Rigorous Finite Element Model Research Articles

Mechanical behavior of large-diameter pipe elbows under low-cyclic loading

Large-diameter steel pipes are often used for transmitting and distributing water, gas, and oil products from the source to the end user. These pipelines are mainly oriented by using pipe el- bows due to their high flexibility along their routes. It is important to understand the mechani- cal behavior of these critical infrastructure components to promote material sustainability. For this purpose, a rigorous 3D finite element model is employed to investigate the mechanical behavior of large-diameter pipe elbows with varying elbow angles such as 90°, 60°, and 30°. Moreover, geometrical and material nonlinearities capture the pipes’ ratcheting behavior even under pressurized and unpressurized scenarios. It is seen that the pipes with a larger elbow angle can endure a higher number of cycles before they reach their limit states. In addition, pipe elbows behave similarly to straight pipes as the elbow angle decreases and becomes more vulnerable to plastic deformations such as kink and buckling under bending loads.

Kinematic bending of piles in made ground

This work explores earthquake-induced kinematic bending in the so far unresolved case of a pile embedded in a two-layer soil with a thin surface layer. The problem is treated analytically by means of a generalised Winkler model, which in addition considers the effect of boundary conditions at the pile head over earlier contributions on the subject. Novel analytical closed-form expressions of the kinematic bending moment at the pile head and at the interface between the two soil layers are provided for both fixed- and free-head piles. The analytical solutions are validated through a rigorous finite-element model, which proves a remarkable accuracy in the static regime. While for interface bending the past literature indications for the dynamic coefficient make the proposed formula accurate for both shallow and deep interface, a novel dynamic interaction factor, describing dynamic effects for pile-head bending, is introduced. A numerical example provides guidance on application of the formulae in real design scenarios.

Novel design of inter-module joint in reinforced concrete modular construction using high-strength fiber-reinforced cementitious composites (HSFRCC)

The development of modular construction is surging in recent years. It has been popularized worldwide due to better quality control, environmental-friendliness and potential construction time and cost savings. A specific challenge associated with reinforced concrete modular construction is the design of inter-module joint. Minimizing the size of in-situ cast joint between prefabricated units is of great importance for improving construction efficiency. In addition, reinforcement details should be as simple as possible to enhance constructability. This study proposes a novel connection design for wall-to-wall inter-module joint using high-strength fiber-reinforced cementitious composites (HSFRCC) and steel plate-induced confining stresses. Small-scale direct tension pull-out tests were performed with specially designed loading fixtures to simulate the bond behavior between HSFRCC and high-yield steel bars in the proposed joint design. The effects of joint material properties, embedment length and confinement conditions were evaluated. Results showed that the combined use of HSFRCC with plate confinement greatly improves bond capacity. The preferred rebar yielding failure (rather than bond failure) was achieved with embedment length of only 8d (d being the bar diameter). A rigorous finite element model was established by incorporating explicit geometry of rebar and mechanical properties of joint material, and the numerical results showed good agreement with test observations. Therefore, the applicability of the FE analysis in facilitating the proposed inter-module joint is demonstrated.

Effect of seismic wave propagation in massed medium on rate-dependent anisotropic damage growth in concrete gravity dams

Seismic modeling of massive structures requires special caution, as wave propagation effects significantly affect the responses. This becomes more crucial when the path-dependent behavior of the material is considered. The coexistence of these conditions renders numerical earthquake analysis of concrete dams challenging. Herein, a finite element model for a comprehensive nonlinear seismic simulation of concrete gravity dams, including realistic soil-structure interactions, is introduced. A semi-infinite medium is formulated based on the domain reduction method in conjunction with standard viscous boundaries. Accurate representation of radiation damping in a half-space medium and wave propagation effects in a massed foundation are verified using an analytical solution of vertically propagating shear waves in a viscoelastic half-space domain. A rigorous nonlinear finite element model requires a precise description of the material response. Hence, a microplane-based anisotropic damage-plastic model of concrete is formulated to reproduce irreversible deformations and tensorial degeneration of concrete in a coupled and rate-dependent manner. Finally, the Koyna concrete gravity dam is analyzed based on different assumptions of foundation, concrete response, and reservoir conditions. Comparison between responses obtained based on conventional assumptions with the results of the presented comprehensive model indicates the significance of considering radiation damping and employing a rigorous constitutive material model, which is pursued for the presented model.

Investigation of mechanical behavior of buried DN110 polyethylene pipe with a scratch defect under land subsidence

In this work, a rigorous 3D finite element (FE) model was established to investigate the mechanical behavior of buried DN110 High-density polyethylene (HDPE) pipe with a scratch defect under land subsidence. Parametric effects, including soil gravity, defect geometric, operating pressure, transition section length, and land subsidence displacement, were investigated to analyze the significance of each factor on mechanical stress field. Then the land subsidence displacement was selected as the evaluation index of the ultimate bearing capacity (UBC) of the pipeline. A model was developed to discuss the function relationship between the land subsidence displacement and other parameters.

Three-dimensional finite element modeling of corrugated metal pipes

The mechanism of the soil-structure interaction in buried corrugated metal pipes (CMPs) depends primarily on the relative stiffness of the flexible buried pipe and the surrounding compacted backfill material. Several geometric, mechanical, and material characteristics control this complex composite interaction, including the shape and dimensions of the pipe, its burial depth, the effect of the backfill compaction, and existing in-situ conditions. Numerical techniques such as the finite element method (FEM) have proved useful in capturing the soil-structure interaction. This paper presents three rigorous three-dimensional finite element models of circular, horizontal ellipse, and arch pipes with spans of 600 mm, 1800 mm, and 6000 mm, respectively. The models developed account for the effect of compaction on the soil behaviour. These models were validated by using three full-scale tests of underground CMPs. The FEM results are compared with recorded measurements of deformation and internal forces in each pipe structure. The effectiveness and suitability of three commonly used constitutive soil models, the Mohr-Coulomb (MC) model, the hardening soil (HS) model, and the hardening soil with small strains (HSs) model, are also examined. The HSs model exhibited the best fit in simulating the interaction of the backfill soil and buried pipes with different types of loading.

DFEM of a post-tensioned precast concrete segmental bridge with unbonded external tendons subjected to prestress changes

Being cost-effective, rapid construction, high transportability, and maintainability, post-tensioned precast concrete segmental bridges (PT-PCSBs) have been one of the most preferred choices for sites that are restrictive to access. Yet, deterioration due to aging, improper maintenance, and unaccounted shrinkage of bridge decks can cause unexpected prestress loss/gain and alter the structural behaviour of PT-PCSBs, especially with unbonded external tendons. In this regard, this paper presents the development of a rigorous and detailed three-dimensional discrete finite element model (DFEM) for a PT-PCSB, of which both global load-deflection behaviour and local responses were verified against experimental results by Takebayashi et al. (1994) and Jiang et al. (2018). The verified model was then adopted to investigate the effects of prestress loss/gain on the structural responses and failure behaviour. Based on the analysis results, a 20% increase/decrease of prestressing can lead to a 10% increase/16.8% decrease of the moment resistance but a 30.2% decrease/36.6% increase of the deflection capacity. At large prestressing level, e.g. 0.874 fpu, significant crushing of concrete can occur in the bottom region of the bridge at ULS that significantly restricts the joint opening, tendon strain development, and ductility. On the contrary, at the extremely low prestressing level of 0.125 fpu, the bridge decompresses under its self-weight and is very unstable under additional load, resulting in very low ductility and moment resistance. Furthermore, for a regular prestress level of around 0.6 fpu, the stress change in tendons from the effective prestress to the stress in ULS matches the recommendations by ACI318 and AASHTO LRFD but is underestimated by EC2. Yet, the ultimate tendon stress which is less sensitive to the prestress change may be more suitable than the stress change in evaluating the ULS behaviour of PT-PCSBs with unbonded tendons.

Fracture response of 1422-mm diameter pipe with double-V groove weld joints and circumferential crack in fusion line

A circumferential crack in the fusion line is a major threat to the structural integrity of the pipeline girth weld because it may possibly lead to fracture failure when subjected to large plastic deformations. For the newly constructed X80 pipeline with an outside diameter (OD) of 1422 mm in China, the complicated configuration of welded joints and the outer diameter are outside the range of the general fracture assessment standards, such as BS 7910. Furthermore, during the pipe manufacture process, pipeline designers merely specify the minimum yield strength limit according to the pipe steel grade. This results in a wide variation range in the choice of yield strength values of the base metal. This also leads to a serious problem in the fracture assessment of pipe girth weld because the weld strength is commonly considered to be either evenly matched or overmatched. The foregoing factors make it difficult to perform an accurate fracture assessment using conventional standards. This study therefore aims to investigate the fracture behavior of 1422-mm OD pipelines with a double-V groove weld and circumferential crack in the fusion line. The specific effect of material mismatch and the realistic operational parameters that influence the fracture capacity of the pipe are examined via a series of numerical investigations. First, a rigorous three-dimensional finite element model to simulate the pipe girth weld is established by utilizing the ABAQUS finite element code package. The weld, which has a canoe-shaped surface crack in the fusion line between the heat-affected zone and root welding metal, is subjected to biaxial loading conditions. The keyhole method is adopted to simulate the crack front as well as the crack tip to eliminate the stress singularity effect during deformation. A refined mesh strategy is introduced to ensure the accurate simulation of the stress and strain fields in the vicinity of the crack tip. Based on the numerical model, the J-integral fracture parameter, which is clearly defined based on a strict mechanical theory, is chosen to characterize the effect of strength matching ratio, internal pressure, and yield to tensile strength ratio on the crack driving force. A new strain-based J-integral prediction formula is proposed based on numerical results.

Using TDA underneath shallow foundations: simplified design procedure

ABSTRACTTire recycling and reuse in North America and worldwide have increased considerably, with the aim of reducing the harmful effects of scrap tires on the environment. Accordingly, the use of tire derived aggregates (TDA) in civil engineering applications is on the rise at an unprecedented rate. In comparison to conventional backfill aggregates, TDA is an inexpensive, lightweight material that costs about 25% of the cost of conventional backfill. Thus, there is an increasing trend of using TDA under shallow foundations as a lightweight backfill alternative. However, limited information exists for the design of shallow foundations built on TDA. Hence, the main objective of this paper is to develop a simplified design procedure for shallow foundations built over a TDA layer. Rigorous finite element models were developed and validated using field tests results. Subsequently regression analyses were used to develop the proposed ultimate bearing capacity equation, taking into account the granular layer thickness, TDA layer thickness, footing width, footing shape, footing depth, and the allowable settlement.

Ultra low-cycle fatigue performance of S420 and S700 steel welded tubular X-joints

The present study is motivated by the need for improving the fatigue performance of offshore wind energy structural systems. In particular, the ultra low-cycle fatigue performance of welded tubular X-joints is examined, motivated by the need of safeguarding the integrity of offshore platforms under extreme loading conditions. The welded specimens are manufactured using hot-rolled tubes of steel grade S420 and S700, and represent X-brace joints of a bottom-fixed offshore wind tubular jacket, with scaling factor of 1:3. Seven specimens are tested under strong fully-reversed cyclic in-plane bending, leading to through-thickness fatigue cracking within less than 100 cycles, simulating extreme loading conditions. The experimental results indicate that X-joints manufactured from both steel grades exhibit similar structural response, in terms of ultra low-cycle fatigue. Rigorous finite element models are also developed, with emphasis on constitutive modeling, to simulate the cyclic loading procedure, providing very good comparisons in terms of load-displacement response and local strain predictions during the initial loading cycles. The experimental data are compared with a large dataset of low-cycle fatigue experiments on welded components, reported in the literature for mild and high-strength steel materials, as well as with existing design provisions. The results indicate similar performance of high-strength steel and mild steel welded connections, and are compared with stress-based and strain-based design methodologies in predicting the number of cycles to failure in the ultra low-cycle fatigue regime.

The Beneficial Role of Piles on the Seismic Loading of Structures

We examine the kinematic response of fixed-head vertical floating piles embedded in continuously nonhomogeneous soils and subjected to upward propagating seismic waves. The problem is explored numerically by means of a rigorous finite element (FE) model of the soil-pile system to quantify the kinematically induced reduction of the horizontal free-field spectral acceleration. Soil stiffness varies continuously with depth according to a generalized power law function. We show that kinematic pile response in the harmonic regime is controlled by a unique dimensionless frequency parameter involving the active pile length in a generalized nonhomogeneous soil. A new, simplified expression for the horizontal kinematic interaction factor Iuis proposed for practical time-domain applications while a novel physical interpretation of the filtering action of piles is reported by introducing the role of pile stiffness in averaging soil motion over an effective pile length. Following a parametric study under transient motion, we propose a set of novel, ready-to-use formulae for a rapid assessment of the pile-induced filtering action. An application of the proposed formulae to clayey soils is finally presented, leading to useful indications for the selection of the pile diameter associated with the maximum filtering potential.

Stress and Deformation Analysis of Buried Gas Pipelines Subjected to Buoyancy in Liquefaction Zones

Buried pipelines are the main means of long distance transportation of natural gas. These pipelines are in high risk crossing liquefaction areas due to large deformations and stresses that may exist in pipe induced by the buoyancy load. In this study, a systematic analytical and numerical analysis were performed to investigate the mechanical behavior of a buried gas pipeline subjected to buoyancy in liquefaction areas. Soil constraints on pipe were considered accurately in the proposed models through soil spring assumptions. Effects of axial forces on pipe’s bending deformation were also considered via the governing equations for beam under bending and tension. Deformation compatibility condition was utilized to derive the axial forces in pipe. The accuracy of the proposed analytical model was validated by comparing its results with those derived by an established rigorous finite element model. In addition, parametric analysis was finally performed using the analytical model to study the influences of pipe diameter, pipe wall thickness, soil spring stiffness and width of liquefaction zone on pipe’s mechanical responses. This study can be referenced in the strength analysis and performance based safety evaluation of buried gas pipelines crossing liquefaction areas.

Effects of Stress–Strain Characteristics on Local Buckling of X80 Pipe Subjected to Strike-Slip Fault Movement

The structural integrity of underground pipelines are subject to a major threat from permanent ground displacements when they cross active tectonic (e.g., strike-slip) faults, because of large strains potentially induced in pipes, leading to pipe buckling and possible rupture. In this paper, the buckling behavior of X80 pipe is studied numerically with an emphasis on the effects of steel stress–strain characteristics. A rigorous mechanics-based nonlinear finite element (FE) model of a buried X80 pipe crossing a strike-slip fault is developed using shell elements and nonlinear springs for the pipe and soil resistance, respectively. The pipe steel material in the FE model is characterized by a novel and versatile stress–strain relationship, which was established to successfully capture both the round-house (RH) type and the yield-plateau (YP) type stress–strain behaviors. This allows investigating the significant effects of the stress–strain characteristics, as observed in this paper, on the buckling behavior of pressurized and nonpressurized pipes.

Numerical Analysis and Strength Evaluation of an Exposed River Crossing Pipeline with Casing Under Flood Load

Pipelines in service always experience complicated loadings induced by operational and environmental conditions. Flood is one of the common natural hazard threats for buried steel pipelines. One exposed river crossing X70 gas pipeline induced by flood erosion was used as a prototype for this study. A mechanical model was established considering the field loading conditions. Morison equations were adopted to calculate distributional hydrodynamic loads on spanning pipe caused by flood flow. Nonlinear soil constraint on pipe was considered using discrete nonlinear soil springs. An explicit solution of bending stiffness for pipe segment with casing was derived and applied to the numerical model. The von Mises yield criterion was used as failure criteria of the X70 pipe. Stress behavior of the pipe were analyzed by a rigorous finite element model established by the general-purpose Finite-Element package ABAQUS, with 3D pipe elements and pipe-soil interaction elements simulating pipe and soil constraints on pipe, respectively. Results show that, the pipe is safe at present, as the maximum von Mises stress in pipe with the field parameters is 185.57 MPa. The critical flow velocity of the pipe is 5.8 m/s with the present spanning length. The critical spanning length of the pipe is 467 m with the present flow velocity. The failure pipe sections locate at the connection point of the bare pipe and the pipe with casing or the supporting point of the bare pipe on riverbed.

Mechanical response of buried polyethylene pipelines under excavation load during pavement construction

Abstract Polyethylene (PE) pipelines are the most popular kinds of urban gas pipelines used in industry. With rapid development of urban pavement construction and increasing demand of natural gas in China. Urban gas pipe accidents caused by pavement mechanical construction have taken place more frequently in recent years, which becomes one main reason for accidents caused by third party damage. Thus, it is of great importance to investigate the failure mechanism of urban pipeline under mechanical excavation for safe operation of urban gas pipelines. A rigorous 3D finite element model was established for mechanical response of PE pipe under mechanical excavation in this study. Soil, tooth of the mechanical excavator and pipe were all modelled by continuum solid elements. Surface to surface contact algorithm was utilized simulating the actual interaction between excavation tooth, pipe and soil. Failure procedure of PE pipeline under excavation load was investigated in detail in a baseline analysis based on two commonly adopted criteria, the stress based failure criterion and the strain based criterion. Parametric analysis was also conducted to discuss the effects of excavation position, pipe diameter, pipe wall thickness and internal pressure on PE pipe's mechanical response. Results show that, large stress and strain will occur at longitudinal ends of the dent in the pipe induced by excavation load. And large ovality will occur in the whole contact area of the pipe. The strain based criteria is more suitable for safety assessment of PE pipe, as the outstanding plastic performance of PE material. The excavation position and internal pressure have little influence on pipe's response, while increasing the pipe diameter and wall thickness can decrease the pipe stress and ovality to a certain extent.

Mechanically Versatile Soft Machines through Laminar Jamming

AbstractThere are two major structural paradigms in robotics: soft machines, which are conformable, durable, and safe; and traditional rigid robots, which are fast, precise, and capable of applying high forces. Here, the paradigms are bridged by enabling soft machines to behave like traditional rigid robots on command. This task is accomplished via laminar jamming, a structural phenomenon in which a laminate of compliant strips becomes strongly coupled through friction when a pressure gradient is applied, causing dramatic changes in mechanical properties. Rigorous analytical and finite element models of laminar jamming are developed, and jamming structures are experimentally characterized to show that the models are highly accurate. Then jamming structures are integrated into soft machines to enable them to selectively exhibit the stiffness, damping, and kinematics of traditional rigid robots. The models allow jamming structures to efficiently meet arbitrary performance specifications, and the physical demonstrations illustrate how to construct systems that can behave like either soft machines or traditional rigid robots at will, such as continuum manipulators that can rapidly have joints appear and disappear. This study aims to foster a new generation of mechanically versatile machines and structures that cannot simply be classified as “soft” or “rigid.”

Numerical and Experimental Validation of a 3D Macro-Model for the In-Plane and Out-Of-Plane Behavior of Unreinforced Masonry Walls

ABSTRACTThe behavior of historical UnReinforced Masonry (URM) buildings subjected to earthquake loading is usually governed by a complex interaction between the in-plane and out-of-plane response of masonry walls. In modern masonry building, the in-plane behavior of masonry walls is generally guaranteed in the structural design, but thin non-structural masonry infills are often vulnerable to out-of-plane actions. A reliable prediction of the combined in-plane and out-of-plane behavior of URM walls requires rigorous nonlinear finite element models, whose complexity and computational cost are generally unsuitable for current engineering applications, motivating the research of alternative numerical approaches. This article presents a study for validation of a 3D macro-model intended to simulate the combined in-plane and out-of-plane behavior of unreinforced masonry walls, against experimental results available in the literature and FEM simulations. The model is based on a three-dimensional macro-element whose kinematics is governed by seven degrees of freedom only. The mechanical behavior of the element is based on a fiber discretization approach that adopts basic material parameters. The performance of the proposed macro-element strategy is assessed by means of nonlinear static analyses performed on masonry walls, for which both numerical and experimental results are available in the literature.

Structural behavior of buried pipe bends and their effect on pipeline response in fault crossing areas

Pipe bends, often referred to as “elbows”, are special pipeline components, widely used in onshore buried steel pipelines. They are sensitive to imposed deformations and their structural behavior is quite flexible and associated with the development of significant stress and strain, which may lead to failure. In the present paper, the mechanical performance of buried steel pipeline bends is investigated first, using rigorous finite element models that account for the pipe-soil interface. Three 36-inch-diameter pipe elbows are considered, subjected to pull-out force and embedded in cohesive soils. The elbows have bend angles equal to 90°, 60° and 30°, and bend radius-over-diameter ratio (R/D) equal to 5. The results show the increased flexibility of the pipeline bend with respect to the straight pipe, and are reported in the form of force–displacement diagrams. Furthermore the deformation limits of each elbow are identified in terms of appropriate performance criteria. The second part of the paper investigates the effect of pipe bends on the response of pipelines crossing active faults using a three-dimensional rigorous finite element model. The numerical results refer to a 36-inch-diameter pipeline crossing a strike-slip fault, and show that the unique mechanical response of pipe bends, in terms of their flexibility, may offer an efficient tool for reducing ground-induced deformations. The three-dimensional model employs the load–displacement curves of the first part of the paper as end conditions through nonlinear springs. Furthermore, the results show that there exist an optimum distance of the elbow from the fault plane, which corresponds to the maximum allowable ground displacement. Therefore, pipeline elbows, if appropriately placed, can be employed as “mitigating devices”, reducing ground-induced action on the pipeline at fault crossings.

Simplified method for the assessment of the seismic response of motorway bridges: longitudinal direction—accounting for abutment stoppers

This paper investigates the effect of abutment stoppers on the seismic response of motorway bridges in the longitudinal direction. A rigorous 3D finite element model of a representative overpass bridge, including the entire bridge–foundation–abutment–soil system, is developed and used as a benchmark. The effect of abutment stoppers is shown to be significant, and must therefore be considered for proper simulation of the seismic response of such bridges. Subsequently, the resistance mechanism of abutments triggered when the bridge deck collides on the stoppers is examined. The model is first validated against theoretical solutions. The abutment cantilever wall is subjected to slightly—but crucially—different loading when the deck collides on the stoppers: the loading is applied at the top of the abutment without any rotational restraint. To gain insights on the key parameters affecting the abutment resistance to such passive loading at the top, a dimensionless analysis and a comprehensive parametric study are conducted, employing an equivalent 2D model of the abutment. The latter is validated against the results of a rigorous 3D model. Based on the results of the parametric study, a simplified model accounting for the effect of abutment stoppers is developed. Its efficiency is assessed on the basis of slow-cyclic pushover and nonlinear dynamic time history analyses, using the full 3D model as a benchmark. Overall, the extended simplified model is shown to offer a reasonable approximation (excellent for cohesive soil) of the seismic performance of typical motorway bridges in the longitudinal direction.

Torsional behavior of a new dumbbell-shaped concrete-filled steel tubes

This paper presents the first study of its kind on the torsional moment behavior of a new dumbbell-shape concrete-filled steel tube (DCFST) system. Experiments were conducted to investigate the effect of the dimensions of the web connecting the two circular CFSTs, in terms of its width-to-diameter (b/D) and its minimum depth-to-diameter (h/D) ratios, on torsional strength (Tu). Torsion tests were also conducted on circular CFSTs (CCFSTs), on hollow tubes and on plain concrete, as control specimens. It was shown that Tu of DCFSTs ranged from 1.3 to 2.2 that of the CCFST, depending on (b/D) and (h/D) ratios. Increasing (b/D) from 0.31 to 0.54 resulted in only 7% increase in Tu. However, a further increase from 0.54 to 1.0 resulted in an additional 31% increase in Tu. The effect of (h/D) on Tu was more pronounced at higher (b/D) ratios. In general, increasing the web width appears to be more effective than web height. A rigorous nonlinear finite element model was developed and a simple mechanics-based design equation have been proposed to predict Tu of CCFSTs and DCFSTs and resulted in an average predicted-to-experimental Tu of 0.98.