Most office buildings and skyscrapers are built with floors and roofs resting atop wall pillars. Their strength comes from the huge weight pressing down. But this strength is a vulnerability in an earthquake as the floors collapse in on themselves. For medium-sized buildings, the best way to quake-proof them is to cut down on the weight. Lighter roofs and floors lower the peak stresses during an earthquake, while constructing concrete floors by pouring them in situ bonds them to the walls. Some skyscrapers have huge roller bearings in the foundations that allow the whole building to slide without cracking. Tuned mass dampers can also be used to counter quakes.