The project , including main bridge and approaches , was substantially completed on the 4 September 1964 when it was opened by H . M . Queen Elizabeth II .
2.2 Evolution of Design
Much of the Forth Road Bridge design was based upon work done originally for the Severn Bridge , which was intended to be built in 1946 .
Attention was paid to aerodynamic stability which influenced the arrangement and shape of stiffening girders and deck . The footways and road deck were separated .
The most important decision to be made was the method of forming the cable . Strands would be too heavy for such a long span and would require special plant for erection . As a structural member , strand has a lower modulus of elasticity compared to plain wire , and a group of strands cannot be compacted into as dense a mass as the parallel wires spun in situ . Economical , space and labour aspects were also considered and it was decided to adopt the spinning process .
Another important decision to be made in the early stages of design is the ratio of cable sag to length of span . A ratio of 1 / 11 was adopted , after laborious investigation including the Severn Bridge estimations .
The originally proposed towers were considerably modified in the course of design . The present system for tower legs is that of welded box sections with interconnecting stiffened plates .
Anchorages were originally contemplated as the gravity ones , however , it was decided to drive tunnels , one tunnel for each cable , filled with concrete .
3.2 Tower Foundations
The base sections of the legs are embedded 10 ft ( 3m ) deep in the upper part of a reinforced concrete pier .
South pier foundations were made with two rectangular caissons , capped by concrete slab on which the pier was constructed . For the north pier a similar slab was made .
3.3 Cables
Attention was paid to the choice of wire diameter , especially to economical and practical requirements and to tensile strength of the wire . It was decided to keep to the standard size of 0.196 in ( 4.98mm ).
It was decided to use 37 strands of 304 ( 19 inner strands ) to 324 , 326 , or 328 ( outer 18 ) wires each , arranged in a hexagon . It was necessary to estimate the final diameter of the cable so that the cable bands could be machined . Percentage of voids was set at 20 % based on American experience . The cable was finally wrapped between the cable bands with steel galvanized wire .
3.4 Saddles
The tower saddle was fabricated in one piece able to keep 32-ton capacity of the erection crane . The cable groove is a single steel casting machined inside and out on the rotary planing machine and they are stepped to suit the vertical hexagon arrangement of the strands .
3 . DETAILED DESIGN 3.1 Tower
The section of each tower leg is composed of five cells formed by three prefabricated boxes ( the outer boxes with variable width ) joined together by four connecting longitudinally stiffened plates . The centre box of each leg forms a point of connection for the transverse bracing .
Correct load and bending moment were checked in sections , the base section was then checked in the free standing condition with various loads . The legs were designed as tapered as the direct load decreases towards the top .
Figure 1 : Main Tower Saddle Details
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