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Author: Ravindranath Salimath Publisher: ISBN: Category : Clay soils Languages : en Pages : 234
Book Description
This thesis presents a better understanding of nonlinear moment-rotation behavior of shallow foundations resting on stiff clay based on the results of 3D finite element analysis and large scale field experiments. In past decades several researchers have carried out both numerical modelling and experimental studies to understand the rocking behavior and earthquake response of shallow foundations. Algie (2011) performed a series of snap-back tests to study the moment-rotation response of shallow foundations on stiff residual clay. The results showed that snap-back testing is an effective tool for obtaining insight into the nonlinear behavior and earthquake response of shallow foundations. Tests also gave a good assessment of magnitude of damping of rocking shallow foundations resting on stiff clay. Another advantage was that the static moment-rotation curves are obtained during the initial pull-back phase of the test and it turns out that the static response provides considerable insight into the dynamic behavior of the foundation. This first half of the thesis comprises of development of a 3D finite element model in , its validation with available experimental data and the analysis results. The moment-rotation behavior predicted from PLAXIS 3D converged with the experimental data and showed considerable nonlinearity once the footing uplift and partial loss of contact occurs. The contact pressure distribution under the footing showed that the footing uplift and nonlinearity is initiated when the applied moment exceeds around 0.45Mult. The analysis results also showed that direction of the applied moment is important as footing exhibited considerably different rotational stiffness about short and long axis. The analyses also showed that effect of footing L/B ratio, static initial factor of safety and the undrained shear strength of the soil considerably affect the non-linear moment-rotation behavior. The evaluation of actual footing contact area during rocking is critical for assessment of seismic performance of the rocking shallow foundations and it depends on the ultimate moment capacity of the footing. Based on numerical modelling results, a new modified hyperbolic equation was proposed to evaluate nonlinear moment-rotation response of footings on stiff clay incorporating the effects of L/B ratio, direction of the applied moment and undrained shear strength. The remaining half of the thesis deals with field experiments on surface shallow foundations resting on stiff residual clay. The experiments were performed on a site located in Silverdale, Auckland. The field experiments included slow-cyclic and snap-back tests on two different sets of footing configurations to simulate static non-linear moment-rotation response and footing rocking. For this study snap-backs were carried out in alternating directions, thus simulating the back and forth motion experienced during an earthquake. The test rig had two parallel foundation strips; in one case the longitudinal axis of the footings was in the plane of the rocking, in the other the footings were oriented perpendicular to the plane of the rocking and placed at the outer edges of the rig. This means that results are obtained for rocking in which there is a gradual uplifting of the foundation strip (foundation longitudinal axis in the plane of rocking), and when the entire footing lifts clear of the underlying soil (foundation strips at the outer edges of the rig). The purpose here was to investigate the validity of the damping mechanism proposed by Housner in his paper on inverted pendulum structures gave the best representation of the foundation damping; so the tests duplicate the partial uplifting of foundations and also foundations that lift completely clear of the underlying soil. The moment-rotation curves from the tests showed that the small strain stiffness is only around 25% of the theoretical stiffness. Also, it was observed the for the case of snap-back tests at higher vertical loads, the damping ratio was generally higher around 20% due to large footing rotation and partial detachment with underlying soil at the time of snap. The direction of rocking and footing L/B ratio also affect the magnitude of damping. Lastly the conclusions from both the finite element analysis and experimental studies are presented in detail. The need of further research and future scope of work is outlined.
Author: Ravindranath Salimath Publisher: ISBN: Category : Clay soils Languages : en Pages : 234
Book Description
This thesis presents a better understanding of nonlinear moment-rotation behavior of shallow foundations resting on stiff clay based on the results of 3D finite element analysis and large scale field experiments. In past decades several researchers have carried out both numerical modelling and experimental studies to understand the rocking behavior and earthquake response of shallow foundations. Algie (2011) performed a series of snap-back tests to study the moment-rotation response of shallow foundations on stiff residual clay. The results showed that snap-back testing is an effective tool for obtaining insight into the nonlinear behavior and earthquake response of shallow foundations. Tests also gave a good assessment of magnitude of damping of rocking shallow foundations resting on stiff clay. Another advantage was that the static moment-rotation curves are obtained during the initial pull-back phase of the test and it turns out that the static response provides considerable insight into the dynamic behavior of the foundation. This first half of the thesis comprises of development of a 3D finite element model in , its validation with available experimental data and the analysis results. The moment-rotation behavior predicted from PLAXIS 3D converged with the experimental data and showed considerable nonlinearity once the footing uplift and partial loss of contact occurs. The contact pressure distribution under the footing showed that the footing uplift and nonlinearity is initiated when the applied moment exceeds around 0.45Mult. The analysis results also showed that direction of the applied moment is important as footing exhibited considerably different rotational stiffness about short and long axis. The analyses also showed that effect of footing L/B ratio, static initial factor of safety and the undrained shear strength of the soil considerably affect the non-linear moment-rotation behavior. The evaluation of actual footing contact area during rocking is critical for assessment of seismic performance of the rocking shallow foundations and it depends on the ultimate moment capacity of the footing. Based on numerical modelling results, a new modified hyperbolic equation was proposed to evaluate nonlinear moment-rotation response of footings on stiff clay incorporating the effects of L/B ratio, direction of the applied moment and undrained shear strength. The remaining half of the thesis deals with field experiments on surface shallow foundations resting on stiff residual clay. The experiments were performed on a site located in Silverdale, Auckland. The field experiments included slow-cyclic and snap-back tests on two different sets of footing configurations to simulate static non-linear moment-rotation response and footing rocking. For this study snap-backs were carried out in alternating directions, thus simulating the back and forth motion experienced during an earthquake. The test rig had two parallel foundation strips; in one case the longitudinal axis of the footings was in the plane of the rocking, in the other the footings were oriented perpendicular to the plane of the rocking and placed at the outer edges of the rig. This means that results are obtained for rocking in which there is a gradual uplifting of the foundation strip (foundation longitudinal axis in the plane of rocking), and when the entire footing lifts clear of the underlying soil (foundation strips at the outer edges of the rig). The purpose here was to investigate the validity of the damping mechanism proposed by Housner in his paper on inverted pendulum structures gave the best representation of the foundation damping; so the tests duplicate the partial uplifting of foundations and also foundations that lift completely clear of the underlying soil. The moment-rotation curves from the tests showed that the small strain stiffness is only around 25% of the theoretical stiffness. Also, it was observed the for the case of snap-back tests at higher vertical loads, the damping ratio was generally higher around 20% due to large footing rotation and partial detachment with underlying soil at the time of snap. The direction of rocking and footing L/B ratio also affect the magnitude of damping. Lastly the conclusions from both the finite element analysis and experimental studies are presented in detail. The need of further research and future scope of work is outlined.
Author: Thomas Brian Algie Publisher: ISBN: Category : Foundations Languages : en Pages : 348
Book Description
The most recent version of the New Zealand design and loadings standard eliminated a clause for the design of rocking foundations. This thesis addresses that clause by presenting a strong argument for rocking shallow foundations in earthquake resistant design. The goals of the research were to perform large scale field experiments on rocking foundations, develop numerical models validated from those experiments, and produce a design guide for rocking shallow foundations on cohesive soil. Ultimately, this thesis investigates the nonlinear rotational behaviour of shallow foundations on cohesive soil. Field experiments were performed on an Auckland residual soil, predominantly clay. The experiment structure - a large scale steel frame - was excited first by an eccentric mass shaker and second by a quick release (snap-back) method. The results show that rocking foundations produce highly nonlinear moment-rotation behaviour and a well defined moment capacity. A hyperbolic equation is proposed in Chapter 4 utilising the initial stiffness and moment capacity to predict nonlinear pushover response. The results show that the initial stiffness should be based on an 'operational soil modulus' rather than a small strain soil modulus. Therefore, the reduction factor from the small strain modulus was around 0.6 for the experiment testing. Additionally, the experiments showed that rocking foundations demonstrate significant damping; snap-back experiments revealed an average damping ratio of around 30%. Experiment data validated two numerical models developed for this study: one, a finite element model in Abaqus and the other, a spring bed model in OpenSEES. The models showed that both forms of nonlinearity in rotating shallow foundations - geometric nonlinearity and material nonlinearity - should be considered in shallow foundation analysis. These models also confirmed the need for an 'operational soil modulus' on shallow foundation rocking, and analysis of varying vertical loads suggested that this reduction factor is dependent on the vertical factor of safety of the foundation. Lastly, two design methods are presented, a displacement-based method and a forcebased method, and two examples of rocking shear walls are given. The displacementbased method is the recommended option, and it is shown that design displacements and rotations compare well to time history analyses performed using the validated OpenSEES model.
Author: Andreas Gerasimos Gavras Publisher: ISBN: 9781658413060 Category : Languages : en Pages :
Book Description
The concept of intentional mobilization of controlled soil inelasticity and foundation uplift, as a rational and economical seismic protection strategy, has much matured as a result of extensive research over the last two decades. This dissertation provides further evidence and tools that contribute toward the implementation of rocking foundations in practice with emphasis on bridges applications. In response to the lack of high-quality experimental data on the seismic response of full-scale rocking foundations, a series of large-scale tests, involving reinforced concrete bridge columns with footings embedded into dense sand, was completed. The experimental program investigated the alignment of the footings to the shaking direction and included varying groundwater table elevations and footing backfill conditions. This study yielded results that corroborate the cumulative centrifuge experiments-based understanding of the dynamic behavior of rocking foundations and confirmed that loose and dry cohesionless backfill soil can ravel under the rocking footing at large rotations, resulting in reduced settlement, reduced stiffness degradation, enhanced energy dissipation, but potentially also causing permanent rotations. A modified beam-on-nonlinear-Winkler-foundation (BNWF) modeling scheme, representing a departure from previous attempts to capture both the vertical and rotational nonlinear responses of a shallow footing, was proposed. The considered scheme is cast around the foundation critical contact area ratio and the trilinear moment-rotation backbone of Deng et al. (2014), and is calibrated to four physical model tests. The calibrated scheme captured successfully the experimental response in terms of moment-rotation envelope, hysteretic energy dissipation and recentering across a range of rotation amplitudes and rocking-induced soil inelasticity. Settlement accumulation was captured for rotation amplitudes less than 0.025 rad but was overestimated at larger rotations. Displacement-based assessment (DBA) guidelines for bridges with rocking foundations were developed. Nonlinear response history analyses of idealized footing-column-mass models, using the calibrated BNWF model, were conducted to determine the fraction of the area-based hysteretic damping that is effective in reducing the seismic displacement demand and evaluate the importance of P-[delta] effects, in refinement of the Deng at al. (2014) study for elastic cantilever bridge columns on rocking foundations. The DBA guidelines were subsequently extended to bridge bents that also include plastic hinging at the top of the column. The bent-level equivalent-linear properties are derived, considering independent foundation-rocking and column-hinging subsystems and constant location for the contraflexure point based on their inelastic capacities. Comparison with a detailed nonlinear pushover analysis-based procedure for representative cases showed that the simplified procedure is practical and sufficient for estimating the total displacement demand, but less accurate at the component level. Lastly, provisions for multi-span bridges that follow a system-level approach for the longitudinal direction and an isolated-bent approach with modified mass and stiffness characteristics for the transverse direction were presented. The guidelines were applied to hypothetically redesign two existing bridges with rocking foundations. Numerical analyses of the full bridges demonstrated the accuracy of the procedure, except for the transverse response of bridges with high strength and energy dissipation at the abutments. Finally, a new rocking shallow foundation performance database, containing dynamic experimental results from five centrifuge and three 1g shaking table test series, was created. Its usefulness was illustrated through example correlations between the peak drift ratio demand of the structures and selected ground motion intensity measures. The fundamental value of the database is that it allows easy access to summary information and derived time series data that can be used to obtain empirical correlations, validate numerical models and design guidelines, and identify future experimental priorities.
Author: D. Milovic Publisher: Elsevier ISBN: 0444597263 Category : Technology & Engineering Languages : en Pages : 637
Book Description
This monograph presents the results of the theoretical analyses of stresses and displacements for shallow foundations subjected to various types of loads. In these analyses not only the classical models but more complex models of soils have been used, such as two-layer half-space, homogenous compressible layer of finite thickness, two-layer compressible layer of finite thickness, anisotropic compressible layer. Contact stresses, settlements, vertical stress distribution, bending moments and shear forces have been determined for foundations of any rigidity. Numerous values of the dimensionless coefficients "I" are tabulated, which can be of use in the solution of practical engineering problems.
Author: Manouchehr Hakhamaneshi Publisher: ISBN: 9781321608755 Category : Languages : en Pages :
Book Description
This dissertation investigates the effect of footing shape, soil type, footing embedment and normalized moment-to-shear ratio on the cyclic performance of shallow footings subjected to rocking. The test results were used to validate modeling parameters and acceptance criteria of rocking shallow foundations in the new ASCE 41-13 standard. The results presented in this study are based on three series of large-scale centrifuge tests and thirty two small-scale centrifuge tests. The large tests included slow cyclic and dynamic shaking loading, but the small centrifuge tests only applied slow cyclic loading to the footings. Previous centrifuge test results have well characterized the behavior of rectangular rocking footings on sand, but few results are available for clayey ground. Gajan et al. (2005) and Deng et al. (2012) have shown that moment capacity of a rocking footing on sand can be accurately predicted using conventional equations. Tests on clay reported herein verify that the same equations hold for moment capacity of rectangular footings on clayey ground. The results reveal that for similar critical contact area ratio (ρac) and rotation demand, footings on clay settle about 20 to 40% less than those on sand. The standard ASCE 41-13 Seismic Evaluation and Retrofit of Existing Buildings includes new provisions for linear and non-linear modeling parameters and acceptance criteria for rocking shallow foundations. The new modeling parameters and acceptance criteria were largely based on model tests on rectangular rocking footings with a limited range of footing length to width ratio (L/B). New model test results are presented, including a systematic variation of L/B and also non-rectangular (H-shaped, C-shaped, and trapezoidal) footings. A tri-linear backbone curve is introduced to model the hysteretic moment-rotation behavior of rectangular and H-Shaped footings. The standard provides equations for rotational stiffness, K50, based on elasticity theory (Gazetas 1991). A simpler empirical method for obtaining the initial stiffness, K50 = 300M(c-foot), is proposed for rectangular footings, where M(c-foot) is the moment capacity of the footing. For H-shaped footings, it is found that K50 varies from 400M(c-foot) to 700M(c-foot). The new ASCE 41-13 provisions are limited to cases with M/VL > 1, which is considered to be a criteria that will ensure that the footing is rocking dominated (i.e., rocking deformations are more significant than sliding deformations). It is shown in this thesis that footings with 0.7M/VL
Author: Hany Shehata Publisher: Springer ISBN: 3030019233 Category : Science Languages : en Pages : 276
Book Description
This volume deals with the advanced analysis of shallow foundations. Several research studies are considered including soil plasticity, cracking, reaching the soil bearing capacity, creep, etc. Dynamic analyses together with stability analysis are also discussed. It gives wide range of topics dealing with the shallow foundations in different parts of the world. The volume is based on the best contributions to the 2nd GeoMEast International Congress and Exhibition on Sustainable Civil Infrastructures, Egypt 2018 – The official international congress of the Soil-Structure Interaction Group in Egypt (SSIGE).
Author: Braja M. Das Publisher: CRC Press ISBN: 1351672444 Category : Technology & Engineering Languages : en Pages : 330
Book Description
Following the popularity of the previous edition, Shallow Foundations: Bearing Capacity and Settlement, Third Edition, covers all the latest developments and approaches to shallow foundation engineering. In response to the high demand, it provides updated data and revised theories on the ultimate and allowable bearing capacities of shallow foundations. Additionally, it features the most recent developments regarding eccentric and inclined loading, the use of stone columns, settlement computations, and more. Example cases have been provided throughout each chapter to illustrate the theories presented.
Author: Ravindranath Salimath
Author: Ravindranath Salimath
Author: Thomas Brian Algie
Author: Sivapalan Gajan
Author: Andreas Gerasimos Gavras
Author: D. Milovic
Author: Manouchehr Hakhamaneshi
Author: 
Author: Hany Shehata
Author: Paul Wesley Mayne
Author: Braja M. Das