Development of a cost-effective and flexible vibration DAQ system for long-term continuous structural health monitoring

In the structural health monitoring (SHM) field, long-term continuous vibration-based monitoring is becoming increasingly popular as this could keep track of the health status of structures during their service lives. However, implementing such a system is not always feasible due to on-going conflicts between budget constraints and the need of sophisticated systems to monitor real-world structures under their demanding in-service conditions. To address this problem, this paper presents a comprehensive development of a cost-effective and flexible vibration DAQ system for long-term continuous SHM of a newly constructed institutional complex with a special focus on the main building. First, selections of sensor type and sensor positions are scrutinized to overcome adversities such as low-frequency and low-level vibration measurements. In order to economically tackle the sparse measurement problem, a cost-optimized Ethernet-based peripheral DAQ model is first adopted to form the system skeleton. A combination of a high-resolution timing coordination method based on the TCP/IP command communication medium and a periodic system resynchronization strategy is then proposed to synchronize data from multiple distributed DAQ units. The results of both experimental evaluations and experimental-numerical verifications show that the proposed DAQ system in general and the data synchronization solution in particular work well and they can provide a promising cost-effective and flexible alternative for use in real-world SHM projects. Finally, the paper demonstrates simple but effective ways to make use of the developed monitoring system for long-term continuous structural health evaluation as well as to use the instrumented building herein as a multi-purpose benchmark structure for studying not only practical SHM problems but also synchronization related issues

Effects of differential axial shortening on outrigger systems in high rise buildings with concrete filled steel tube columns

Concrete Filled Steel Tube (CFST) columns are popular in high rise buildings due to their superior strength, seismic and fire resistance capacities and construction simplicity. Structural framing systems in high rise buildings are commonly coupled with reinforced concrete outrigger and belt systems to facilitate lateral load resistance. When axial shortenings of vertical elements occur due to time dependent phenomena of creep, shrinkage and elastic deformations, the horizontal stiff elements balance the shortening differentials in the vertical elements and cause load redistributing among them dynamically. This can result in high transfer stresses induced in the stiff outrigger and belt systems which need to be considered in design or mitigated during construction. To plan mitigation strategies such as the time to connect the shear core to the structural frame to effectively reduce time dependent transfer stresses, it is necessary to quantify current and future differential axial shortenings. This paper first quantifies the differential axial shortening (DAS) between the shear core and columns, considering effects of construction sequence, time dependent material properties and reinforcement and then quantifies the transfer stresses built up in outrigger and belt systems in CFST high rise buildings. This information will be useful in mitigating the adverse effects of these high transfer stresses

Prestress evaluation in prestressed concrete plate-like structures

Condition assessment and capacity evaluation of existing structures using their vibration responses has been subjected to extensive research for many years. Prestressed concrete structures have been one of the main focuses of those studies. In the case of prestressed concrete structures, effective prestress force is the most important parameter for their best performance and yet currently there is no effective method in identifying the prestressing force in an existing prestressed concrete structure. Effect of prestress is different for different types of structural elements and has to be treated accordingly for its accurate quantification. This paper presents a new approach to evaluate the effective prestress force of plate-like structures with simply supported boundary conditions using their vibration responses. The proposed method quantifies the prestress effect with a reasonable good accuracy, even with noisy measurements using both periodic and impulsive excitations. Prestress estimation can be done using collected data from as less as two measurement locations

Effects of CFRP layer orientation on strengthening of hollow steel elements

U ovom se radu proučavaju utjecaji orijentacije slojeva CFRP traka (polimera ojačanih ugljičnim vlaknima) na poboljšanje nosivosti ojačanih čeličnih kružnih šupljih elemenata. Provedeno je ispitivanje savijanjem uz djelovanje sila u četiri točke, a elementi su opterećeni na savijanje do sloma. Poboljšanje nosivosti ojačanih cijevnih čeličnih elemenata analizirano je s obzirom na silu otkazivanja, krutost, kompozitno djelovanje i vrstu sloma. Nosači armirani CFRP trakama s dva uzdužna sloja i jednim po obodu pokazuju bolje ponašanje od nosača armiranih s jednim uzdužnim slojem i dva sloja po obodu.This paper studies the effects of orientation of CFRP (carbon fibre reinforced polymer) strip layers on the improvement of bearing capacity of strengthened steel-made circular hollow elements. The four point bending test was conducted, and elements were subjected to bending until failure. The improvement of bearing capacity of strengthened tubular steel elements is presented in terms of failure load, stiffness, composite beam action, and modes of failure. Beams strengthened with CFRP strips with two longitudinal layers and one layer along the periphery performed better than the beams reinforced with one longitudinal layer and two layers along the periphery

Computation-effective structural performance assessment using Gaussian Process-based finite element model updating and reliability analysis

Structural health monitoring data has been widely acknowledged as a significant source for evaluating the performance and health conditions of structures. However, a holistic framework that efficiently incorporates monitored data into structural identification and, in turn, provides a realistic life-cycle performance assessment of structures is yet to be established. There are different sources of uncertainty, such as structural parameters, computer model bias and measurement errors. Neglecting to account for these factors results in unreliable structural identifications, consequent financial losses, and a threat to the safety of structures and human lives. This paper proposes a new framework for structural performance assessment that integrates a comprehensive probabilistic finite element model updating approach, which deals with various structural identification uncertainties and structural reliability analysis. In this framework, Gaussian process surrogate models are replaced with a finite element model and its associate discrepancy function to provide a computationally efficient and all-round uncertainty quantification. Herein, the structural parameters that are most sensitive to measured structural dynamic characteristics are investigated and used to update the numerical model. Sequentially, the updated model is applied to compute the structural capacity with respect to loading demand to evaluate its as-is performance. The proposed framework's feasibility is investigated and validated on a large lab-scale box girder bridge in two different health states, undamaged and damaged, with the latter state representing changes in structural parameters resulted from overloading actions. The results from the box girder bridge indicate a reduced structural performance evidenced by a significant drop in the structural reliability index and an increased probability of failure in the damaged state. The results also demonstrate that the proposed methodology contributes to more reliable judgment about structural safety, which in turn enables more informed maintenance decisions to be made

Reliability-based load carrying capacity assessment of bridges using structural health monitoring and nonlinear analysis

For assessment of existing bridges, load rating is usually performed to assess the capacity against vehicular loading. Codified load rating can be conservative if the rating is not coupled with the field data or if simplifications are incorporated into assessment. Recent changes made to the Australian Bridge assessment code (AS 5100.7) distinguishes the difference between design and assessment requirements, and includes addition of structural health monitoring (SHM) for bridge assessment. However, very limited guidelines are provided regarding higher order assessment levels where more refined approaches are required to optimize the accuracy of the assessment. This paper proposes a multi-tier assessment procedure for capacity estimation of existing bridges using a combination of SHM techniques, advanced nonlinear analysis, and probabilistic approaches to effectively address the safety issues on aging bridges. Assessment of a box girder bridge was carried out according to the proposed multi-tier assessment, using data obtained from modal and destructive testing. Results of analysis at different assessment tiers showed that both load carrying capacity and safety index of the bridge vary significantly if current bridge information is used instead of as-designed bridge information. Findings emerged from this study demonstrated that accuracy of bridge assessment is significantly improved when SHM techniques along with reliability approaches and nonlinear finite element analysis are incorporated, which will have important implications that are relevant to both practitioners and asset managers

Structural Deterioration Detection Using Enhanced Autoregressive Residuals

This paper presents a study on detecting structural deterioration in existing buildings using ambient vibration measurements. Deterioration is a slow and progressive process which reduces the structural performance, including load-bearing capacity. Each building has unique vibration characteristics which change in time due to deterioration and damage. However, the changes due to deterioration are generally subtler than changes due to damage. Examples of deterioration include subtle loss of steel-concrete bond strength, slight corrosion of reinforcement and onset of internal cracks in structural members. Whereas damage can be defined as major sudden structural changes, such as major external cracks of concrete covers. Herein, a deterioration detection method which uses structural health monitoring (SHM) data is proposed to address the deterioration assessment problem. The proposed novel vibration-based deterioration identification method is a parametric-based approach, incorporated with a nonparametric statistical test, to capture changes in the dynamic characteristics of structures. First, autoregressive (AR) time-series models are fitted to the vibration response time histories at different sensor locations. A sensitive deterioration feature is proposed for detecting deterioration by applying statistical hypotheses of two-sample f-test on the model residuals, based on which a function of the resulting P-values is calculated. A novel AR model order estimation procedure is proposed to enhance the sensitivity of the method. The performance of the proposed method is demonstrated through comprehensive simulations of deterioration at single and multiple locations in finite element models (FEM) of 3 and 20-storey reinforced concrete (RC) frames. The method shows a promising sensitivity to detect small levels of structural deterioration prior to damage, even in the presence of noise

Modelling techniques for structural evaluation for bridge assessment

Load assessment of existing bridges in Australia is evaluated mainly using beam line model and the grillage analogy to examine the structural integrity of bridge components due to live loadings. With the majority of existing bridge networks designed for superseded design vehicular loading, the necessity to utilise more rigorous analysis methods to assess the load effects of bridges is indispensable. In this paper, various vehicular loading cases on a grillage model of a box girder bridge and its equivalent finite element model (FE) are considered, and their applicability for bridge assessment using structural health monitoring (SHM) as defined in the new revision of AS 5100.7 is studied. Based on numerical analyses, it was observed that component-level load effects in the two models have notable differences, irrespective of vehicle speed, position and loading. However, when global-level load responses are compared, the discrepancy in analysis outputs drops dramatically. The modelling ratios developed in this paper are practical and will be applicable with any modelling techniques for bridge assessment under vehicular loading on both a global and component-response basis. It was also observed that FE is more efficient in terms of model updating and damage simulation, and hence more appropriate for implementation of SHM techniques. The proposed flowchart suggested for heavy load assessment incorporates detailed and simple modelling approaches aligned with experimental data obtained by SHM techniques, which can be used for periodic and long term monitoring of bridges. It can enhance the proper determination of bridge condition states, as any conservative estimation of bridge capacity may result in unnecessary load limitations

Vibration characteristics and damage detection in a suspension bridge

Suspension bridges are flexible and vibration sensitive structures that exhibit complex and multi-modal vibration. Due to this, the usual vibration based methods could face a challenge when used for damage detection in these structures. This paper develops and applies a mode shape component specific damage index (DI) to detect and locate damage in a suspension bridge with pre-tensioned cables. This is important as suspension bridges are large structures and damage in them during their long service lives could easily go un-noticed. The capability of the proposed vibration based DI is demonstrated through its application to detect and locate single and multiple damages with varied locations and severity in the cables of the suspension bridge. The outcome of this research will enhance the safety and performance of these bridges which play an important role in the transport network