Skyscrapers, one of man’s greatest achievements, are known for their strength and sturdiness. A large skyscraper has an image of safety and security, due to its’ masculine appearance and sturdy reputation.
Without sturdy and effective load balancing designs, today’s skyscrapers could have never been built. These designs are a precise balance of economics, engineering, and construction. Internally supported structures skyscrapers do not need guy wires or any type of external support.
The structural design is of paramount importance due to the multitude of people and valuable commodities skyscrapers shelter. Engineers must test for every possible known situation in which the building’s integrity may be jeopardized. Although engineers cannot be completely confident their design will withstand the test of time and nature, they can ensure that their design provides a reasonable margin of safety against known hazards. When skyscrapers do fail, engineers must investigate if the failure was due to preventable cause or could not have been anticipated.
The weight of a skyscraper mainly consists of dead load, the load exerted by the building itself. Any extra weight from people, furniture, vehicles, etc. is known as live load. In addition, wind and other unexpected sources can be load providers. The design of a skyscraper is mainly dictated by how the total load is to be distributed. Skyscraper designs are categorized as steel frames, shear walls, concrete core, or tube designs.
Shear Walls
In a shear wall design, the weight of the structure is distributed through the walls. These structures are often made of steel reinforced brick or cinder block–materials with high compressive strength. The shear wall design is primarily used in small projects such as urban brownstones or suburban housing. As the load exerted on the building increases, shear walls must increase in bulk, meaning skyscrapers would need considerably large walls. Because of this, for tall buildings, this system is only used in conjunction with other supporting systems.
Steel Frame
When one thinks of a low-rise skyscrapers, the steel frame design comes to mind. This design is characterized by a large steel box, containing smaller steel boxes inside. This 3D grid is simple and efficient for most low-rises, but has its’ drawbacks for high-rise structures. As the building’s height increases, the space between steel beams must decrease to compensate for the extra weight, resulting in less office space and the need for more material.
Tube Frame
The tube design is a recent innovation used to maximize floor space and increase resistance to lateral force in any direction. The buildings skin (outside) consists of closely aligned supporting columns. This design only leaves about one-half of the building’s exterior left for windows. Depending on the designer’s outlook, this can be an advantage or disadvantage. The decreased window space helps those who suffer acrophobia (a fear of heights) comfortably occupy the space; however, it decreases the visibility and openness offered by other designs.
The tube frame design was made popular by the World Trade Centers, whose ultimate failure, some believe was due to the tube frame design.
Concrete Core
This is the most common design for modern skyscrapers as it is fast to build and provides a strong center. All the utilities, elevators, and stairwells are centralized in this design, making it easier for building modifications and repair. This design can be dangerous. If a part of the core is damaged, everything above that section will be cut off from ground access. This happened in the World Trade Center towers during the September 11, 2001 terrorist attacks, making it impossible for many people to escape the burning towers.
9/11 Changes
The terrorist attacks on September 11, 2001 called for a change of design in skyscrapers. Some of the more noticeable changes include widened and decentralized stairwells as well as sturdier bases to provide more street level protection. Some smaller changes include the use of better heat resistance coating on metal supports and different alloy combinations for stronger steel.