221 research outputs found
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
Liquefaction-induced ground deformation and damage to piles in the 1995 Kobe Earthquake
A significant geotechnical feature of the 1995 Kobe earthquake was the widespread and massive liquefaction of reclaimed fills in the port area of Kobe. The liquefaction resulted in cyclic ground displacements of inland fills of 30-40 cm while lateral spreading towards the sea occurred in the waterfront area with a magnitude of 1-4 m at the quay walls. The excessive ground movements caused numerous failures and damage to pile foundations in the waterfront area. This paper summarizes the outcome of detailed field, laboratory and analytical investigations and highlights the key features of the liquefaction during the Kobe earthquake. Particular attention is given to liquefaction-induced ground displacements and to their effects on the performance of pile foundations
Effects of fines on undrained behaviour of sands
A series of monotonic and cyclic triaxial tests were performed on a
sand with fines sourced from Christchurch, New Zealand. The sand was sieved and
then mixed to give three soils with different fines contents. The undrained tests were
used to examine the effects of fines on the strain softening behaviour under
monotonic loading and liquefaction resistance in cyclic loading. Two reference states
were used as a basis for evaluation of the effects of fines: the relative density and the
steady state line within the state-concept framework for sand characterization. The
addition of fines to the sand base caused downward movement of the steady state
line in the Dr-p' plane (e-p' plane), and this effectively increased the potential for
strain softening or flow deformation. Samples prepared at an identical relative
density showed decreasing cyclic strength with increasing fines content. Conversely,
samples at an identical initial state relative to the steady state line showed increasing
cyclic strength with the fines content
Prediction of spatially distributed seismic demands in specific structures: Ground motion and structural response
The efficacy of various ground motion intensity measures (IMâs) in the prediction of
spatially distributed seismic demands (Engineering Demand Parameters, EDPâs) within a
structure is investigated. This has direct implications to building-specific seismic loss
estimation, where the seismic demand on different components is dependent on the location of
the component in the structure. Several common intensity measures are investigated in terms
of their ability to predict the spatially distributed demands in a 10-storey office building,
which is measured in terms of maximum interstorey drift ratios and maximum floor
accelerations. It is found that the ability of an IM to efficiently predict a specific EDP
depends on the similarity between the frequency range of the ground motion which controls
the IM and that of the EDP. An IMâs predictability has a direct effect on the median response
demands for ground motions scaled to a specified probability of exceedance from a ground
motion hazard curve. All of the IMâs investigated were found to be insufficient with respect
to at least one of magnitude, source-to-site distance, or epsilon when predicting all peak
interstorey drifts and peak floor accelerations in a 10-storey RC frame structure. Careful
ground motion selection and/or seismic demand modification is therefore required to predict
such spatially distributed demands without significant bias
Intensity measures for the seismic response of pile foundations
In this study the efficacy of various ground motion intensity measures for the seismic
response of pile foundations embedded in liquefiable and non-liquefiable soils is investigated.
A soil-pile-structure model consisting of a two-layer soil deposit with a single pile and a
single degree-of-freedom superstructure is used in a parametric study to determine the salient
features of the seismic response of the soil-pile-structure system. A suite of ground motion
records scaled to various levels of intensity are used to investigate the full range of pile
behaviour, from elastic response to failure. Various intensity measures are used to inspect
their efficiency in predicting the seismic demand on the pile foundation for a given level of
ground motion intensity. It is found that velocity-based intensity measures are the most
efficient in predicting the pile response, which is measured in terms of maximum curvature or
pile-head displacement. In particular, velocity spectrum intensity (VSI), which represents the
integral of the pseudo-velocity spectrum over a wide period range, is found to be the most
efficient intensity measure in predicting the seismic demands on the pile foundation. VSI is
also found to be a sufficient intensity measure with respect to earthquake magnitude, sourceto-
site distance, and epsilon, and has a good predictability, thus making it a prime candidate
for use in seismic response analysis of pile foundations
Effects of soil-foundation-structure interaction on seismic structural response via robust Monte Carlo simulation
Uncertainties involved in the characterization and seismic response of soil-foundation-structure systems along with the inherent randomness of the earthquake ground motion result in very complex (and often controversial) effects of soil-foundation-structure interaction (SFSI) on the seismic response of structures. Conventionally, SFSI effects have been considered beneficial (reducing the structural response), however, recent evidence from strong earthquakes has highlighted the possibility of detrimental effects or increase in the structural response due to SFSI. This paper investigates the effects of SFSI on seismic response of structures through a robust Monte Carlo simulation using a wide range of realistic SFS systems and earthquake input motions in time-history analyses. The results from a total of 1.36 million analyses are used to rigorously quantify the SFSI effects on structural distortion and total horizontal displacement of the structure, and to identify conditions (system properties and earthquake motion characteristics) under which SFSI increases the structural response
Prediction of spatially distributed seismic demands in specific structures: Structural response to loss estimation
A companion paper has investigated the effects of intensity measure (IM) selection in
the prediction of spatially distributed response in a multi-degree-of-freedom structure. This
paper extends from structural response prediction to performance assessment metrics such as:
probability of structural collapse; probability of exceeding a specified level of demand or
direct repair cost; and the distribution of direct repair loss for a given level of ground motion.
In addition, a method is proposed to account for the effect of varying seismological properties
of ground motions on seismic demand that does not require different ground motion records to
be used for each intensity level. Results illustrate that the conventional IM, spectral
displacement at the first mode, Sde(T1), produces higher risk estimates than alternative
velocity-based IMâs, namely spectrum intensity, SI, and peak ground velocity, PGV, because
of its high uncertainty in ground motion prediction and poor efficiency in predicting peak
acceleration demands
Improved seismic hazard model with application to probabilistic seismic demand analysis
An improved seismic hazard model for use in performance-based earthquake engineering is presented. The model is an improved approximation from the so-called 'power law' model, which is linear in log-log space. The mathematics of the model and uncertainty incorporation is briefly discussed. Various means of fitting the approximation to hazard data derived from probabilistic seismic hazard analysis are discussed, including the limitations of the model. Based on these 'exact' hazard data for major centres in New Zealand, the parameters for the proposed model are calibrated. To illustrate the significance of the proposed model, a performance-based assessment is conducted on a typical bridge, via probabilistic seismic demand analysis. The new hazard model is compared to the current power law relationship to illustrate its effects on the risk assessment. The propagation of epistemic uncertainty in the seismic hazard is also considered. To allow further use of the model in conceptual calculations, a semi-analytical method is proposed to calculate the demand hazard in closed form. For the case study shown, the resulting semi-analytical closed form solution is shown to be significantly more accurate than the analytical closed-form solution using the power law hazard model, capturing the 'exact' numerical integration solution to within 7% accuracy over the entire range of exceedance rat
Interpretation from large-scale shake table tests on piles undergoing lateral spreading in liquefied soils
doi: 10.1016/j.soildyn.2005.02.018Results from a benchmark test on full-scale piles are used to investigate the response of piles to lateral spreading. In the experiment, two single piles, a relatively flexible pile that moves together with the surrounding soil and a relatively stiff pile that does not follow the ground movement have been subjected to large post-liquefaction ground displacement simulating piles in laterally spreading soils. The observed response of the piles is first presented and then the results are used to examine the lateral loads on the pile from a non-liquefied soil at the ground surface and to evaluate the stiffness characteristics of the spreading soils. The measured ultimate lateral pressure from the crust soil on the stiff pile was about 4.5 times the Rankine passive pressure. The back-calculated stiffness of the liquefied soil was found to be in the range between 1/30 and 1/80 of the initial stiffness of the soil showing gradual decrease in the course of lateral spreading
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