Luyện tập tổng hợp

In the growing cities of the nineteeth century, such as Chicago and New York, businesses needed offices near the center of town. Because there was only so much available space, people were forced to build upwards. However, certain innovations in materials and technology were needed before buidings could become the skyscrapers of today. The earliest buildings were built using bricks and mortar, which were quite heavy. When building upward, each floor had to carry the weight of the floors above it, with the lowest level carrying the weight of the entire building. Thus, the ground floors of those buildings had extremely thick walls – some up to eight feet thick! – and the consequent loss of space this entailed defeated the purpose of building tall. In addition, the rooms were rather dark, as too many windows would have weakened the structure. The introduction of iron and steel, which were much lighter and many times stronger, solved this problem. Interconnected beams and columns forming a skeleton now supported buildings and many times stronger, solved this problem. Interconnected beams and columns forming a skeleton now supported buidlings and transferred their weight to foundations resting on the earth’s bedrock layer. Because walls need not bear the weight of buildings anymore, more windows could also be installed to let light in. Skyscrappers face not just the vertical force of weight, but also the horizontal force of strong winds that could topple them. Some engineers solved this problem by buidling a stiff inner core to resist the tremendous force, while others moved the columns and beams to the perimeter, essentially turning the building into a hollow, rigid tube that is just as strong while weighing much less. Finally, tall buildings would have meant many stairs to climb, and it was the invention of the elevator at around that time that made buildings not just tall, but also practical for the people using them.