The plan for the towers was organized in a 21 foot grid that was represented with steel columns placed at the intersections of the grid. By using it for structure, Mies simplified the steel frame to its purpose while using it to express the grid. Because of its purpose Mies wanted the inner character of the steel skeletons to show. This became a problem due to fire regulations, but Mies made it his goal to make these skeletons visible despite this fact. He achieved this by covering concrete shells with steel plates, thus keeping the uniformity of the steel columns.