The third step also involves building the steel skeleton. This steel frame has outer walls(or curtain walls) that support almost the weight of the building. The weight from all the floors gets transferred directly to the vertical columns. This sends forces through the steel skeleton down into the building's base. In modern skyscrapers, these exterior curtain walls are made almost entirely of glass which gives more floor space for tenants to live or work. Once the skeleton has been built, the fourth step is much easier. The floors are the