50 & 20 Years Ago
Bridge over the River Tagus
Taken from Building with Steel, 1965
The £50 million suspension bridge at present being built across the Tagus River at Lisbon is reputed to be one of the most outstanding engineering and construction projects ever undertaken. The bridge will have the longest suspended span in Europe and the fourth longest in the world. It is also the longest bridge in the world designed for combined road and rail traffic and has the highest continuous trusses. It has the highest bridge towers in Europe and the world’s deepest pier.
It consists of a 3,232 ft. centre span flanked by 1,586 ft. side spans, and has a clear height above water of 230 ft. The amount of steelwork used is in the region of 80,000 tons. It is expected that building the bridge and approaches will take 51 months, which means that the project should be finished early in 1967. The steelwork already nears completion and work on the approaches is well advanced.
Although at present only intended for road traffic, the bridge is so designed that if required a lower deck can be added with minimum effort to carry a double-track railway. This would involve the installation of a secondary cable system to take part of the additional live load without the necessity of altering the stiffening trusses. This combination road-rail design has been made practical by using for stiffening trusses considerable quantities of heat treated constructional alloy steel of 55-67 tons sq. in. of tensile strength.
The main cables are massive: practically 2 ft. in diameter and constructed from 37 strands, each is made from 304 galvanised high strength steel wires of 0.192 in. diam. There are 11,248 wires in each cable, giving a total length of 33,676 miles of cable wire, an impressive figure.