Effective Stress Method Research Articles

Characteristics of Gravelly Granite Residual Soil in Bored Pile Design: An In Situ Test in Shenzhen

Granite residual soil is widely distributed in south China and is treated as a special soil. Its design parameters in rotary drilling bored piles are a matter of debate due to lack of in‐situ pile load tests. Back‐analysis of test piles is a reliable means of studying the geotechnical capacity of granite residual soil for pile design. In this study, a series of in situ tests was conducted comprising six full‐scale instrumented test piles in gravelly granite residual soil in Shenzhen to consider the effects of different construction methods. The six piles were constructed with three different rotary drilling methods. Two commonly used design methods were investigated in the back‐analysis: the SPT and effective stress methods. The results of the loading tests and strain gauges were used to obtain the back‐analyzed parameters of the ultimate shaft resistance and ultimate base resistance for gravelly granite residual soil with these two design methods.

09.01: Numerical methods for the fatigue assessment of welded joints – influence of misalignment and geometric weld imperfections

ABSTRACTThe fatigue design life of welded joints is increasingly assessed by using numerical methods, such as the structural (hot‐spot) stress method and the effective notch stress method. When compared to the classical design approach using nominal stress S‐N design curves, these methods offer the advantage of flexibility and a wider scope of application. However, a number of questions arise when these methods are used to assess geometrically imperfect welded joints, such as joints with plate misalignments or excessive weld convexity or concavity. In these cases, the classical S‐N curves are known to cover imperfections up to the common tolerance classes for fatigue‐prone welded joints (e.g. in accordance with ISO 5817 class B). For the numerical methods, differing and conflicting recommendations exist on how to account for the geometric imperfections in the welded joints, with little or no background to these recommendations available. In this paper, a study is presented in which two standard welded joints (butt welds between plates of equal and unequal thickness; T‐joints with fillet welds) are analysed with the help of the structural hot‐spot and the effective notch stress approach. Various levels of geometric imperfection up to tolerance limits, and the resulting fatigue life predictions, are compared to the nominal stress approach predictions and the underlying test results from the literature. Since the nominal stress approach curves are based on reliable statistical data and desired survival probabilities for these known, standard cases, this methodology allows one to determine the correct application of the numerical methods to cases with geometric imperfections. This information may be used for a pertinent refinement of design recommendations for these methods, as well as for cases where these methods are applied to fitness‐for‐purpose assessments.

Time-dependent bearing capacity of a jacked pile: An analytical approach based on effective stress method

Complex stress changes occur in the soil adjacent to a jacked pile during pile installation, equalization and loading, but the corresponding effects on the bearing performance of the jacked pile are not well understood at present. This paper performs a theoretical study on the time-dependent bearing capacity of a jacked pile by considering the entire stress history of the surrounding soil. The pile installation process is modeled by the undrained expansion of a spherical cavity at the pile tip and a cylindrical cavity around the pile shaft, respectively. A K0-based anisotropic modified Cam-clay (K0-AMCC) model is employed in the cavity expansion solution to evaluate the stress state of the surrounding soil immediately after pile installation. The dissipation of the jacking-induced excess pore water pressure is modeled by the governing equation of radial consolidation theory. The change of the effective stress in the surrounding soil is obtained by solving the governing equation and taking the soil relaxation effects into consideration. A time-dependent empirical reduction factor is introduced to incorporate the thixotropic effect on the friction angle between the soil and pile interface. An analytical approach, based on the effective stress method, is presented to predict the time-dependent bearing capacity of published centrifuge model pile tests and field pile tests. The predicted results agree well with the measured data. It demonstrates that the present approach can be applied to capture the time-dependent bearing capacity of a jacked pile in clay effectively.

NUMERICAL METHODS FOR THE FATIGUE ASSESSMENT OF WELDED JOINTS: INFLUENCE OF MISALIGNMENT AND GEOMETRIC WELD IMPERFECTIONS

The fatigue design life of welded joints in steel structures is increasingly assessed by using numerical models and methods, such as the structural (hot-spot) stress method and the effective notch stress method. When compared to the classical design approach using nominal stress S-N design curves, these methods offer the advantage of flexibility and a wider scope of application. However, a number of questions arise when these methods are used to assess geometrically "imperfect" welded joints, such as joints with plate misalignments or excessive weld convexity or concavity. In these cases, the classical S-N curves are known to cover imperfections up to the common tolerance classes for fatigue-prone welded joints (e.g. in accordance with ISO 5817 class B). For the numerical methods, differing and conflicting recommendations exist on how to account for the geometric imperfections in the welded joints, with little or no background to these recommendations available. In this paper, a study is presented in which two standard welded joints (butt welds between plates of equal and unequal thickness; T-joints with fillet welds) are analysed with the help of the structural (hot-spot) stress and the effective notch stress approach, considering various levels of geometric imperfection up to the tolerance limits, and the resulting fatigue life predictions are compared to test results from the literature and the nominal stress approach predictions. Since the nominal stress approach curves are based on reliable statistical data and desired survival probabilities for these known, standard cases, this methodology allows one to determine the correct application of the numerical methods to cases with geometric imperfections. This information may be used for a pertinent refinement of design recommendations for these methods, as well as for cases where these methods are applied to fitness-for-purpose assessments - e.g. because the nominal stress approach is not applicable.

Fatigue life assessment of large scale T-jointed steel truss bridge components

Among current approaches for fatigue strength assessment, the effective notch stress method is widely employed by practising engineers designing welded joints. This is particularly important in the situation where the nominal stress and structural stress cannot be easily quantified. In this paper, the applicability of the so called effective notch stress approach on large-size T-joints in truss bridges is investigated through a comprehensive experimental programme supported by numerical analysis. A series of large-scale fatigue tests on prototype large-size T-joints with cope holes were conducted. These types of joints are normally used in fully welded truss bridges. Furthermore, a simple parametric study was conducted using finite element analysis to investigate the effect of plate thickness and cope-hole radius on effective notch stress. Comparison of the results with commonly used design guidance documents reveals that the effective notch stress approach provides a conservative estimate of the fatigue strength of the specimens tested in this experimental programme.

Cavity Expansion Analysis to Predict Side Shear Set-Up in Clayey Soils

In this study, fifty driven piles located in the Santos Coastal Plain (“Baixada Santista”), Brazil, have been dynamically tested at various times after installation, indicating gain of capacity over time. The average side shear resistance, related to a low-OCR clayey layer – known as SFL soil , was evaluated over time, indicating side shear set-up within a relatively narrow range of values (between 2.0 and 2.5) approximately 20 days after installation. The cavity expansion theory, which considers an ideal cohesive soil, has been used to predict the effective radial stress over time, considering two main parameters: undrained shear strength and horizontal coefficient of consolidation. Finally, the average of measured pile shaft capacities were compared to the unit shear resistances predicted by the Method (effective stress method). The theoretical values of side shear set-up are fairly similar to the results of the analyzed load tests, which seems to support the viability of the presented method.

On the accuracy of nominal, structural, and local stress based approaches in designing aluminium welded joints against fatigue

This paper investigates the accuracy and reliability of nominal stresses, hot-spot stresses, effective notch stresses, notch-stress intensity factors (N-SIFs) and material length scale parameters in estimating fatigue lifetime of aluminium welded joints. This comparative assessment was based on a large number of experimental data taken from the literature and generated by testing, under either cyclic axial loading or cyclic bending, a variety of aluminium welded structural details. Whenever it was required, stress analyses were performed by solving bi-dimensional linear-elastic finite element models. The obtained results demonstrate that the effective notch stress method, the N-SIF approach, and the Theory of Critical Distances (TCD) provide a more accurate fatigue life estimation in comparison with the other methodologies. In this context, the TCD was seen to be easier to adopt, requiring less computational effort than the effective notch stress method and the N-SIF approach. Finally, based on the experimental results being re-analysed, a unifying value of 0.5mm is proposed for the TCD critical distance, with this value allowing aluminium welded connections to be designed accurately irrespective of joint geometry’s complexity.

Application of Neuber’s effective stress method for the evaluation of the fatigue behaviour of magnesium welds

In this study, the microstructural length of magnesium welded joints was determined by using experimental data acquired from three different weld geometries and Neuber’s stress averaging method. The aim was to determine microstructural length, which is assumed to be a material constant that is identical for all Mg-materials, so that it can be employed for effective stress and fictitious notch radius rf calculations. By considering the worst case scenario, rreal=0, this fictitious notch radius can be used as a new reference radius rref for magnesium welded joints.Using the reference radii from two different hypotheses rref=1mm and rref=0.05mm, calculations were performed for notches and data were employed in the calculations. Also FE-analyses were utilised in order to identify the stress gradients for weld geometries. Effective stresses were calculated using experimental data along with the stress gradients in Neuber’s stress averaging method.Finally, S-N lines were derived by correlating effective stresses with fatigue life for the worst case with R=0.5 to account for possible high tensile residual stresses. The scatters of these S-N lines were calculated and compared for every ρ∗ iteration. The length value that resulted in the minimum scatter corresponded to the microstructural length. The identified microstructural length was ρ∗=0.12mm. Until now, there were no published results for microstructural length of magnesium welded joints and the findings in this paper can be used to replace the current recommendations regarding magnesium welded joints.

Effective notch stress approach for welds having low stress concentration

The effective notch stress approach for fatigue assessment of welded joints according to the IIW-recommendations is a widely accepted procedure and has already become part of many technical rules and guidelines. The method provides additional means for evaluation of welded joints besides classical nominal and structural stress concepts. In contrast to the latter concepts, effects of many local weld parameters can only be studied analytically using the effective notch stress approach. The effective notch stress approach requires use of a standardized idealization of the weld detail and use of continuum mechanics theory for stress analysis (i.e. use of solid elements for finite element models). The idealization with respect to geometry includes modeling of all weld toes and weld roots using a reference radius together with an associated S-N curve. For joints facing low stress concentration, like butt welds or welds at thin sheets, the method requires checking the fatigue behavior of the base material as well. This sometimes is not considered in using the effective notch stress method, because it is not so obvious. Explanation of background information and a reanalysis of an example from literature is presented here for explanation and demonstration of this concept and of specific needs for welds with low stress concentration. Also the selection of the size of the reference radius will be discussed in this paper by using different radii for the case investigated here. Finally the application limits of the S-N curves as given in the IIW-recommendations is discussed since the selected example is slightly out of the range of application of the S-N curves.

Origin and distribution of abnormally high pressure in the Mahanadi Basin, east coast of India

Drilling deep wells in the Mahanadi Basin of the east coast of India is highly challenging because of the variations in pore pressure in the Miocene formations. We have observed that the wells drilled in the northern part of the study area have more drilling hazards due to the presence of high pore pressure (modular dynamic test measurements of up to 18.5 ppg) when compared with wells drilled in the southern part of the basin. In the northern part, pore pressure prediction assuming disequilibrium compaction (DC) underpredicts by approximately 2–3.5 ppg when compared with observed pressures; however, in the southern part, pressure prediction matches the observed pressures in the drilled wells. Analysis of sonic velocity-density crossplots suggests that along with DC, some other secondary mechanism also plays an important role in generating excess overpressure in the northern part of the study area; however, the well data do not indicate the presence of an established secondary mechanism, such as fluid expansion, clay conversion, or cementation. The prime difference between the northern and southern areas is the presence of multiple canyon cuts in the northern part and the observation that very high overpressures occur below these canyon cuts. Hence, an attempt was made to ascertain the relationship between the presence of canyon cuts and the observed high pressure with the help of burial history modeling that incorporates the canyon cut features. Pressure estimation based on this approach closely matches the observed pressures in the drilled wells. This very high overpressure observed in the northern part is most likely generated by the combined effect of porosity rebound (due to overburden removal) along with persistence of overpressures that developed prior to erosion. This burial history modeling approach helps in recognizing and understanding the impact of erosional canyon cut events on generation of excess overpressure in the study area. Furthermore, effective stress methods that take secondary pressure generating mechanisms (unloading) into account are used to quantify the difference in pore pressure.

Statistical analysis of the effective stress method and modifications for prediction of ultimate bond strength of soil nails

Uncertainty in the predicted ultimate pullout strength of soil nails can be significant due to the complexity of nail–soil interactions, inherent variability in soil properties and the effects of nail installation. The paper first presents a statistical evaluation of the accuracy of ultimate bond strength of soil nails using the effective stress method (ESM) equation that has been adopted in Hong Kong. A total of 113 ultimate nail capacity data points from field pullout tests were collected from the literature and used to estimate the accuracy of the current ESM. Based on the available data, the current ESM default pullout model is found to be excessively conservative (on average) by at least a factor of three. The spread in prediction accuracy measured by the coefficient of variation (COV) of bias is in the range of 36–43 % after removing anomalous test data. Here, bias is the ratio of measured to predicted pullout load capacity. In addition, the accuracy of the current ESM equation for prediction of nail bond strength is shown to be dependent on the magnitude of predicted ultimate bond strength and magnitude of nominal vertical effective stress which is undesirable. The paper examines four candidate-modified bond strength equations with empirical coefficients that have been back-fitted to measured bond strengths to improve the overall accuracy of the equation and to reduce or remove the undesirable dependencies noted above. One equation with an empirically corrected stress-dependent term is judged to be the best candidate model based on the mean of bias values, spread (COV) of bias values, lack of dependencies and simplicity. Finally, the relative contributions of random variability in soil shear strength to measurement bias in bond strength (prediction accuracy) for each soil type are computed for the best bond strength model. Analysis of the contribution of soil shear strength to prediction accuracy showed that the combination of variability due to factors other than soil shear strength was greater than the variability in soil shear strength alone, where the latter is defined by the soil secant friction coefficient.

Axial friction response of full-scale pipes in soft clays

The axial friction response of subsea pipelines in soft clays is a very important aspect for designers of subsea pipelines but the response is not well understood so far. There is a pressing need for the comprehension of the response. In this paper, model tests are performed using full-scale pipes coated with polyethylene (PE) to study the effects of the set-up period, the pipe diameter, the buried depth of the pipe, the shear strength of soft clays and the loading rate on the axial friction response of pipelines in soft clays. The variations of the axial friction coefficient are analyzed using the effective stress method based on model test results. The results show that the axial friction resistance increases with the increasing pipe diameter but the effect of the pipe diameter on the axial friction coefficient can be neglected. The ultimate axial resistance also increases with the increase of the buried depth of pipelines, the undrained shear strength of soft clays and the loading rate. The axial friction coefficient increases with the increasing loading rate. However, the axial friction coefficient decreases with the increasing buried depth. The method to determine the axial friction coefficient is developed by analyzing model test results, which considers the effects of the diameter, the buried depth, the undrained shear strength of soft clays and the loading rate. The study results not only extend the industry data base but also supply a basis to determine the axial friction coefficient of PE-coated pipes in soft clays for ocean engineering geological investigations.

Modeling lateral enlargement in dam breaches using slope stability analysis based on circular slip mode

Evaluation of breach flood of landslide or artificial dams is usually performed by combining the hydraulic modeling of the breach flow and geotechnical analysis of the breach channel stability. This paper is a continuation of the previous work, which mainly focused on the hydraulic aspects of a dam breach flood. Efforts have been made to improve the related slope stability analysis approach that traditionally adopts a simple wedge failure mode. The improvements includes a vertical cut at the slope toe due to soil erosion, an approach to determine the critical slip surface, the effective and total stress methods dealing with different dam materials, and a procedure to model the stepped failures of the breach bank due to continuous toe cutting. Using VBA programming, an Excel spreadsheet entitled DBS-IWHR has been developed to perform the stability analysis. This spreadsheet has been incorporated into another spreadsheet entitled DB-IWHR for the calculation of the flood hydrograph. The developed model has been tested by back analysis of the Yigong landslide dam breached at the Tibetan Plateau in China in 2000 with a flood peak of 94,013m3/s. The calculated results of the final breach base level and the peak discharge are in good agreement with the field data. Further, the results are shown to be insensitive to the variations in the geotechnical parameters used in the model.

Fitness-for-service assessment of defected welded structural details by experimental evaluation of the fatigue resistance S-N curve

During the fabrication of two different structures (a ship and a bridge), made by mild construction steel, some unacceptable anomalies were detected; the structures were almost completely fabricated and a repair of the defected structural details was impractical due to the extent of the defected zones. A fitness-for-service assessment has therefore been carried out, in both cases, by performing fatigue tests on representative samples, cut off from the structure and by evaluating the fatigue resistance curve. A re-assessment of the structural details in question was undertaken, on the basis of the experimental S-N curve, evaluating the fitness for service of the “defected details”. The experimental tests showed that the detected anomalies had no influence on the fatigue strength of the welded joints and that the examined details (a butt joint and a cruciform joint) behave in accordance with the relevant FAT classes. The experimental program has also given the opportunity to compare different methodologies for fatigue assessment: the Effective Notch Stress Method, the Fracture Mechanics Method and the Peak Stress Method. The different procedures have been applied to the tested specimens, and the expected fatigue life was compared with the experimental one.

Comparison of local stress based concepts — Effects of low-and high cycle fatigue and weld quality

This paper presents a study of the ability for two stress based methods, the effective notch stress method and the structural stress approach, to estimate the fatigue life in the low cycle and the high cycle fatigue regime, considering the weld quality. Two different non-load carrying joint configurations were considered, cruciform joint and T-joint. The conducted fatigue analysis shows that both methods are capable to estimate the fatigue life with good accuracy within the low cycle and the high cycle regime. The effective notch stress generally shows a smaller scatter, it also considers increased weld quality with good accuracy, in contrast to the structural stress approach.

Insight into Failure Mechanism of Large-Diameter Monopile for Offshore Wind Turbines Subjected to Wave-Induced Loading

Wu, K.; Fan, Q., and Zheng, J., 2015. Insight into failure mechanism of large-diameter monopile for offshore wind turbines subjected to wave-induced loading.In this paper, the refined elasto-plastic finite element model based on effective stress method is developed for the large-diameter monopile for offshore wind turbines subjected to cyclic wave-induced loading. An improved dynamic Cam-clay constitutive model is utilized to simulate the behavior of seabed soil under cyclic loading. The finite element package is used for analyses in plane strain case and its capability in simulating the wave-induced seabed responses is successfully verified using the analytical solution available. Then the contact pairs algorithm is employed to simulate nonlinear interaction behavior of the contact between the monopile and soil. The lateral boundary in the finite element model is considered according to repeatability principle. By using the refined finite element model established, the failure mechanism and distribution of skin friction acting on the outer wall of monopile is evaluated.

Investigations on the effect of weld penetration on fatigue strength of rib-to-deck welded joints in orthotropic steel decks

A common practice for the manufacture of orthotropic steel decks in Japan is to use 75% partial joint penetration welds between closed ribs and deck plates. To evaluate the effectiveness of the proposed 100% penetration on improving the fatigue strength of rib-to-deck welded joints in orthotropic steel bridge decks, four full-scale orthotropic deck specimens were subjected to laboratory testing. Specimens, consisting of a 12-mm-thick deck plate and 6-mm-thick rib, with one closed rib, were fabricated with 75% and 100% penetration. Fatigue test results showed that fatigue cracks initiated from the weld toe inside the rib in the 100% penetration specimens, but from weld root inside the rib in the 75% penetration specimens. To investigate this fatigue behavior, strain measurements were taken at 5 mm from the rib-to-deck weld line. Results of finite element analysis using the effective notch stress method indicate that a deeper partial penetration results in a slightly higher effective notch stress at the weld root of the partial penetration weld. The effective notch stress at the crack initiation location with 100% penetration is lower than that obtained with partial penetration. The open angle appears to have a significant effect on the effective notch stress at the upper weld toe when using 100% penetration. Therefore, the proposed 100% penetration appeared to have a positive effect on enhancing the fatigue resistance of rib-to-deck welded joints.

Fatigue strength assessment of HFMI-treated butt welds by the effective notch stress method

Experimental fatigue data for high-frequency mechanical impact (HFMI)-treated butt welds with low stress concentrations have been collected and analysed using the effective notch stress (ENS) method given by the International Institute of Welding (IIW). The aim of this study is to suggest a more reliable fatigue assessment procedure for HFMI-improved butt welds with low stress concentration factors. In total, 165 published test results for butt welds subject to R = 0.1 and R = 0.5 axial loading are presented. K n, min values for the ENS approach are suggested in order to avoid computational problems due to low stress concentrations at the weld toe. Minimum notch stress concentration values are suggested based on the material yield strength (f y ). All the data is presented with respect to previously proposed and verified material f y correction method. For HFMI-treated butt welds in the ENS system, the use of the K n, min values with the f y correction representing one fatigue class (approximately 12.5 %) increase in strength for every 200-MPa increase in f y has been proposed and verified. © 2014 International Institute of Welding.

Fatigue design of axially-loaded high frequency mechanical impact treated welds by the effective notch stress method

The effective notch stress method (ENS) as defined by the International Institute of Welding is widely used by design engineers to assess the fatigue strength of welded components. This paper provides a comprehensive evaluation of published data for welded joints improved by high frequency mechanical impact (HFMI) treatment. The goal is to verify already-known fatigue classes for the ENS with the available axially-loaded fatigue data. In total, 280 experimental test results obtained from longitudinal, cruciform and butt welds subject to stress ratio of R=0.1 axial loading are evaluated. Notch stress concentration factors (Kn) for each joint geometry are analysed based on the finite element method. Calculated Kn and reported nominal stress values are used to determine local stresses. Fatigue strength assessment of the all available data is performed by the previously-proposed and verified correction procedure for yield strength (fy). A formerly-defined minimum Kn values as a function of fy is used for butt welds. The already-known fatigue classes are found to be conservative with respect to available fatigue test data.

Effective stress method to be used in beam finite elements to take local instabilities into account

In the fire situation, Bernoulli beam finite elements are the workhorse used in numerical calculation model for simulating the behaviour of the structure. Such finite elements treat all sections as class 1 (stocky) sections whatever the slenderness of the plates that make the section, allowing the development of a full plastic stress distribution in the section which leads to complete plastic redistribution along the members in the structure. This type of element is thus not adapted for modeling structures that contain slender sections of class 2, 3 or 4. This document presents a new approach to take into account local instabilities in slender sections using beam finite elements. The new approach is based on an effective constitutive law of steel. The effective law is not symmetrical with respect to tension and compression because, in tension, the stress-strain relationship is not modified whereas, in compression, the stress-strain relationship is modified.