Understanding Shallow Foundations: Types and Applications
Shallow Foundations
Shallow foundations are a common type of foundation used when the soil at a shallow depth has sufficient bearing capacity to support the structure’s loads.
1. Footings
Foundation footings are commonly used in areas where the ground has medium to high resistance relative to the structure’s loads and is homogeneous enough to minimize differential settlement.
1.1 Isolated Footings
Isolated footings are individual, typically square concrete pads used to support single columns or posts.
1.2 Eccentric Footings
Eccentric footings are also individual footings, usually rectangular, designed to handle eccentric loads.
1.3 Tied Footings
Tied footings are individual footings connected by a tie beam, designed to prevent horizontal movement between footings or to provide additional stability.
1.4 Footings and Foundation Beams
Foundation beams are structural elements that distribute wall loads to isolated footings. They are used for various reasons, including load distribution and settlement control.
1.5 Continuous Footings
Continuous footings, or strip footings, are used for closely spaced columns or walls, providing a continuous support along their length.
1.6 Combined Footings
Combined footings support two or more columns, often used when columns are closely spaced or when one column is near a property line.
2. Slabs
Slab foundations are used over the entire structure’s surface in areas with less resilient, less homogeneous, or less resistant soil. They increase the contact surface and reduce differential settlement.
Slab foundations can be more economical than individual footings if the total area of the footings exceeds half the building’s footprint, due to reduced concrete thickness, reinforcement, excavation, and formwork.
2.1 Slab Thickness
Uniform thickness slabs offer simplicity in construction. Formwork savings can offset the increased concrete volume if loads and spans are not significant.
2.2 Slabs with Capitals
Capitals increase the slab thickness under columns, improving bending and shear resistance. Capitals can be above or below the slab, with below-slab capitals simplifying excavation and providing a flat basement surface.
2.3 Ribbed Slabs
Ribbed slabs feature major ribs and secondary ribs between columns. Ribs can be above or below the slab. Above-slab ribs require more complex formwork and may necessitate lightweight fill and independent flooring to create a flat surface.
Below-slab ribs can be formed directly in the excavation.
2.4 Floating Slabs
Floating slab foundations are used for structures sensitive to settlement in poor soil conditions. The foundation size is designed so that the weight of the excavated soil is similar to the structure’s load. This minimizes changes to the subgrade surface, theoretically reducing settlement.