The Forth Bridge/Wind Pressure and Wind Gauges

by Wilhelm Westhofen

Wind Pressure and Wind Gauges.

The wind pressure to be provided for in the calculations for bridges in exposed positions is 56 lb. per square foot, according to the Board of Trade regulations, and this twice over the whole area of the girder surface exposed, the resistance to such pressure to be by deadweight in the structure alone.

The most violent gales which have occurred during the construction of the Forth Bridge are given with the pressures recorded on the wind gauges in the annexed Table, No. I.

TABLE No. I — Records of Wind Gauges on Inchgarvie during Violent Gales

Year.	Month and Day.	Pressure in Pounds per Square Foot.					Direction of Wind.
		Revolving Gauge.	Small Fixed Gauge.	Large Fixed Gauge.	In Centre of Large Gauge.	Right-hand Top of Large Gauge.
1883	December 11	33	39	22	—	—	S.W.*
1884	January 26	65	41	35	—	—	S.W.*
1884	October 27	29	23	18	—	—	S.W.
1884	October 28	26	29	19	—	—	S.W.
1885	March 20	30	25	17	—	—	W.
1885	December 4	25	27	19	—	—	W.
1886	March 31	26	31	19	—	—	S.W.
1887	February 4	26	41	15	—	—	S.W.
1888	January 5	27	16	7	—	—	S.E.
1888	November 17	35	41	27	—	—	W.
1889	November 2	27	34	12	—	—	S.W.
1890	January 19	27	28	16	—	—	S.W.
1890	January 21	26	38	15	—	—	W.
1890	January 25	27	24	18	23½	22	S.W. by W.

* These data are unreliable, owing to faulty registration by the indicator needle, as will be explained. They were altered after this date. The barometer fell to 27.5 in. on that occasion—over ¾ in. within an hour.

It is worthy of observation that only one gale from easterly direction is recorded—January 5, 1888—but there have been a number of gales from that quarter registering between 15 lb. and 16 lb. and up to 20 lb. per square foot, and, of course, the same from other directions.

The pressure gauges which were put up in the summer of 1882 on the top of the old castle on Inchgarvie, and from which daily records have been taken throughout, were of very simple construction. As the object was to ascertain only the maximum pressures which the structure would eventually have to resist, the maxima only were taken. The most unfavourable direction from which the wind pressure can strike the bridge is at right angles to the longitudinal axis, or nearly due east and west, and two out of the three gauges were fixed to face these directions, while a third was so arranged as to register for any direction of wind. There was no provision made for registering intermediate pressures, nor particulars as to direction of wind or times of occurrence, except in so far as the records were taken generally at 9 a.m. every day.

WIND GAUGES ON INCHGARVIE

The principal gauge (Fig. 17) is a large board—20 ft. long by 15 ft. high, or 300 square feet area—set vertically with its faces east and west. The weight of this board is carried by two rods suspended from a framework surrounding the board, and so arranged as to offer as little resistance as possible to the passage of the wind, in order not to create eddies near the edge of the board. In the horizontal central axis of the board there are fixed two pins, which fit into the lower eyes of the suspension-rods, the object being to balance the board as nearly as possible. Each of the four corners of the board is held between two spiral springs, all carefully and evenly adjusted so that any pressure exerted on either face will push it evenly in the opposite direction; but on such pressure being removed, the compressed springs will force the board back to its normal position. To the four corners four wires are attached, uniting in pyramidal formation in one point, whence a single wire passes over a pulley to the registering apparatus below. This in the original arrangement consisted of two levers at right angles to one another, the the shorter one—about one-third of the other—being acted on by the wire from the windboard, the longer one with an index pointer registering the amount upon prepared paper slips. The indication by the pointer was thus about three times the amount of travel of the wire up and down. In order to ascertain, to some extent, how far great gusts of wind are quite local in their action, and exert great pressure only upon a very limited area, two circular spaces—one in the exact centre, and one in the right-hand top corner about—18 in. in diameter, were cut out of the board, and circular plates inserted which could register independently the force of the wind upon them.

By the side of this large square board, at a distance of about 8 ft., another gauge—a circular plate of 1+¹⁄₂ square feet area, facing east and west—was fixed up with separate registration. This was intended as a check upon the records given by the large board.

Another gauge of the same dimensions as the last, but with the disc attached to the short arm of a double vane, so that it should face the wind from whatever direction it might come, was set up (see Fig. 18.) This also had a separate registering apparatus arranged in the same manner as the first two described.

Fig. 17 gives two views of the large wind-board board with the two independent gauges at A and B. Fig. 18 shows the revolving wind gauge, consisting of a circular disc hung in four bent springs, and rods which centre in the point M. Here a horizontal crossbar connects the two opposite springs, and to this bar, immediately opposite the centre of the hollow vertical spindle P, a small chain is attached, which passes through a slot in P over the pulley N, and right down the centre to the registering apparatus below. The double vane is fixed upon P, which revolves in the socket column R. The small fixed gauge is of the same construction and area, the only difference being that the vane is absent, and P is a fixture in R and cannot turn. The circular disc of course faces west.

It is needless to say that these gauges were not expected to give very accurate records of the wind pressures which occurred since they were put up, but they give a sufficiently approximate idea of what the structure will hereafter have to encounter in the way of wind.

Upon one occasion the small fixed board appeared to register 65 lb. to the square foot, a registration which caused no little alarm and anxiety. Mr. Baker, however, declined to accept the figures recorded, and on investigation found that, with the lever multiplication-pointer, it was not difficult to obtain high figures, owing to the lever acquiring momentum from a suddenly applied force such as a strong gust of wind would produce, thereby overshooting the mark. Thus a blow of about 20 lb. applied smartly to the gauge would register 65 lb., and would probably have registered more but that the pointer could go no further. This occurred in January, 1884, and since then the recording apparatus has been altered, the horizontal lever being done away with, and a vertical bar—suspended directly from the wire of the gauge—being substituted.

Since then the highest pressures recorded have only been 35 lb., 41 lb., and 27 lb. respectively.

A gale on March 31, 1886, gave the following results:—

Upon the small fixed gauge	31	lb. to the sq. ft.
Upon the„ revolving gauge„	26	lb. to the sq. ft.„
In the centre of the large board	28½	lb. to the sq. ft.„
In the„ upper corner of the large board„	22	lb. to the sq. ft.„
All over the large board	19	lb. to the sq. ft.„

These figures seem to indicate that the higher wind pressures come more in gusts and sudden squalls than in a steady and even pressure extending over a large area.

After the central towers had been carried up to the full height, two additional revolving gauges—one at the north-east and one at the south-west corner of each tower—were put up and records taken and compared with those given by the other gauges. The records confirm most distinctly the results of the smaller gauges inserted upon the large board—for the pressures recorded vary as much as 10 lb. and 12 lb. between the different piers—sometimes the one, sometimes the other, showing the higher registration.