The area of earthquake engineering includes the study of soil-structure interaction (SSI). It is critical to remember that the structural response is primarily caused by soil-structure interaction forces, which have an effect on the structure. This is a type of earthquake excitation. When compared to free-field ground motion, a committee of engineering research works with the investigation of soil-structure interaction only when these forces have a substantial effect on the basement motion. The motion recorded on the surface of the soil without the involvement of the structure is known as free-field ground motion. The interaction of three interconnected systems determines the structural reaction to an earthquake.
The approach of analysing the collective reaction of the three linked systems listed above for a particular ground motion is called soil-structure interaction analysis. The soil-structure interaction can be characterised as the process through which the soil’s response influences the structure’s motion, and the structure’s motion affects the soil’s response. This is a situation in which structure and ground displacements occur independently of one another. The main interaction forces that might arise for any structure are soil-structure forces. However, they are not capable of altering soil motion under all circumstances.
Considerations in Soil-Structure Interaction Effects
When a structure’s foundation is assumed to be rigid, it is said to have no soil-structure interaction effects. Even though the interaction force has an impact on the foundation, this situation is now considered. The interaction forces’ impact on soil motion will be determined by: The strength of the force The soil foundation’s flexibility The value of interaction forces can be estimated using the base mat acceleration and the structure’s inertia. For a given soil site and free-field seismic excitation, the heavier the structure, the greater the soil-structure interaction effects. The majority of civil structures, whether they are built on hard or soft soil, exhibit no signs of SSI impacts.
As previously stated, the SSI effects are more prevalent in heavy structures, such as dams and nuclear power plants (NPP) reactor buildings. We can deduce that the research of soil interaction in earthquake engineering was primarily created and applied to these domains of building. The soil flexibility is another factor in the soil-structure interaction effects. The softer the soil, the greater the likelihood of SSI impacts. This is for a structure and a location that are both subject to free-field seismic stimulation. The soil shear module is equal to the product of the mass density of the soil and the square of the shear wave velocity. In practise, the soil mass density will be around 2,0 t/m3. As a result, the primary.
Soil-Structure Interaction and Structural Response
According to traditional beliefs, the soil-structure interaction has beneficial implications on structural response. In most structural design regulations, the effect of SSI is recommended to be ignored in the seismic analysis of the structure. This proposal is based on the misconception that the SSI improves the structure’s reactivity and hence increases the safety margins. When the impacts of soil structure interaction are taken into account, a more flexible structural design can be achieved. This contributes to the structure’s natural period being extended. When compared to a rigid construction, this creates a more stable framework. Incorporating SSI effects into structural design aids in increasing the structure’s damping ratio.
For conservative design procedures, this study is limited or ignored. The SSI analysis is really difficult. The structure analysis will be less complicated as a result of the neglection. This disproves the popular belief that SSI effects are beneficial to structures. In fact, SSI has the potential to harm structures. The SSI effect should not be overlooked when designing the superstructure and substructure. What consequences does soil structure interaction have?The SSI’s impacts are largely focused on its negative consequences. Even while studies have shown that designing based on soil structure interaction lengthens the time period, lengthening the time period is not necessarily a good thing.
When seismic waves travel through soft soil sediments, they become longer. This causes the natural period to rise, resulting in resonance. This occurs when a vibration has a long period.As the natural period lengthens, so does the demand for ductility. This could lead to irreversible deformation and soil failure, worsening the structural seismic response even more. When a structure is subjected to seismic force (seismic excitation), there is contact between the soil and foundation, causing ground motion to vary. There are two sorts of phenomena or consequences that might result from soil structure interaction (As per FEMA P-750, NEHRP).
The free-field motion is the soil displacement induced by earthquake ground motion. The foundation, which is located on the dirt, does not follow this free field motion. The inability of the foundation to sink with the free field motion of the earth causes the kinematic interaction. Interaction of Inertial Forces The inertial interaction is the additional deformation in the soil induced by the transmission of inertial force to the soil by the superstructure. The kinematic effect of SSI is more obvious when the ground shaking is low. The period lengthens and the radiation damping increases as a result of this. When the shaking becomes more intense, the soil modulus deterioration limits the radiation damping.
The inertial dampening is more noticeable in this case. As a result, excessive displacements near the ground surface will occur. The piling foundations will be harmed as a result of this.The analysis and research from previous and recent earthquakes demonstrate that the following factors influence the overall response of the structure:The foundation’s response The soil’s reaction When huge constructions are subjected to earthquakes, the SSI have become a major cause of failure. The Hanshin Expressway was destroyed in 1995 by the Kobe earthquake.The following are the factors that are linked to the above-mentioned effects:
Stiffness and Damping of the foundation
Moments, torsion, and base shear are all produced when a vibrating structure creates inertia force. These are the forces at work at the soil-foundation contact, causing displacements and rotation. The produced displacement and rotation are a result of the soil and foundation’s elasticity. This flexibility is the foundation of the entire structure’s stability. Energy is dissipated as a result of the displacements caused. This has an impact on the overall dampening of the system. The inertial interaction effects are named by the fact that all of these effects are more anchored with structural inertia. There are differences between free-field motions and foundation input motions.Because of the following factors, these movements may differ: Interaction of Kinematics Displacements between the foundation and the free field in relation to each other.
The rigid foundation elements that are placed above or below the ground surface cause the foundation motions. In the lack of structure, and the foundation inertia generates the kinematic interaction, this is done to have divergence from the free field motion. Deformation of the Foundation The flexural, axial, and shear deformations are caused by the forces and displacements imparted to the foundation elements by the superstructure or the soil medium.
Analysis and Applications in Design
These are the requirements for which the foundation’s components must be designed. These impacts are particularly pronounced in the case of rafts and piles, which serve as foundations.Soil Structure Interaction Analysis Two methods of analysis can be used to measure the above-mentioned interactions. They are the following: Substructure Approach to Direct Analysis Direct Analysis.
Various formulations based on the substructure method are addressed, ranging from a discrete model with springs, dashpots, and masses to boundary-element methods with convolution integrals involving either the dynamic-stiffness coefficients or the Green’s functions in the time domain via the iterative hybrid-frequency-time-domain analysis procedure with the iterative hybrid-frequency-time-domain analysis procedure with the iterative hybrid-frequency-time-domain analysis procedure with the iterative.
A simple three-dimensional soil–structure interaction (SSI) model is proposed.
Conventional design practice for ordinary structures usually neglect soil-structure interaction effects.
Explanation: Soil structure is an important factor which influences many soil properties such as permeability.