Resistance of headed studs subjected to fatigue loading Part II: Analytical study

Most design codes consider the static and fatigue resistance of composite steel-concrete structures with separate verifications for ultimate limit state and the limit state of fatigue. For headed shear studs both verifications are based on worldwide performed experimental investigations with push-out specimens. The determination of the fatigue life of headed shear studs in recent European codes is based on the S-N curve developed from the statistical analysis of a great number of cyclic push-out tests. The S-N curve considers the shear stress range as the only loading parameter. In addition, the maximum shear force is limited at serviceability limit states in order to avoid significant inelastic behaviour. However, recent researches showed that beside the shear stress range, also the peak load and the static strength of the shear studs influence the fatigue life. Based on the results of the 71 push-out tests presented in the earlier companion paper, international push-out tests to determine the fatigue life of headed shear studs in solid concrete slabs are sorted and reanalyzed and analytical expressions are developed to predict the fatigue life and the reduced static strength after high cycle preloading. Furthermore, the linear damage accumulation hypothesis according to Palmgren and Miner is modified and improved to consider load sequence effects and non-linear behaviour.     

Zur Frage der Ermüdungsfestigkeit von Kopfbolzendübeln

Fatigue resistance of headed shear studs. The design concepts in current national and international standards for headed shear studs in composite structures under fatigue loads are based on the assumptions, that the ultimate shear resistance of headed shear studs and the load‐deflection behaviour under service loads are not affected by high cyclic loading up to the theoretical value of their life time. These assumptions do not represent the real behaviour of headed shear studs subjected to fatigue loading. The paper deals with the results of the project C8 within the scope of Collaborative Research Centre 398 funded by the German Research Foundation. Based on a comprehensive program of experimental investigations new damage models to describe the effect of high cyclic loading on the local behaviour of headed shear studs are developed. On this basis the interaction between the local damage at the studs and the global behaviour of composite beams is investigated.     

钢桥面-薄层CRRPC组合结构栓钉连接件抗剪疲劳性能研究

采用推出试验和弯曲疲劳试验,研究钢桥面板与薄层CRRPC铺装层间栓钉连接件抗剪疲劳性能。基于Von Mises材料破坏准则和双线性本构关系,对 推出试验模型进行有限元分析、静力试验和疲劳试验,得到不同加载比和界面联接方式下的滑移-加载循环次数曲线,以及栓钉连接件抗剪承载力;开展足 尺组合梁弯曲疲劳试验,考察组合桥面系统中栓钉整体抗剪疲劳性能。研究结果表明：加载比是影响栓钉抗剪疲劳寿命关键因素之一,对于推出疲劳试验, 加载比建议取0.4;钢板与CRRPC间的黏结效应是影响疲劳寿命另一重要因素,对于存在黏结效应的推出试件,在0.5的加载比下等效疲劳寿命可以达到92.2 万次,但无黏结时,疲劳寿命仅为16.9万次;控制栓钉间距是提高组合桥面抗剪疲劳性能的有效方式,在等效实桥应力幅下,即使钢板-CRRPC间不存在黏结 效应,栓钉布距为125mm×125mm,经过8888.9万次疲劳加载,组合梁界面仍没有发生疲劳破坏。     

Static and fatigue behavior of large stud shear connectors for steel-concrete composite bridges

For the design of shear connection in high shear areas of steel-concrete composite bridges, large shear studs can be an excellent alternative. Through the push-out tests on large stud shear connectors up to 30 mm diameter, which are beyond the limitation of current design codes, static and fatigue behavior was investigated and compared with design equations. The ultimate strength of the shear connection showed that the design shear strength in Eurocode-4 and AASHTO LRFD gives conservative values for large studs. The fatigue endurance obtained from the tests was slightly lower than the current design codes in Eurocode-4. Based on the push-out test results on large studs, partial composite beams with about 38% degree of shear connection were fabricated and static tests were performed. The ultimate strength and horizontal shear load redistribution of partial composite beams, which had parameters of stud shank diameters and distribution, were evaluated and group failure in shear span was observed. The ultimate strength of the shear connection had a value about 1.59 times larger than that from push-out tests.     

Resistance of headed studs subjected to fatigue loading: Part I: Experimental study

In steel-concrete composite structures the transfer of longitudinal shear forces at the interface between steel and concrete is mostly realized by headed shear studs. Especially in bridges due to traffic loads these shear connectors are subjected to high-cycle loading and the fatigue resistance governs the design. In this first part of two companion papers a series of experimental work with standard EC4 push-out specimens is presented. The main purpose of these tests was to determine the fatigue life and a possible reduction of the static strength of the headed shear studs subjected to unidirectional cyclic loading. A further aspect was to examine the effects of the loading sequence and damage accumulation on the fatigue life. The results of the experimental investigations show that due to a crack initiation at the stud foot at 10%-15% of the fatigue life, an early reduction of the static strength is caused. Furthermore tests to examine the effects of the loading sequence on the fatigue life revealed that the linear damage accumulation hypothesis according to Palmgren and Miner on which the present design codes are based do not describe the real behaviour.     

Neue Untersuchungen zum Ermüdungsverhalten von Kopfbolzendübeln

New investigations on the fatigue behaviour of headed shear studs. In current national and international standards the determination of the ultimate load capacity and the fatigue life of headed shear studs subjected to high‐cycle loading take place with separate and independent verifications at the ultimate limit state, serviceability limit state and fatigue limit state, respectively. The effect of predamage due to fatigue loading is not considered. The paper deals with new research based on a comprehensive program of experimental investigations which considers the crack propagation through the stud foot and the local damage of concrete surrounding the stud as relevant consequences of high cycle loading. New design methods to predict the fatigue life and the residual strength after high cycle loading as well as the development of an improved damage accumulation hypothesis are the main subjects of this paper.     

Behavior of post-installed shear connectors under static and fatigue loading

This paper describes a series of tests investigating methods to develop composite action in existing non-composite floor systems. Three types of 22-mm diameter post-installed shear connectors were tested under static and fatigue loading. Test results are compared with previous research results on 19-mm diameter, post-installed shear connectors as well as with conventional welded shear studs. Based on the test results, preliminary design equations are proposed for the static and fatigue strength of post-installed shear connectors. These post-installed shear connectors showed a significantly higher fatigue strength than conventional welded shear studs. The superior fatigue strength of these post-installed shear connectors enables the strengthening of existing bridge girders using significantly fewer shear connectors than possible with conventional welded shear studs.     

Trag‐ und Ermüdungsverhalten liegender Kopfbolzendübel unter Quer‐ und Längsschub

Structural and fatigue behaviour of horizontally lying shear studs subjected to vertical and longitudinal shear. In consideration of structural, manufacturing and economic aspects new interesting composite cross sections for buildings and bridges consisting of reinforced concrete and structural steel lead to a horizontally lying arrangement of headed shear studs influenced by edge proximity. Compared to common vertical shear studs the relatively small distance of horizontally lying shear studs from the edge of the slab leads to different bedding conditions of the connectors within the surrounding reinforced concrete material, which results in differing structural and fatigue behaviour of this type of shear connection. For future applications of horizontally lying shear studs on the basis of experimental and theoretical investigations design rules for the shear connection subjected to monotonic vertical shear, monotonic vertical and longitudinal shear and cyclic longitudinal shear are presented.     

Structural performance of composite joints using bent studs

A composite truss bridge is an efficient structural type where the merits of both the steel member and the prestressed concrete member are combined. The design of a connection for the truss bridge is of particular interest due to complex details resulting from concentrated forces at the composite joint. In this paper, experimental investigations were conducted to evaluate the performance of the joint with bent studs welded on a gusset plate. Push-out tests for static and fatigue loadings on a group stud connection were conducted to evaluate the direct shear strength and fatigue endurance of the joint. Because the joints are subject to various loading conditions, flexure-shear tests were also performed to verify the design provisions for the joints under a combination of tension and shear. In order to enhance the structural behavior of the connection, a bent shaped stud was proposed to increase the pull-out strength of the concrete slab. Test results showed that it is adequate to use Eurocode-4 for the evaluation of a shear connection which satisfies the requirement of minimum stud spacing specified in Eurocode-4. Since the shear load is concentrated at the joint structure of composite truss bridges, additional confining reinforcements are needed to enhance the horizontal shear strength of the shear connection. The design provision on the interaction of tension and shear for the joint structure gave conservative results. When the pyramid failure surface was assumed, internal studs had little effect on the pull-out strength of the joint. Constructability of the joint can be enhanced by decreasing the number of connectors and by strengthening the concrete slab through simplified design checks assuming four failure modes and their interactions.     

阵列式布设内栓钉的钢管混凝土K型节点足尺模型疲劳性能研究

为研究阵列式布设内栓钉的钢管混凝土K型(CFST KS)节点疲劳性能,进行CFST KS节点和不布设内栓钉的钢管混凝土K型节点(CFST K)足尺模型的疲劳试验和有限元研究,对比分析了2种节点的热点应力分布、疲劳裂纹扩展过程和疲劳破坏模式,研究了内栓钉布设区域与排布方式对热点应力的影响,采用基于热点应力的S-N曲线探讨疲劳寿命的判定标准与疲劳寿命评价方法。结果表明：CFST KS节点的热点应力宜采用3点外插法计算,最大热点应力位于冠点处;CFST KS节点的疲劳性能演化过程可以分为疲劳裂纹的萌生、扩展和破坏三个阶段,CFST KS节点较CFST K节点在裂纹扩展阶段荷载反复次数更高,在裂纹破坏阶段疲劳裂纹的扩展速率更低,由于布设了内栓钉,疲劳裂纹扩展至鞍点附近后不再继续沿着相贯线方向扩展,而往主管环向且与主管轴线呈一定角度发展;建议采用CFST K节点冠点应变峰值变化15%对应的荷载反复次数作为疲劳破坏的判定基准;提出了基于热点应力的CFST K节点疲劳设计S-N曲线,并依据S-N曲线预测在同等热点应力幅下,布设内栓钉可以使CFST K节点的疲劳寿命提高45%;减少支管侧内栓钉的轴向间距和径向间距可以提高节点的疲劳性能,非支管侧的栓钉布置、栓钉的尺寸以及栓钉的轴向布置范围对疲劳性能的影响较小,栓钉的轴向布置范围对栓钉自身的受力状态有所影响;基于CFST KS节点静力与疲劳性能提出了内栓钉的合理布置形式。     

静力和疲劳荷载下后锚固抗剪连接的性能研究

介绍了一系列试验研究方法,以研究将现有非复合楼板系统改建为复合楼板系统之后的复合作用。在静力和疲劳荷载下,对直径22mm的三种类型的后锚固抗剪连接进行了测试。将测试结果与先前对19mm直径的后锚固抗剪连接,以及传统的焊接抗剪螺栓的研究结果进行了对比。基于试验结果,对后锚固抗剪连接的静力和疲劳强度的设计提出了初步的计算公式。相比传统的焊接抗剪螺栓,后锚固抗剪连接表现出了极高的抗疲劳强度。这样,可以采用比传统焊接抗剪螺栓更少的抗剪连接,实现对现有桥梁的加固。     

闭口型压型钢板-混凝土组合板疲劳性能试验研究

为研究闭口型压型钢板-混凝土组合板的疲劳性能,完成14块闭口型压型钢板-混凝土组合板的疲劳试验研究。试验主要考察组合板端部栓钉布置、组合板板厚、疲劳荷载水平及组合板加载剪跨比等4个主要因素对组合板疲劳破坏模式及疲劳损伤程度的影响。试验过程中对组合板在各个关键阶段的动挠度、动滑移、压型钢板及混凝土应变、组合板残余挠度及组合板剩余承载能力等参数进行测量和分析。在试验研究基础上,根据组合板疲劳破坏形态及损伤发展情况,着重对组合板的疲劳性能进行较全面研究,初步提出组合板疲劳性能水平评估方法及组合板疲劳设计建议。试验研究结果表明,无栓钉组合板的疲劳性能明显差于带栓钉组合板,布置栓钉可大大提高组合板疲劳性能;无栓钉组合板的设计疲劳荷载上限可以取为40%的静力极限荷载,而带栓钉组合板的设计疲劳荷载上限可以取为60%的静力极限荷载。研究结果为闭口型压型钢板-混凝土组合板疲劳设计计算提供有利依据。     

Neue Untersuchungen zum Ermüdungsverhalten von Verbundträgern aus hochfesten Werkstoffen mit Kopfbolzendübeln und Puzzleleiste

New investigations on the fatigue behavior of composite beams made of high strength materials with two different kinds of shear connection. Scope of the research project [1] was the investigation of the fatigue behaviour of different shear connectors (headed studs ∅︁ 22 mm and puzzle‐strip) together with high strength materials (steel S 460 and concrete C 80/95). For both types of shear connectors cyclic push‐out‐tests as well as large scale experiments with composite beams under cyclic loading have been carried out. The headed studs failed after a mean lifetime of 2,3 million cycles, the residual strength after 1 million cycles was reduced by about 25% compared to the static push‐out strength. In the puzzle‐strip cracks were visible, however, the residual strength was not reduced at all. Further, the push‐out tests with headed studs show a slight improvement of the fatigue behaviour using high strongth concrete (HSC). The cyclic beam tests show that the prediction of the fatigue according to EC 4 – as for specimens with normal strength materials – is not satisfactory. A good prediction of the crack propagation (and fatigue) can rather be achieved by using a crack‐slip‐relationship [6]. Also for shear joints with continuous puzzle‐strips a threshold value can be determined by fracture mechanics to which no further crack propagation (theoretically) occurs. However, this depends on the state of the cutting‐induced heat treatment of the surface material and on the surface roughness.     

Experimental study on mechanical behavior of shear studs in assembled monolithic steel-concrete composite beam

为研究装配整体式钢-混凝土组合梁中栓钉抗剪连接件的受力性能,设计了10个栓钉抗剪连接件,对其进行推出试验,得到了现浇混凝土板和预制混凝土板中栓钉抗剪连接件在单调和重复荷载作用下的界面剪力-滑移曲线以及破坏形态。结果表明：预制混凝土板中栓钉受剪承载力比现浇混凝土中栓钉受剪承载力略低,均为栓杆剪断和栓钉根部焊缝破坏;重复荷载作用下峰值界面剪力对应的界面滑移明显大于单调荷载作用下峰值界面剪力对应的界面滑移;重复荷载作用下的峰值界面剪力与单调荷载下的峰值界面剪力相当,其界面剪力-滑移曲线基本一致。     

栓钉锈蚀的钢-混凝土组合梁疲劳性能研究

为研究栓钉锈蚀后负弯矩区钢-混凝土组合梁的疲劳性能,分别进行1个试件的静力试验和5个试件的疲劳试验研究。通过静力试验获得组合梁的极限荷载,为疲劳试验提供依据。通过疲劳试验测试分析了不同栓钉锈蚀率组合梁在经历不同的疲劳循环加载次数后的材料应变、跨中挠度、残余变形及刚度等。试验结果表明,栓钉锈蚀率的增加使组合梁疲劳寿命下降,使负弯矩区组合梁在经历相同疲劳加载次数后的残余变形增加。组合梁刚度在疲劳加载过程中有一定程度的退化,弯曲刚度的退化与栓钉的锈蚀率无明显关系,滑移刚度的退化会随着栓钉锈蚀率的增加而加剧。     

高温下钢筋桁架楼承板中栓钉抗剪性能研究

对15个试件进行常温和高温下推出试验,研究钢筋桁架楼承板中栓钉的抗剪性能,得到混凝土楼板和栓钉不同位置处的温度分布以及栓钉受剪承载力随温度的退化规律。试验结果表明,钢筋桁架楼承板中栓钉在常温和高温下的破坏均为栓钉剪断破坏,栓钉根部混凝土局部压碎,但是与平板混凝土板中栓钉的破坏位置不同,所研究的栓钉剪断破坏的位置在钢梁上翼缘处,而不是在栓钉根部焊缝处,这也在很大程度上导致了钢筋桁架楼承板中栓钉受剪承载力比平板混凝土板中栓钉低。钢筋桁架楼承板中栓钉的受剪承载力和刚度均随温度的升高而降低。通过对试验数据的分析,提出了钢筋桁架楼承板中栓钉高温下受剪承载力和荷载-滑移曲线的计算方法。     

集簇式栓钉抗剪承载力折减系数计算方法

为给装配式钢-混组合结构梁桥集簇式栓钉抗剪连接件的设计提供参考,深入研究了集簇式栓钉抗剪连接件的受力特性。采用ABAQUS有限元软件建立推出试验有限元模型,对群钉效应及其主要影响因素进行了有限元参数化分析,并给出综合考虑混凝土强度、栓钉排数、栓钉纵向间距影响的集簇式栓钉连接件抗剪承载力折减系数计算公式。结果表明:受到群钉效应的影响,集簇式栓钉连接件的单钉平均抗剪承载力有较大程度的折减并且钉群受力呈现明显的不均匀性; 随着混凝土强度的提高,φ22×200栓钉连接件抗剪承载力、抗剪刚度逐渐增大; 当栓钉排数从3排增加至7排时,群钉的单钉平均抗剪承载力以及栓钉抗剪承载力折减系数逐渐降低,同时钉群受力的不均匀程度大幅提升; 当栓钉纵向间距由4d(d为栓钉直径)增加至8d时,群钉的单钉平均抗剪承载力以及栓钉抗剪承载力折减系数呈现增大趋势; 提出的集簇式栓钉连接件抗剪承载力折减系数计算公式计算值与有限元值吻合良好,可以为装配式钢-混组合结构梁桥集簇式栓钉抗剪连接件的设计提供理论依据。     

H型钢腹板焊接栓钉的部分外包混凝土组合构件纵向受剪性能试验研究

通过13个在单调荷载作用下H型钢腹板焊接栓钉的部分外包混凝土组合构件的推出试验,对其破坏形态、荷载-滑移特性和纵向剪力传递性能进行了研究,分析了栓钉数量、栓钉直径、栓钉布置方式、型钢翼缘宽度、腹部混凝土箍筋布置方式及加载方式等对受剪承载力的影响。试验结果表明：H型钢腹板焊接栓钉的部分外包混凝土组合构件的主要破坏形态为外包混凝土劈裂破坏和栓钉剪断破坏;典型的荷载-滑移曲线可分为3个特征阶段,分别为无滑移阶段、荷载上升段及荷载下降段;腹部栓钉有效地提高了部分外包混凝土组合构件的纵向受剪承载力,且受剪承载力随栓钉直径的增大和栓钉数量的增加而增大;腹部栓钉纵向布置优于横向布置,栓钉纵向偏心布置对受剪承载力影响较小;宽翼缘型钢能更好地约束腹部混凝土,提高其受剪承载力;腹部箍筋不同布置方式对极限荷载的影响较小;通过分析构件的受力机理和纵向剪力传递模式,提出了H型钢腹板焊接栓钉的部分外包混凝土组合构件纵向受剪承载力算式,计算结果与试验结果吻合较好。     

Longitudinal shear test on steel-concrete composite beam with two rows of shear studs

为研究带双排栓钉的组合梁的纵向受剪性能,进行了7个带双排栓钉的钢-混凝土组合梁试件和1个带单排栓钉的钢-混凝土组合梁对比试件的纵向受剪试验。研究栓钉横向或纵向间距、横向配筋率、混凝土强度、栓钉排布方式等因素对带双排栓钉的钢-混凝土组合梁纵向受剪性能和混凝土板破坏形态的影响,并分析了各研究参数对峰值荷载和极限滑移量的影响。结果表明：带双排栓钉的钢-混凝土组合梁试件破坏模式主要表现为“八”字形裂缝的剪切破坏;栓钉横向、纵向间距、混凝土强度以及混凝土板配筋率是影响钢-混凝土组合梁受剪承载力的主要因素;受剪承载力均随着栓钉横向间距、纵向间距、混凝土强度以及混凝土板的横向配筋率的增加而增大。     

Fatigue life estimates for a slender orthotropic steel deck

Fatigue cracks in several types of welded joints and geometrical details have been reported to occur in a large number of slender orthotropic decks of existing steel bridges in many parts of the world. Some of these cracks are initiated very early, a few years after the bridge is brought into service. That is why it is said that fatigue life estimation for the welded joints of orthotropic decks is not an easy designing task. This is so because the random variation of local stresses is sensitive to many factors, in particular to those involved in the dynamic interaction between the vehicle’s tyres, the pavement and the steel structure.This paper presents and discusses the main causes for the observed cracks and the outcome of the ultimate fatigue life estimates for typical welded and geometrical details of a slender orthotropic deck with trapezoidal cross-section longitudinal ribs. This is the deck of an existing long-span steel bridge, which has been strengthened by adding a reinforced concrete layer fixed with shear studs to the steel deck top plate. This was done to avoid continued and extensive repair work and to enhance the fatigue life expectancy of the steel deck structure. The fatigue life estimation is done with the aid of refined numerical modelling and in situ experimental strain measurements and also by taking into account all the built-in structural alterations, changes in volume of traffic and in vehicles loading which have occurred during this bridge’s 32 years of service life.