of those that were small.

Mr. J. S. WILSON thought that the interesting and somewhat unexpected result deduced by Dr. Stanton from his experiments-that the high pressures during wind-storms acted simultaneously over very large areas could be more easily accepted if some of the indications to the contrary could be explained. Some measurements made during the building of the Forth Bridge, given below, supplied one important indication of such a character, and to sustain his conclusions he hoped Dr. Stanton had some explanation of the phenomenon. During the building of the Forth Bridge the wind-pressure on specially-made pressure-plate gauges had been carefully measured and recorded, because the Board-of-Trade recommendation that a wind-pressure of 56 lbs. per square foot should be allowed for in the design was a matter of great importance, and the engineers responsible for the design were very doubtful as to the necessity of providing for so high a pressure. For comparison with those pressures, careful measurements were also made to ascertain how much the cantilevers were deflected laterally by the wind. When the southern cantilevers of the Queensferry pair of cantilevers had been built out about 500 feet, a good opportunity for measurement was available. This long arm exposed to the wind had then its end

near the granite abutment at the Queensferry shore, but received no Mr. Wilson. support either vertically or laterally from it, so that any lateral or vertical movement of the bridge could be measured with facility. In addition to this, the force necessary to deflect the end of the cantilever laterally was measured by means of hydraulic jacks put between the box end of the cantilever and the masonry of the pier. Many measurements, in which the cantilever was pushed laterally various distances up to 1 inch, gave a mean figure of about 24 tons total load to move the end 1 inch. On the 21st March, 1889, while the South Queensferry southern cantilever was standing free with respect to the abutment, during a north-east gale, when the wind would strike the structure of the bridge approximately at right angles to its longitudinal axis, the pressures recorded at the high-level gauges were the following:On the gauge, 2 square feet in area, fixed to the top platform in the centre of the Queensferry cantilever at 381 feet above high water, 15 lbs. per square foot was registered. On the similar gauges fixed on the tops of the Garvie and Fife cantilevers, pressures of 27 and 31 lbs. per square foot respectively were registered. During this gale no lateral movement of the cantilever end could be observed, but there was an up and down see-saw movement of about 0.75 inch. On two other occasions during strong winds a vertical oscillation of about 1 inch was measured, but the horizontal movement was only about 0.25 inch. The estimated maximum horizontal deflection under a 56-lbs.-per-square-foot wind-pressure was calculated to be 11 inches. If a pressure of only about 15 lbs. per square foot had acted simultaneously over the whole exposed area, the deflection would have been 2 to 3 inches, and it was difficult to account for the very small deflection, except on the assumption that the high pressures did not act simultaneously over the whole area of the bridge; and that was the view taken by the late Sir Benjamin Baker.1

The AUTHOR, in reply, observed that he was fully in agreement The Author. with the suggestion that the research should be extended to bridgespans exceeding 250 feet, and that observations on such spans should be made at wind-speeds exceeding 60 miles per hour. He might say that, since the reading of the Paper, the authorities in charge of the Forth Bridge had been approached for permission to install wind-pressure gauges on the structure of the bridge, and a sympathetic reply had been received, to the effect that, subject to approval

1 These tests are reported in "Notes on the Erection of the Forth Bridge," p. 67 (1890), by F. E. Cooper, M. Inst. C.E., the type-written MS. of which is in the Institution Library.-SEC. Inst. C.E.

The Author. by the Directors of the appliances to be installed, such permission would be granted. The design of the apparatus was now in hand. The information already existing as to the variation of windpressure with height was, he thought, sufficient for all purposes of design, but a useful check on the data could be made in the forthcoming experiments on the Forth Bridge.

With regard to the remarkably small lateral deflection of one of the cantilevers of the Forth Bridge on the 21st March, 1889, when the wind-gauge was registering a pressure of 15 lbs. per square foot, to which Mr. Wilson called attention as affording strong evidence that high pressures during wind storms did not act over the whole of a large surface, the Author had not questioned the existence of wind conditions of this nature-in fact several of the records given in the Paper showed evidence of their existence. On the other hand he had obtained definite evidence, of which the records were given in the Paper, of the existence of winds of high velocity in which the lateral variation of velocity over a front of 250 feet was practically negligible, and had pointed out that such phenomena must be taken into account in the choice of a wind-pressure factor for structures. The question whether such lateral uniformity might exist for wider spans and at higher speeds than 60 miles per hour was a matter for future research.

13 January, 1925.

BASIL MOTT, C.B., President,
in the Chair.

The PRESIDENT said it was with great regret that he had to announce the death of one of the Honorary Members of The Institution, Sir William Edmund Garstin, G.C.M.G. It was not necessary for him to say anything as to the work of Sir William Garstin in Egypt, because it was well known to all. He was Inspector-General of Irrigation there, and was associated with Sir Benjamin Baker in the Assuan dam and other works. The Council had passed the following resolution :

66

That the Council record with deep regret the death of Sir
William Edmund Garstin, G.C.M.G., G.B.E., who has
been an Honorary Member of The Institution since
March, 1917."

On the recommendation of the Council, the members present elected by acclamation as an

Honorary Member.

Senator GUGLIELMO MARCONI, G.C.V.O., D.Sc., LL.D.

The Council reported that they had recently transferred to the class of

Students.