B3/2003Y Fall of an intermediate floor structure intended as permanent at a service station site in Orivesi on 25 August 2003
An old service station building in Orivesi was being extended. The steelwork of the extension had been installed, and hollow-core concrete slabs had been lifted onto it as an intermediate floor structure - i.e., the floor of the second storey. On the morning of Monday, 25 August 2003, a six-centimetre-thick surface concreting was performed onto the area of the hollow-core slabs. At approximately 11:30 a.m., the workers had begun returning from their lunch break and went to work under the area of the hollow-core slabs. Suddenly, the area of the hollow-core slabs, approximately 200 square metres of surface area, fell while five men were in the danger zone. Three of the men managed to move away, but two remained under the fallen slabs. One man was seriously injured. Another died, despite the efforts of rescue workers, who arrived fairly quickly at the scene.
The steelwork had been manufactured in a small machine shop, transported to the site, and assembled and installed there. The hollow-core slab section with its horizontal steel beams fell when three of the six consoles supporting the horizontal beams and hollow-core slabs came off the pillars. The consoles had been fixed to the pillars with fillet welds, but these failed completely. The welds hardly penetrated the pillar, and the load borne by each joint was only a fraction of its design capacity. Apparently, the technical reason was that the welding voltage used had been slightly too high and the welding torch had been held in a slightly incorrect position.
The error was possible because the procedures for establishing the compliance of the construction products, listed in the construction orders and standards, had not been followed. The supplier of the steel structures was permitted to manufacture and install the structures without proving their compliance. A welding procedure specification (WPS), a valid certificate of the welder’s qualification - applicable to the welding job in question - and a welding record concerning the manufacture would have served as proof according to the standards. Neither did the monitoring by the authorities ascertain that the compliance of the load-bearing structural parts and of the construction products used in them had been established before the installation.
To avoid similar accidents, the investigation commission recommends that the organisations in the of steel construction branch establish procedures for determining and proving the compliance of welded steel structures in order to aid in the supervision of construction and assist the construction organisation and the manufacturers of structures.
Furthermore, the investigation commission recommends that the construction supervision authority remind the developer during the early stages of a project of his duty of solicitude and of methods of establishing the compliance of products supplied by contractors and subcontractors. In this way, developers would be better prepared to monitor the capability of the construction organisation formed by them to produce structures in compliance with the standards.
In the welding tests performed during the inquiry, it was noted that one proof of a good welded joint is a bright streak left on the edge of the weld. This observation should be studied in more depth, and - as applicable - the subject should be added to the instructions for visual inspection of welded joints.