Fire station conversion
Conversion of a fire station into a house on Stieltjeslaan in Utrecht
In the south-west wall on the garden side, the client wanted as many openings as possible into an extension. We chose two adjacent openings of about 4.5 m, framed by two portal frames with downstand beams. A soil investigation showed that the ground had poor bearing capacity and that the water table was deep enough to excavate well down, so that the ground floor could be lowered and a thick structural slab used. We chose cost-effective foam concrete: the slab then insulates well and, thanks to its thickness, is strong enough to carry the considerable column forces via the downstand beams (the bending strength of the slab increases with the square of its thickness). This means no expensive driven piles are needed under the portal frames or the extension columns to transfer the loads to the ground. The slab was designed as a plate on an elastic foundation. The original, ninety-year-old stepped footings of the south-west wall provide additional security in supporting the portal frames. Because virtually all column-to-beam connections were made fully moment-resisting, the columns and beams could be built from the slender HE180A section. Besides using little expensive steel, this also leaves a large clear space between the HE180A sections. The client also wanted large openings in the wall between the portal frames, perpendicular to the south-west wall, where it meets the external walls; this turned out not to be feasible, because without costly, heavy temporary works the wind load could not be properly transferred to the foundations. Large openings reduce the compression from self-weight. This reduced compression is less able to offset the tension caused by wind. If the wall perpendicular to the side wall were also given large openings, too little compression would remain to offset the wind tension adequately.