4.2 Geotechnical Earthquake Engineering
Effects of Dynamic Soil-Structure Interaction on Performance Level of Moment Resisting Buildings Resting on Different Types of Soil
Hamid Reza Tabatabaiefar, Behzad Fatahi & Bijan Samali
ABSTRACT: In this study, two structural models comprising five and fifteen storey moment resisting building frames are selected in conjunction with three different soil deposits with shear wave velocity less than 600m/s. The design sections are defined after applying dynamic nonlinear time history analysis based on inelastic design procedure using elastic-perfectly plastic behaviour of structural elements. These frames are modelled and analysed employing Finite Difference approach using FLAC 2D software under two different boundary conditions namely fixed-base (no soil-structure interaction), and considering soil-structure interaction. Fully nonlinear dynamic analyses under the influence of different earthquake records are conducted and the results of inelastic behaviour of the structural models are compared. The results indicate that the inter-storey drifts of the structural models resting on soil types De and Ee (according to the Australian standard) substantially increase when soil-structure interaction is considered for the above mentioned soil types. Performance levels of the structures change from life safe to near collapse when dynamic soil-structure interaction is incorporated. Therefore, the conventional inelastic design procedure excluding SSI is no longer adequate to guarantee the structural safety for the building frames resting on soft soil deposits.
Lateral Loading Test on Spread Foundation in an Existing Reinforced Concrete School Building
Toshikazu Kabeyasawa & Toshimi Kabeyasawa
ABSTRACT: Lateral loading test on the spread foundation of an existing school building was conducted in April 2010 to identify the lateral stiffness of the foundation. A part of the building was separated through the foundation to the roof level, to which static or impact load was applied horizontally at the base foundation level. The objective, plan, testing method and the detailed test results are reported in this paper. The test results were compared with analytical model based on boring investigation into the soil.
Undrained Cyclic Shear Behaviour of Partially Saturated Decomposed Granite Soil
R.P. Orense, M. Hyodo & N. Yoshimoto
ABSTRACT: Past large-scale earthquakes have shown that even unsaturated soils can undergo flow-type deformation similar to that observed in liquefied fully-saturated ground. Therefore, it is necessary to understand the shear behaviour of partially saturated soils when subjected to cyclic loading. For this purpose, undrained cyclic triaxial tests were performed on decomposed granite soil under different saturation ratios and confining pressures to examine its cyclic shear characteristics. Since the apparatus used could not measure suction directly, we estimated the magnitude of suction using the pore air pressure determined from the volume change of the specimen based on Boyle-Charles law. From the test results, the mechanism of liquefaction in partially saturated soil was explained.
Estimation of Steady State of Sands Containing Crushable Materials
Yasuyo Hosono & Mitsutoshi Yoshimine
ABSTRACT: Mica is known to have thin plate shape and have high crushability and contractiveness. The authors performed undrained triaxial shear tests and examined the steady state line (SSL) of sands containing mica. At first, we plotted void ratio versus effective mean stress to examine the SSL of sands with varying mica contents. When the mica content was increased, the SSL shifted to an upper location in the plot. At the same void ratio, the degree of compression is different for each mica contents, and it was inadequate to estimate the SSL using void ratio only. Secondly we tried to arrange by the SSL using relative density, but the maximum and minimum void ratios including the effects of confining stress, so it was also difficult also to estimate the SSL using relative density. In this paper, we performed compaction tests on the material, and then we found that the steady states were more or less uniquely related to the degree of compaction irrespective of mica contents, expect 0% of mica content.
Snap-Back Testing for Estimation of Nonlinear Behaviour of Shallow and Pile Foundations
M.J. Pender, T.B. Algie, R.P. Orense, L.M. Wotherspoon & N.M. Sa’Don
ABSTRACT: We are working on the development of methods for analysing the earthquake response of foundations that make use of Soil-Foundation-Structure-Interaction (SFSI) as a means of incorporating nonlinear soil deformation effects and nonlinear geometrical effects into the earthquake resistant design of foundations. There are three challenges in this work. First, to incorporate adequately the nonlinear response of the soil during the earthquake. Second, to account for geometrical nonlinearity during the earthquake - that is loss of contact between various parts of the foundation and the underlying and/or adjacent soil. Third, to obtain appropriate values for the soil parameters which describe the nonlinear response of the foundations. The main thrust of this paper is to show how snap-back testing is a most effective means of evaluating nonlinear soil behaviour. We consider that snap-back testing is more convenient than using a shaking machine which applies sinusoidal excitation. The results from rocking of a shallow foundation and cyclic lateral loading of a single pile enable damping and stiffness to be estimated at increasing levels of lateral loading.
Implications of Soil Variability for Performance Based Shallow Foundation Design
J.C.W. Toh, M.J. Pender & R. McCully
ABSTRACT: This paper considers the earthquake response of shallow foundations supported on saturated clay. It examines the implications of natural variability in the undrained shear strength and stiffness of a cohesive soil for the earthquake response of a shallow foundation. Calculating the response of a simple structure-foundation system to seven different earthquake records, all scaled to the hazard spectrum given in NZS1170, the paper shows the implications of soil variability for the maximum actions imposed on a shallow foundation, the natural period of the system, and permanent foundation displacements. The paper concludes that, despite the level of soil variability considered covering the range expected in natural soil, variations in the scaled earthquake records have a much larger effect on calculated permanent foundation displacements.
Recommendations for the Calibration of Nonlinear and Equivalent Linear Ground Response Analyses for Soft and Liquefiable Sites.
Eng Sew Aw, Marc Ryan, Kevin Burlingham, Zhi-Liang Wang, John Egan & Annmarie Behan
ABSTRACT: Seismic site response analyses are frequently performed to help characterize site-specific amplification effects of soil profiles when subjected to earthquake loading for critical structures such as nuclear power plants and lifeline infrastructure. Equivalent-linear models (EQL), as implemented in SHAKE, are commonly used in practice due to their simplicity, but cannot capture nonlinearity (NL) due to large strains or excess pore pressure development leading to liquefaction within the soil deposits. More sophisticated NL models such as the bounding surface hypoplasticity model, as implemented in SUMDES, can capture nonlinear effects and liquefaction behaviour of soils, but require significantly more calibration with field and laboratory tests. This paper discusses the advantages and limitations of the EQL and NL models for soft and liquefiable soils using lessons learned from case studies. EQL analyses may significantly overestimate design accelerations and underestimate the soil displacement as compared to the NL model; these deviations may affect the structure and foundation design. Recommendations are provided on how to perform the NL and EQL analyses. Both models are sensitive to the selection of soil properties, material damping, and low strain viscous damping. These recommendations should provide the user with the knowledge and confidence in evaluating the application of NL and EQL models.