Combined footing

Definition

A combined footing is a type of shallow foundation that supports more than one column or wall and distributes their loads over a larger area of soil. This design helps in preventing excessive settlement and ensures stability when the columns are close together or when one column bears significantly more load than the others. Combined footings are particularly useful in situations where individual footings might overlap or where the soil has low bearing capacity.

5 Must Know Facts For Your Next Test

1. Combined footings can be designed as either slab-type or strap-type, depending on the arrangement of the columns and the load distribution.
2. They help reduce the risk of differential settlement between adjacent columns by spreading the load evenly across the footing.
3. The design of combined footings must consider factors like soil bearing capacity, column loads, and local building codes.
4. When using combined footings, proper reinforcement is essential to manage shear and bending moments effectively.
5. Combined footings are often used in situations where space is limited, making it impractical to construct separate footings for each column.

Review Questions

• How does a combined footing improve stability compared to individual footings?
  • A combined footing improves stability by distributing loads from multiple columns over a larger area of soil, which helps prevent excessive settlement. In cases where columns are closely spaced or have differing loads, a combined footing reduces the risk of differential settlement that can lead to structural issues. By consolidating the support into one footing, it also simplifies construction and minimizes excavation work.
• What factors should be considered when designing a combined footing for a building?
  • When designing a combined footing, several factors must be taken into account, including the bearing capacity of the soil, the load from each column or wall, and the spacing between them. It's important to assess potential settlement issues and ensure that the footing is adequately reinforced to handle shear and bending moments. Local building codes and environmental considerations also play a critical role in determining design specifications.
• Evaluate the advantages and disadvantages of using combined footings in urban construction settings.
  • Using combined footings in urban construction has several advantages, including efficient use of space, reduced excavation costs, and minimized differential settlement among closely located columns. However, they may require careful engineering assessments due to varying load conditions and soil properties. Additionally, if not designed properly, they can lead to complex reinforcement needs and increased risk of structural failure under unforeseen loads. Balancing these factors is essential for successful implementation.

Related terms

shallow foundation: A type of foundation that transfers building loads to the ground within a depth of about 3 meters, commonly used for structures with lower load requirements.

strip footing: A continuous foundation that supports a wall, distributing its load along the length of the wall to prevent settlement.

mat foundation: A large concrete slab that supports multiple columns and walls, used when soil conditions require spreading loads over a wide area.