Ribbed post-tensioned slabs are often needed in areas with problem soils, but they require costly excavation to form the stiffening beams. If site conditions permit, a more economical construction method can be used to reduce the number of stiffening beams. This method is often referred to as a hybrid or donut slab: a combination of a ribbed perimeter and a uniform thickness interior. PLASTIC CLAY CONDITIONS In some areas slabs on grade are supported by problem soils containing plastic clays. These fine-grained soils have a high potential for shrinking and swelling. Because of this, changes in moisture content can change soil volume, enough to cause the soil to move, either pushing the slab or receding from it. DESIGN OPTIONS Designing slabs to be constructed on plastic soils can be complex. The most common approach has been a post-tensioned design resulting in a slab ribbed in two directions and analyzed as a monolithic unit. The enhanced T-beam action characteristic of stiffened (ribbed) post-tensioned slabs helps to control the movement and differential deflection commonly associated with shrinking and swelling soils. However, on very large slabs with any horizontal dimension greater than 100 feet, the soil moisture changes generally have an effect only near the slab perimeter. Using a hybrid type of slab can be an effective and economical solution, since this slab design combines stiffening elements where the volume change exists (the slab perimeter) and a uniformly thick slab where no changes exist (the slab interior).