Kelio istorija.Tolesnė tiltų statybos raida

Abstract

The first use of cement in bridges since the Romans occurred in 1796 on the isle of Sheppey Bridge in England. Concrete was first used for bridges in France in 1840 for the construction of a 14 m arch bridge over the Garonne Canal at Grisolles. Unreinfor-ced concrete has no significant tensile strength and thus presents little adverance over masonry blocks. The lack of tensile strength is overcome by adding steel reinforcing bars to carry any tensile loads. Reinforced mortar has been found in Roman roofs and in the Great Wall of China. The modern introduction of the idea possibly began with Jacques Soufflot, who used iron bars to prevent cracking in the masonry dome of the Church of St. Genevieve in Paris in the late eighteen century. Prestressed concrete came into its own following the cement shortages after World War II, but owes its development to the experiences Freyssinet had in 1910-1911 on his 72 m span flat concrete arch bridge. The bridge was destroyed in 1940 during the Second World War, but it very nearly did not even make the First World War; it sagged some 130 mm soon after construction when the concrete deformed under load and over time, a phenomenon known as creep. The first movable bridges were drawbridges over defensive moats. This defensive role was a supplementary function of many early bridges. A second reason for using movable bridges was to avoid building much higher bridges to permit the passage of shipping. The drawbridge operates by relatively simple technology compared with swing, traversing, and lift bridges.