The central support structure of a skyscraper is its steel skeleton . Metal beams are riveted end to end to form vertical columns . At each floor level, these vertical columns are connected to horizontal girder beams.
Many buildings also have diagonal beams running between the girders, for extra structural support.
This concentrates the downward force caused by gravity into the relatively small areas where the columns rest at the building's base. This concentrated force is then spread out in the substructure under the building. In a typical skyscraper substructure, each vertical column sits on a spread footing . The column rests directly on a cast-iron plate , which sits on top of a grillage . The grillage rests on a thick concrete pad poured directly onto the hard clay under the ground.