The Forth Bridge/Raising of the Approach Viaduct Girders, and Underbuilding of the Piers

by Wilhelm Westhofen

Raising of the Approach Viaduct Girders, and Underbuilding of the Piers.

The height to which the piers of the approach viaducts had to be carried was 130 ft. 6 in. above high water, and the question how best to deal with both the erection of the girders and the building of the piers was ultimately settled by deciding to put the girders together at any convenient level, and make the lifting of these and the building of the masonry a simultaneous operation.

The fifteen spans of 168 ft. each, of a total weight of slightly over 3000 tons, or, roughly, 200 tons per span, were built under a sub-contract by Messrs. P. and W. McLellan, of Glasgow, the contract including their being put together and rivetted up on the staging provided at the Forth Bridge.

The girders are of the ordinary double-lattice girder type, consisting of two parallel girders at 10 ft. centres, having trough-shaped top and bottom booms and side bracings consisting of intersecting diagonal struts and ties. The trains run on the top, the troughs of the two outside rails forming the top booms of the girders. Each span is divided into eight bays of 21 ft. each, which is also the height of the girders, and at the intersection of struts and ties a vertical support is carried upwards to the top boom. A cross-bearer occurs every 7 ft., or three to each bay, and carries not only the two troughs for the inside rails, but also projects beyond the two outside troughs for a distance sufficient to form a 4-ft. path on each side, and to support the brackets of the wind fence. The floor is made up of buckle plates. The bottom booms of the girders are braced laterally by lattice girders intersecting at centre, and there are also vertical cross-bracings of double angles at suitable distances.

Two spans are made continuous, and expansion joints are provided over every second pier. The ends of all the girders rest on sliding bedplates, no rollers being used. The details of these girders call for no special remark. They were originally intended to be built of wrought iron, but, in view of the cheapness and excellent quality of steel, the latter material was ultimately adopted.

Various circumstances combined to fix the height of the staging on which these girders were put together.

On the Fife shore the high ground upon which the cantilever end pier and piers 10 and 11 were founded necessitated the putting up of staging to a height of 41 ft. above high water; the four piers 10 to 13 having by September 30, 1883, been brought up to 37 ft. above high water.

On this staging the girders were erected and rivetted up, the last length next to the north cantilever end pier and nearly half the length of the span between pier 13 and the abutment being left out, however, for the time being. The cantilever end pier had been built up to a considerable height, and between pier 13 and the abutment a public road passed at a considerable height above the then level of the staging.

As the mode of raising the girders will be more fully described in connection with the south approach viaduct, it will be sufficient to state here that, in order to raise the end of the girders nearest the cantilever end pier, a set of strong columns were built up and lengthened by degrees as the lifting proceeded; and upon these the hydraulic cylinders were placed. The mode of raising was similar to that employed for the large platforms of the central towers, and fully described there. Upon piers 10, 11, 12, and 13 the lifting proceeded in the usual way, but, not far from the abutment, a set of columns similar to those at the

TABLE No. IX.—Viaduct Piers and Cantilever End Piers.—Tabular Statement of Quantities
All Figures for Concrete and Rubble in Cubic Yards—for Granite in Cubic Feet

—					Foundations.		Granite Masonry Piers.				—
					Concrete.	Rubble.	Concrete.	Rubble.	Granite.	Freestone Bond Courses.
South Viaduct.
Pier No.	1	...	...	...	246	nil	42	314	5526	not stated
Pier No.„	2	...	...	...	176	nil	322	318	5591	1793
Pier No.„	3	...	...	...	151	nil	413	607	10002	3425
Pier No.„	4	...	...	...	73	nil	540	607	10002	3392
Pier No.„	5	...	...	...	72	nil	622	607	10002	3513
Pier No.„	6	...	...	...	80	nil	685	607	10002	5912
Pier No.„	7	...	...	...	228	nil	936	607	10002	5993
Pier No.„	8	...	...	...	320	nil	973	607	10002	5946
Pier No.„	9	...	...	...	706	nil	973	607	10002	5987
South Cantilever End Pier				...	2167	295	1753	3420	43984	39078	162 of blue brick
North Viaduct.
North Cantilever End Pier				...	205	nil	774	2320	32944	17305	37 of blue brick
Pier No.	10	...	...	...	69	nil	90	598	11610	not stated
Pier No.„	11	...	...	...	52	nil	130	611	11814	not stated„
Pier No.„	12	...	...	...	273	nil	304	508	11756	1954
Pier No.„	13	...	...	...	340	nil	419	610	11789	1019

TABLE No. X.—Showing Quantities of Masonry Piers, Arches, and Abutments certified for up to November 30, 1889.

—			Cubic Yards.	Cubic Feet.
Concrete	...	...	64,315
Rubble	...	...	48,353
Rock-faced granite		...	...	494,642
Dressed granite		...	...	140,756
Bond courses	...	...	...	105,180
⁠Total	...	...	112,671	740,578
Grand total, 140,000 cubic yards.
Note.—The quantities of masonry, &c., for abutments and arches are not given in the detail statement above, South Cantilever End Pier but they are included in the totals here stated. Some 7500 cubic feet of granite for completion of the cantilever end piers have to be added.

HEATING FURNACE FOR PLATES.

other end were used. When the girders had been raised to a point above the cantilever pier and the abutment (as far as these had by then been built) the two ends were joined on, and the completed girders raised up to their final position. The first lift of these girders was made about the middle of October, 1885, to level 42 ft. above high water, and the last lift to level 130 ft. 6 in. above high water on February 15, 1887.

The connections of the permanent way on this viaduct with that on the cantilever at one end and with that on the masonry arches at the other end are made by means of expansion joints, identical with those used at the fixed ends of the Fife and Queensferry central connecting girders, and fully described in connection with the latter.

On the Queensferry shore, owing to the rising ground between pier 3 and the south abutment, the girders had also to be built at different levels, and were only joined when the various portions had arrived at the same level. The seven spans from pier 3 to south cantilever end pier were all built on staging erected between the piers, and rising to a few feet above the masonry, in order to allow the lifting rams and lifting girders to be placed between piers and viaduct girders.

Piers 3 to 5 had been brought up to level 18 ft. above high water early in January, 1884, pier 6 in March, 1884, pier 7 in October, 1884, pier 8 and cantilever pier in February, 1885, and pier 9 in June, 1885. The delays which occurred were due to difficulties and to a larger amount of work in connection with the construction of the cofferdams and the building of foundations and lower portions of these piers; but so soon as any of them were completed, the staging between was built, and a start made with the erection and rivetting of the viaduct girders. Thus it was possible to have everything ready and to commence lifting early in the month of May, 1886.

Piers 2 and 1 had in the mean time had their foundations laid and their masonry raised to about 43 ft. above high water, while the abutment on top of the hill was raised to 115 feet above high water.

For the purpose of raising the viaduct a box girder of great strength, about 22 in. deep; 2 ft, 6 in. wide, and some 24 ft. in length, was placed directly under the girder end-posts—that is, along the centre of the piers. During the raising of the girders temporary connections had been made between girders at those places where expansion and contraction could take place. Immediately under each main girder a hydraulic ram, 14 in. in diameter and 16 in. stroke, was inserted in the box girder, and was made a fixture in it. There was thus a length of girder of about 3 ft. outside each cylinder for packing. All the cylinders were connected with force-pumps, which could produce a pressure of 35 cwt. per square inch, the pumps being placed within the bottom booms of the viaduct girders, which were completely decked over and used as a road, being fenced on each side, and having two narrow-gauge lines laid down on it, during the time of lifting. Round each pier a timber platform was suspended from the girders, so arranged that its planking could be brought up close to the masonry as the piers diminished in size with the ascent. On this platform—which is clearly shown in Plate VIII.—were also stowed the hardwood packings and other necessaries for the lifting work. All these things put a great deal of extra weight on the girders, but as neither buckle-plates nor wind-fence had been put on as yet, the weight did not greatly exceed that of the finished girders. The mode of procedure was as follows: Hardwood packing in slabs of varying thickness, from 1 in. up to 12 in., was placed under the girders in two or three places, and by means of long wedges, one worked from each side, every lift was closely followed up to prevent sudden drops, should the hydraulic presses give way. When a lift of 1 ft. had thus been made, the girder was packed and the ram was fleeted—that is, drawn back into the cylinder—and a hardwood block with a strong steel plate on top was underlaid and a further lift made.

At first it was attempted to raise the whole mass of over 1400 tons at once on all points, but this was not found to work very well, and it was then arranged to work 3 in. at the time only, and lift on each pier in succession. No accumulator was used,but the pumps were just kept going until the girders were up. As a rule the lifts were of 3 ft. 6 in. at the time for two courses of masonry, but at times a foot more or so was done, according to requirements.

With the last lift on each occasion the packing was so arranged as to occupy the least room upon the masonry last done, and the masons then at once proceeded to set the granite facing and build the rubble backing in every place that they could get at, taking good care to leave the work so as to make sufficient bond with the remainder. After allowing forty-eight hours for the setting of the new work, the weight of the girders was shifted on to packings laid on the new work, and the remaining of portions the two or three courses, as the case might be, were then built in. After due allowance for the setting of this work also—another forty-eight hours generally—a fresh lift

was made.

All materials for building were raised to the girder level by means of two hoists stationed at either end, and the materials so raised were run on trollies by boys or drawn by Shetland ponies to the places where they were required. Thus granite blocks, Arbroath rubble, and mortar, were all prepared on the jetty below and lifted up.

BENDING PRESS FOR PLATES AND MACHINE FOR PLANING ENDS OF CURVED PLATES.
Fig. 66. Machine for Planing Edges of Curved Plates.

To assist the masons in their work of laying granite and rubble an overhead traveller was placed to each side of every pier. All these travellers were worked by friction gear and clutches, both for hoisting and lowering, or for travelling, an endless wire rope being run along the girders from end to end, driven by the same engines which drove the pumps during lifting.

When the seven spans had thus been raised to 47 ft. above high water, the next two spans from pier 3 to pier 1—which in the meantime had been erected on staging passing over the Edinburgh-road—were joined on, and the further lifting proceeded with in the same manner as before. The last span from abutment to pier 1 had meanwhile been erected on staging at level 119 ft. above high water, and was joined on when the other girders reached that level, the complete viaduct being then raised through the remaining 11 ft.

The first lift at South Queensferry was made from level 23 ft. above high water in May, 1886.

Level 47 ft. above high water was reached August 10, 1886.

Level 91 ft. above high water was reached February 15, 1887.

Level 119 ft. above high water was reached June 14, 1887.

Level 130 ft. 6 in. or top was reached August 7, 1887.

No mishap of any kind occurred in the course of this work, and in no case did the granite facing or rubble backing show the least sign of giving way during the underbuilding of the piers under this considerable load.

After the girders had reached the tops of the piers the cross girders and hydraulic rams were removed and the bedplates substituted.

In all cases the thirteen viaduct piers had the hearting constructed of concrete up to the level at which the girders were built, but from the time lifting was done Arbroath rubble was used exclusively, as it was feared that the concrete could not set sufficiently hard in the time allowed between the lifts, while the Arbroath rubble masonry set firmly and solidly in about thirty hours.

Very little staging was used in this mode of erecting, most of the timber being used over and over again, for as soon as a girder was rivetted up and could carry itself across the span the staging was removed from under it, and was transferred a span further forward.

After the piers were in position the temporary connections at the joints where provision had been made for expansion and contraction were removed, and the remaining buckle-plates on the top and the wind-fence on both sides were put on.

After several coats of paint had been put on the girders they were left until the time when the permanent way was required to be laid down. One of the views on Plate VIII. shows the girders at full height.