A raft beam is a structural element specifically incorporated into a raft foundation to enhance its strength and load-bearing capacity.
Understanding Raft Beams
To understand what a raft beam is, it's helpful to first know about the raft foundation itself.
Context: The Raft Foundation
According to structural engineering principles, a raft foundation, also known as a mat foundation, is a layer of reinforced concrete covering the whole area of the building, or even of several buildings. This large concrete slab distributes the weight of the structure over a wide area, making it suitable for sites with low bearing capacity soil or when building heavy structures.
The Role of Raft Beams
While the raft slab itself provides foundational support, it often requires additional strengthening. The reference indicates that a raft foundation may be strengthened by increased thickness of concrete in areas of heavy loading, such as under columns, or by a system of main and secondary beams.
Therefore, raft beams are integral parts of this strengthening system. They are essentially beams cast monolithically with, or directly beneath, the raft slab.
- Main Beams: These are typically larger and run in primary load paths, often under lines of columns or walls.
- Secondary Beams: These are generally smaller and connect the main beams, further stiffening the slab.
Why Use Raft Beams?
Incorporating beams into a raft foundation offers several advantages:
- Increased Stiffness: Beams significantly increase the rigidity of the raft, reducing differential settlement, which is crucial for structural integrity.
- Improved Load Distribution: They help transfer concentrated loads (like those from columns) more effectively across the wider area of the raft slab.
- Reduced Slab Thickness: By using beams, the overall thickness of the raft slab between beams might be reduced compared to a plain flat slab raft designed for the same load.
- Enhanced Punching Shear Resistance: Beams under columns provide excellent resistance against punching shear failure, where a column could potentially push through the slab.
In essence, raft beams act like stiffeners, turning the flat slab raft into a more robust, three-dimensional structure capable of handling heavier loads and more challenging ground conditions.
