Assessment of seismic performance of soil-structure systems

Invited LectureThree different approaches for assessment of seismic performance of pile foundations (and soilstructure systems in general) are discussed in this paper. These approaches use different models, analysis procedures and are of vastly different complexity. All three methods are consistent with the performance-based design philosophy according to which the seismic performance is assessed using deformational criteria and associated damage levels. It is shown that even though the methods nominally have the same objective, they focus on different aspects in the assessment and provide alternative performance measures. Key features of the three approaches and their unique contribution in the assessment of seismic performance of soil-structure systems are demonstrated using a case study

Design Prediction and Performance of Piles for Seismic Loads

Pile foundations are regarded as a safe alternative for supporting structures in seismic areas. The performance of piles depends on soil profile, pile and earthquake parameters. The soils may also be prone to liquefaction. In non-liquefying soils the shear modulus degrades with increasing strain or displacements. Material damping increases with increasing strain or displacement. Stiffness of single pile and pile groups are needed for different modes of vibration; e.g., vertical vibrations, horizontal sliding in x or y direction, rotation about x or y axis and torsion. Group action is generally accounted for by including interaction factors. Pile response in any mode of vibration is determined from principles of structural dynamics. In liquefiable soils, the liquefaction may lead to substantial increases in pile cap displacements above those for the non-liquefied case. Down-drag due to liquefied soil may also pose problems. After liquefaction, if the residual strength of the soil is less than the static shear stresses caused by a sloping site such as a river bank, lateral spreading or down slope displacements may exert damaging pressures against the piles as observed during the 1964 Niigata and the 1995 Kobe earthquakes. The paper presents state of the art on analysis and design of piles subjected to seismic loading

Seismic Performance and Design of Bridge Foundations in Liquefiable Ground with a Frozen Crust

INE/AUTC 12.3

Prediction of pile response to lateral spreading by 3-D soil-water coupled dynamic analysis: shaking in the direction of ground flow

Numerical predictions of a series of shake table tests are presented in this paper in order to examine the accuracy of a 3-D effective stress analysis in predicting the behavior of piles subjected to liquefaction-induced ground flow. For a rigorous assessment of the analysis, “Class B” predictions are reported in which numerical and constitutive model parameters were set before the event, and the target motion was used as an input motion in the analysis. Modeling of the stress-strain behavior of sand, identification of the initial stress state and critical numerical parameters in the 3-D seismic analysis of the soil-pile system are discussed in detail. Combined effects of kinematic loads due to large lateral ground movement and inertial loads on pile behavior are examined through a series of tests using different shaking direction, excitation amplitude and mass of the footing (load from the superstructure). By and large, very good agreement was obtained between the predicted and measured peak responses of the pile foundation, whereas the analysis underestimated the displacements of the sheet-pile wall and was less accurate in predicting the residual deformation of the foundation piles. Reasons for these discrepancies and limitations of the analysis method are discussed

The Effects of Long-Duration Subduction Earthquakes on Inelastic Behavior of Bridge Pile Foundations Subjected to Liquefaction-Induced Lateral Spreading

Effective-stress nonlinear dynamic analyses (NDA) were performed for a large-diameter reinforced concrete (RC) pile in multi-layered liquefiable sloped ground. The objective was to assess the effects of earthquake duration on the combination of inertia and liquefaction-induced lateral spreading. A parametric study was performed using input motions from subduction and crustal earthquakes covering a wide range of motion durations. The NDA results showed that the pile head displacements increased under liquefied conditions, compared to nonliquefied conditions, due to liquefaction-induced lateral spreading. The NDA results were used to develop a displacement-based equivalent static analysis (ESA) method that combines inertial and lateral spreading loads for estimating elastic and inelastic pile demands

A Simplified Approach for the Evaluation of Kinematic Pile Bending

There are two sources of loading of the pile by the earthquake: “inertial” loading of the pile head caused by the lateral forces imposed on the superstructure, and “kinematic” loading along the length of the pile caused by the lateral soil movements developed during the earthquake. Seismic codes prescribe that piles have to be designed for soil deformations arising from the passage of seismic waves which impose curvatures and thereby lateral strains on the piles along their whole length. Accepting these lines, the new Italian seismic normative (NTC, 2008) specify that kinematic effects should be taken into account in the design of pile foundations. Pseudo-static approaches for the seismic analysis of pile foundations are attractive for practicing engineers because they are simple when compared to difficult and more complex dynamic analyses. Thus, in the paper a simplified numerical model for the analysis of the behavior of a single pile subjected to static loadings and/or to lateral soil movements based on the “p-y” subgrade reaction method has been adopted. The approach involves two main steps: first a free-field site response analysis is carried out to obtain the soil displacements along the pile; next a static load analysis is carried out for the pile subjected to the maximum free-field soil displacements at each node along its length and the static loading at the pile head based on the maximum ground surface acceleration

The Effects of Long-Duration Subduction Earthquakes on Inelastic Behavior of Bridge Pile Foundations Subjected to Liquefaction-Induced Lateral Spreading

Effective-stress nonlinear dynamic analyses (NDA) were performed for a large-diameter reinforced concrete (RC) pile in multi-layered liquefiable sloped ground. The objective was to assess the effects of earthquake duration on the combination of inertia and liquefaction-induced lateral spreading. A parametric study was performed using input motions from subduction and crustal earthquakes covering a wide range of motion durations. The NDA results showed that the pile head displacements increased under liquefied conditions, compared to nonliquefied conditions, due to liquefaction-induced lateral spreading. The NDA results were used to develop a displacement-based equivalent static analysis (ESA) method that combines inertial and lateral spreading loads for estimating elastic and inelastic pile demands