B1/2005Y A Threat of Roof Collapse in a Shopping Centre in Kuopio on 18 March, 2005
On Friday 18 March 2005, cracks were observed on the concrete roof beams of a Prisma commercial centre located in the town of Kuopio. The building dates from 1996. The situation was regarded as posing a sufficient threat of collapse that the decision was taken to evacuate all staff from the building. There were no customers in the commercial centre at the time. The emergency services were alerted of the apparent danger of collapse and emergency service units were dispatched to the building. The centre was isolated by the police and the fire brigade ensured that the building was empty. A building surveyor present on-site determined that the centre should remain closed for the time being. The damaged beams were fitted with additional supports during the weekend and the centre was re-opened to customers in the following week.
The load-bearing roof structure of the building consists of pre-tensioned single-span ridge beams supporting pre-tensioned TT slabs. The beam rows are spaced 12 metres apart and the beam supporting pillars are spaced 18 metres apart. The beams are 18 metres in length, 1.35 metres in height and span 17.76 metres.
The damage to the pre-tensioned concrete beams was caused by the absence of binding hooks from the longitudinal beam reinforcement extensions and the top of the beam. The purpose of the binding hooks is to fasten the iron reinforcement bars on the upper beam flange in the sideways position. The stress state of the upper flange of the pre-tensioned concrete beam changes from a state of tensile stress in the installation state to a state of compressive stress in the in-service limit state. The tensile beam reinforcement thus subsequently converts to compressive beam reinforcement. If the compression reinforcement bars are not held fast with binding hooks, intermittent alterations in the direction of compression stress of the upper flange at the ridge results in a shear tendency in the ridge area.
To avoid similar damage occurrences the Board of Inquiry recommends that the openness to interpretation of drawings should be eliminated and prefabrication factories manufacturing pre-tensioned concrete beams should be provided with precise information concerning the required position of binding hooks for beam upper flange reinforcement bars.
Furthermore, the Board recommends that beam heights in accordance with span-width curves must not be reduced without absolute due reason. If shorter beams are used, their dimensioning and steel reinforcement design should verified by a specialist in ridge beam engineering.