Bridges and Other “static” Structures
What challenges are there in building skyscrapers?
The first challenge is to design a foundation that can support the tremendous weight of a large building. The best way is to dig down to the bedrock. This can be as close as about 21 meters (70 feet) in New York City to almost 61 meters (200 feet) in Chicago. If the distance is short holes can be bored and concrete piers can be formed in the holes. More frequently a caisson is required. This is a large hollow waterproof structure that is sunk through the mud, pulling it into and then out of the top of the caisson. A third method is go build a large steel and concrete underground pad that “floats” on the top of a hard clay layer.
The load that the foundation must support includes the weight of the building, its furnishings and equipment, and the changing load of occupants. In addition to the loads, strong winds must also be considered.
The walls of early tall buildings were constructed of masonry that supported the weight of the building. The 16-story 65.5-meter (215-foot) high Monadnock Building in Chicago, built from 1889 to 1891, required 1.8-meter (6-foot) thick walls at the base. It was so heavy that it sank, requiring steps to be constructed between the sidewalk and the ground floor. The second half of the building used a steel frame on which masonry was attached, allowing much wider windows to be used.
The steel frames can be bolted, riveted, or welded together. When the 59-story, 279-meter (915-foot) tall Citigroup building was constructed in New York City from 1974 to 1977 the frame was bolted together, but later computer models showed that if hurricane-strength winds struck the building it would be in danger of collapse. As a hurricane moved up the eastern seaboard in 1978 workers hurriedly welded plates over the bolted joints. Luckily the hurricane moved out to sea, sparing New York.
Another effect of winds on tall buildings is to make them sway back and forth. While a variety of braces can reduce the sway, they add weight to the building. Another method is now used. The Citigroup building has a 400-ton concrete damper at the top. The damper moves back and forth, opposing the wind-driven motion of the building and reducing sway. Dampers, both liquid and solid, are used in tall buildings, towers, off-shore oil drilling platforms, bridges, and skywalks. The 210 meter (690 foot) Burj al-Arab hotel in Dubai has 11 mass dampers. The dampers can also mitigate the effect of earthquakes.
Transporting large numbers of people into and out of upper floors is a challenge to those who design the elevator systems. As was demonstrated in the collapse of the World Trade Center buildings on September 11, 2001, stairways can be used in emergency situations, but the simultaneous movement of occupants down and firefighters up the stairways caused severe problems.
Another consideration is the safety of occupants in case of fire. Some buildings have entire floors designed to be especially fire-resistant so that people could gather there and be safer than on other floors.