B2/2000Y Collapsing of ceiling of supermarket at Pudasjärvi, on 27 December, 2000
On 27 December 2000 at 21.24 hours an accident occurred in a KKK Supermarket at Pudasjärvi, Finland. In the accident the ceiling of the supermarket and the heating pipes, refrigeration-apparatus pipes, water pipes, ventilation ducts, electric wires and shelves fixed in the ceiling, collapsed. The last customers had left a while ago as the supermarket had been closed about 20 minutes before the accident.
At the time of the accident there were two cleaning ladies in the supermarket, working in its different ends. They heard a clicking sound and then a steadily strengthening crashing sound from the ceiling in the meat counter end of the market. Then the ceiling and the dressing boards hanging therefrom started to jerk downwards. At this moment one of the cleaning ladies ran outside through the back-door while the other one hid in a wind box. The cleaning ladies having escaped safely, the ceiling with its installations collapsed over the entire 1710 m2 surface area of the market. According to the cleaning ladies, the collapsing occurred in about one minute, with further minor collapses taking place even several days thereafter. The corresponding damage and loss were assessed at about FIM 7 million.
The accident was caused by a total negligence or insufficient consideration of the entity in the planning, design and building of the ceiling and the mountings and suspensions therein. All participating workers and experts only carried out their own specific tasks without worrying about the remaining questions. In fact all subcontractors were known as experienced and highly professional, which probably contributed to the general conviction of everything being in order. As is the regular practice in building business, the planning, design and realization of a ceiling and its mountings and suspensions were contracted to several different enterprises.
The direct technical cause of the accident was the insufficient capacity of the nail joint between the 50x50 mm ceiling strips and the roof trusses, to carry the loads of the ceiling. As a rule the nail joints featured two 90x3.1 mm machine nails that ensure the joints a specific capacity of 326 N/m2 to resist suspension loads, as calculated in conformity with the norms and standards adopted in the building business. However according to the calculations made by the Accident Investigation Board of Finland, in a certain studied area the suspension loads including the dead weight of the ceiling, totalled 447 N/m2 on average. In addition some smaller areas of the ceiling were charged by important concentrated loads generated by the suspensions. An adequate structure necessarily has to feature an over 1.5-fold capacity as related to the loads, with consideration of the safety coefficients.
The nail joints had been realized as contrary to the plans, as the planner of the structure had equipped the joint with three 100x3.4 mm French nails. However in practice the planned joint failed to be viable, as in our days almost all work sites use machine nails with a maximum length of up to 90 mm. Furthermore the joint area between the strip and the roof truss is not big enough - in terms of the relevant norms and standards - to permit three nails to be fixed therein. The specific capacity of the designed nail joint to resist suspension loads would have been 677 N/m2 which would also have been insufficient for the carrying of the concentrated loads.
The investigations conducted yielded an uncertainty prevailing in the building business as for the capacity of the machine nails. Therefore the Accident Investigation Board recommends further studies to be made on nails shot by a pneumatic nail machine. Furthermore the Building Regulations in Finland (Suomen rakentamismääräyskokoelma) should also include instructions for an assessment of the nails in question in the planning and design phase. Moreover the Accident Investigation Board recommends that the resources engaged in supervision of building work be increased, possibly needed technical support be ensured, and the licence granting practices be harmonized. Finally the Accident Investigation Board proposes that the requirements adopted in the fire resistance rating and the responsibilities of the principal planner, be defined in detail.