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While constructing a residential building with the flat-slab structure in 2008, the foundation slab, columns and walls of the underground floor were subjected to inclination and cracks. Damage was mainly caused by complex ground and water conditions which were not included in the structural design, and particularly by the building foundations at the depth of 2.4 m below the stabilised groundwater table. As part of a team of two surveyors, I presented the method of stabilizing and reinforcing the building structure.
The expert opinion focused on a six-storey flat-slab structure of the residential building with overall dimensions of 44 x 25 m and height of 20 m. Many faults were made while constructing the building: ground and water conditions were incorrectly classified and mistakes were made in the MES model representing the design (assuming, among other things, too large finite elements and an incorrectly concentrated grid). Values of punching forces exerted on the foundation slab and flat floor slabs in the vicinity of the columns, were underestimated in the calculations. Assumed wind loads were not sufficiently precise which, in turn, had an impact on calculating the insufficient reinforcement of reinforced concrete columns. The above faults were probably caused by young age and the lack of experience of the structure designer, and by the site manager who underrated the complex ground and water conditions.
Fig. 1. View of a case foundation of the constructed residential building
Fig. 2. MES model representing the flat-slab structure of reinforced residential building
The structure was very unstable during the construction. Soil under the foundation slab was very loose even though sheet piles were used during the construction. Results of geodetic measurements of the slab lowering were 90 mm, slab displacement up to 30 mm, and the lowering difference between columns was up to 40 mm. One of initial stages of repairing the structure was concrete topping of the foundation slab to balance buoyant force of water with the structure weight. Then, the foundation was reinforced with jet grouted piles having a diameter of 50 cm and the length of ca. 6 m. When the stabilisation stage of ground was completed, columns and floor areas close to columns were reinforced against punching. The building was commissioned in 2010 and it has been safely used so far. The above emergency state of the structure was discussed in details in the paper .
author: Mariusz Jaśniok
company owner of CorrTEST