Weight is the biggest problem in the construction of tall buildings. The load on the lower sections of a very tall building is only slightly less than that of the foundations. For this reason, the top section of a skyscraper is made using lighter materials. Most multistory buildings have frames consisting of steel girders that become thinner toward the top.
After weight, the next greatest force acting on a tall structure is the pressure of moving air. Winds tend to blow much harder on the upper floors then lower down. To resist wind pressure and prevent the top of a building from moving too much, thick steel beams are used to stiffen the outer walls. Another way to reduce sway is to make a skyscraper narrower at the top than at the bottom. This lessens the area of the walls where the wind speeds are highest, so that the sideways force on the top parts of the building is lowered. Now narrower the top means lighter in weight.
