Numerical assessment of the seismic response of an earth embankment on liquefiable soils

This study presents a numerical assessment of the seismic behaviour of an earth embankment founded on liquefiable foundation soils during earthquake loading. Analysis was carried out using an effective stress-based, fully coupled, finite element method. The behaviour of the sandy soil is described by means of a cyclic elastoplastic constitutive model which was developed within the framework of the Armstrong–Frederick type non-linear kinematic hardening concept. The numerical method and the analysis procedure are briefly outlined and as an example, the seismic response of an earth embankment on a saturated sand foundation is assessed. Based on the numerical results, the distinctive patterns of seismic response of the embankment are discussed. Special emphasis is given to the computed results of excess pore water pressures, co-seismic and post-seismic deformations, and accelerations during the seismic excitation. It has been found that the numerical model can capture fundamental liquefaction aspects of the embankment foundation system and produce preliminary results for its seismic assessment.      

Effects of specimen density and initial cyclic loading history on correlation between shear wave velocity and liquefaction resistance of Toyoura sand

Some previous studies have shown a good correlation between the shear wave velocity, V_s, and the cyclic resistance ratio, CRR. Recently, however, a V_s-based liquefaction assessment method has become an alternative and supplementary method to the conventional N_SPT-based method. It is known that the CRR is influenced not only by the specimen density, but also by the soil fabric. Unfortunately, there are concerns that different combinations of the effects of the specimen density and the soil fabric may generate different relations between V_s and the CRR even if the tested specimens are of the same soil material. In the current study, a series of V_s measurements and undrained cyclic triaxial tests is performed on Toyoura sand specimens with different soil fabrics for three different specimen densities. The fabric of the specimens is varied by applying initial cyclic loading. The results of the V_s measurements indicate that the V_s of the specimen is affected by the initial cyclic loading histories, and the results of the undrained cyclic triaxial tests show that there is a good correlation between V_s and the CRR. However, the correlation varies depending on the specimen density even when the tested material is Toyoura sand only. In other words, the soil-type specific correlation between V_s and the CRR depends on the specimen density. Therefore, the results indicate that both V_s and the specimen density are necessary parameters for an accurate assessment of the CRR.     

Behaviour of prestressed geotextile-reinforced sand bed supporting a loaded circular footing

In the past, the beneficial effects of prestressing the geosynthetic in reinforced soil foundations have been studied mathematically. It is timely to experimentally investigate the degree of improvement generated by prestressing the geosynthetic layer for several embedment depths of a footing resting on a reinforced sand bed. Therefore, laboratory physical model tests and finite element analyses were conducted to study the behaviour of prestressed geotextile-reinforced sand bed supporting a loaded circular footing. The addition of prestress to the geotextile reinforcement results in significant improvement to the settlement response and the load-bearing capacity of the foundation. For a surface footing, the load-carrying capacity at 5 mm settlement for the prestressed case (with prestress equal to 2% of the allowable tensile strength of the geotextile) is approximately double that of the geotextile-reinforced sand without prestress. The beneficial effects of the prestressed geotextile configuration were evident for greater footing depths, in comparison with unreinforced and reinforced (without prestress) counterparts. Experimental and numerical results were also used to validate a few empirical relationships, which are commonly used for solving soil-structure interaction problems. The results obtained from finite element analysis using the program, PLAXIS are generally found to be in reasonabaly good agreement with experimental results.     

Seismic behavior of CFRSTC composite frames considering slab effects

This paper reports on an experimental and numerical investigation conducted on the seismic behavior of concrete-filled rectangular steel tubular columns (CFRSTC) composite frames. The experimental study was conducted by subjecting two full-scale composite frames to simulated seismic loads. Both frames were composed of CFRSTC and steel beams. One specimen was placed on a reinforce concrete (RC) floor slab and the other was not. The purpose of the test was to investigate the elasto-plastic performance of the CFRSTC composite frame system and to examine the effects of composite action on the behavior of composite frames. The test results showed that the stiffness, strength and energy-dissipating capacity of the CFRSTC frame increased significantly with the presence of the floor slab. Compared with a bare steel beam, the composite beam experienced a decrease in the rotation capacity from 0.046 rad to 0.026 rad. The shear deformation of the panel zone grew because of the composite action, which delayed the fracture of the beam. Finite element (FE) models were established to simulate the tested frames. The results of the FE model fit well with that of the test model in terms of stiffness, strength, hysteretic behavior and component deformation.     

Design of steel portal frame buildings for fire safety

This paper describes a study into the fire behaviour of steel portal frame buildings at elevated temperatures using the finite element programme SAFIR. The finite element analysis carried out in this report is three dimensional and covers different support conditions at the column bases, the presence of axial restraints provided by the end walls, different fire severities within the building, different levels of out-of-plane restraint to the columns and the effect of concrete encasement to the columns. From a large number of analyses, it is shown that the bases of the steel portal frames at the foundations must be designed and constructed with some level of fixity to ensure that the structure will deform in an acceptable way during fire, with no outwards collapse of the walls. The analyses also show that to avoid sidesway (i.e. collapse outwards) it is not necessary for steel portal frame columns to be fire-protected unless the designer wishes to ensure that the columns and the wall panels remain standing, during and after the fire.     

Comparison of sand liquefaction in cyclic triaxial and simple shear tests

The liquefaction resistance and correction factors K_σ and K_α of Nakdong River sand obtained from cyclic triaxial (CTX) tests were compared with those determined by cyclic simple shear (CSS) tests to ascertain the importance of the reduction factor C_r and correction factors K_σ and K_α in liquefaction evaluations, especially in view of the lack of comparative liquefaction assessments based on different laboratory test apparatuses. All samples used for the comparisons were obtained from the same type of sand by using similar preparation methods and they were subjected to similar stress states to minimize the number of factors influencing the comparison results; moreover, the apparatuses used in the two tests were manufactured by the same company and all tests were conducted by a single operator. It was found that the liquefaction resistance in CTX tests was always greater than that in CSS tests. Furthermore, C_r varied from 0.63 to 0.36, and it depended on the relative density D_r and initial static shear ratio α. K_σ, which increased with the normal effective stress σ′_nc in CTX tests, was identical to K_σ observed in CSS tests when α was increased up to 0.1. By contrast, K_α in the CSS tests was 58%–97% of K_α measured in the CTX tests, and it depended on the combined effect of D_r, σ′_nc, and α. The relationship between K_α and α in both CTX and CSS tests was well represented by a parabolic function. Moreover, the differences in K_α values between the CTX and CSS tests were also found to be a parabolic function of α. This information can be used for converting CTX (or CSS) values into equivalent CSS (or CTX) values.     

Wave-induced liquefaction and instability of offshore monopile in a drum centrifuge

This paper discusses the instability of an offshore monopile in association with wave-induced liquefaction of sand beds. Centrifuge wave tests in a drum channel were performed with viscous scaling introduced such that the time-scaling laws for fluid wave propagation and consolidation of the soil were matched. The relationships between liquefaction and the start and development of monopile instability in a sand bed under wave loading were investigated. The monopile started significant structural rocking when liquefaction occurred and progressed to one-third of the monopile embedment depth. The residual displacement of the pile increased markedly with the downward progress of the liquefied zone, eventually leading to the collapse of the pile. The characteristics of the onset of liquefaction around the monopile were also investigated. It was found that liquefaction first took place at the sides of the monopile rather than at the front or rear presented to the direction of the travelling waves, which highlights the importance of the rotation of the principal stress axes induced in the sand bed under the passage of progressive waves. The experimental results further demonstrated the effect of embedment of the pile in a dense layer. It was found that embedding the pile in the dense layer with thickness equal to or more than half of the pile embedment depth was effective in preventing the significant inclination and collapse of the pile. The present study also clarified the effects of scour protection around the monopile on wave-induced liquefaction and pile instability. It was found that the scour protection increased the liquefaction resistance depending on the diameter ratios of the scour protection and the pile, however, it could not prevent the collapse of the monopile once liquefaction occurred. These results show the importance of soil stratification on the wave-seabed-structure interaction in light of wave-induced liquefaction.     

强夯引起地基震动衰减规律的数值模拟

从弹性地基空间轴对称动力问题出发 ,建立强夯计算模型 ,运用轴对称有限元法对单点单次夯击下地基的振动过程进行数值模拟。结合工程算例 ,详细分析了强夯施工时地基土的位移、速度和加速度随时间的变化过程及其峰值的空间分布 ,进而分析强夯引起地基震动衰减规律以及相应的减振隔振措施 ,为控制强夯地基处理施工产生的振动影响提供了一些有益的参考     

QLY25轮胎起重机吊臂有限元分析

本文用美国ＣＶ公司ＣＡＤＤＳ／ＳＴＲＥＳＳＬＡＢ建模分析软件、对ＱＬＹ２５轮胎起重机吊臂进行有限元分析，以验证其结构其刚度。     

Modelling of residual stress relaxation around cold expanded holes in carbon steel

In cases of cold expanded holes, taking residual compressive stresses into account in strength estimation, without considering their relaxation under cyclic load, leads to inaccurate mathematical models for the prediction of fatigue life of structural components. This article presents the outcomes from experimental and numerical investigations of residual stress relaxation around cold expanded holes in medium-carbon steel under cyclic tension. An approach to experimental modelling of the relaxation is proposed. This approach reduces to minimum the volume of the experiment and at the same time provides a relatively simplified mathematical model applicable to engineering practice. A generalized mathematical model of residual stress relaxation has been obtained and it predicts the relaxation as a function of the number of cycles, parameters of the cycle and hole expansion rate. A metallographic analysis has been made for clarifying the physical nature of the residual stress relaxation. On this basis two FE models have been developed for quantitative estimation of the errors from the basic formulations in the study as well as for estimation of the residual stress relaxation. The experimental and the numerical outcomes are in a good agreement.     

ROBOT软件在特种结构计算中的应用

结合实例介绍有限元结构分析软件ROBOT在特种结构空间分析中的应用。     

Numerical modelling of the structural fire behaviour of composite buildings

This paper describes numerical models constructed to simulate the response of composite steel/concrete building floors under fire conditions. In particular, this study deals with two of the fire tests recently undertaken on a full-scale multi-storey building at Cardington, UK. The analysis is carried out using a structural analysis program which accounts for both geometric and material nonlinearities, and which includes temperature-dependent constitutive models for steel and concrete materials. The approaches used to represent the various structural details are discussed, and the procedure employed for incorporating the experimentally measured temperature profiles and histories is outlined. For the two tests considered in this investigation, the numerical results are in general agreement with the experimental data, particularly in terms of the magnitude of vertical deformations induced in the floors at elevated temperatures. Close examination of the numerical and experimental findings provides an insight into the complex interactions that occur in the structure at elevated temperatures. Most significantly, the influence of the restraint to thermal expansion of the heated floor area, which is provided by the surrounding parts of the structure, is shown to be of paramount importance. The increasing confidence that can be placed in numerical models as well as the improved understanding of the structural fire response may be used in developing more realistic and cost-effective design methods which are based on the actual structural response rather than that of isolated members.