Sub-pressure slabs are structural elements designed to withstand upward forces, such as the hydrostatic pressure generated by groundwater. These slabs are commonly used in the foundations of buildings located in areas with high water tables or where there is a risk of water accumulation beneath the structure. The pressure exerted by groundwater can cause flotation (the tendency of the structure to lift or displace due to upward force) and positive buoyancy, which requires reinforcement arrangements in different positions than usual.
During the structural design phase of the Porto 7 building, located in Chapecó, Santa Catarina, the presence of a high water table was detected, resulting in an upward hydrostatic pressure equivalent to a water column of 3.5 meters (or 3.5 tons-force per square meter).
To counter these forces, a slab with a thickness of 35 centimeters was specified. This slab was designed to support both the typical gravitational loads of residential garages and the upward water buoyancy loads acting against gravity. The reinforcement detailing took into account the simultaneous and independent action of these forces, ensuring an adequate structural response in all situations.
Due to the constant presence of groundwater, the use of concrete with crystalline additives was specified to provide suitable protection against the aggressive environment. However, it required strict control of deformations in the slab, which could not exceed specific limits to ensure the effectiveness of waterproofing. The formation of cracks also needed to be carefully monitored, preserving the integrity of the final structure.
The structural analysis also needed to cover each phase of construction separately. In the initial stages, the reduced weight of the partially completed structural elements is insufficient to maintain equilibrium and prevent the flotation of the structure. To address this limitation, additional measures were necessary, such as the use of foundation elements designed to withstand tensile forces (tiebacks), properly anchored into the ground.
The correct application and dimensioning of sub-pressure slabs are crucial for ensuring the structural stability and longevity of buildings. As urbanization grows and underground construction increases, the importance of these slabs continues to rise, demanding increasingly innovative and effective solutions from engineers to meet these challenges.
