The Erection Cables.
A fundamental design constraint of the Sydney Harbour Bridge was that it should be built without centering, and the erection procedure of working out from both shores with the cantilevers held back by cables anchored into rock is widely known. Those cables were a major part of the cost of the work, though very little trace of the erection method is still visible in the finished bridge.
The method ultimately used was not what was intended when the Dorman Long Tender was submitted. At that time, it was envisaged that a tall post would be erected above the end panel of the half-arches. It would be stayed back to the rock with a lesser number of cables as the greater leverage gained from the tall post would need a smaller force to hold up the weight of the steel structure. This was the method used at Hell Gate in New York, a very similar, though much smaller, bridge a decade before.
The reasons which drove Dorman Long to abandon this scheme in favour of the scheme eventually used included the cost of the cables and the near impossibility of designing the points of attachment to the upper chord of the bridge arch. Although the first scheme only used 96 lines of cable at each end post, as opposed to 128 in the adopted plan, each would have to be terminated three times with an extra connection either side of the post as well as the link onto the bridge. The end sockets on the cables were expensive and 96 times three was a lot more costly than 128 times one. There were also difficulties in maintaining accurate loads in all the cables as the half arches lengthened and increased in weight.
Cross section of cable, with legend on binding identifying it as supplied to Sydney Harbour Bridge. Thos & Wm Smith were not the recorded suppliers of wire ropes to the bridge, but were one of a number of firms which merged in June 1924 to form British Ropes. The central wire in this sample is the same diameter (0.160 inches) as all other 216 surrounding wires. In the cables used on the bridge the central wire was larger at 0.200 inches. Charles Horne photo from his collection of bridge artefacts.
Anchorage-cables were manufactured with wire drawn by Dorman Long by British Ropes Limited and Wright Ropes Limited in England.They were sent to Sydney in coils, each coil weighing about 9 tons and having an internal diameter of 6 feet, an external diameter of 9 feet 6 inches, and a depth of 2 feet, the length of the cable being about 1,200 feet. The lengths of cables varied according to their position, and they were made to the correct lengths plus a margin of a few feet. No spare cables were supplied, and although two hundred and fifty-six cables had to be shipped and placed in position none was injured.
Prior to the placing of an anchorage-cable, a manila pilot-rope was threaded along the intended course through the various saddles and supports, and this was used to draw a steel wire haulage-rope, 3¾ inches in circumference, with a working tensile strength of 5 tons.
Each cable had to pass through two pylon-saddles, two tunnel saddles, six intermediate supports, and round the correct groove in the tunnel; it was therefore necessary to exercise considerable care in watching its course. The coil of anchorage-cable was set up on a wooden drum mounted on a shaft, formed from a 14-inch steel pipe, and having a powerful hand-operated brake, and placed on the east side of the abutment tower. The end of the haulage-rope was fixed to the anchorage cable by a "stocking" grip, which contracted under tension, and which enabled the connection between the two to be made quickly, and without increasing the diameter to an extent which would interfere with the passage of the cable through the 4½-inch diameter tubes of the saddles. For pulling the haulage-rope a 3½-ton winch was fixed on the west side of the abutment-tower behind the saddle. When the haulage commenced, and the heavy anchorage-cable moved down the slope to the base of the tunnel, it was necessary to check downward movement with the brake on the drum carrying the coil of cable. After that considerable skill was required to adjust the action of the brake and the hauling winch to ensure a steady tension without overloading the winch. The operators in charge of the brake and winch were in communication with each other and with observers along the route of the cable by telephone, and after some experience the process worked smoothly, cables being reeved at the rate of four per day.
Clamps were then secured to each end of the cable, so as to bear on the upper ends of the tubes through the saddles When these clamps were fixed the cable between the pylon-saddles was in its correct position, and under an initial tension of 5 tons. This was sufficient to enable its true length between these points to be estimated correctly. The free ends, each about 130 feet in length, were accessible in their unstressed condition, and were then marked for cutting and socketing. To provide for errors in the lengths of the cables at this stage a margin of 3 inches long or short had been allowed, but in practice it was found that the actual error in length was within 1 inch.
As each anchorage-cable was run through its course, the manila pilot-rope was attached to it and drawn through the course of the next cable to be placed, a disconnection being required at each point of support and at each saddle. Thus, when a cable was hauled completely round, the pilot-rope occupied the course of the next cable to be reeved. The pilot-rope was then used to pull back the haulage-rope, which was used in turn for pulling the next anchorage cable into place. End-Connection
42. Cables locked against the pylon saddle, with the eight at lower left being the temporary connction to the end post. The two cables draped across the centre of the image are in the process of being connected to the 'permanent' link. Note the timber spool from which the cables are unwound to be drawn through the anchorage. 11 June 1929. MHNSW NRS 12685.
Sockets. - As the cables were reeved in position they were laid out in correct sequence over timber·bolsters, with the free ends hanging down over the face of the abutment-tower.
On each side of the tower over these free ends a covered shed was built for socketing the cables. In each shed four socketing-benches were set up, so that the processes of socketing could be kept in continuous operation, the workmen dealing with each process operating upon successive cables. The first step was to lift the free end of the cable, and to secure it in clamps below the socketing-bench.
The socket was placed over the end of the cable, and dropped down clear of the free end and on to a pair of 2-inch diameter screws, arranged to correspond with the holes on either side of the socket.
The end of the cable projecting above the socket was then burnt off to the correct length, and bound with soft wire of about 16 gauge, 13 inches from the end. The wires of the cable were next separated and spread out down to this binding, seventy wires out of the total two hundred and seventeen being turned back through 180 degrees 1 inch from the end, and thoroughly cleaned. This spreading ensured that the cable-end would lock in the conical interior of the socket. The clamps were then removed, allowing the cable-end to drop into the socket, and were then again secured to the cable. By means of the two 2-inch screws the socket was forced hard on to the cable-end; the socket was then filled with white metal made of 86 per cent. lead, 3 per cent. tin, and 11 per cent. antimony, the socket being heated by blow-lamps to a temperature of approximately 200° F. to keep the white metal fluid, and to assist its flow into the spaces between the wires. The socket was then allowed to cool, after which it was lowered down over the front of the abutment-tower ready for lifting into position when required for the anchorage.
The form of socket used is shown below, and was only decided upon after much designing and testing, justified by the large number, namely five hundred and twelve sockets, required for the work.
The sockets as finally designed weighed only 65 per ·cent. of those originally proposed, and during many tests to destruction of cables no failure took place through weakness of the sockets. They were of cast steel, to British Standard Specification No. 30 (1907), Steel Castings for Marine Purposes," grade "B"; ultimate tensile strength 25-32 tons per square inch.
Photo 22 and 29
Photos 30 and 31
During the first stage of erection of the half-arch the crane stood on the sloping girders, or ramp, built up immediately behind the end-posts, thus occupying the space eventually occupied by the link-plates connecting the anchorage-cables. It was therefore necessary to provide temporary anchorages clear of the ramp to support the arch structure until the first panel was completed, and the erection-crane had moved on to it. The ramp girders could then be removed, freeing the space required for the link-plates. This temporary anchorage was formed of eight of the final anchorage-cables, which were connected to two pins in a position below and outside the space taken by the inner group of three link-plates of the final or "permanent" anchorage, and were supported by a bracket or extension behind the end-post, the cables being connected to pins by U-bolts. This bracket also served to support the ramp-girder and the link-plates. The total pull on these cables amounted to about 400 tons, and the line of the resultant was about 5 feet below panel-point 29, and 4 feet from the centre-line. The stresses due to this eccentricity had to be carefully investigated. After the crane had moved forward the inner groups of link-plates were attached, and sufficient cables of the final anchorage connected to them. The temporary anchorage was no longer required, and the cables were transferred to their final positions. It was then possible to erect the three outer link-plates of each anchorage.
Photo 43 Photo 44
During the early stages of erection, the stretch of the anchorage-cables on either side of the half-arch, due to the lifting of members of the arch and transverse travelling of the crane jib, would have caused significant distortions of the permanent structure. Until several panels were erected the half-arch possessed insufficient rigidity to preserve its form, and to avoid any risk of disturbance or twist of the structure, diagonal struts were built up from the pylons against the tops of the end-posts, and the tension in the anchorage was adjusted to ensure that the tops of the end-posts were always bearing against these struts; thus any movement of the permanent steelwork, and therefore any distortions other than those due to its own deformations, were avoided.
The stresses in the structural steelwork at this stage were small, and these deformations were not significant.
As erection of the arch proceeded additional anchorage-cables, corresponding with the amount of steelwork erected, were connected from time to time. It was always necessary to produce a tension in the cables exceeding the pull exerted by the erected steelwork, and provision had also to be made for changes in the length of the cables due to temperature variations. Great care had to be exercised in connecting up and stressing the cables to avoid either overloading the struts, or the alternative risk of allowing the erected structure to separate from them. The struts were made up from the post members of the bridge (posts 8-9), with special ends attached by fitted bolts to the holes for the permanent connections at the ends.
The stress in the struts was observed by means of fixed extensometers on each at the lower ends, and accessible from the pylons, to ensure that it conformed with the calculated amount corresponding to each stage of erection.
For the connection of the anchorage-cables to the end member of the top chord, it was at first proposed to build up a series of eight transverse pins supported in the permanent structure of the end connection at joint 29, with an attachment riveted above the chord to increase the depth sufficiently to contain eight pins placed symmetrically about the centre line. It was, however, very difficult to contrive the details of this connection, and the arrangement also lacked an articulation at the point of connection; this would have tended to stress the cables differently as the half-arch moved forward in the later stages of erection, thus necessitating adjustments of the cable tensions to ensure uniformity. The stresses in the cables at this stage would have been high, and their adjustment correspondingly difficult. It was therefore decided to adopt the form of connection shown in the drawings adjacent, and although this required the construction in the shops of some very difficult structural work, it avoided uncertainty and possible risks during erection, and facilitated the final operations of lowering the two half-arches together.
The anchorage-cables were connected to eight lines of pins, sixteen cables in each line, the pins being supported by six link-plates connected to the chord members by a single line of pins; this provided symmetry and balance under all conditions of cable-tension and forward movement of the structure. The pins connecting the link plates were 27 inches in diameter, and divided into two lengths, each supported practically symmetrically in two of the four webs of the chord. The total pull on each pair of 27-inch pins eventually reached 14,000 tons, and to sustain this within the limits of material practicable to construct, it was necessary to stress the material of the chords and anchorage connections practically up to the full permissible limits. The stresses allowed were as follows, the material being silicon-steel:-
It is of interest to note that on completion of the structure the temporary material was removed without difficulty, and showed no sign of distress as a consequence of these stresses.
As the trusses of the approach-spans were spaced the same distance apart as those of the arch, it was necessary in order to enable the anchorage-cables to clear them to spread out the anchorages, and the axis of the pins was therefore not normal to that of the arch structure
The inclination to the normal was about 2 degrees at the south end. At the north end it was 4 degrees in order to broaden sufficiently the base of the anchorage at ground level to clear the greater breadth of the north approach on the west side caused by its curvature. The pin holes were bored at these inclinations, and the transverse component of the tension from the link-plates was transferred to the top and bottom flange-plates of the member by the material of the webs, the thickness of which amounted to 10½ inches. The holes were made 1/16 inch larger than the diameter of the pins.
There were six plates at each connection, the two plates at the centre being separate but in contact, so that the system was divided into two groups, each of three plates, symmetrically disposed about the axis; each outer plate of each group carried one-fourth of the tension on the group, and the central plate one half. At the outer ends these link-plates supported eight lines of pins 11 inches in diameter, each line being divided into two lengths, one in each group of three link-plates. In the centre of each of these lengths a saw-cut was made in the pin to reduce its bending strength, and to avoid the transfer of an undue share of the load on them to the centre link-plates.
Semi-cylindrical grooves corresponding with the anchorage bolts were formed in the pins to maintain them at the correct spacing, and to allow the bolts to be as near as possible to one another.
Each rope terminated in a socket as already described, and was supported by two anchorage-bolts, 3 inches in diameter with V-threads of ¼-inch pitch, with hexagon nuts at both ends, the nut at the outer end bearing on spherical-seated washers on the sockets.
Each pair of nuts at the inner end rested on a steel saddle bearing on the 11-inch pin, the length of bolt and screw-thread allowing for the adjustments of the lengths required for making the connection of the cable, the lowering of the half-arches and the disengaging of the cables. The grouping of the connections with the link-plates disposed the end saddles of the cables in four internal compartments, each compartment containing thirty-two ropes in eight vertical rows of four. This arrangement facilitated the systematic attachment and adjustment of the cables, and although the connections were so compact that clearances were very small in any direction, no serious difficulty was experienced in carrying out the operations during connection and adjustment.
Each "central" and "outer" link-plate of each group weighed about 20 tons and 11 tons respectively, and to support the outer ends of these, and to provide a means of adjusting them in the exact required points, a shelf or bracket was built up behind the end posts extending across their full width. The inner length of the 27-inch pin was lifted up in the top transverse portal-member connecting panel-points 29 E. and 29 W., and the outer length was supported in a frame outside the chord member; both were then driven to the centre without any pilot end. The 11-inch pins were then placed in position, and the connection was ready for the reception of the anchorage cables. These cables were connected in groups in accordance with instructions carefully drawn up to ensure that the total tension, after joining each group, was the correct figure which would allow of the next stage of erection without overstressing the raking struts or allowing the structure to move away from them, and also that at each stage all the cables had the same tension. The margins permissible were small; accurate calculations were therefore necessary for the preparation of these instructions, and particular care was taken to ensure that they were carried out. Each cable was connected at a measured tension by drawing it up with a calibrated hydraulic jack, working against a temporary saddle across the free ends of the anchorage bolts. When this tension was reached the inner nuts were screwed home. The alignment of the cables also provided an unmistakable check on the uniformity of the tensions, as any variation revealed itself at once by the difference of sag of one particular cable when compared with that of the remainder.
At the time when details of the cable-connections were designed, there was little recorded experience of the strength of the forms of construction proposed to be used.
On the other hand, owing to the great number and cost of these connections, it was essential to work to the highest safe stresses. A great number of full-size tests were made in the contractors' testing machine at Middlesbrough, and the details of construction adopted were the outcome of these investigations. The tests showed the high quality of the cables, and proved that the design adopted for the connections was satisfactory.
The intended maximum load in each cable was 128 tons, but no cable failed below a breaking load of 350 tons. In every test to destruction, although there was evidence of distress in the end connections, failure always occurred in the cable. These tests also provided information as to the effective modulus of elasticity of the wire cable used. This was found to be approximately 8,500 tons per square inch of the section of the wires during the first application of the tension, and subsequently increased to 9,000 tons. This information was, of course, essential in order to determine the position of the half-arches during the process of erection. Another property of interest investigated was the stretch of the wire cables with time. It was found that if the cables were subjected to a constant tension of 100 tons, equivalent to 22·9 tons per square inch, in addition to the original elastic extension when the load was applied they extended approximately 0·002 inch per foot of length in 40 days, practically one-half of this extension occurring in 10 days.
At the end of 40 days extension practically ceased. The tests were valuable in checking the design of the socket already described and in settling the number of wires to be turned back in the socket. Each cable had two hundred and seventeen wires, and as there were five hundred and twelve rope ends there were over a hundred thousand wire ends. A certain proportion of these had to be bent back one wire at a time, an operation which could only be done by hand. As the result of the tests this proportion was fixed at 32 per cent., although it was originally assumed that all the wires would be turned back.
The steel was manufactured, and the wire drawn by Messrs. Dorman, Long and Company, Limited; the cables were made by British Ropes, Limited, and Wright Ropes, Limited, and no fault was observed in any cable.
Once the two half arches had met the Creeper Cranes retraced their path back to the ends of the span, erecting the deck as they went. Similar precautions against any failure of the haulage gear as had been used on the outward journey were required. The tensioning winches for the heavy link chains remained at the crown of the arch, rather than following the cranes down. Since the chains were only the length of one panel of the truss they had to be extended as the cranes moved away using sections of the now removed supporting cables.
