Crack identification method for tapered cantilever pipe-type beam using natural frequencies

A method to detect location and size of a crack in tapered cantilever pipe-type beam using changes in natural frequencies is presented. The boundary conditions of cantilever beam are applied to a general solution for vibrating tapered beam. Then, an equivalent bending stiffness for cracked beam is used to obtain the natural frequencies. Numerical simulations are carried out to construct a set of training patterns of a neural network, and committee of neural networks is employed to identify the crack. Crack identifications are carried out for the 16 damage cases numerically and for the 3 damage cases experimentally. The identified crack locations and sizes agree reasonably well with the exact values.     

低温下钢结构裂缝损伤识别方法(Ⅱ)

利用截面抗弯刚度的降低来模拟这种裂缝损伤,并以简支梁为研究对象,分别针对单位置及其多位置损伤,利用有限元分析软件计算得到简支梁的位移模态,再通过应变和位移之间具有二阶导数的关系,对简支梁进行了应变模态分析。     

Damage detection method for large structures using static and dynamic strain data from distributed fiber optic sensor

A method for damage detection applicable to large slender steel structures such as towers of large-scale wind turbines, long-span bridges, and high-rise buildings is presented. This method is based on continuous strain data obtained by distributed fiber optic sensor (FOS) and neural network (NN) analysis. An analytical model for cracked beam based on an energy balance approach was used to train a NN. The continuous static strains and the natural frequencies obtained from the distributed FOSs were used as the input to the trained NN to estimate the crack depths and locations. An experimental study was carried out on a cracked cantilever beam to verify the present method for damage identification. The cracks were inflicted on the beam, and static and free vibration tests were performed for the intact case and the damage cases. The distributed FOSs were used to measure the continuous strains. The damage estimation was carried out for the 5 damage cases using the NN technique. It has been found that the identified crack depths and locations agree reasonably well with the inflicted cracks on the structure.     

Wavelet‐Based Detection of Beam Cracks Using Modal Shape and Frequency Measurements

Abstract: When a structure is subjected to dynamic or static loads, cracks may develop and the modal shapes and frequencies of the cracked structure may change accordingly. Based on this, a new method is proposed to locate beam cracks and to estimate their depths. The fault‐induced modal shape and frequency changes of cracked structures are taken into account to construct a new hybrid crack detection method. The method includes two steps: crack localization and depth estimation. The locations of the cracks are determined by applying the wavelet transform to the modal shape. Using the measured natural frequencies as inputs, the depths of the cracks are estimated from a database established by wavelet finite element method. The effectiveness of the proposed hybrid two‐step method is demonstrated by numerical simulation and experimental investigation of a cantilever beam with two cracks. Our analyses also indicate that the proposed method performed reasonably well at certain level of noise.     

Numerical simulation of crack propagation with equivalent cohesive zone method based on virtual internal bond theory

The virtual multi-dimensional internal bond (VMIB) model is developed within the framework of the virtual internal bond (VIB) theory. The VIB theory considers a solid as a three-dimensional network consisting of randomized material particles at the microscopic scale, which are bonded with VIBs possessing “normal stiffness”. In VMIB, however, the material particles are bonded with virtual normal bonds possessing both normal stiffness and shear stiffness. The macroscopic constitutive relationship is derived in terms of the normal and shear stiffness of the bonds. In this paper, we use the VMIB to simulate propagation of the pre-existing cracks with the equivalent cohesive zone (ECZ) method. To make the cracked body a seamless continuum, an ECZ is embedded into the pre-existing crack. The ECZ has the same microstructure as the surrounding material, but an initial internal deformation is assigned to it to ensure that the interaction between the two crack faces is negligible. Thereafter, the ECZ can behave as a pre-existing crack. For the ECZ possesses the same microstructure with the surrounding material, the ‘mended’ cracked body can be described with a uniform constitutive relation. The properties of the pre-existing cracks are implicitly incorporated into the constitutive relationship of VMIB. The advantages of this method are that: (i) the meshing process can be carried out regardless of the geometrical integrity of cracked material; (ii) remeshing is not necessary when crack propagates and (iii) the fracture criterion is embedded into the constitutive relationship. The example simulation shows that the method is efficient and feasible.     

碾压混凝土拱坝诱导缝的半解析有限元计算

在碾压混凝土诱导缝缝端建立奇异超级解析单元，将该解析元与有限元相结合，在单裂纹平面半解析有限元基础上，建立了多裂纹平面半解析有限元程序，计算了碾压混凝土三点弯曲梁断裂参数和拱坝诱导缝等效强度随高程、缝大小和间距等的变化情况，并同其他文献结论和计算模型进行了对比和验证。     

无支撑半刚性连接钢框架的简化分析

用变刚度的螺旋弹簧模拟梁柱节点半刚性连接的弯曲特性 ,建立了半刚性连接钢框架梁单元的刚度矩阵。通过分别引入横梁修正线刚度和修正转动刚度 ,将等效代替框架法和无剪力分配法推广应用于无支撑半刚性连接钢框架的结构分析 ,其计算十分简便     

钢筋混凝土梁开裂后刚度退化研究

编制了钢筋混凝土梁截面弯矩-曲率关系计算程序.采用该程序计算了不同混凝土强度等级、不同截面尺寸和不同配筋钢筋混凝土梁截面的弯矩-曲率关系.根据弯矩-曲率关系的计算结果,研究了混凝土开裂至纵向受力钢筋屈服这一阶段梁截面抗弯刚度的变化规律,给出了估计钢筋混凝土梁开裂后截面抗弯刚度的实用方法.可供既有混凝土结构性能检测评定时应用.     

A new method for the design of tall buildings: The mit design method

A new method for the design of tall and super-tall buildings is presented in this paper. The method is based on transforming the stiffness properties of the building to equivalent beam properties. The equivalent beam is allowed to deform both in bending and shear to capture the true behavior of the building. The method is a stiffness based approach that later requires checks for strength. This is quite appropriate since the design of tall and super-tall buildings is governed by stiffness and not by strength. Knowing the load and assuming a desired shape of the beam, the required shear and bending rigidities of the beam can be calculated. The method is robust and easy to apply, and is particularly suited for space truss systems. The simplicity and efficiency of the method are illustrated with an example.     

交叉加劲钢板深梁弹性屈曲分析

以交叉加劲钢板深梁为研究对象,利用有限元软件ANSYS分析其弹性屈曲性能,讨论了抗弯刚度比、跨高比、钢板深梁厚度对其弹性屈曲性能的影响;考虑钢板深梁在钢框架的弯剪受力特性,根据板的经典理论建立了交叉加劲钢板深梁屈曲荷载计算公式,提出了等效屈曲系数。结果表明:交叉加劲钢板深梁的临界屈曲荷载随抗弯刚度比增大而增大,但达到门槛刚度比后,增大幅度急剧减小,得到门槛刚度比约为10;临界屈曲荷载随跨高比和板厚的减小而减小,等效屈曲系数随板厚减小而增大;等效屈曲系数与跨高比关系曲线由二次抛物线形向波浪形渐变,交叉加劲钢板深梁受力特性由剪切主导向弯曲主导过渡。     

Modal properties of beam-and-block pre-cast floors

This article describes a combined experimental and analytical approach to investigating modal properties of beam and block (B&B) building floors. A rarely performed dynamic testing of a representative sample of four nominally identical B&B open plan suspended ground floors in a real-life building has been carried out using state-of-the-art modal testing featuring a pair of simultaneously operating electrodynamic shakers and multi-channel data acquisition. The testing demonstrated that the floors have considerable transverse stiffness able to engage sizeable parts of the floor area in a manner similar to orthotropic plates. Although nominally identical, the four floors have somewhat different modal properties. In general, although open plan, the B&B floors tested developed considerable levels of non-proportional damping. This is possibly a consequence of friction between non-monolithic floor elements (in the beam-to-block and/or block-to-block contacts). By back analysis, using experimentally measured static stiffness and modal properties, a simple mathematical model of a rectangular simply supported orthotropic plate was verified as an applicable method for the calculation of modal properties of B&B floors tested. However, somewhat surprisingly, for this method to work, it was necessary to assume approximately 2.5 times higher bending stiffness than that of just the beams in the B&B floors, which are the only structural elements in this type of construction. Transverse stiffness was estimated to be 8% of this enhanced main stiffness. This is probably due to composite action of the in situ cast non-structural sand and cement screed and/or engaged friction stiffness between non-monolithically connected beams and blocks.     

Ultimate shear strength of intact and cracked stiffened panels

The present paper focuses on the ultimate shear strength analysis of intact and cracked stiffened panels. Several potential parameters influencing the ultimate shear strength of intact panels are discussed, including the patterns and amplitudes of initial deflection, the slenderness and aspect ratios of the plates, and the boundary conditions defined by the torsional stiffness of support members. An empirical formula for the ultimate shear strength of intact stiffened panels is proposed based on parametric nonlinear finite element analyses in ANSYS. Furthermore, the ultimate shear strength characteristics of cracked stiffened panels are investigated in LS-DYNA with the implicit method. Three types of cracks are considered, namely vertical crack, horizontal crack and angular crack. A simplified method is put forward to calculate the equivalent crack length. And the formula for the ultimate shear strength of cracked stiffened panels is derived on the basis of the formula for intact stiffened panels.     

芳纶纤维筋有黏结部分预应力混凝土梁受弯性能研究

对以芳纶纤维筋为有黏结预应力筋、环氧涂层钢筋为非预应力筋的部分预应力混凝土梁的受弯性能进行研究,共进行了10根梁从混凝土开裂、裂缝开展直至梁受弯破坏的全过程试验。测定了梁的开裂弯矩,梁截面应变分布,变形的发展,裂缝出现、发展及分布情况,提出区分芳纶纤维筋破断及混凝土压坏的界限等效配筋率或界限中和轴高度,推导出芳纶纤维筋拉断和混凝土压坏同时发生,混凝土压坏、芳纶纤维筋未拉断及芳纶纤维筋拉断、混凝土未压坏等情况的受弯承载力计算方法,并提出芳纶纤维筋有黏结部分预应力混凝土梁短期刚度及裂缝宽度的计算方法。     

基于模态刚度的预应力混凝土梁损伤识别方法研究

阐明了模态刚度在损伤识别研究中的重要意义,并对11根多级损伤状态的预应力混凝土梁进行动力试验研究。通过对梁模态分析发现,由于噪音污染等多种因素的影响,仅凭各梁实测模态刚度数值的直观分析很难对梁的多级损伤状态进行有效的识别。为此,提出了以模态刚度变化率为损伤指标的BP神经网络和PNN神经网络的损伤识别方法,并利用实测数据验证所提方法的实用性。研究表明,两种神经网络分类器识别方法均能够有效应用于实际中,且具有很高的损伤识别精度,为结构损伤识别方法研究提供了新思路。     

多梁式混凝土桥梁结构刚度的裂缝特征——动力分解综合识别法

基于有限模型梁的静、动力交替试验研究,分析了钢筋混凝土梁的基频、静刚度、动刚度在“正常使用”阶段的变化规律,建立静、动刚度比与结构内力、自振基频的回归关系式,在对混凝土梁进行非线性分析的基础上,建立了结构裂缝统计参数与结构特征参数之间的回归关系。基于模型桥的静、动力交替试验研究,分析了多梁式T型结构在“正常使用”阶段的变形、应力、内力、频率的变化规律和裂缝开展过程及其统计参数。提出易被工程技术人员所接受的多梁式结构预测评估的动力分解法——根据外观裂缝检查成果和动力试验参数将桥跨结构分解为单片梁,获得各梁目前状态下的动刚度及静刚度,并得到模型试验成果的验证。     

A Simplified Approach for Fire Resistance Design of High Strength Q460 Steel Beams Subjected to Non-uniform Temperature Distribution

Presented in this paper is an analytical investigation on fire resistance of high strength Q460 steel beam subjected to non-uniform temperature distribution. Based on the critical bending moment associated with overall flexural stability and results obtained from the previous experimental investigation on the mechanical properties of Q460 steel at elevated temperature, an equivalent stiffness method is established to evaluate the fire resistance of the beam with the consideration of the influence of temperature gradient across the section of the beam. Lateral torsional buckling resistance, critical temperature and overall stability coefficient are obtained for flexure of high strength Q460 steel beams at elevated temperature. A 3-D nonlinear finite element model, which is capable of accounting for temperature gradient and predicting critical bending moment of Q460 steel beam at elevated temperature, is developed. Results from the finite element simulations are compared with the results determined by the proposed equivalent stiffness method and there is a good agreement between the results of the two methods with the maximum difference of 6%. Using the equivalent stiffness method, parametric studies were carried out to investigate effects of steel grade and temperature distribution pattern on fire resistance of high strength Q460 steel beams. Accordingly, a simplified design approach was proposed to predict the critical temperature and overall stability coefficient of Q460 steel beams subjected to non-uniform temperature distribution. The simplified approach is applicable to high strength Q460 steel beams with cross section dimensions ranging from 175 mm to 350 mm and 250 mm to 500 mm for flange width and section height, respectively.     

预应力混凝土钢管桁架叠合板施工阶段短期刚度研究

预应力混凝土钢管桁架叠合板是近几年出现的一种新型叠合板,叠合板在施工阶段的受力性能非常重要。设计了4个预应力混凝土钢管桁架叠合板试件,并对其进行受弯载荷试验。给出了4个试件的荷载-跨中挠度曲线。同时考虑钢管对混凝土的约束,按照等效截面刚度法推导出了施工阶段的短期刚度计算公式,计算了简支板的跨中挠度。在构件抗弯刚度未发生严重退化时,跨中挠度的计算结果与试验结果较为吻合。     

梁柱节点处锚固方式对粘钢梁抗弯性能影响研究

为探究梁柱节点处锚固方式对粘钢加固梁抗弯性能的影响,对3个采用新型锚固方式的粘钢加固试件以及采用《混凝土结构加固设计规范》(GB 50367—2006)提供的锚固做法对比试件进行单向静载试验,对规范做法给出改进措施并利用ABAQUS建模分析。结果表明:(1)柱端U形箍板与加固钢板间距离对粘钢梁抗弯性能影响明显;(2)采用角钢加锚栓进行锚固的试件抗弯性能良好;(3)采用斜向植筋和加肋锚固钢板进行锚固的试件,其抗裂能力及抗弯刚度与计算值接近,但由于加固钢板剥离,抗弯承载力略低。     

Constrained Ant Colony Optimisation Algorithm for the layout and size optimisation of sanitary sewer networks

The incremental solution building capability of Ant Colony Optimisation Algorithm (ACOA) is used in this paper for the efficient layout and pipe size optimisation of sanitary sewer network. Layout and pipe size optimisation of sanitary sewer networks requires optimal determination of pipe locations, pipe diameters and pipe slopes leading to a highly constrained mixed-integer nonlinear programming (MINLP) problem presenting a challenge even to the modern heuristic search methods. A constrained version of ACOA equipped with a Tree Growing Algorithm (TGA) is proposed in this paper for the simultaneous layout and pipe size determination of sewer networks. The method is based on the assumption that a base layout including all possible links of the network is available. The TGA algorithm is used in an incremental manner to construct feasible tree-like layouts out of the base layout, while the constrained ACOA is used to optimally determine the cover depths of the constructed layout. Proposed formulation is used to solve three hypothetical test examples of different scales and the results are presented and compared with those produced by a conventional application of ACOA in which an ad-hoc engineering concept is used for layout determination. The results indicate the effectiveness and efficiency of the proposed method to optimally solve the problem of layout and size determination of sewer networks.     

钢纤维增强钢丝网混凝土T形梁正截面强度计算及裂缝分析

根据有关试验结果 ,研究了钢纤维增强钢丝网混凝土T形梁的正截面强度及裂缝计算方法。结合某用钢纤维增强钢丝网混凝土建造的桥梁 ,进行了钢纤维增强钢丝网混凝土T形梁的正截面强度及裂缝计算。结果表明 ,钢纤维增强钢丝网混凝土T形梁具有较高的正截面抗弯强度和良好的抗裂性能。