It's to allow for expansion and contraction of the steel - depending on the temperature of the place where it's constructed. Like other metals, steel expands in warm weather and contracts in cold. Leaving one end of the bridge unfixed, allows the main girder to 'flex' with the temperature changes.
The towers are built first. Steel ropes are suspended from the towers. Special machines spin these into strong steel cables. Next, long steel cables called hangers are attached to the suspending bridge. Sections of the deck are lifted into place and fixed to the hangers.
Use the coefficient of thermal expansion. This is a measure of how much a unit length of steel would expand per each unit increase in temperature. There are different kinds of steel so you may need to know its composition.
The Verrazano-Narrows bridge connects Brooklyn and Staten Island and is named for Giovanni da Verrazzano. The bridge is constructed from concrete pilings, steel wire cables, iron eyebars, steel girders, and steel towers.
The expansion of the American steel industry and railroads during the civil war was made possible by Andrew Carnegie. Mr. Carnegie was a Scottish American industrialists who is known for leading the steel industry expansion.