Pile foundation is a popular type of deep foundation used for transferring structural loads, such as axial load and lateral load, into the deeper layers of solid soil. To select and design an appropriate type of pile, it is crucial to have a good understanding of the types of loads acting on piles and their transfer mechanism.
Axial loads generate compressive or tensile forces that act parallel to the foundation’s axis. When the pile is vertical, the axial load is equivalent to the applied load in the vertical direction. On the other hand, lateral loads cause moments and shear, leading to lateral deflection in the pile foundation. This lateral deflection, in turn, activates lateral resistance in the adjacent soil.
It is important to note that lateral loads can cause significant damage to the foundation if not addressed properly. Therefore, it is crucial to consider the type and magnitude of lateral loads while designing the pile foundation. By doing so, the lateral resistance provided by the soil can be optimized to prevent excessive lateral deflection and damage to the foundation.
1. Axial Loads
When a load is applied to a structure, it may either be compressive or tensile in nature. In the case of a compressive load, which is directed downward, deep foundations rely on friction resistance and toe bearing resistance to counteract the load. This is depicted in Figure 1.
However, if the load is tensile, meaning it is directed upward, the resistance is generated by side friction and the weight of the foundation itself, as shown in Figure 1. For deep foundations that have an enlarged base, uplift loads can also be countered by bearing along the ceiling of the base.
Axial loads, which consist of dead loads, live loads, snow and ice loads, are transferred from the superstructure to the pile foundation. These loads are then borne by the deep foundation, which must resist the forces in order to maintain the stability and safety of the structure as a whole.
Dead and Live Loads
When designing a building, it is necessary to take into account the weight of the structure itself, as well as any additional loads it may bear. Dead loads, which refer to the weight of the building materials and permanent fixtures, can be calculated once the structural designer has provided all the necessary details about the superstructure’s design. On the other hand, live loads, which are dynamic and vary according to the intended use of each space, are typically determined using applicable codes.
However, in cases where such information is not readily available, an initial estimate of loading for each floor of a high-rise building can be made. This estimate usually ranges from 10 to 15 kilopascals per storey. It is important to note that this is just an initial estimate and should be adjusted based on more accurate information as it becomes available.
When it comes to pile foundations, the self-weight of the foundation is determined by several factors. These include the thickness of the raft, the dimensions and number of piles used, and the unit weight of the concrete. By taking these factors into account, it is possible to calculate the self-weight of the pile foundation and ensure that it is sufficient to support the weight of the building above.
2. Lateral Loads
Deep foundations are subject to lateral loads that can induce shear and moment within them. These lateral loads result in lateral deflections of the foundation, which cause lateral resistances to be mobilized in the surrounding soil. The stiffness of both the foundation and soil play a crucial role in determining the magnitude of these deflections and resistances, and ultimately, the load-bearing capacity of the foundation.
Pile foundations, in particular, typically rely on passive soil resistance on the face of the cap, shear on the base of the cap, and passive soil resistance against the pile shafts to resist lateral loads. However, among these sources of resistance, only the passive soil resistance against the pile shafts is typically considered reliable.
To summarize, lateral loads can induce shear and moment in deep foundations, resulting in lateral deflections and resistances in the surrounding soil. The load-bearing capacity of the foundation is dependent on the stiffness of both the soil and foundation. Pile foundations typically rely on passive soil resistance against the pile shafts as the most reliable source of resistance against lateral loads.
Wind Loads
Fig. 3 illustrates how wind loads can exert a notable eccentric loading on a foundation plan. When designing structures, it is important to consider the impact of wind loads, which can be estimated using specific guidelines. As a general rule, wind loads can be calculated as 1.5% of the dead load or 2kPa pressure for structures up to 200m in height. However, if a structure exceeds this height, wind tunnel testing is typically required to determine the appropriate wind pressure.
Various standards have been established to guide the process of estimating wind loads, including ASCE7 and AS1170.2–2011. These standards outline procedures for assessing wind loads on structures and can provide valuable guidance for engineers and architects. By following these guidelines, professionals can design structures that can withstand the forces of wind loads and maintain their stability over time.
Earthquake Loads
When designing a pile foundation, it is important to take into account the impact of earthquake loads. Similar to wind loads, earthquake loads can cause significant eccentric loading on the foundation plan, mainly in a horizontal direction. To ensure a safe and stable foundation, these loads must be carefully considered during the design process.
One of the key factors to consider when designing for earthquake loads is the inertial effects caused by the loads applied to the pile by the supporting structure. This includes factors such as kinematic effects, which are caused by ground movements generated by the earthquake, as well as the potential for the loss of soil support during the earthquake due to liquefaction or partial loss of soil strength.
In order to accurately compute earthquake loads, response spectra and dynamic structural analysis are typically used. This involves analyzing the response of the structure to a range of different earthquake ground motions and determining the resulting structural response.
By carefully considering all of these factors and utilizing the appropriate analysis tools, designers can ensure that their pile foundations are capable of safely and effectively withstanding earthquake loads. This is essential for ensuring the safety and stability of the structure and protecting against potential damage or collapse during seismic events.
Loads Due to Earth Pressures
The computation of loads caused by earth pressure is of significant importance in the design of substructure systems and basement walls. This process can begin at an early stage by utilizing earth pressure theory. However, a more detailed and final design requires the consideration of soil-structure interaction.
The accurate determination of loads caused by earth pressure is crucial in the design of basement walls and substructure systems. To this end, earth pressure theory can be applied at the initial stage of the design process. Nevertheless, a comprehensive and final design necessitates the application of soil-structure interaction.
Designers of substructure systems and basement walls must pay close attention to the impact of earth pressure loads. Earth pressure theory can offer insights into this phenomenon at the outset of the design process. Still, a thorough and conclusive design necessitates the incorporation of soil-structure interaction.
Loads Due to Ground Movements
When analyzing the lateral loads acting on a pile foundation, ground movement is a significant factor that must be taken into account. To properly understand the impact of ground movement on the foundation system, it is important to consider the interaction between the two. Rather than attempting to directly convert ground movement into an equivalent force, it is preferable to evaluate the magnitude of the ground movements themselves.
Consequently, it is essential to consider the relationship between the foundation system and the source of ground movement. By doing so, engineers can gain a better understanding of how the foundation system will respond to lateral loads. Additionally, this approach allows for a more comprehensive analysis of the various factors that may contribute to ground movement and the corresponding loads on the foundation.
In summary, when evaluating lateral loads on a pile foundation, it is crucial to consider the impact of ground movement. Rather than attempting to convert ground movement to an equivalent force, engineers should focus on evaluating the magnitude of the ground movements and their interaction with the foundation system. This approach will allow for a more accurate assessment of the loads acting on the foundation and enable the development of effective strategies for managing the associated risks.
3. Other Loads
Structural design of buildings requires consideration of various sources of loading, beyond just the typical weights of the building materials and intended occupancy. These additional sources of loading include snow, ice, thermal effects, major impacts, and explosions. Standards have been established to outline the requirements for considering these loads during the design process.
These standards aim to ensure that buildings are designed to withstand a range of potential loads, including those that may be less predictable or occur less frequently than others. Snow and ice loading, for example, may be of particular concern in areas with harsh winter weather, while thermal effects may need to be considered in regions with significant temperature variations. Major impacts and explosions are less common sources of loading, but can cause significant damage if not taken into account during the design phase.
Overall, the inclusion of these additional sources of loading in the design process helps to ensure that buildings are constructed to be as safe and durable as possible, capable of withstanding a range of potential stressors over their lifespan.