4.6 Approach Viaducts
The north and south viaducts support the carriageways , footpaths , and cycle tracks . The south viaduct of eleven spans has a total length of 1420 ft ( 433m ) and the north viaduct of six spans has a total length of 820 ft ( 250m ). Reinforced concrete piers support the deck structure in spans varying from 110 ft to 16O ft ( 33.5m to 51.5m ). The deck structure is continuous and consists of two steel box beams with transverse beams and outriggers having a reinforced concrete slab working in composite action with the steelwork . The piers consist of two vertical shafts of T-section with the tapering legs of the Ts joining at the top of the pier over an arch of 17 ft ( 5.2m ) radius .
The first four spans of the south viaduct were erected by mobile cranes operating at ground level . The S7-S6 sections were lifted by an erection mast . The main beams were cantilevered forward from pier S7 . Span S6- S5 was erected similarly , lifted by mobile cranes .
Erection of the North viaduct started the same way as the south , using falsework supports for the first two spans . After that a 20-ton capacity crane was built on the main beams . Its wheels moved on the top flanges . All remaining steelwork was erected with it by cantilevering forward from each pier in turn . The continuous slab was concreted between expansion joints . It is bonded to the steelwork by T-section shear connectors welded on top flanges .
4.7 Superstructure 4.7.1 Towers
Access to the piers to both towers was provided by rockfilled causeways from the land . For the bases , shop welded boxes were assembled , connected by high strength bolts , and concreted . All except the bases and lowest sections of each tower were erected by means of a climbing structure .
As each box section was landed it was immediately connected to the one below by high tensile steel tie rods and 3 in ( 7.6cm ) dia . high strength grip bolts . The three boxes forming each tower leg were interconnected by means of four vertical cover plates secured with 4 in ( 10.2cm ) dia . close tolerance bolts . Internal diaphragms , access ladders and hoist supports were assembled and the diagonal bracing or cross girders erected and grip bolted . Weather hoods were put on the cross bracing connexions and final painting of bolt heads , butt joints , etc , completed .
When the climbing structure reached its eleventh and final position , the upper cross girder of the tower , which up to then had formed part of the climbing structure , was unshipped and re-erected in its permanent position .
The working platform with two special 10-ton derricks , which were required for assembly of catwalks and cable spinning , were erected . These cranes then dismantled the climbing structure and its 32-ton derrick .
4.7.2 Supply and Erection of Cables
The two main cables of the bridge were each specified to be made of 11 618 galvanized hard drawn wires , erected by the process of cable spinning . It is a highly complex process which involves the use of much specialized plant , such as reeling and unreeling machines , tramway drives , compacting and wrapping machines and also expert knowledge . At that time it had never been used outside the United States so the Contractors sought advice on all details there .
7 450 tons of galvanized high tensile steel wire 0,196 in ( 4.98mm ) in dia . were delivered by rail to the site . 1 500 tons ( i . e . about 3 300 coils ) were stored directly at site , the remaining 6 000 tons were stored under cover in Middlesbrough .
Over the top of the side towers and main towers temporary footbridges / catwalks were built . They enabled men to adjust the wires and subsequently to compact and wrap them .
Loops of wire were carried over the span on a grooved spinning wheel attached to a tramway drive . The wires were unreeled by eight electrically powered unreeling machines - they help reduce the tension in the wire . Two wheels shuttled to and fro across the river , each trip laying ten miles of wire . The adjustments of individual wires were carried out during spinning . The wires of the last trip completed a strand of 314 wires ( average number ).
" All the cable spinning was done by men working on two eight-hour shifts from 8:00 a . m . to 4:00 p . m . and from 4:00 p . m . to midnight . In order to permit uninterrupted spinning there were no stops for tea or meal breaks . Tea and hot soup were delivered periodically by means of an urn strapped on the back of a tea boy and meals were eaten between the passing of the wheels . In spite of their exposed position , high over the Forth on bitter , cold nights , little if any shelter could be provided for the cable spinners on account of the necessity for keeping obstructions to wind down to a minimum ". ( 4.52 , p . 463 , 1965 FRB )
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