The Forth Bridge/The Circular Granite Piers

by Wilhelm Westhofen

The Circular Granite Piers.

The foundations below low-water level of ten out of the twelve circular piers, vary both in size and considerably in depth, but above that level they are exactly alike. The two exceptions are the two north piers on Fife already described. Their foundations start at 7 ft. below high water, and they are only 45 ft. in diameter under the necking course. In all other respects they do not differ from the remaining ten piers. In all these the granite masonry starts at low-water level, or 18 ft. below high-water level, with a diameter of 55 ft.; rises with a regular straight batter of 1 in 10+¹⁄₂ to a height of 12 ft. 8 in. above high water, where the diameter is 49 ft., and terminates in a necking and a coping course with a somewhat rounded top at exactly 18 ft. above high water. The courses of granite, of which there are nineteen, are rock-faced, while necking and coping are of dressed granite, and they vary in thickness from 21 in. in the lower to 16 in. in the upper courses, while above those the necking is 19 in. and the coping about 3 ft. in. thickness.

TABLE No. VIII.—CIRCULAR GRANITE PIERS.—TABULAR STATEMENT OF QUANTITIES.

All Excavation—Concrete, Rubble, and Brickwork in Cubic Yards. Granite in Cubic Feet. Steel, Iron, and Cast Iron Tons, Cwts. and Qrs.

Piers. Q.–Queensferry. G.–Inchgarvie. F.–Fife.	Excavation	In Foundations below Low Water.	In Circular Pier.	Steel.	Iron.	Cast-Iron Anchor Plates.
Concrete.	Brickwork.	Rubble.	Rubble.	Granite.	Brick-work.	Cutting Edge of Caisson &c. Plates.	Holding down Bolts.	Caissons.	Bolts.
										Tons	cwts.	Tons	cwts.	qra.	Tons	cwts.	Tons	cwt.	Tons	cwt.
Q.N.E.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	Mud and boulder clay.	6827	9801	Nil	Nil	2376	13,621	Nil	52	7	9	6	3	399	0	9	9	12	12
Q.N.W.	6271	7547	1105	"	2376	14,543	"	52	7	9	6	3	399	0	9	9	12	12
Q.S.E.	6651	7361	Nil	"	2305	13,237	"	52	7	9	6	3	338	5	9	9	12	12
Q.S.W.	6372	7089	"	"	2305	13,237	"	52	7	9	6	3	340	7	9	9	12	12
G.N.E.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	Whinstone rock.	755	Nil	"	236	2300	13,237	"	Nil		9	6	3	24	3	16	14	12	12
G.N.W.	324	Nil	"	608	2300	13,237	"	Nil		9	6	3	38	10	17	1	12	12
G.S.E.	1054	5635	626	Nil	2300	13,327	"	95	16	9	6	3	334		13	6	12	12
G.S.W.	831	6576	626	Nil	2300	13,237	"	95	16	9	6	3	360	11	13	6	12	12
F.N.E.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right\}\,}$	Whinstone rock.	438	Nil	Nil	264	854	6484	"	Nil		8	12	2	Nil		8	12	12	12
F.N.W.	444	"	"	481	494	6842	350	"		8	12	2	"		8	14	12	12
F.S.E.	1004	"	"	836	2300	13,237	Nil	"		9	6	3	48	2	20	9	12	12
F.S.W.	823	"	"	200	2300	13,237	"	"		9	6	3	21	17	16	13	12	12

The latter two courses are of Cornish granite, the others of Aberdeen granite, both light grey in colour. The blocks of rock-faced granite have the edges dressed to the batter of the pier, and to horizontal and vertical joints. The courses are alternately headers and stretchers, with a bond of not less than 9 in. The joints are not more than ¹⁄₄ in. wide, and are pointed from the outside with pure cement.

The hearting is principally of flat bedded Arbroath rubble, but a large number of whinstone blocks roughly squared were also built in. In building the pier the rubble masonry closely followed the setting of the granite, and both vertical and horizontal bond was strictly observed. Between the concrete in the foundation and the rubble masonry in the piers bond was also established by large blocks of whinstone squared to obtain proper bedding. (See Figs. 54 and 55.) At this point a wrought-iron belt 56 ft. in. in diameter, 18 in. in depth and ³⁄₄ in. thick, made in sections and rivetted together, was placed and built in. A second belt of similar strength, but only 43 ft. in diameter, was built in about 2 ft. below high water, and a third belt of double strength, or 1+¹⁄₂ in. thick and 39 ft. in diameter, was built in just behind the coping course. All these are shown in Fig. 54.

When the level 7 ft. below high water was reached a temporary timber stage was erected and carried to the level of the underside of the fixed bedplate about 17 ft. 6 in. above high water. Upon this a templet of the bedplates made of light angles and ¹⁄₄-in. plate in four sections bolted together, which had all the holes for the holding-down bolts in it, was laid, correctly centred and screwed down. The bolts with anchor plates supported by nuts were then carefully set up and built into the rubble masonry, a space of a few inches being left round each bolt to admit of subsequent adjustment. This is very distinctly seen in Plate VII.—Inchgarvie north-east pier. The building of the pier was now continued, the position of templet and holding-down bolts being frequently checked to insure correctness.

From the plan it will be seen that the coping course consists of alternate headers and stretchers.

The top or crown of the pier is slightly spherical, and is built up of blocks of dressed Aberdeen granite from 17 in. to 18 in. in thickness. The blocks fitting into and adjoining the coping course were all cut to wooden templets sent, the remainder being arranged in straight courses. The blocks project from 6 in. to 12 in. under the bedplate, a recess being cut out to receive the latter. The remaining space under the bedplate is made up with Arbroath rubble masonry, and in one case with a tolerably thick layer of Staffordshire blue bricks built in cement.

The mortar used in building the piers was throughout of one part cement and two parts of sand, and was mixed in a pugmill close by. At first hand cranes only were used in the building, but steam cranes were substituted as being more handy and expeditious. Both granite blocks and rubble were handled and set by means of pointed chain-clips, holes being picked for the purpose.

The holding-down bolts in each pier were forty-eight in number, in four rows of twelve each. They are set at 2 ft. 9 in. and 7 ft. respectively to each side of the centre line, and are longitudinally about 3 ft. apart. Owing to the inward set of the bottom members in cantilevers, rather more than half of the bolts are set in a line parallel to the centre line of the bridge, the remainder following the deviation of the bottom member. In order to bring the largest possible mass of masonry into play, the four centre-bolts of each outside row are bent outwards, as shown in Fig. 54, and to prevent any tearing action upon the masonry which would be produced by the bolts being drawn tight at top, cast-iron shoes are inserted at the point of kinking, and these are held together by a pair of angle-bars to each pair of bolts. (Figs. 56, 57, 58, and 59.) The holding-down bolts are of a special steel. They are 2+¹⁄₂ in. in diameter, with an enlargement to 3 in. at both ends, where a screw thread is cut upon them. They are about 25 ft. long. The anchor plates are 2 ft. square with a long boss, stiffened by four diagonal ribs, and are held by an ordinary nut. The bolts received several coats of tar before being built in.

When the masonry had been carried to within about 8 ft. of the top the templet had to be removed to allow the rubble to be placed in position, but it was replaced from time to time, and the position of the bolts frequently checked. As the heads of these bolts fitted in the lower bedplate without any play whatever, it was necessary that their position should be absolutely correct.

When the masonry had got up close to the under side of the bedplate the bolts were again set with the greatest care, and the spaces left round them were filled up with cement grout to within about 4 ft. of the top.

Immediately underlying the bedplate, and with a view of making a perfectly level bed for the same, cast-iron blocks, 12 in. square and 4 in. thick, with a hole which only just admitted the head of the bolt, were placed, carefully levelled by instrument and set in cement, the spaces all round and between these being levelled to the thickness of 1 in. with cement. In addition to the round hole in each block, a slot was cut and a taper wedge driven hard into this and against the screw thread. This was done to prevent torsion in the bolts when the large upper nuts required to be drawn tight. On the bed thus prepared the lower bedplate was laid in the manner hereafter described. The heads of the holding-down bolts, as well as square washers, nuts, &c., are shown further on in connection with the bedplates.