Investigation on Ultimate Strength of STS304L Stainless Steel Welded Connection with Base Metal Fracture Using Finite Element Analysis

Many studies on the application of stainless steels as structural materials in buildings and infra-structures have been performed thanks to superior characteristics of corrosion resistance, fire resistance and aesthetic appeal. Experimental investigation to estimate the ultimate strength and fracture mode of the fillet-welded connections of cold-formed austenitic stainless steel (STS304L) with better intergranular corrosion resistance than that of austenitic stainless steel, STS304 commonly used has carried out by authors. Specimens were fabricated to fail by base metal fracture not weld metal fracture with main variables of weld lengths according to loading direction. All specimens showed a block shear fracture mode. In this paper, finite element analysis model was developed to predict the ultimate behaviors of welded connection and its validity was verified through the comparison with test results. Since the block shear behavior of welded connection due to stress triaxiality and shear-lag effects is different from that of bolted connection, stress and strain distributions in the critical path of tensile and shear fracture section were investigated. Test and analysis strengths were compared with those by current design specifications such as AISC, EC3 and existing researcher’s proposed equations. In addition, through parametric analysis with extended variables, the conditions of end distance and longitudinal weld length for block shear fracture and tensile fracture were suggested.     

焊接空间网架结构全寿命期风致疲劳分析

针对焊接空间网架结构在服役期内,随使用年限增加而导致结构刚度逐渐降低,加剧结构风致疲劳损伤,提出对结构进行全寿命期风致疲劳分析,以研究结构整个服役期风致抗疲劳性能,保证结构使用安全。通过分析焊接节点与杆件之间连接刚度随裂缝损伤变化的关系,建立网架结构连接刚度可变模型;通过高周疲劳试验,获得节点与杆件之间焊材的应力-寿命(S-N)曲线及疲劳参数;对结构进行风致响应分析,得到结构杆件连接处的应力时程,利用雨流计数法对应力时程进行循环计数,采用Basquin模型的疲劳寿命估算公式和Miner线性累加损伤理论,得到杆件连接处的累积疲劳损伤值和疲劳寿命。以武汉体育中心游泳馆屋顶网架结构为背景,分析了该结构在无焊接缺陷、有10%的焊接缺陷随机分布和最不利分布三种工况下的100年使用期内,随服役期增加、结构刚度不断降低时的所有杆件疲劳损伤程度。结果表明：网架结构在服役早期,杆件连接处就出现了疲劳损伤和连接刚度降低,随使用年限增加,结构连接刚度进一步降低,疲劳损伤程度也不断加剧,发生连接疲劳破坏的杆件数量大幅增加;特别是有焊接初始缺陷时,结构刚度降低更快,发生连接疲劳破坏的杆件数量更多;即使在良态风环境下,也有杆件连接处发生了疲劳破坏。     

高强度钢材疲劳性能研究进展

疲劳破坏是钢结构失效的主要形式,也是工程界与学术界关注的重点。在总结了国内外高强钢母材、焊缝连接和螺栓连接的疲劳性能研究现状基础上,介绍了疲劳寿命理论计算方法,结合大量试验数据分析统计了高强度钢材疲劳寿命发展规律,并基于国内外不同的疲劳设计规范对高强钢母材及连接接头的抗疲劳能力进行了评估。结果表明：随着钢材强度等级的提高,高强钢母材表现出较好的疲劳性能,规范设计曲线偏于保守,明显低估了高强钢母材的抗疲劳性能;受焊接工艺和焊接质量影响,高强钢焊缝连接的疲劳强度提高幅度较小,ANSI/AISC 360-10和EN 1993-1-9规范曲线能够较好地评估Q460D与Q690D焊缝连接的疲劳特性,并具有足够的安全储备;螺栓连接的疲劳强度受预紧力、摩擦面处理、成孔方式等众多因素影响,已有研究表明随着钢材强度的提高,疲劳强度亦有改善,ANSI/AISC 360-10和BS 7608-2014设计曲线适用于Q460螺栓连接疲劳寿命计算,具有足够安全保障,对于Q690钢螺栓连接疲劳性能评估,规范方法偏于保守。随着高强度钢材在实际工程中的应用增多,需开展大量不同等级高强钢母材和连接形式的疲劳试验,补充Q460及以上强度钢材的疲劳设计方法和细部连接构造。     

Fatigue assessment of a composite railway bridge for high speed trains. Part I: Modeling and fatigue critical details

Steel bridges for high speed trains may sustain excessive fatigue damage due to stronger dynamic effects induced by the increased train speed. Dynamic tests were carried out on a composite railway bridge for high speed trains. A detailed finite element (FE) model of the bridge was established and validated by the dynamic test results. Six types of structural details in the bridge were considered for fatigue evaluation. The stress history of each concerned detail during a single train passage was generated by the validated FE model. The stress spectrum, obtained through Rainflow cycle counting of stress history, was used to calculate the fatigue damage of each detail, based on the detail category specified S–N design curve and the Palmgren–Miner damage rule. Among various structural details, the load carrying fillet weld around the gusset plate of the diagonal bracing at the bridge bearing is predicted to be the most fatigue critical detail. In this paper, a general methodology for determination of fatigue critical details is presented, which can serve as a basis of enhanced fatigue evaluation by using local stress approaches. In “Part II: conditions for which a dynamic analysis is needed” as the continuation of this paper, fatigue assessment will be investigated based on the dynamic stresses predicted by different approaches, i.e. static analysis considering dynamic amplification factor, direct dynamic analysis with a moving load model or a train–bridge interaction model.     

Design of screwed steel sheeting connection at ambient and elevated temperatures

Cold-formed profiled steel roof sheeting can be directly connected to the top chord of a steel roof truss through self-tapping screws. At ambient temperatures, neither EN 1993-1-8 nor EN 1993-1-3 can be used directly for this type of connection. Besides, no design rules are available in EN 1993-1-2 for designing screwed connections in fire. A 3D Finite Element (FE) model for a single-lap shear screwed connection is developed using ABAQUS software. After the validation by tests, the model is used to predict the ultimate resistance of connections at both ambient and elevated temperatures. Further, the effects of edge and end distances on the connection resistance are investigated. Based on the analyses results, revised design equations for predicting the connection resistance are proposed. The design resistance is calibrated by testing and FE analyses results according to the procedure given in EN 1990 and the partial safety factor is derived.     

常温和高温下钢板螺栓连接的设计

通过自攻螺钉可以将冷弯压型钢板与钢屋架的上弦杆直接连接。常温下,无论是EN1993-1-8规范还是EN1993-1-3规范都可以直接作为此类连接的设计规范。此外,在EN1993-1-2规范中,没有针对火灾中螺栓连接的设计条文。利用ABAQUS软件,建立了一个单剪螺栓连接的三维有限元模型。通过试验证实此模型的有效性之后,采用该模型计算常温和高温下螺栓连接的极限承载力。并且,分析了边缘效应和末端距离对连接承载力的影响。根据EN1990规范,考虑了分项系数,在试验和有限元分析结果的基础上,确定承载力设计值。     

Design of screwed steel sheeting connection at ambient and elevated temperatures

Cold-formed profiled steel roof sheeting can be directly connected to the top chord of a steel roof truss through self-tapping screws. At ambient temperatures, neither EN 1993-1-8 nor EN 1993-1-3 can be used directly for this type of connection. Besides, no design rules are available in EN 1993-1-2 for designing screwed connections in fire. A 3D Finite Element (FE) model for a single-lap shear screwed connection is developed using ABAQUS software. After the validation by tests, the model is used to predict the ultimate resistance of connections at both ambient and elevated temperatures. Further, the effects of edge and end distances on the connection resistance are investigated. Based on the analyses results, revised design equations for predicting the connection resistance are proposed. The design resistance is calibrated by testing and FE analyses results according to the procedure given in EN 1990 and the partial safety factor is derived.     

Assessment of the dynamic and fatigue behaviour of the Panaro railway steel bridge

The functionality maintenance of European infrastructures like bridges is acquiring more and more importance due to the huge economic losses related to the interruption of their regular service. In particular, fatigue represents one of the most common failure modes occurring in steel and steel–concrete composite bridges: most failures in steel structures are related to fracture and fatigue. Railway bridges endure millions of stress cycles during their life and they are expected to be highly vulnerable to such phenomena. Phenomena like ‘vibration induced’ and ‘distortion induced’ fatigue are still partially uncovered by actual design codes and they represent critical aspects for the assessment of existing bridge remaining life and for the design of new bridges. The European research project FADLESS ‘Fatigue damage control and assessment for railways bridges’, funded by the Research Fund for Coal and Steel, aims at defining innovative technical guidelines for the assessment and control of existing and new bridges with regard to fatigue phenomena induced by vibrations and distortions produced by train passages. The project combines experimental and numerical techniques to study fatigue cracking induced by vibration and distortion phenomena taking into account the dynamic train–bridge interaction effects and the actual traffic spectra on European railway lines. In the present paper, preliminary analyses performed on the Italian case study, the Panaro Bridge, are being reported upon. Results obtained by standard fatigue assessment according to Eurocode rules were compared with the actual fatigue cracks found in the deck secondary components. A preliminary critical review of adopted fatigue assessment methodologies was carried out. Moreover, experimental tests were designed and performed on the bridge in order to identify global and local vibration modes and to evaluate the strain time-histories under train passages of critical details. Finally, experimental global/local mode shapes were compared to numerical results from the preliminary FE bridge model.     

Fatigue reliability assessment of retrofitted steel bridges integrating monitored data

This paper focuses on fatigue reliability assessment of retrofitting distortion-induced cracking in steel bridges integrating monitored data. The fatigue reliability assessment of the connection details is based on the approach used in the AASHTO standard design specifications with all necessary information from finite element modeling (FEM) and structural health monitoring (SHM). Both in-plane traffic loading and out-of-plane relative displacements are considered along with different connection boundary conditions. The primary cause of the observed fatigue cracks before retrofitting is identified as the out-of-plane relative displacements, while the potential fatigue cracking re-initiation after retrofitting depends on the boundary conditions and critical locations that can be identified from the validated FEM. When the identified critical locations are different from the SHM sensor locations, the original monitored data may be modified by using a spatial adjustment factor (SAF). The proposed approach is illustrated by using an actual bridge monitored by the Advanced Technology for Large Structural Systems (ATLSS) Center, a National Engineering Research Center at Lehigh University.     

Fatigue analysis of long-span suspension bridges under multiple loading: Case study

Long-span suspension bridges are often subject to multiple types of dynamic loads, especially those located in wind-prone regions and carrying both trains and road vehicles. Fatigue assessment shall be performed to ensure the safety and functionality of the bridges. This paper proposes a framework for fatigue analysis of a long-span suspension bridge under multiple loading by integrating computer simulation with structural health monitoring system. By taking the Tsing Ma Bridge in Hong Kong as an example, a computationally efficient engineering approach is first proposed for dynamic stress analysis of the bridge under railway, highway and wind loading. The fatigue-critical locations are then determined for key bridge components, and databases of the dynamic stress responses at the critical locations are established. The time histories of dynamic stresses induced by individual loading during the design life of the bridge are generated based on the databases. The corresponding stress time histories due to the combined action of multiple loading are also compiled. Finally, fatigue analysis is performed to compute the cumulative fatigue damage over the design life of 120 years. The results indicate that it is necessary to consider the combined effect of multiple loading in the fatigue analysis of long-span suspension bridges.     

Fatigue life evaluation of welded joints considering the local multiaxial stress/strain states and numerically determined residual stresses/strains

This paper puts forward a kind of calculation method of estimating fatigue crack initiation life by considering welding residual stresses as initial stresses. The numerical method could be used to quantitatively analyze the influence of residual stresses on the cumulative fatigue damage. In order to gain the distribution of weld-induced residual stresses, the FE analyses as well as measurements were carried out. Based on critical plane approach, analysis of damage parameters was performed considering both welding residual stress and biaxial loads. Subsequently, validity of the proposed numerical method is verified by analyzing fatigue test results for several typical welded joints. It is concluded that the result of the present method is closer to the experimental value than that obtained with empirical formula method in the estimation of fatigue crack initiation life of welded structures.     

Finite element modeling of bolted connections in thin-walled stainless steel plates under static shear

The recently performed experimental study indicates that the current Japanese steel design standards (AIJ) cannot be used to predict accurately the ultimate behavior of bolted connections loaded in static shear, which are fabricated from thin-walled (cold-formed) SUS304 austenite stainless steel plates and thus, modified formula for calculating the ultimate strength to account for the mechanical properties of stainless steel and thin-walled steel plates were proposed. In this study, based on the existing test data for calibration and parametric study, finite element (FE) model with three-dimensional solid elements using ABAQUS program is established to investigate the structural behavior of bolted shear connections with thin-walled stainless steel plate. Non-linear material and non-geometric analysis is carried out in order to predict the load–displacement curves of bolted connections. Curling, i.e., out of plane deformation of the ends of connection plates which occurred in test specimens was also observed in FE model without geometric imperfection, the effect of curling on the ultimate strength was examined quantitatively and the failure criteria which is suitable to predict failure modes of bolted connections was proposed. In addition, results of the FE analysis are compared with previous experimental results, failure modes and ultimate strengths predicted by recommended procedures of FE showed a good correlation with those of experimental results and numerical approach was found to provide estimates with reasonable accuracy.     

半刚性连接组合梁框架足尺模型模拟地震振动台试验

为了研究半刚性连接组合梁框架在地震作用下动力特性以及破坏模式,为半刚性框架抗震设计及在地震区的使用提供依据,进行了1个足尺半刚性连接组合梁框架结构模型振动台试验研究。通过试验,分析了小震、中震和大震作用下半刚性连接组合梁框架结构的动力特性、位移反应、加速度反应、半刚性组合节点内力反应、结构破坏模式。研究结果表明:半刚性连接组合梁框架具有较好的抗震性能,在罕遇地震作用下(1.20g),整体结构仍然没有明显损坏,结构破坏形式为半刚性组合节点和柱脚处产生较大塑性变形,半刚性连接框架结构完全可以在高抗震设防烈度地区使用。对试验框架建立有限元分析模型,进行结构的非线性动力时程分析,计算结果与试验结果较为接近。     

Numerische Untersuchungen zur Ermüdungsbeanspruchung vorgespannter Ringflanschstöße mit Imperfektionen

Numerical investigations on the fatigue‐relevant bolt stresses in preloaded ring flange connections with imperfections. For the structural design of preloaded bolted ring flange connections in tower‐like steel structures (e.g. chimneys or wind turbine tubular towers), the fatigue assessment of the bolts is very important. Gaps between the contact surfaces before preloading (so‐called flange imperfections) may have a negative influence on the fatigue‐relevant bolt stresses. A FE‐model for imperfect ring flanges was built up and validated by means of the results of large‐scale tubular bending tests on flange connections. With the validated FE‐model, a parametric study on imperfectly simulated L‐flange connections having realistic dimensions of typical wind turbine towers was performed. The results enable differentiating the various gaping forms with regard to their negative influence on the bolt fatigue. This influence may not be neglected when assessing the fatigue safety of the bolts.     

Dynamic stress analysis for fatigue damage prognosis of long-span bridges

For fatigue damage prognosis of a long-span steel bridge, the dynamic stress analysis of critical structural components of the bridge under the future dynamic vehicle loading is essential. This paper thus presents a framework of dynamic stress analysis for fatigue damage prognosis of long-span steel bridges under the future dynamic vehicle loading. The multi-scale finite element (FE) model of the bridge is first developed using shell/plate elements to simulate the critical structural components (local models) and using beam/truss elements to simulate the rest part of the bridge (global model). With the appropriate coupling of the global and local models, the multi-scale FE model can accurately capture simultaneously not only the global behavior in terms of displacement and acceleration but also the local behavior in terms of stress and strain. A vehicle traffic load model is then developed for forecasting the future vehicle loading based on the recorded weigh-in-motion (WIM) data and using the agent-based traffic flow microsimulation. The forecasted future vehicle loading is finally applied on the multi-scale model of a real long-span cable-stayed bridge for dynamic stress analysis and fatigue damage prognosis. The obtained results show that the proposed framework is effective and accurate for dynamic stress analysis and fatigue damage prognosis.     

Design and behavior of recentering beam-to-CFT column connections with super-elastic shape memory alloy fasteners

This study investigates the performance of new composite (steel-concrete) moment connections through numerical simulations. The innovative aspects of this research lay in the use of end-plate connections between steel beams and concrete-filled tube (CFT) columns that utilize a combination of low-carbon steel and shape memory alloy (SMA) components. In these new connections, the intent is to use the recentering effect provided by super-elastic SMA tension bars to reduce the level of building damage and residual drift after a major earthquake. The low-carbon steel components provide excellent energy dissipation. The analysis and design of these structures is complicated because the connections cannot be modeled as being simply pins or full fixity ones; they are partial restraint (PR). A refined finite element (FE) model with sophisticated three dimensional (3D) solid elements was developed to perform numerical experiments on the PR-CFT joints with a view to obtaining the global behavior of the connection. Based on the behavioral information obtained from these FE tests, simplified connection models were formulated using 2D joint elements with nonlinear spring components. The behavior of entire connections under cyclic loads was examined. The results were compared with the connection behavior obtained from the 3D FE simulations and corresponding connection tests. Good agreement was found between the simple and sophisticated models, confirming the robustness of the approach.     

Fatigue tests and design of thin CHS-plate T-joints under cyclic in-plane bending

A review of existing fatigue design guidelines has been undertaken and shows that there are no fatigue design recommendations for circular hollow section-to-plate T-joints subjected to in-plane bending. This paper therefore reports on the high cycle fatigue behaviour of welded thin-walled circular hollow section (CHS) tubes of thicknesses less than 4 mm that are fillet-welded onto a plate to form a base plate moment connection. The resultant thin CHS-Plate T-joints are subjected to cyclic in-plane bending load to determine their fatigue strength at different stress ranges. The S–N data from the failed thin CHS-Plate T-joints is analysed using the least-squares method of statistical analysis to obtain a design S–N curve. The derived design S–N curve for the thin CHS-Plate T-joints is compared to existing design S–N curves and also that of square hollow section (SHS)-to-plate T-joints under in-plane bending in current fatigue design guidelines. The thin CHS-Plate T-joints are found to have a better fatigue strength compared to the existing S–N curve for SHS-Plate T-joints under in-plane bending. An appropriate design S–N curve from the group of existing design S–N curves is adopted for the design of thin CHS-Plate T-joints.     

Q690D高强钢螺栓连接疲劳性能试验研究

为探究Q690D高强度钢材螺栓连接的疲劳性能,对Q690D高强钢母材、有孔板和螺栓连接三组试件进行疲劳试验,拟合了S-N设计曲线,并与现行规范进行比较,对其疲劳特性及疲劳寿命给予评价。试验结果表明：Q690D高强钢母材、有孔板、螺栓连接疲劳极限比GB50017理论计算值分别提高了170%、76.02%、47.76%,比AISC360理论计算值分别提高了200%、131.77%、70.49%,说明Q690D高强钢螺栓连接具有较高的抗疲劳能力。讨论了应力集中与螺栓预紧力对试件疲劳强度的影响,应力集中系数越大,疲劳强度越小,螺栓预紧力能缓和应力集中程度,可间接提高疲劳寿命。基于零塑性累积应变率假设得到了疲劳损伤公式,应力集中在一定程度上可以反应损伤发展,损伤曲线可以解释疲劳破坏机理。     

Finite element analysis of precast hybrid-steel concrete connections under cyclic loading

In this paper, a nonlinear finite element (FE) analysis of hybrid-steel concrete connections is presented. The detailed experimental results of the four full-scale hybrid-steel concrete connections with limited seismic detailing have been discussed in a different paper. However, due to the inherent complexity of beam-column joints and the unique features of the tested specimens, the experimental study was not comprehensive enough. Therefore, in this paper, an analytical investigation based on the FE models and using the DIANA software is presented. The FE models were validated using the experimental results of the hybrid-steel concrete connections tested in Nanyang Technological University, Singapore. The critical parameters influencing the joint’s behaviour, such as the axial load on column, the connection plate thickness, and the continuation of beam bottom reinforcement, are varied, and their effects, especially implications on code specifications, are studied.     

Assessment procedure and rehabilitation criteria for the riveted railway Adige Bridge

Traffic patterns increasing and the degradation of existing railway steel bridges has lead to the need of an assessment of their remaining fatigue life, and deciding whether to retrofit or supply structure replacement. This paper deals with the structural assessment of an actual case study, the Adige Bridge, which connects the Rovigo and Padua provinces in northern Italy. The bridge has been in service since 1886 and the overall length is about 161m through three spans. As a reference the innovative procedures to estimate the remaining fatigue life of bridges outlined in the JRC-ECCS document Assessment of Existing Steel Structures: Recommendations for Estimation of Remaining Fatigue Life, has been applied for the case study, together with other codes and technical instructions. Stress data, obtained by a 3D finite element model, were used to estimate the remaining fatigue life. Assessment results, obtained by considering different traffic estimations, point out that most of the identified critical details have an infinite remaining safe life, but at the same time some members appear critical. Appropriate retrofitting criteria are proposed to support the designer in common damage situations.