Soil Structure Interaction [SIG]
- Dogukan Kilic
Analyzing a pile under lateral loading is a complex procedure because the soil's reaction at any point on the pile depends on the pile's deflection. Conversely, the pile's deflection relies on the soil's resistance. As a result, determining the response of a pile under lateral loading falls into a category of soil-structure interaction issues. It is essential to meet the conditions of compatibility and equilibrium between the pile and the soil, as well as between the pile and the superstructure. Therefore, in order to meet this equilibrium condition and properly analyze a pile foundation, a nonlinear relationship needs to be applied.
To perform nonlinear analysis in stage construction, click 'Construction > Construction Stage' in the navigation tree view, then activate the Nonlinear option.
Lateral soil-structure interaction is modeled with nonlinear p-y curves. Axial interactions are modeled with nonlinear t-z curves for soil skin-friction and q-z curves for soil end bearing (tip) resistance.
Lateral Soil Resistance (p-y curves)
The Fig. 1 displays the stresses acting on a cyclindrical pile under the affect of lateral loading. When unloaded, there is a uniform distribution of unit stresses perpendicular to the pile wall, depicted in Fig. 1a. As the pile deflects by a distance y, the stress distribution resembles Fig. 1b, with a resisting force p. Stresses decrease behind the pile and increase in front, where some unit stresses include both normal and shear components as displaced soil attempts to move around the pile.
To model the lateral resistance of soil p-y curves are assigned to the pile nodes. p-y curve is a function of pile dimensions, soil properties and depth below the ground surface. When it comes to the finite element analysis, each pile node has to be assigned with a unique p-y curve according to these parameters.
OpenBrIM has a library of soil material models for p-y curves. OpenBrIM can automatically assign these curves to nodes based on the pile's dimensions and soil properties at specific depths. However, engineers also have the option to input custom curves for the piles if desired.
Axial Soil Resistance (t-z & q-z curves)
The stress-strain behavior of pile can be explained by three main mechanism: soil skin-friction, soil end-bearing (tip) and axial deformation of the pile. The t-z curve simulates the nonlinear stress-strain behavior in soil by using nonlinear stiffness curves, known as t-z curves for soil skin friction and q-z curves for soil end bearing (tip) resistance.
To model the soil skin friction, the pile is divided into segments, each comprising two pile elements and one soil spring. Each soil spring represents the effect of skin friction between the pile and the soil. The pile elements are arranged such that the soil displacement used to calculate the skin friction is based on the midpoint of the adjacent pile elements. In other words, one soil spring accounts for half the length of each connected element.
To represent the soil end bearing (tip) resistance, cross-sectional area at the tip of the pile is multpilied with unit tip bearing resistance (q).
Similar to the p-y curves, OpenBrIM has a library of soil material models for t-z and q-z curves. OpenBrIM can automatically assign these curves to nodes based on the pile's dimensions and soil properties at specific depths. However, engineers also have the option to input custom curves for the piles if desired.