Collapse of a precast concrete beam for a light roof. Importance of elastomeric bearing pads in the element’s stability

• The collapse in construction of a 24.9 m span precast concrete Y-beam is studied.
• Second order effects were very important due to the bearing pad arrangement used.
• Sensitivity and probabilistic analyses are conducted to investigate the parameters’ influence.
• A 90° plan rotation of the bearings would increase stiffness, reducing the second order effects.

The importance of second order effects has been widely studied in slender concrete structures subjected to compressive stresses, although their effects are not generally taken into account in other fields of structural engineering; this is the case of the overall stability of structures flexible supports, as elastomeric bearing pads. In this paper, the collapse in construction of a 24.9 m span precast concrete Y-beam for the light roof of a sports hall is studied. The roof consisted of a series of these beams simply supported at each end by plain neoprene bearing pads. After a worker had climbed up to the last placed element, the element over-turned producing a partial collapse of the light roof. A simplified analysis, without considering second order effects, indicated that there was an over-turning safety factor (OSF) of 2.67. However, a more detailed analysis showed that with the bearing pad arrangement used, rotational stiffness was drastically reduced, causing the second order effects to gain importance. In spite of the OSF the safety of the element is shown not to be adequate. Upon considering the actual flexibility of the bearing and the large rotations, a reduction of the OSF to neutral equilibrium is explained. Sensitivity and probabilistic analyses are conducted in order to investigate the influence of each parameter and the most likely failure cause. A 90° plan rotation of the bearings would increase stiffness, remarkably reducing the second order effects and would achieve an OSF of 3.99 and a reliability index of 7.9.