Hybridbrücken mit blockverleimtem Brettschichtholz

Hybrid bridges with log‐glued‐laminated timber. In Germany, timber as load‐bearing element of bridges is only used for the construction of foot‐ and bicycle viaducts. The development of new timber‐based products and modern technologies gives interesting chances for timber bridge‐building in future. Main beams with high stiffness and stability of dimension can be produced by log‐glued lamination of timber. Efficient hybrid cross‐sections of superstructures as timber‐concrete‐composite constructions can be made by the combination of beams consisting of log‐glued‐laminated timber and concrete deck slab. For road bridges, these hybrid cross‐sections could be used alternatively to conventional methods of construction. The following paper gives a review of the development status of timber‐concrete‐composite construction in bridge building and shows static and constructive specifics.     

Experimentelle und numerische Untersuchungen zum Brandverhalten von Brettsperrholzplatten

Experimental and numerical analysis of the fire behaviour of cross‐laminated timber panels. Cross‐laminated timber panels represent an interesting technical and economical product for modern timber structures and the use of large prefabricated cross‐laminated timber panels for wall and floor assemblies has become increasingly popular for residential timber buildings. The fire behaviour of cross‐laminated timber panels has been experimentally and numerically studied during two different ongoing research projects carried out at the Institute of Structural Engineering of ETH Zürich and the Trees and Timber Institute CNR‐IVALSA in Trento, Italy. The paper presents the main results of the experimental and numerical analyses. Particular attention is given to the comparison of the fire behaviour of cross‐laminated timber panels and homogeneous timber plates. The results of the analyses performed have shown that the fire behaviour of cross‐laminated timber panels depends on the behaviour of the single layers. If the charred layers fall off, an increased charring rate needs to be taken into account. The same effect is observed for initially protected timber members after the fire protection has fallen off. Thus the fire behaviour of cross‐laminated timber panels can be strongly influenced by the thickness and the number of layers. Further vertical structural members (walls) may show a better fire behaviour in comparison to horizontal members (slabs).     

Bewertung der Weiterverwendung einer alten Stahlbrücke auf der Grundlage bruchmechanischer Methoden

Evaluation of the further use of an old steel bridge on the basis of fracture‐mechanics methods. By the example of the bridge at the Dornheimer Weg in Darmstadt, which was built at the beginning of the 20^th century, the application of a fracture‐mechanics analysis procedure for the further use of old steel bridges is presented in this paper. Therefore the so‐called critical components of the bridge are determined, and for these elements the cross‐section items are represented by fracture‐mechanics models (cracked plates). By the assumption of a starting damage in form of a crack the remaining endurance of the component up to achieving the crack length leading to the failure can be determined by using a fatigue crack growth calculation. As far as possible a refinement of the models can be made, in order to avoid a too conservative estimation of the remaining endurance after the first run of the procedure. After iterative application of this method the final calculated endurance of the critical components arises, and it is possible to make a recapitulatory evaluation for the further use of the bridge.     

混凝土结构裂缝扩展的双G准则

断裂力学基本上有两种分析裂缝稳定性的方法 :应力强度因子法和能量法。在混凝土断裂性能的描述上两者应该是等效的。针对目前大多数混凝土断裂模型以应力强度因子型的断裂韧度作为判定准则 ,本文尝试从能量的角度出发 ,结合线弹性断裂力学和虚拟裂缝区上的黏聚力分布 ,以能量释放率G作为断裂性能判定参数 ,建立了混凝土结构裂缝扩展的双G准则。与双K断裂参数相对应 ,双G准则引入了两个重要的裂缝扩展判定参量 :起裂断裂韧度GiniIc 和失稳断裂韧度GunIc 。其中 ,起裂断裂韧度GiniIc 对应于起裂荷载Pini和初始裂缝长度a0 ;失稳断裂韧度GunIc 对应于极值荷载Pmax和临界有效裂缝长度ac。根据线性渐进叠加假定 ,可以把 (Pini,a0 )和(Pmax,ac)代入线弹性断裂力学相对应的公式直接求得GiniIc 和GunIc 。考虑到起裂荷载Pini的不易确定性 ,本文引入黏聚力分布引起的能量损耗GcIc,通过三者GiniIc 、GunIc 和GcIc的关系 ,给出了起裂断裂韧度GiniIc 和失稳断裂韧度GunIc的适用计算公式 ,并通过三点弯曲梁实验得到了验证。     

Ideelle Biegedrillknicklasten von Kreisbogenbindern aus Brettschichtholz mit gebundener Kippachse

Bifurcation loads of circular curved beams of glued‐laminated timber with continuous lateral support. On the basis of the energy method in connection with the static stability criterion a specific finite element formulation is developed with which bifurcation loads of curved beams of glued‐laminated timber can be determined easily. Because of the small numeric effort this procedure is excellently suitable for parameter studies. Extensive single investigations on the basis of two or three‐dimensional finite element models become normally superfluous. In the last part of the paper the results of a study are presented at circular curved beams of glued‐laminated timber. On condition of a continuous lateral support at the upper chord bifurcation loads are determined and presented graphically.     

Einsatz bruchmechanischer Methoden zur Bestimmung der Restnutzungsdauer von Windenergieanlagen

Application of fracture mechanics to the assessment of remaining life‐span of wind turbines. The assessment of cracks in steel tube towers of wind turbines by using models based on fracture mechanics is described. The presented concept can be used to determine the remaining life‐span of wind turbines. Maximum allowable crack dimensions are determined. Growth of the cracks up to these critical dimensions is investigated. For that purpose semi elliptic surface cracks are assumed in the welded joint between door frame and tower hull. Solutions for the stress intensity factor are used to describe the stress field at the crack tip. Input stresses are determined by linear‐elastic finite element calculation. The Failure Assessment Diagram (FAD) is used to determine critical crack dimensions. Crack growth is estimated considering stress ratio and s‐shaped dependency of crack growth rate on cyclic stress intensity.     

层状岩体中洞室围岩层裂及破坏的数值模拟研究

非均质性是岩石类材料的基本特性,均质度的不同使得岩石表现出不同的力学特性和失效特征。基于RFPA?8程序,对层状岩体中洞室围岩的层裂及破坏进行了数值模拟,得到了层裂结构的层裂板厚度。同时,研究结果表明,对于岩石材料,其均质度对受压岩体内裂纹的演化具有重要意义。对于不同均质度范围,洞室围岩将表现出较高均质度的层裂破坏或较低均质度的局部弱化失效两种特点。     

带有裂纹构件的旧钢桁桥的安全评估

采用断裂力学理论对老化钢桁桥的安全性能进行评估并对最终承载能力予以估算,已越来越多地被工程界接受。为了得到试样破坏时的最终承载能力,对三组I型试样进行三点弯曲试验,并推出断裂韧性KIC以及带有一定长度裂纹的构件允许应力[σ]的计算式。采用现场无损测试和磁探法来测定钢构件的裂纹情况。通过对现场测试结果与理论计算结果的比较,可以得出,采用断裂力学方法对带有裂纹构件的老化钢桁桥的安全性能进行评估的必要性。     

Dynamic crack propagation in plates weakened by inclined cracks: an investigation based on peridynamics

Peridynamics is a theory in solid mechanics that uses integral equations instead of partial differential equations as governing equations. It can be applied to fracture problems in contrast to the approach of fracture mechanics. In this paper by using peridynamics, the crack path for inclined crack under dynamic loading were investigated. The peridynamics solution for this problem represents the main features of dynamic crack propagation such as crack bifurcation. The problem is solved for various angles and different stress values. In addition, the influence of geometry on inclined crack growth is studied. The results are compared with molecular dynamic solutions that seem to show reasonable agreement in branching position and time.     

岩石中断裂力学的研究

在断裂力学的基础上,研究了岩石破坏类型和受压裂缝的扩展,并对微裂缝演化进行了探讨,提出利用断裂力学中的裂纹尖端应力和应变场的分布情况,可以预测和制止岩体的失稳。基于能量平衡建立岩石裂纹的扩展条件,进而导出断裂稳定性准则。指出岩石断裂力学中存在的一些问题,并对研究要点进行了总结,为岩石断裂问题研究提供了理论依据。     

混凝土I-型细观断裂模型及其在材料层次#br# 尺寸效应中的应用

混凝土材料的断裂破坏本质上是内部微细裂纹在荷载作用下不断萌生、扩展以及贯通的结果,断裂裂缝在细观层次上则是由砂浆裂缝、界面裂缝以及骨料裂缝3部分组合而成。文章基于细观力学和断裂力学基本理论,建立一类能够同时考虑细观裂缝在混凝土材料内部扩展过程中绕过骨料和穿透骨料发展的混凝土I 型细观断裂模型。与已有试验数据对比表明,文章模型能够有效预测混凝土断裂能等宏观力学参数随细观组分力学性能的变化规律。进而,基于建立的细观断裂模型,初步分析混凝土材料层次的强度尺寸效应,结果表明：当砂浆力学性能确定时,混凝土材料的名义强度与骨料强度和界面强度正相关;界面的力学性能能够显著影响混凝土材料名义强度等宏观力学参数随骨料尺寸的变化规律;高性能混凝土强度随骨料尺寸增大而增大,普通性能混凝土强度随骨料尺寸增大而减小。文章模型分析方法以期为基于性能设计的混凝土配合比研究奠定理论基础。     

Ingenieurholzbau für mehrgeschossige Industriebauten – Konzept und Detaillösungen

Timber construction engineering for multiple storey buildings A multiple storey industrial building was erected in Berlin with timber and timber‐concrete‐composite technology. The load bearing system made of glued laminated timber has a height of four floors in visual quality and with a fire resistance level of R90. The design was done by Roswag architects, the load bearing system with timber and TCC ceilings was developed by Stephan Holzbau GmbH. A special challenge are the joints of the post and lintel construction. The high load impact from the posts had to be transfered by the continuing timber lintels. Therefore, a special solution was found in a polymer compound grout with a very high compression resistance. Drill holes, perpendicular to the grain, were filled with the grout to produce integrated columns. Hence, the impact to the cross section of the glulam lintel is low and the load bearing system transfer still works without limitations. Due to the pre‐fabrication and computer‐aided wood‐working in the workshops of Stephan Holzbau it was possible to meet the very high standards of visual appearance and precision of the construction. Furthermore, the cost of the joints could be reduced by about 30 % compared to usual steel details.     

切削和侵入作用下层理岩层破碎机理浅析

层理岩层是石油钻井中经常遇到的一类地层,特别是在页岩气开采过程中,所钻页岩属于典型的层理岩层。研究层理岩层在钻齿作用下的破碎机理对于控制井眼轨迹、优化钻进参数、提高钻进效率都有很大指导作用。在以往岩石破碎机理研究中,一般多假设岩石介质为各向同性;但是对于层理岩层,由于其表现出强烈的强度各向异性特征,各项同性假设不再适用。本文利用离散单元方法(PFC2D)建立了钻齿与层理岩层相互作用模型。研究了层理岩层在钻齿切削和侵入过程中的破碎情况、裂纹的扩展情况以及钻齿受力情况。研究表明:切削力根据层理倾角分为高切削力区和低切削力区;液柱压力的存在对裂纹扩展有抑制作用,增大切削力;在岩石侵入过程中,层理的存在改变了裂纹的走向,当主裂纹扩展至层理处时,裂纹不再向岩石内部扩展,而是朝着层理的方向扩展。     

Prediction of the fatigue life of slender web plates using fracture mechanics concepts

The results of a series of fatigue tests, on slender plate girders subjected to repeated shear loading, are summarised and used to establish a lower bound fatigue strength curve for the welded web boundary, based on geometric or principal surface stress ranges. The propagation of a semi-elliptical surface crack, through the thickness of a plate, is studied using fracture mechanics concepts. An extensive parametric study indicates that the initial crack size and aspect ratio, the geometric stress concentration at the weld toe, and the plate thickness, all have a significant influence on fatigue strength. It is concluded that the fracture mechanics approach requires specification of a number of parameters which are difficult to determine in practice. However, having specified a realistic crack size and aspect ratio and an approximate stress concentration at the weld toe, the solution can be calibrated against available experimental fatigue strength curves.     

断裂力学钢筋混凝土结构中裂纹发展过程分析

基于断裂力学的基本原理，从改变结构中裂纹受力状况入手，论述了各种状态的应力强度因子，对钢筋混凝土中裂纹扩展状况及与混凝土应力强度因子的关系进行了分析，采取必要的措施降低裂纹的应力强度因子，达到显著效果。     

考虑T应力的岩石压剪裂纹起裂机理

传统断裂理论在研究岩石压剪裂纹起裂机理时,往往仅考虑裂纹尖端应力场Williams展开式中的r~(1/2)奇异应力项,而忽略了非奇异应力项(T应力)的影响,造成理论预测值与试验结果不符。在对压剪应力下裂纹受力特征进行分析的基础上,将T应力引入传统断裂力学的最大周向应力准则,提出了考虑T应力的修正最大周向应力准则。同时考虑压剪应力下的裂纹应力传递特点,在上述准则中又引入裂纹面法向刚度及切向刚度等变形参数。最终建立了能够同时考虑岩石性质和裂纹几何参数(如裂纹倾角、长度等)、强度参数(裂纹面摩擦系数)及变形参数(裂纹面法向及切向刚度)的最大周向应力准则,更好地反映了岩石压剪裂纹起裂机理。算例表明由该方法计算得到的翼裂纹起裂角与试验结果吻合较好,同时通过参数敏感性分析发现裂纹尖端相对临界尺寸对翼裂纹起裂角的影响最大。     

低温冻结岩体单裂隙冻胀力与数值计算研究

寒区岩体工程中含水裂隙随温度降低会发生水冰相变产生冻胀力,内部冻胀力会驱动裂隙发生Ⅰ型扩展,从单裂隙入手,基于弹性力学、渗流力学和相变理论,建立了考虑水分迁移下的冻胀力求解模型,冻胀力不仅随着水分迁移通量的增加而迅速降低,还与岩石基质以及冰体的力学强度参数有关。采用等效热膨胀系数法对低温裂隙中水冰相变下热力耦合应力场进行了模拟分析,并与理论模型计算结果进行对比,结果表明冻胀力解析解与数值解吻合较好;结合断裂力学,利用应力外推法得出了裂隙尖端应力强度因子的数值解,与理论解析解及半解析解也具有较好的一致性,最后通过实例验证了等效热膨胀系数法的可靠性,可为研究低温裂隙岩体冻融损伤与裂隙扩展研究提供参考。     

Correlation between tension softening relation and crack extension resistance in concrete

Changes of the material fracture energy consumption with crack propagation can be described by a crack extension resistance curve, one of the fundamental fracture criteria in crack mechanics. Recently, experimental observation of the fracture behavior in concrete was used to develop a new fracture criterion, the crack extension G_R resistance curve, to analyze crack propagation during the entire concrete fracture process. The variation of the crack extension resistance is mainly associated with the energy consumption in the fracture process zone ahead of the stress-free crack tip. The crack extension resistance is then a function of the softening curve, which is a basic mechanical property in the fracture process zone. The relationship between the softening curve and the crack extension G_R resistance curve is then analyzed based on results of three-point bending beams tests. The results indicate that the characteristic points of the crack extension resistance G_R curve is closely related to the characteristic point on used tension softening curve.