A New Approach for Health Monitoring of Structures: Terrestrial Laser Scanning

Abstract:   A new approach is presented for health monitoring of structures using terrestrial laser scanning (TLS). Three-dimensional (3D) coordinates of a target structure acquired using TLS can have maximum errors of about 10 mm, which is insufficient for the purpose of health monitoring of structures. A displacement measurement model is presented to improve the accuracy of the measurement. The model is tested experimentally on a simply supported steel beam. Measurements were made using three different techniques: (1) linear variable displacement transducers (LVDTs), (2) electric strain gages, and (3) a long gage fiber optic sensor. The maximum deflections estimated by the TLS model are less than 1 mm and within 1.6% of those measured directly by LVDT. Although GPS methods allow measurement of displacements only at the GPS receiver antenna location, the proposed TLS method allows measurement of the entire building's or bridge's deformed shape, and thus a realistic solution for monitoring structures at both structure and member level. Furthermore, it can be used to create a 3D finite element model of a structural member or the entire structure at any instance of time automatically. Through periodic measurements of deformations of a structure or a structural member and performing inverse structural analyses with the measured 3D displacements, the health of the structure can be monitored continuously.     

Internal force monitoring and estimation of a long‐span ring beam using long‐gauge strain sensing

The internal force information of curved beam members is important evidence for the stress state evaluation and design verification of long‐span space structure during construction. Many structural analysis methods for curved beams have been proposed, but most are used to handle the complex geometric shapes. For stress state evaluation of long‐span steel‐concrete composite (SCC) ring beam, this article presents a method that employs a sectional strain distribution model (SSDM) and a fiber model to estimate the internal force distribution depending on the long‐gauge strain. The SSDM of the ring beam used in the article is based on a mechanical model of two‐dimensional (2‐D) plane bending deformation and a three‐dimensional (3‐D) solid finite element model of curved beam members. The application range of the SSDM is defined by the divided different stress areas of ring beam based on the 3‐D solid finite element model. Combined with the established SSDM and the different stress areas, the axial force and bending moment along the span are then identified separately based on the fiber model. In consideration of field monitoring, the influence of different sensor layout on the establishment of SSDM and the result of internal force identification is discussed. The results of the numerical studies show that the proposed methodology can identify the internal force distribution accurately. Internal force of long‐span ring beam from Jiangsu Grand Theater during construction is also identified through the proposed method by using the strain monitoring data.     

Terrestrial laser scanning harnessed for moisture detection in building materials – Problems and limitations

Since the 1980s, Terrestrial laser scanning is successfully adopted in geodesy for contact-free measurements. Collecting dense point-clouds by using TLS is proven as increasingly useful in several other quasi-geodetic, structural, and civil engineering applications. In the study, the newest trend of harnessing TLS is discussed in association with assessing the properties of a scanned object as opposed to its geometrical location. The most promising area of the aforementioned application of TLS is moisture detection in buildings and structures. The present study involved a thorough research programme dedicated to this topic as described in previous publications. Different scanners utilizing visible green and infrared laser beam were harnessed in the research programme. Such aspects of scanning porous construction materials as roughness, colour and presence of water are analysed. Based on the experience, the possibilities and limitations of harnessing TLS for moisture detection in building materials are discussed in the study.     

Analytical Model for Estimation of Maximum Normal Stress in Steel Beam‐Columns Based on Wireless Measurement of Average Strains from Vibrating Wire Strain Gages

In this article, an analytical model is developed for estimation of the maximum normal stress in a beam‐column member subjected simultaneously to an axial load and bending moments. The analytical model is derived by defining the relationship between the maximum strain and average strains measured wirelessly from vibrating wire strain gages. On the basis of the experimental tests on a two‐dimensional steel frame structure with beam‐column members, the performance of the estimation model is evaluated by comparing the maximum strains directly obtained from electrical strain gages and the estimated maximum strains from the model. Using the model, a practical monitoring of the maximum stress in steel beam‐columns is made possible by measuring three average strains at arbitrary locations.     

Fractal Dimension‐Based Damage Detection Method for Beams with a Uniform Cross‐Section

Abstract: For civil structures, damage usually occurs in localized areas. As fractal dimension (FD) analysis can provide insight to local complexity in geometry, a damage detection approach based on Katz's estimation of the FD measure of displacement mode shape for homogeneous, uniform cross‐sectional beam structures is proposed in this study. An FD‐based index for damage localization (FDIDL) is developed utilizing the difference of angles of sliding windows between two successive points, which is expressed in FD. To improve robustness against noise, FDIDL is calculated using multisliding windows. The influence of the spatial sampling interval length and the number of 2‐sampling sliding windows on sensitivity to damage and robustness against noise is investigated. The relationship between the angle expressed in FD and the modal strain energy is established and thereby an FD‐based index for the estimation of damage extent (FDIDE) is presented. The two damage indices are applied to a simply supported beam to detect the simulated damage in the beam. The results indicate that the proposed FDIDL can locate the single or multiple damages, and FDIDE can reliably quantify the damage extent. The optimal spatial sampling interval and the number of sliding windows are investigated. Furthermore, the simulation with measurement noise is carried out to demonstrate the effectiveness and robustness of the two defined FD‐based damage indices. Finally, experiments are conducted on simply supported steel beams damaged at different locations. It is demonstrated that the proposed approach can locate the damages to a satisfactory precision.     

Spritzbeton — der etwas andere Beton

Shotcrete — a special Concrete Analysis of Material Behaviour and Tests Shotcrete is one of the main support measures when doing excavational works according to the “New Austrian Tunnelling Method” (NATM). The peculiarity of a support made of shotcrete is that the young shotcrete, which runs through the hydration process, is subjected to rather large constraints. The estimation of the load bearing capability of shotcrete in comparison to the actual degree of loading needs the knowledge about its time dependent strength and the actual stress state. The problematic stress monitoring using hydraulic stress cells is replaced by the introduction of a time‐dependent stress — strain relationship and displacement resp. strain monitoring. This approach enables the calculation of the actual stresses via the strains which are derived from the monitoring data.     

基于激光影像的物体三维点云获取系统

三维点云获取系统能够快速地获取目标物体的几何信息,生成大量点云,将目标的真实三维形态在计算机中可视化的展现出来。本文提出了一种新的三维点云数据获取的方法,应用在自主开发的基于激光影像的物体三维点云获取系统中,即标定激光面映射目标表面点的一维坐标,利用单像摄影测量后方交会和一维坐标的联合解算,得出目标点三维伪坐标。通过坐标逆向旋转恢复,得到真实的三维坐标数据,据此完整地建立目标物体的三维可视化模型。     

基于三维激光扫描测量技术的井架变形观测

本文基于三维激光扫描测量技术,应用Trimble GX 200地面三维激光扫描仪对山东某矿井架进行了多次井架点云数据采集。首先设计了扫描方案,对获取的点云数据滤波去噪处理并进行了扫描精度检验,其次用目标板的中心拟合坐标分析了井架的垂直变形,最后通过平面拟合方程分析了井架倾斜变形。结果显示相对于传统测量方法,三维激光扫描仪获取大量点云数据能较好地分析井架整体的变形,为矿山井架安全量测提供一种高效、高精度应用方法。     

A Two‐Dimensional Plum‐Blossom Sensor Array‐Based Multiple Signal Classification Method for Impact Localization in Composite Structures

The growing use of composite materials on aircraft structures has attracted much attention for the impact monitoring as a kind of structural health monitoring method. Uniform linear sensor array (ULSA)‐based multiple signal classification (MUSIC) technology is a promising method because of its directional scanning ability and easy arrangement of the sensor array. However, the monitoring range of ULSA‐based MUSIC method is 0°–180°, and its beamforming properties degrade at angles close to 0° and 180°. Besides, the ULSA‐based MUSIC methods proposed require the knowledge of the direction dependent velocity profile obtained by additional experiments. This article presents a novel two‐dimensional (2‐D) plum‐blossom sensor array (PBSA)‐based MUSIC method. First, the velocity propagating at the specific direction is estimated by impact signal itself using PBSA directly. Second, 2‐D PBSA‐based MUSIC method well realizes omnidirectional 0°–360° impact localization of composite structures. Experimental results show its successful performance on epoxy laminate plate and complex composite 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.     

Development of non-contact scanning vibration measurement system for real-scale structures

In order to rationalize vibration monitoring in structures, this paper addresses a non-contact scanning vibration measurement system employing laser doppler vibrometers. Generally, in the case of measurement for concrete members or dirt-adhering steel members, monitoring by laser doppler vibrometers is extremely difficult due to low laser reflectance. In this study, it is experimentally verified that the return or reflected laser beam quantity depends on subtle differences in the surface condition of a measurement object. Therefore, by searching the vicinity of the insufficient measurement point for the optimum point which provides a maximum reflected laser beam quantity, a remarkable improvement of measuring accuracy for real-scale structures can be achieved. As an example, vibrations of steel girders and reinforced concrete deck of actual bridges are measured, so as to verify the measuring accuracy of the developed system, and eigen local mode shapes of the members are able to be identified.     

Integrated multi‐type sensor placement and response reconstruction method for high‐rise buildings under unknown seismic loading

Structural health monitoring system has been implemented on high‐rise buildings to provide real‐time measurement of structural responses for evaluating their serviceability, safety, and sustainability. However, because of the complex structural configuration of a high‐rise building and the limited number of sensors installed in the building, the complete evaluation of structural performance of the building in terms of the information directly recorded by a structural health monitoring system is almost impossible. This is particularly true when seismic‐induced ground motion is unknown. This paper thus proposes an integrated method that enables the optimal placement of multi‐type sensors on a high‐rise building on one hand and the reconstruction of structural responses and excitations using the information from the optimally located sensors on the other hand. The structural responses measured from multi‐type sensors are fused to estimate the full state of the building in the modal coordinates using Kalman filters, from which the structural responses at unmeasured locations and the seismic‐induced ground motion can be reconstructed. The optimal multi‐type sensor placement is simultaneously achieved by minimizing the overall estimation errors of structural responses at the locations of interest to a desired target level. A numerical study using a simplified finite element model of a high‐rise building is performed to illustrate the effectiveness and accuracy of the proposed method. The numerical results show that by using 3 types of sensors (inclinometers, Global Positioning System, and accelerometers), the proposed method offers an effective way to design a multi‐type sensor system, and the multi‐type sensors at their optimal locations can produce sufficient information on the response and excitation reconstruction.     

A Refined Methodology for Durability‐Based Service Life Estimation of Reinforced Concrete Structural Elements Considering Fuzzy and Random Uncertainties

Abstract: A reliable method for service life estimation of the structural element is a prerequisite for service life design. A new methodology for durability‐based service life estimation of reinforced concrete flexural elements with respect to chloride‐induced corrosion of reinforcement is proposed. The methodology takes into consideration the fuzzy and random uncertainties associated with the variables involved in service life estimation by using a hybrid method combining the vertex method of fuzzy set theory with Monte Carlo simulation technique. It is also shown how to determine the bounds for characteristic value of failure probability from the resulting fuzzy set for failure probability with minimal computational effort. Using the methodology, the bounds for the characteristic value of failure probability for a reinforced concrete T‐beam bridge girder has been determined. The service life of the structural element is determined by comparing the upper bound of characteristic value of failure probability with the target failure probability. The methodology will be useful for durability‐based service life design and also for making decisions regarding in‐service inspections.     

Concurrent flexural strength and ductility design of RC beams via strain‐gradient‐dependent concrete stress–strain curve

The authors have previously conducted an experimental study that showed that strain gradient would improve the maximum concrete stress and strength of reinforced concrete (RC) members under flexure. As a continued study, the authors herein will extend the investigation of strain gradient effect on flexural strength and ductility of RC beams to higher strength concrete up to 100 MPa by theoretical analysis. In this study, the flexural strength of RC beams is evaluated using nonlinear strain‐gradient‐dependent stress–strain curves of concrete applicable to both normal‐strength and high‐strength concrete. On the basis of this, a parametric study is conducted to investigate the combined effects of strain gradient and concrete strength on the flexural strength and ductility of RC beams. It was evident from the results that both the flexural strength and ductility of RC beams would be improved with strain gradient considered. From the results, two formulas are proposed for the strain‐gradient‐dependent concrete stress block parameters α and β. A constant value of 0.0032 is proposed for the ultimate concrete strain in flexural strength design with strain gradient effect considered. Lastly, for practical engineering design purpose, design formulas and charts have been presented for flexural strength and ductility of RC beams incorporating strain gradient effect. Copyright © 2015 John Wiley & Sons, Ltd.     

Online‐Monitoring und Berechnung der Betonspannungen infolge thermischen Zwangs für ein Trogbauwerk am Hauptbahnhof Berlin

Online‐monitoring and calculation of the concrete stresses caused by thermal restraint for a trough structure at the main station of Berlin. The trough structure N at the main station of Berlin guides the North‐South‐lines from the lower‐most plane of station to ground level [1]. The high groundwater pressure required the coupling of the trough slab with the back‐anchored underwater concrete slab. Hence, high thermal restraint will arise. The assessment of restraint required several assumptions in addition to standard specifications. Because of these assumptions, the Deutsche Bahn decided the online‐monitoring of important restraint data. Measurements started with the casting of the trough slab and will continue during regular operations. In [1], the results of measurement of temperatures, relative displacements and strains in the injection piles are reported. This report deals with the measurement of concrete stresses in the trough slab. Measurements are being performed by the MPA Braunschweig, thereby applying so‐called stressmeters. In a massive structure, the restraint stresses during early age of concrete depend on temperature and rapidly changing properties of concrete. After initial compression, the stress at the lower edge of trough slab changes into tension. This stress of about 1.3 MPa at the age of 8 weeks is seemingly being exported into the phase of hardened concrete as a constant supplement. The bottom stress increases to ≈ 2.5 MPa in the first winter. Cracking at the bottom is likely. From then on, the stress follows the seasonal change of temperature. At the upper edge of the trough slab compressive stresses prevail. The in‐situ measurement in two planes renders local values of stress. However, the distribution of stresses across the slab's section and the restraint actions cannot be derived with them. Hence, the restraint stresses were also calculated. The calculation is based on the measured temperatures and on material models derived from tests. The restraint was estimated for a cut‐out from the structure. The non‐linear stresses in the center of the slab were computed with the three‐layer‐method. The comparison between measured and calculated stresses renders information on the forecasting quality of the applied structural and material models.     

龙滩水电站左岸地下厂房区三维地应力场反演分析

在龙滩地下厂房区工程地质和试验洞变形监测结果综合分析的基础上，建立地质概化和试验洞施工开挖模型；采用地应力场随埋深变化呈线性分布的假设，并根据地应力测试成果，确定地应力场的分布区间。在此基础上，采用均匀设计方法和三维显式有限差分法，通过模拟试验洞的开挖变形，对龙滩水电站厂房区岩体地应力场进行了三维变形反演分析。对比计算位移和监测位移，计算各方案的位移目标函数，获得了岩体地应力场沿深度的分布规律。比较监测与计算位移，两者在量值上相当，在变形趋势上也基本相同，而且实测与计算所得的地应力值较吻合，表明所获得的岩体地应力场是基本合理的，同时说明此反演方法简便、有效。     

Modelling natural stresses in the arc syncline and comparison with in situ measurements

In the Provence coal basin, the HBCM company mines lignite at great depths. Several measurements of the natural stresses were carried out using flat jack and hydraulic fracturing. A strong stress anisotropy and high stresses were observed. To interpret these measurements, large scale numerical modelling is used. This study presents a comparison between the predicted natural stresses and those measured during field investigations. Three models are successively developed and loaded on the basis of assumptions about current tectonics and historical tectonics. Calculations lead to a better explanation of the orientations and magnitudes of the principal stresses which are not in contradiction with the observation of a NE-SW stress deviation in the vicinity of the area where the measurements were carried out. It appears that the natural stresses in the Arc depend mostly on the major geological structures in comparison to the loading type. The results indicate that there is a relatively good match between predicted and measured stress magnitudes, and orientations. Finally, on the basis of a good estimation of the tectonic loading stages, it is possible to explain the distribution of the natural stresses in theArc syncline, even though the magnitudes of the tectonic stresses were unknown.     

Separation and extraction of bridge dynamic strain data

Through comparing the measured data of dynamic strains due to loading and temperature by the stain gauge and temperature sensor at the same location, the information in the strain data was divided into three parts in the frequency domain by using the defined index named power spectral density (PSD)-ratio index. The three parts are dominated respectively by temperature varying, stresses, and noises and thus can be distinguished from the determined the separatrix frequencies. Also, a simple algorithm was developed to separate the three types of information and to extract the strain caused mainly by structural stresses. As an application of the proposed method, the effect of strain deformation and noises on the fatigue assessment was investigated based on the separated data. The results show that, the determined values of separatrix frequencies are valuable for the monitoring data from other bridges. The algorithm is a multiresolution and hierarchical method, which has been validated as a simple and effective method for data analyses, and is suitable for the compression and preprocessing of the great amount monitoring data and easy to be integrated into the structural health monitoring (SHM) soft system. The strain due to temperature varying attributes a little to the errors of fatigue assessment; however, the noises or random disturbance existed in the monitoring data have much responsibility for the errors, and the main reason is that the random disturbance shifts the real strain/stress amplitude picked up by real structural stress or strain.     

非饱和黄土的三维有效应力

以杨凌非饱和黄土为对象进行了真三轴试验 ;根据笔者的非饱和土有效应力新表达式提出了三维应力状态下有效应力参数的确定方法 ,并计算出相应的有效应力与应变之间的关系曲线。     

Ermittlung der Grenztragfähigkeit von Mehrschraubenverbindungen bei Normalkraft‐Biegemoment‐Interaktion

Determination of the ultimate bearing capacity of multi‐bolt connections with normal force – bending moment – interaction. This paper will present a general design method for single or multi‐bolt connections of beams with arbitrary thin‐walled cross sections, suitable for application in computer programs. The design method is based on the classical strain iteration algorithm for the determination of the stress distribution in cross sections. In this case, the ultimate capacity of bolted connections will be obtained using an iterative numerical determination of the elastic‐plastic stress distribution in the connection elements. The numerical method will be derived in two steps – the first step is the numerical determination of the stress distribution in the connection for a given combination of internal forces and the next step is the determination of the ultimate bearing capacity of the connection. Furthermore, an analytical design method for a multi‐bolt tube connection will be derived. Finally, results of numerical and analytical calculations will be compared with corresponding test results.