Foundation Design in 8 Steps

The design of foundations plays a crucial role in structural engineering because these structures are responsible for supporting the entire building and transferring the loads to the ground. In the following paragraphs, we will share some steps to carry out this process effectively.

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What are foundations?

Foundations are the structures that are built below ground level to support and distribute the load of a building or structure. They are designed to safely transfer the vertical and horizontal stresses of the building to the underlying ground, ensuring the stability and durability of the entire structure.

Foundations are necessary because the superficial ground is not able to directly support the load of the building without undergoing deformations and instability. Foundations distribute the weight of the building over a larger area and transfer the loads to the soil layers most suitable to support them.

Types of foundations

There are different types of foundations, which are chosen based on the characteristics of the ground, the type of structure, and the stresses to be supported. Generally, foundations can be divided into two main categories:

• shallow foundations: used when the ground, in the superficial part, is strong enough to support the load of the building without the need to reach deeper layers. This category includes:
  • raft foundations, consisting of a large reinforced concrete slab that covers the entire building’s support area and aims to evenly distribute the load of the superstructure on the underlying ground;
  • inverted beam foundations, characterized by beams, usually inverted T-shaped, whose static behavior is opposite to that of beams in elevation and whose function is to transmit the actions from the building’s load-bearing structure to the ground;
  • plinth foundations, consisting of isolated or properly connected elements, positioned below the pillars to increase their support base and thus allow a more uniform distribution of structural loads.
• deep foundations: used when the superficial ground is not sufficiently strong and it is necessary to reach more stable layers at greater depths. This type includes:
  • pile foundations, consisting of vertical elements, commonly made of steel or concrete, which are “driven” into the ground to reach more resistant layers;
  • micropiles, similar to piles but with a reduced diameter, are often used in situations where it is difficult to reach great depths with traditional drilling equipment.

Types of foundations

Requirements for foundation design

When designing foundations, it is essential to ensure the functionality and safety of the superstructure. This involves analyzing the ultimate limit states (ULS) and serviceability limit states (SLS) that may occur during the nominated life of the work, i.e., conditions beyond which the foundation system can no longer guarantee the desired performance.

The ultimate limit states may be related to soil collapse and the structural resistance of the foundation itself. This means that the foundation soil must be able to support the expected loads without exceeding its limit load, and the structural elements of the foundation must be designed to withstand these loads. It is necessary, therefore, to carry out bearing capacity checks of the soil and design the reinforcements appropriately.

In addition to ultimate limit states, it is important to consider safety against serviceability limit states. Serviceability limit states refer to the movements and deformations of the soil that could affect the stability and functionality of the superstructure. At this stage, the expected settlements in the foundation soil are evaluated, and their compatibility with the overlying building is verified. Serviceability limit state checks may be more restrictive than ultimate limit states checks, as they focus on the long-term stability of the foundation work. Therefore, they can influence the dimensions and the design of the foundation itself.

It is essential to emphasize that the foundation design process requires a thorough evaluation of the geotechnical conditions of the site, the properties of the foundation soil, and the functional requirements of the building. Structural engineers must consider various factors, such as load distribution, soil bearing capacity, expected settlements, and seismic stresses, to ensure the long-term safety and stability of the foundations.

How to carry out foundation design?

The foundation design process is divided into several phases, which may require iterations and adjustments. During each phase, various approaches and modeling techniques are used to analyze the superstructure, the foundation itself, and the underlying soil. This process is iterative, meaning that phases may be repeated or modified based on new information or emerging needs during the design process.

The steps to follow when designing a foundation structure can be outlined as follows:

1. preliminary investigations: the initial phase involves conducting both field and laboratory investigations to gather detailed information on the mechanical characterization of the foundation soil. The goal is to understand how the soil will react under the weight and stress of the future building structure and define a project stratigraphy;
2. determination of loads: this phase involves determining the distribution and magnitude of the loads transmitted from the superstructure to the foundation structure. Both permanent loads and those due to overloads and seismic forces must be considered. This phase is crucial to ensure that the foundation is designed and sized correctly to support the superstructure safely and stably, even in extreme loading situations such as those due to earthquakes;
3. selection of foundation type: based on the information collected in the previous phases, the type of foundation that best suits the soil characteristics and structural requirements is chosen, and a preliminary sizing is carried out. The depth of the foundation structure’s laying plane is also defined. This depth depends not only on the specific soil characteristics but also on the building’s dimensions and the presence of any underground structures such as basements or parking lots;
4. calculation of the limit load: this is a fundamental phase in foundation design as it provides an estimate of the maximum load that the soil-foundation system can support before yielding. By dividing the limit load by a safety factor that takes into account various uncertainty factors in the design process (such as soil property variability, calculation accuracy, and the presence of temporary or exceptional loads), a preliminary estimate of the load the foundation can withstand under normal conditions of use is obtained. This value forms the basis for the actual sizing of the structure and ensures that the foundation is robust enough to support all expected loads without the risk of settlement or collapse;
5. settlement evaluation: this phase involves assessing the total and differential settlements of the foundation soil to determine how much it will settle under the weight of the structure and any applied loads. Excessive settlements can cause damage to both structural and non-structural elements and compromise the overall stability and functionality of the building. The calculation results are then compared with the allowable settlements that the structure can tolerate, taking into account specific design requirements and safety regulations;
6. structural analysis: in this phase, the interaction between the soil and the foundation structure is studied, considering the actual stiffness of the structure and the behavior of the soil. The main objective is to accurately assess how the foundation structure will respond to applied stresses, considering its ability to distribute and support loads uniformly on the underlying soil. During structural calculation, various factors are considered, including the foundation’s geometry and characteristics, the materials used in its construction, the applied loads, and the soil conditions. Through mathematical models and advanced structural analyses, the behavior of the foundation under load is evaluated, and any critical points requiring attention or corrections in the design are identified;
7. operational planning: this phase focuses on the practical procedures for realizing the foundation, evaluating various aspects such as excavation design, managing groundwater levels to ensure optimal working conditions, installing necessary structural reinforcements to protect surrounding structures from potential damage, and so on. Detailed instructions, guidelines, and technical specifications are defined to ensure the correct execution of the foundation works;
8. control scheduling: in this phase, a control plan is developed, which includes conducting checks and inspections both during the construction of the work and during its operational life. This plan includes a series of ongoing checks aimed at ensuring that the works are carried out in accordance with the approved design and current regulations. In addition, periodic checks are planned during the operational phase of the work to monitor the stability and safety of the foundations over time. These control activities are crucial to ensure that the work is built correctly and maintains its performance over time, ensuring the safety and durability of the structure.

Foundation design with EdiLus

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• advanced analysis: the FEM solver allows you to perform sophisticated and accurate structural analyses to evaluate the response of the foundation system to different loads and stresses. You will obtain detailed results on the behavior of the soil and foundations, including settlements, deformations, and internal stresses;
• BIM modeling: thanks to BIM technology, you can create three-dimensional models of your foundation works intuitively and accurately. This will allow you to visualize and analyze the structure comprehensively and in detail, facilitating collaboration among team members and improving project understanding;
• design optimization: the software will enable you to explore different design solutions and evaluate the performance of each to find the most efficient solution in terms of cost, construction times, and structural performance;
• regulatory compliance verification: the system includes features for verifying foundations according to applicable technical regulations and design codes. You can perform automatic checks to ensure that your foundations meet the safety and stability requirements.

Take advantage of technology to improve the efficiency and accuracy of your foundation processes. Start exploring the features of this system today to achieve efficient, reliable, and industry-compliant results right from the start.