BUILDING FOUNDATIONS; WHAT YOU SHOULD KNOW

The foundation of a building structure is fundamentally that part of a building structure that transfers the weight of a building to the ground. The building is a very heavy structure. It’s weight is as a result of its self weight and the weight of the properties and the people that use the structure. The whole weight is then passed to the ground to carry.

We now wonder; can the ground carry all these weights? Yes it can!. Remember all the materials used for the construction of buildings are taken from the earth. We are just redistributing the materials of the earth when we build. No amount of building will push the earth off its orbit round the sun.

If the ground can carry all the weight of our building structures, why do we then worry about the foundation of our buildings? Our buildings are safe when its weight can be carried by the ground without appreciable movement.

From physical observations we know that when a heavy object is placed on a soft material it bends or compresses it. The ground is a soft material and so when the weight of our buildings is placed on it, it compresses it. 

If the movement due to compression is small our building is safe but when it bends a lot, our building structure sitting on it suffers. This settlement or deformation under the weight of the building is what we know as consolidation.

 When excessive it often leads to a building failure. Our media spaces like 9jabrief [1] are ripe with incidences of building collapse. The building control agencies take proactive steps to forestall these failures, one of such steps involves the strict regulation of Issuance of building permits as seen in Nigeria by the Enugu state government[2] 

Building foundations are crucial to the stability of the building, It is therefore imperative that we have a basic knowledge of this important component of our buildings.

Key Takeaways:

  • Foundations are essential for building stability.
  • Understanding foundation types and design principles is crucial for construction professionals.
  • Proper soil testing and engineering calculations are necessary for safe and durable foundations.
  • Regular maintenance and inspections are essential to ensure long-term foundation health.

The Foundation Design Process

The design of a building’s foundation is a critical step in the construction process. It requires careful consideration of various factors to ensure the structure’s stability and longevity. Here’s a breakdown of the key steps involved in foundation design:

1. Soil Investigation:

  • Site visit: Structural engineers conduct a thorough inspection of the construction site to assess the surrounding terrain, soil conditions, and any potential hazards.
  • Soil sampling: Samples are taken from different depths to determine the soil’s properties, such as its bearing capacity, compressibility, and permeability.
  • Laboratory testing: The soil samples are analyzed in a laboratory to determine their mechanical characteristics and suitability for supporting a building’s foundation.

2. Building Load Calculation:

  • Structural analysis: Engineers calculate the total load that the foundation will need to support, including the weight of the building itself, its contents, and any live loads (e.g., people, furniture, equipment).
  • Dead load: This refers to the weight of the building’s structural elements, such as walls, floors, and roof.
  • Live load: This includes the weight of occupants, furniture, and other movable objects.

3. Foundation Type Selection:

  • Based on soil conditions and building load: The engineer selects the most appropriate foundation type to ensure adequate support and prevent excessive settlement.
  • Common foundation types: Shallow foundations (e.g., slabs, rafts, piers) are suitable for stable soil conditions, while deep foundations (e.g., piles, caissons) are used in unstable or deep soil.

4. Foundation Design Calculations:

  • Structural analysis: Engineers use computer-aided design (CAD) software and engineering principles to calculate the required size, depth, and spacing of foundation elements.
  • Settlement analysis: Calculations are made to estimate the potential settlement of the foundation under the building’s load.
  • Bearing capacity: The foundation must be designed to withstand the maximum expected load without exceeding the soil’s bearing capacity.

5. Permit and Regulatory Compliance:

  • Building codes: Foundation design must comply with local building codes and regulations to ensure safety and structural integrity.
  • Permits: Necessary permits and approvals must be obtained from local authorities before construction can begin.

6. Foundation Construction:

  • Excavation: The construction site is excavated to the required depth for the foundation.
  • Formwork: Forms are constructed to shape the foundation elements, such as footings, piers, or piles.
  • Reinforcement: Steel reinforcement is placed within the forms to provide structural strength.
  • Concrete pouring: Concrete is poured into the forms to create the foundation elements.
  • Curing: The concrete is allowed to cure and gain strength before backfilling and construction of the superstructure can begin.

The foundation design process is a critical step in ensuring the stability and longevity of a building. By carefully considering soil conditions, building loads, and regulatory requirements, engineers can design foundations that provide adequate support and minimize the risk of structural failure.

Soil bearing capacity

Every soil surface will settle when the weight of a building is placed on it. The amount of load a square metre of the soil can carry without appreciable settlement is known as the load bearing capacity of the soil.

If this value is high, it means that the soil can carry a lot of weight on a small area of land. If however the value is small it means the soil can only carry a small amount of weight on a small area of land.

How the building foundation works

A foundation transfers the weight of a building structure to the ground. The foundation does this by spreading the weight of a building over an area of land. The more the area of land the weight is spread, the less the pressure on the soil and the less the settlement.

 If the load is spread out well enough the pressure on the ground as a result of the weight of the building will be less than the soil bearing capacity of the soil.

The soil will therefore be able to carry the weight comfortably. The more we spread the weight of the building, the wider the foundation of the building and the more the cost of the foundation.

When the bearing capacity is very low (requiring a very wide foundation) it is often cheaper to take the foundation deeper into the ground till a soil strata of acceptable bearing capacity is found.

Choice of a foundation type for a building 

The choice of a foundation type for a building is determined by the weight of the structure and the soil bearing capacity. A foundation spreads the load of a building over a large area of land to enable the ground carry it without considerable settlement.

 

Strip foundation:

This is the foundation type used to transfer the weight of load bearing walls to the ground. The strip foundation is a concrete slab placed under the walls to transfer the weight of the wall or load from the wall to the ground.

In order to effectively distribute load to the ground without any need for steel reinforcement, the dimensions of the strip foundation are carefully chosen. First  we determine the needed width of the foundation.

This is done by dividing the wall load by the soil bearing capacity. When this value is less than the three times the wall width we have a narrow strip.

It is important to ensure that the width of the foundation slap is three times the width of the wall and that its depth is same as the wall width to ensure its whole mass is effective as distributing load to the ground. A strip is usually adequate in small building structures that are situated in places with a good bearing capacity. 

BUILDING FOUNDATIONS; WHAT YOU SHOULD KNOW

A strip foundation [3]

Pad footing with ground beams:

A pad footing is a reinforced concrete slab placed unto columns to distribute the loads from the column evenly on the ground. A pad footing with ground beams is a situation where all the columns are resting on pad footings and the pads at the position they connect with the columns all connected together by ground beams.

The beams ensure that all the loads from walls are transferred to the pad footings. They also provide lateral stability to the building’s foundation by connecting them together as one unit. In this kind of foundation, only the pad footings distribute the weight of the building to the foundation. 

WHAT YOU SHOULD KNOW

A pad foundation with ground beams [4]

Strip and Pad foundation:

This is the commonest foundation. In this kind of foundation the columns rest on the reinforced conrete slabs (pad footings) while the walls of the building rest on the strip foundation. This foundation is usually adequate for multi-storey buildings resting on a soil with a good bearing capacity.

Wide strip and Pad:

This is a slight variant of Strip and Pad. In the case of a wide strip, the width of the strip slab is more than three times the wall width. The wide strip by implication covers a wider area of land thereby distributing the building load better than the wide strip.

This is prevalent in places where the soil bearing capacity is very low such that the narrow strip does not provide enough bearing area. It can also be as a result of a larger load from the building, a case were the wall supports considerable load.

Wide strip can also result in foundations were deep ground beams are used to spread assist the column in transferring load to the foundations.

 In such case while the columns rest on pad footings, the deep beams rest on wide strips to form inverted T- beams. The wider the area of the foundation the lower the earth pressure as a result of the load.

This is to enable the ground to be able to support the load without appreciable settlement. Unlike narrow strip foundations, wide strip foundations do contain reinforcement bars.

BUILDING FOUNDATIONS; WHAT YOU SHOULD KNOW

A wide strip foundation [5]

Raft foundation:

In cases were the pad footings (pad foundation) in a building are too wide such that they overlap with each other, a raft foundation is considered.

A raft foundation is essentially a large reinforced concrete slab placed under the whole building. Raft foundations are necessary in buildings situated in areas with very low bearing capacity or in buildings that are very heavy such as multi-storey buildings.

They become necessary when the weight of the building needs to be spread over a very large area, an area comparable to the entire surface area upon which the build is to be situate. 

BUILDING FOUNDATIONS; WHAT YOU SHOULD KNOW

A raft foundation [6]

Pile foundation:

There are cases where after the load or weight of the building is spread across the entire building area, the pressure from the building will still be higher than what the ground can support.

This occurs in places with very low bearing capacity or for buildings with considerable weight such as high rise buildings. In such cases the soil strata that can carry the building is located at a good distance beneath the soil. 

 Excavating the soil to such a distance to locate our foundation and start the building is very costly. Hence reinforced concrete or steel pillar/columns are drilled or forced to such depths to transfer the weight of the building to the soil in that strata.

This kind of foundation is called a pile foundation. The piles can be end bearing when they transfer loads to a better load bearing soil strata, they can be referred to as friction piles when they rely on the friction between the pile and the surrounding soil in carrying loads.

 WHAT YOU SHOULD KNOW

A pile foundation [7]

Conclusion

In conclusion, the foundation of a building is a critical component that ensures its stability and longevity. While the earth can support the weight of our structures, excessive compression can lead to settlement and potential failure.

By understanding the principles of foundation design and adhering to building regulations, we can safeguard our buildings and prevent costly structural damage.

 It is imperative for both professionals and homeowners to prioritize the proper design and construction of building foundations to ensure the safety and durability of their structures.

References

[1] https://www.9jabrief.com 

[2] https://9jabrief.com/view_feed?feed__key=35039 

[3] https://viyaconstructions.com/strip-foundation-or-strip-footing/

[4] https://fpmccann.co.uk/wp-content/uploads/2020/09/FP-McCann-Precast-Concrete-Structural-Ground-Beams.jpg

[5] https://constructiontermsx.com/wide-strip-foundation-definition/#google_vignette 

[6] https://vgeotechexperts.com/raft-foundation-its-advantages-and-applications/ 

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