Knowledge of the core of any structure begins with its foundation, so builders need to understand the importance of its base elements. Pad foundations are in greatest demand for all forms of foundation solutions due to their capacity to support concentrated loads, especially in buildings and structures. Here, we will talk about the pad foundations, their types, advantages, disadvantages, and the circumstances under which they are chosen.
What is the Pad Foundation
A pad foundation, also known as a pad footing, is a type of shallow foundation designed to support individual point loads, such as those from columns in framed buildings. It typically consists of a square or rectangular reinforced concrete block that spreads the load from the column to the ground below, ensuring the bearing pressure is within the safe limit of the soil’s capacity.
Although usually constructed at shallow depths, the depth and size of the pad may be increased in cases of heavier loads or weaker soil conditions. In some designs, ground beams connect multiple pad footings to enhance structural stability and distribute loads evenly.
Types of Pad Foundations
Pad foundations can be of various forms, such as:
1. Plain Concrete
Plain concrete pad foundations are constructed without any steel reinforcement and are most appropriate for cases where the load on the building is extremely low. Such a foundation will be economical for light loading conditions. However, as loads rise, the pad must be very thick to be stable, which may not be economical. In the pad’s design, its thickness will be equal to the distance between the face of the vertical structure member (column) and the edge of the pad on both sides to ensure symmetrical load transfer.
2. Reinforced Concrete
Pad foundations are reinforced to spread over a larger area and maintain a shallow overall depth. They are square, so the construction and installation of the reinforcement cage are simpler. However, a rectangular pad is employed if the load is unevenly distributed or at an angle (eccentric or inclined). Since the reinforcing provides additional strength, these foundations do not necessarily have to be as thick as regular concrete.
3. Continuous Pad Foundation
A continuous pad foundation is utilised where several columns are placed alongside one another. Rather than each column having a separate pad, the foundation is prolonged to link them. The reinforcing bars continue throughout the foundation length with greater strength and stability. This kind of setup adds overall building stiffness, compensates for irregular ground movement, and minimises differential settlement between columns.
4. Combined Pad Foundation
Combined pad foundations are generally employed where a building is near a boundary, where it is not feasible to extend individual footings due to space constraints. In this case, two or more pads, typically for an external and internal column, are connected to share and transfer the load. This configuration allows for a more equal weight transfer, particularly when one side (such as the outer column) cannot support a full-size pad because of obstructions such as subsurface structures or shape constraints.
5. Pad and Ground Beam Foundation
Using the ground beams links pad footings together in a pad and ground beam foundation system. Ground beams enable the individual pad footings to be a unified system that contributes overall strength and stability. It is particularly useful when loads are off-centre or uneven, as the beams can evenly split the forces throughout the foundation.
Pros and Cons of Pad Foundation
Pros
- Pad foundations are quick to build.
- They don’t cost much to construct compared to other foundations, such as a pile foundation.
- They are suitable when the ground is firm within 3 meters of the surface.
- These are ideal for building like warehouses or shops with wide roofs, but only one floor.
Cons:
- They should not be used where the ground can’t carry much weight.
- They only work when strong ground is near the surface.
- If water builds up under the foundation, it can push the pad upwards.
- Water pressure from below can harm both the pad and the column it supports, making the building unsafe.
When to Use Pad Foundations?
- Pad foundations are usually employed in buildings that support a concentrated load, e.g., isolated columns, whose load can be redistributed.
- They are suitable for buildings that support moderately light construction and where the ground is stable.
- Pad foundations are ideally used where the foundation depth is not an issue and the ground beneath can effectively transfer the structural load. In the case of larger or more complicated buildings, a deeper foundation is necessary.
Construction Process of Pad Foundations
Building pad foundations involves a series of essential steps to ensure the structure is safe, stable, and built to last:
1. Site Assessment
Before anything is designed, the ground is completely checked. This includes testing the soil’s strength and whether it might shift or settle over time. This site survey step helps engineers decide how large and strong the foundation needs.
2. Design Planning
After understanding the ground conditions, engineers start designing the pad foundation. They consider how much weight the structure will carry and how the soil will respond. Specialised software often determines the right measurement, depth, and reinforcement.
3. Excavation and Fixing of Formwork
After the design, digging starts to make room for the foundation. The digging depth depends on the type of soil, the potential for ground freezing during colder months, and how deep the foundation needs to be placed beneath the surface. A wooden or metal framework is set to give the pad the shape per the design.
4. Reinforcement Installation
Steel bars are inserted within the formwork. They help prevent cracks and improve the structure’s ability to handle bending forces. The rebar is handled cautiously to fulfil the design requirements and provide sufficient spacing and overlap.
5. Pouring of Concrete
Concrete is finally poured into the mould once everything is set. Power vibrating tools ensure no bubbles in the poured concrete mix, and it’s tightly packed.
6. Finishing and Curing
At last, smoothing or levelling is completed, so the surface is prepared for the rest of the structure to be constructed. The curing period is based on weather and mix type.
Design Considerations of Pad Foundation
Between various foundations of pads being discussed, the choice depends on the following considerations:
- The soil’s load-bearing capacity
- The load to be transferred
- Column arrangement
- Site conditions
- Accessibility
- Subsurface conditions
- Water table depth
- Cost considerations
Conclusion
Pad foundations are an easy-to-use and affordable solution that suits most types of construction. It provides ease, affordability, and dependability. Knowing when and how to implement a pad is important for any construction. By assessing the soil conditions, load demands, and area availability, engineers can create pad foundations that support any construction. If you plan to use a pad foundation, consult an expert to ensure the best result for your project.