Side tower saddles are mounted on a 13 ft ( 4m ) rocker that allows longitudinal movements of the cable and the force due to the inclination from the vertical is very small . Changing stress in the cable which passes over the saddle is absorbed by friction between the wires and the metal sprayed surfaces of the saddle grooves .
Figure 2 : Side Tower Saddle Details
Splay saddles are mounted on rockers . The cable groove is a steel casting . Vertical component of the deflexion is maintained to the rear ; the strands do not deflect horizontally without a vertical force to hold them down .
Figure 3 : Splay Saddle Details
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3.5 Hangers
The cable bands have two grooves on the upper surface for a pair of hanger ropes . The connection to the top chord of the stiffening truss is by a common socket . Where the rope enters the socket , a split cap and rubber gasket is fitted to exclude moisture .
3.6 Suspended Structure
It is an open rectangular framework 78 ft ( 23.78m ) wide and 27 ft 6 in ( 8.38m ) deep braced on all four sides . Its stiffness is designed for torsion ( especially for aerodynamic stability ) and for vertical and horizontal bending . In the side spans most of the lateral wind force is carried by the suspended structure , but on the main span 60 % of the total wind force is transferred to the cables by the lateral inclination of the hangers .
3.7 Side Towers
The two 150-ft high ( 45.7m ) high reinforced concrete side towers carry the rocker saddles over which the cables are deflected from the side span down to the anchorages .
3.8 Design Standards
The live loading adopted was as specified in British Standard 153 Part 3 : 1954 . The bridge was designed for a maximum wind speed of 110 mile / h ( 177 km / h ) at deck level .
3.9 Structural Analysis and Calculations
For details , please refer to the original paper ( 1965 FRB ). However , please note that " It is impossible to record within the compass of this Paper all the calculations leading up to or substantiating the design " ( 2.102 , p . 57 , 1965 FRB ).
3.10 Tests on Model Members
Seventeen types of model member were tested to destruction as pin-ended struts .
4 . CONSTRUCTION 4.1 Conditions
The Forth Road Bridge is situated much further north than any major suspension bridge in the world at the the time . The site is exposed to south-westerly and easterly winds , and speeds of 70 mile / h ( 113 km / h ) during gale conditions are common . The maximum wind speed recorded on the Forth Railway Bridge anemograph during the construction period was 103 mile / h ( 166 km / h ).
As the free standing towers oscillated under the winds during erection , a damping device was devised and installed .