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    Cyclic Large Strain and Induced Pore Pressure Models for Saturated Clean Sands
    (ASCE-AMER SOC CIVIL ENGINEERS, 2012) Cetin, K. Onder; Bilge, H. Tolga
    Semiempirical probabilistic models are described to assess cyclic large strain and induced excess pore-water pressure responses of fully saturated clean sands. For this purpose, available cyclic simple shear and triaxial tests were compiled and studied. The resulting r(u) versus gamma, and gamma versus N databases are composed of 101 and 84 cyclic test data, respectively. Key parameters of the proposed r(u) and gamma models are defined as critical shear strain, relative density, effective confining stress, and equivalent number of loading cycles. Consistent with the maximum likelihood methodology, model coefficients were estimated by maximizing the likelihood function. For comparison purposes, the compiled database was again used to evaluate the performance of existing r(u) models. Both for comparison and calibration purposes, for each framework, two separate sets of limit-state models were used: model implemented with (1) the original and (2) the updated model coefficients. The model performances are assessed by simple statistics (i.e., mean and standard deviation) of residuals. It is concluded that existing models produce inconsistently biased predictions that vary in the range of 2.5 to 70%. The successes of the proposed and existing models are also assessed for the validation database composed of additional 10 cyclic test results. In addition to (1) repeated improved predictions, (2) differentiating contractive or dilative cyclic soil responses, and (3) incorporation of strain-dependent modulus degradation effects, the main advantage of the proposed methodology is the probabilistic nature of model predictions, which enables the incorporation of the model uncertainty into pore pressure generation predictions. DOI: 10.1061/(ASCE)GT.1943-5606.0000631. (C) 2012 American Society of Civil Engineers.
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    Performance-Based Assessment of Magnitude (Duration) Scaling Factors
    (ASCE-AMER SOC CIVIL ENGINEERS, 2012) Çetin, K. Önder; Bilge, H. Tolga
    More pronounced at relatively smaller magnitude events, significant variations to an extent of a factor of two in magnitude (duration) scaling factors (MSFs) explain the need to further study this issue, which is also recognized and recommended by the National Center for Earthquake Engineering Research Group. Inspired from this gap, the main motivation of this study is defined as to (1) comparatively assess the validity of existing magnitude scaling models and the accuracy of their predictions; (2) develop robust and practical to use semiempirical magnitude scaling models applied on CRR: separate sets for strain (cyclic mobility) or excess pore pressure (cyclic or flow liquefaction) problems. The writers' excess pore water pressure and shear strain accumulation models were used for the assessment of magnitude (duration) scaling factors. On the basis of the proposed framework, it is concluded that (1) MSFs are not only a function of number of equivalent loading cycles but increase with increasing r(u) or gamma(max) thresholds and decreasing dilational response (i.e., decreasing relative density and/or increasing effective stress states) of soil layers, (2) significantly different set of MSFs than the NCEER recommendations can be estimated for different combinations of gamma(max) (or r(u)), N-1,N-60,N-CS, sigma'(nu, 0), (3) for the assessment of critical structures (e.g., nuclear power plants), where significantly smaller shear strain performance targets are needed, use of existing models may produce significantly higher MSFs, leading to unconservative estimates of cyclic mobility potential. DOI: 10.1061/(ASCE)GT.1943-5606.0000596. (C) 2012 American Society of Civil Engineers.