Targeting at a factor 20 reduction of the environmental load of buildings in 2040, it is necessary and logical to start by reducing the environmental load of the most important aspects of a building. The bearing structure is responsible for the second largest part of the environmental load of the building; the energy consumption is responsible for the greater part, assuming the life span of a building is 75 years (van den Dobbelsteen & van der Linden, 2000)2. The bearing structure consists of the foundation, columns, beams, floor and roof structure, and sometimes walls and facades. The horizontal elements of the bearing structure, comprising of the floor, roof structure and beams, are causing about 80% of the environmental load of the total bearing structure. Particularly the floor & roof structure play a very remarkable role (Arets, 2000). Therefore, optimization of the floor structure will lead to an important environmental improvement of the whole office building. The question which structural material is ecologically or economically best, depends on the span the building design needs. For each floor type, for structural spans between 4.8 and 16.8 m, the environmental load and cost have been determined, resulting in a clear overview of best solutions for different spans.