Proof Load Testing Research Articles

Damage assessment of reinforced concrete beams including the load environment

Quantitative condition assessment of structures has been traditionally using proof load test leading to an indication of the load-carrying capacity. Alternative approaches using ultrasonic, dynamics etc. are based on the unloaded state of the structure and anomalies may not be fully mobilized in the load resisting path and thus their effects are not fully included in the measured responses. This paper studies the effect of the load carried by a reinforced concrete beam on the assessment result of the crack damage. This assessment can only be performed with an approach based on static measurement. The crack damage is modelled as a crack zone over an area of high tensile stress of the member, and it is represented by a damage function for the simulation study. An existing nonlinear optimization algorithm is adopted. The identified damage extent from a selected high level load and a low load level are compared, and it is concluded that accurate assessment can only be obtained at a load level close to the one that creates the damage.

Reliability-Based Code Calibration for Axial Ultimate Bearing Capacities of Single Bored Piles in Taipei Basin

ABSTRACTIn Taipei, many pile proof load tests were not conducted to failures but only to a multiple (e.g., 2) of the design load. This leads a difficulty of incomplete information: For these test results, the ultimate bearing capacities of the test piles are unknown. This paper addresses the issue of calibrating resistance factors of piles for the axial ultimate bearing capacities based on the incomplete information from these tests. A simplified probabilistic method is proposed to resolve this issue. A local pile test database of Taipei is presented, and the analysis results show that the inclusion of the incomplete pile load test data helps in calibrating the resistance factors. Moreover, it is found that the calibrated resistance factors for the axial ultimate bearing capacities are consistent to the safety factors that are adopted in the current Taiwan design code. This paper also addresses another important issue for the pile design in Taipei: the end bearing capacity in the sandstone layer is very uncertain. A maximum likelihood method is taken to identify the best estimate for this end bearing capacity. The conclusion of this paper may be useful for reliability-based designs for the axial ultimate bearing capacities of single bored piles in the Taipei region.

Characteristic dynamic traffic load effects in bridges

When formulating an approach to assess bridge traffic loading with allowance for Vehicle-Bridge Interaction (VBI), a trade-off is necessary between the limited accuracy and computational demands of numerical models and the limited time periods for which experimental data is available. Numerical modelling can simulate sufficient numbers of loading scenarios to determine characteristic total load effects, including an allowance for VBI. However, simulating VBI for years of traffic is computationally expensive, often excessively so. Furthermore, there are a great many uncertainties associated with numerical models such as the road surface profile and the model parameter values (e.g., spring stiffnesses) for the heavy vehicle fleet. On site measurement of total load effect, including the influence of VBI, overcomes many of these uncertainties as measurements are the result of actual loading scenarios as they occur on the bridge. However, it is often impractical to monitor bridges for extended periods of time which raises questions about the accuracy of calculated characteristic load effects. Soft Load Testing, as opposed to Proof Load or Diagnostic Load Testing, is the direct measurement of load effects on bridges subject to random traffic. This paper considers the influence of measurement periods on the accuracy of soft load testing predictions of characteristic load effects, including VBI, for bridges with two lanes of opposing traffic. It concludes that, even for relatively short time periods, the estimates are reasonably accurate and tend to be conservative. Provided the data is representative, Soft Load Testing is shown to be a useful tool for calculating characteristic total load effect.

Recent Advances in Performance Evaluation and Flexural Response of Existing Bridges

This paper presents a comprehensive overview of recent developments in the performance evaluation and flexural response of existing bridges from fundamental concepts to advanced topics. The focus is particularly on the structural evaluation to enhance the quality assessment of constructed bridge superstructures, taking into account a synthesis of the most important contributions to the flexural behavior of existing bridges, such as dynamic responses and lateral load distributions. This state-of-the-art paper provides a critical review of published literature and existing assessment methodologies in conjunction with corresponding analysis techniques, including technical comments on the code provisions. Detailed descriptions of the important parameters influencing flexural responses of existing bridges are discussed, such as the geometric effect, loading configuration, present condition of a bridge, and contribution of secondary structural elements. The review also includes the investigation methodologies, namely, the diagnostic and proof load tests. Finally, the current research needs to further advance the performance evaluation technologies for existing bridge superstructures are recommended.

Bayesian approach to proof loading of quasi-identical multi-components structural systems

The present article considers a special class of structural systems for which condition control has proven to constitute a real problem, namely attachments such as e.g., fasteners of façade systems, anchors of retaining walls, rivets in riveted connections, riser attachments of offshore platforms, and many other similar types of structures. Such structural systems are comprised of a large number of quasi-identical components under relatively homogeneous conditions for what concerns both resistances and loads. The problem of condition control is addressed in the context of Bayesian reliability updating; utilizing proof-load tests of a few individual components for the purpose of gaining knowledge about all components. The optimization of proof-load test strategies in regard to the number of components to be tested and the load intensity to be applied is discussed. Two examples are given to illustrate the use and the benefits of the proposed methodology.

Acoustic emission from wire ropes during proof load and fatigue testing

The paper describes an experimental investigation into the enhancement of proof and fatigue testing procedures for wire ropes by incorporating data from acoustic transducer signals. During proof load tests on a selection of damaged wire ropes, it is shown that acoustic emission increases significantly after the damage is inflicted, even though ropes continue to pass the proof test. The relationship between acoustic emission signal characteristics and wire breaks is investigated and it is found that the most effective acoustic signal discriminators are energy and amplitude. Finally, the pattern of acoustic emission signals during a fatigue test is studied and a suggestion is made for a filtering technique to improve the recognition of imminent failure.

Instrumentation and Response Measurement of a Double‐Lane Bailey Bridge During Load Test

During the past decades very significant advances in analytical techniques have led to the development of extremely powerful and versatile methods of structural analysis that are generally computer-based. Within the linear elastic range, these methods are supposed to make predictions with reasonable accuracy. These advanced methods of analysis (finite element, finite strip, grillage analysis, etc.) are used for both the design and strength evaluation of bridges. It has frequently been found that significant discrepancies exist between the predicted and observed responses, even when the loading is within the linear elastic range of the bridge. The discrepancies between the analytical and measured responses were subsequently found to be not due to inadequacies of the methods of analysis, but rather due to the presence of behavioural factors that could not be included in the mathematical modelling because of difficulties in their quantification. The theoretical evaluation of the bridge requires accurate information about material properties, support behaviour, contribution of non-structural members and other factors. Many a time, simplified assumptions are made to account for these parameters in analysis. It makes one believe that even highly rigorous methods of analysis cannot be relied upon unquestionably to predict the actual response of a bridge. There is no better way to understand the shortcomings of the mathematical models used for design (or) evaluation of bridges than to investigate the behaviour of bridges through load testing. Bridge testing is rapidly becoming a major tool in evaluation and may be seen as an acceptance proof test. Load testing provides several benefits from a safety point of view. Load testing helps to assess the behaviour of the bridge under specified loading condition and also to evaluate the load carrying capacity of particular structural configuration. This involves instrumenting the bridge at various critical locations and measurement of strain and deflection responses under the test loads. Selection of response locations, sensor types, installation techniques and data acquisition system are all very important and challenging in actual load testing of bridges. Load testing can have several benefits in evaluation, provided the tests are simple to perform and of reasonable cost. The results must also be able to be interpreted in a manner to realistically assess the behaviour and load carrying capacity of the bridge. Two types of bridge testing are available: (i) proof load testing, and (ii) diagnostic/behaviour tests. In the proof load test, a large load level is used, which assures that there is sufficient strength capacity for the bridge. Several problems exist with this type of test. The test is costly and a high level of risk is involved if the test programme is not properly planned. Hence, many bridge owners are reluctant to undertake such tests. The diagnostic/behaviour test is the most common type of bridge test in which some load, often much below the service load, is used and the response measured for the tested load level. The bridge response for higher classes of loading can be obtained by extrapolation. Behaviour tests are carried out either to study the mechanics of bridge behaviour or to verify other methods of analysis, the objective in the latter case being that after verification, the methods can be used with confidence in the design and evaluation of bridges. One double-lane Bailey bridge, 24.38 m (80 ft) span, was instrumented with 50 strain gauges and 14 dial gauges at various critical locations to obtain the structural responses under static loading. This paper describes the instrumentation details, loading arrangement, testing procedure and measurement of bridge response during the load test. The maximum load permitted on the bridge as per Indian Road Congress (IRC) guidelines was estimated from this investigation.

Reliability Verification Using Proof Pile Load Tests

Proof pile load tests are an important means to cope with uncertainties in the design and construction of pile foundations. In this paper, a systematic method to incorporate the results of proof load tests not conducted to failure into the design of pile foundations is developed. In addition, illustrative acceptance criteria for driven piles based on proof load tests are proposed for use in a reliability-based design. Finally, modifications to conventional proof test procedures are studied so that the value derived from proof tests can be maximized. Whether or not a proof test is conducted to failure, its results can be used to update the probability distribution of the pile capacity using the method proposed in this paper. Hence, contributions of the proof test can be included in foundation design in a logical manner by considering several load test parameters such as the number of tests, the test load, the factor of safety, and test results. This adds value to proof load tests and warrants improvements in the procedures for acceptance of pile foundations using proof load tests. A larger test load for proof tests, say 1.5 times the predicted pile capacity, is recommended since it will yield more information about the capacity statistics and thus allow for more economical designs.

Understanding Capacity Rating of Bridges from Load Tests

The paper discusses a few issues related to capacity rating of existing bridges using field test results. The principles of proof and diagnostic load tests for capacity evaluation are presented through discussing the rating formulas for both tests. The proof load rating is proven a lower bound method. The diagnostic rating is proven the same as linear extrapolation, meaning an upper bound approach in nature. Two different methods used in the literature in calculating load distribution factors are discussed. The two methods are proven theoretically equal in an ideal case, but could be significantly different for field bridges due to the effects of many field factors. To distinguish the two methods, one is defined as load distribution factor ~LDF! and the other as girder distribution factor ~GDF!. An example is used to demonstrate the discussions. The results indicate that, on one hand, the measured LDF, though calculated from measured strains, cannot fully reflect the capacity conditions of field bridges. It reflects only a relative load distribution among girders. On the other hand, the GDF approach may overestimate the bridge capacity, because it includes all the beneficial effects. Some beneficial effects may not be reliable for ultimate capacity and thus need to be removed from the capacity rating.

Safety Factors for Assessment of Existing Structures

Current structural codes have been developed mainly for new design and are not efficient for assessment because of significant differences between these situations. One of the main differences is the possibility to update information about an existing structure. This paper proposes a way in which updating can be taken into account when developing code safety formats (partial factor and load and resistance factor design). Updating information about individual structural properties such as material strengths by carrying out on-site inspections is discussed. Uncertainties associated with on-site testing including “model” uncertainty are considered and a method for treating these uncertainties in the context of the partial factor format is described. Results indicate that “model” uncertainty may have a significant effect on partial factors. Furthermore, we consider updating information about the resistance of the whole structure by proof load testing or by taking into account the previous safe performance of ...

Approach to Formulation of Nondestructive Quality Assessment Criterion of High-Strength Steel Lifting Hooks

The present investigation attempts to formulate the analytical quality assessment criteria of lifting hooks made of 4140 grade (400 °C temper) high-strength steel using linear elastic fracture mechanics (LEFM) procedures. According to the existing standard specifications, the proof load test is mandatory for the acceptance/rejection of these components. Also, there is a provision that the hooks should be checked by nondestructive testing (NDT) techniques and must be free from obvious defects, but there is no stipulation of acceptance limit of defect, if any. It is therefore not realistic and is insufficient, because sometimes tested hooks are found to fail in actual service. In the present paper, acceptance limit that is, the maximum allowable defect sizes for dynamic loads with various stress ratios are quantitatively estimated from the computed defect growth curves. The maximum allowable operating cycles for nondestructively detectable minimum defects are tabulated, from which a logical quality assessment criteria has been formulated for the assessment of the fatigue life of the hooks. This investigation is helpful for the safe operation of the hooks.

Residual stress fields and fatigue analysis of autofrettaged parts

Compressive residual stresses improve the fatigue life of cyclically loaded components. Therefore, they are often introduced intentionally, e.g. by rolling, shot peening, proof load tests, or autofrettage. This paper addresses the calculation of residual stresses due to autofrettage and the resulting increase of the endurance limit. The deformation behaviour of materials unloaded from an elastic–plastic state is dependent on the maximum strain. Unloading curves can be determined experimentally using smooth specimens. In inhomogeneously stressed components each material element is subjected to a different maximum strain value. Thus, the unloading stress–strain behaviour is also inhomogeneous. In order to model this effect in finite element (FE) stress analyses a different unloading elastic–plastic material behaviour is attached to each FE depending on its maximum equivalent strain value reached in loading. Residual stress distributions using this procedure are compared to results from application of different methods proposed in the literature. The calculated residual stress field serves as input for calculating the endurance limit of the component. A fatigue crack growth arrest criterion is used for its prediction. The necessary stress intensity factors due to loading and residual stresses are calculated using the weight function method. Experimentally determined and predicted endurance limits are compared.

Proof load testing for bridge assessment and upgrading

Bridge deterioration with time and ever increasing traffic loads raise concerns about reliability of aging bridges. One of the ways to check reliability of aging bridges is proof load testing. A successful proof load test demonstrates immediately that the resistance of a bridge is greater than the proof load. This reduces uncertainty in the bridge resistance and so increases the bridge reliability. The paper considers a reliability-based calibration of intensities of proof loads for aging bridges to verify either an existing or increased load rating taking into account possible bridge deterioration. Intensities of proof loads are calibrated based on a consistent target reliability index. The influence of test risk, dead to live load ratio, and uncertainties associated with dead and live loads and bridge resistance is considered. The results presented in the paper relate to short and medium span bridges.

Fitness-for-Purpose Evaluation of Bridges Using Health Monitoring Technology

Evaluation of in-field performance of bridges is dependent on many assumptions. Those associated with the analytical model (boundary conditions, influence of curbs, membrane action, the model, etc.) and the resistance model (material properties, condition, fatigue damage, etc.) are commonly recognized as the sources for the differences between theoretical strength and in-service strength as demonstrated by proof load testing worldwide. Likewise there are many assumptions associated with the loading model and the related load factors. Bridge health monitoring technology is providing the opportunity to monitor the in-service performance of bridges. A methodology is presented for undertaking a fitness-for-purpose evaluation (FPE) of bridges based on health monitoring and theoretical data. The methodology is set in the context of limit state codes and illustrated by four case studies, two from Australia and two from New Zealand. Each of the bridges is steel, lowly rated theoretically, and located on relatively low-trafficked routes. The outcome of the FPEs indicates that the structures are safe to remain in service under current loading conditions although interventions are suggested in the relatively short term. Health monitoring was also able to identify the reasons for the significant differences between the theoretical and the health monitoring results. Many of the reasons are related to the specific loading conditions at the site and could not readily have been identified using theoretical approaches, behavioral testing, or proof load testing. Although this technology has its limitations, it is providing better information for those involved in the decision-making process and helping to target actions based on risk.

Control of Live Load on Bridges

Application of field testing for an efficient evaluation and control of live-load effects on bridges is described. A system is considered that involves monitoring of various parameters, including vehicle weight, dynamic load component, and load effects (moment, shear force, stress, strain) in bridge components, and verification of the minimum load-carrying capacity of the bridge. Therefore, an important part of the study is development of a procedure for measuring live-load spectra on bridges. Truck weight, including gross vehicle weight, axle loads, and spacing, is measured to determine the statistical parameters of the actual live load. Strain and stress are measured in various components of girder bridges to determine component-specific load. Minimum load-carrying capacity is verified by proof load tests. It has been confirmed that live-load effects are strongly site specific and component specific. The measured strains were relatively low and considerably lower than predicted by analysis. Dynamic load factor decreases with increasing static load effect. For fully loaded trucks, it is lower than the code-specified value. Girder distribution factors observed in the tests are also lower than the values specified by the design code. The proof load test results indicated that the structural response is linear with the absolute value of measured strain considerably lower than expected. Field tests confirmed that the tested bridges are adequate to carry normal truck traffic.

Local design, testing and manufacturing of small mixed airfoil wind turbine blades of glass fiber reinforced plastics: Part I: Design of the blade and root

Wind energy has attracted a great deal of attention in recent years in Jordan as one of the possible alternative renewable energy resources. Almost of the local research and development activities in this field were directed to explore, develop, and optimal utilization of energy systems. The time has come to establish a link between local scientific (academic) work and local industries to produce a usable technology which will increase the local share in an inevitably emerging wind energy industry in Jordan. To achieve this goal, a well founded manufacturing base is required. The most important component of a Wind Energy Converter is the rotor. The efficiency of a rotor is characterized by its profile (airfoil section) and the corresponding aerodynamic design. A selection procedure of airfoil section and the aerodynamic design of the blade for a small wind turbine are discussed and implemented in this paper (Part I). It is found that for small blades up to 5 m long, two different air foils mixed at the outer third of the span will be sufficient and demonstrated good strength and aerodynamic characteristics. As a composite material, the Glass Fiber Reinforced Plastic was used in designing the rotor blade. This rotor was then installed on 15 kW grid-connected-pitch-controlled machine. A static proof load test indicated that this blade could withstand loads ten times the normal working thrust, and a field performance test showed that the rotor blade has a 41.2% measured average power coefficient.

Role of Load History in Reliability-Based Decision Analysis of Aging Bridges

Bridges deteriorate with time due to aggressive environments and are subject to ever-increasing traffic loads. This results in reduction of their reliability, which may eventually fall below an acceptable level. A successful proof load reduces the uncertainty associated with resistance of the bridge and so increases its reliability. Also, if a bridge has survived for T years of service, its resistance is higher than any of the prior imposed loads and so is influenced by its load history. Thus, the reliability of service-proven (i.e., older) bridges would increase. However, the influence of deterioration and increases in traffic loads may negate this expected increase. The paper considers the effects of load history (proof loads and prior service loads) on the reliability of aging bridges. The influence of bridge age and the magnitude of proof loads on updated estimates of bridge reliability is examined. It was found that proof load testing may not be cost effective if the costs of bridge failure (unsucces...

Proof Load Testing of Deteriorated Steel Girder Bridges

Proof load tests were carried out on three deteriorated steel girder bridges with simple supports. Selected bridges are more than 60 years old, and their span lengths range from 11.0 to 15.5 m. Two military tanks were used to load the bridges up to a predetermined proof load level, which depended on the maximum allowable legal load. The midspan moment was increased in several steps by gradually moving the tanks closer to the midspan. Each tank weighed over 530 kN. Stresses and deflections in steel girders were measured at selected locations. Small stresses and a linear response were considered as the indication of a safety reserve. All bridges were able to sustain the applied proof load. Observed stresses and displacements were considerably smaller compared with those from analytical models. The composite action between concrete slab and steel girders was observed even at the maximum load level, although no shear connectors were provided. Nonstructural members such as parapets, railings, and concrete faca...

Time-Dependent Reliability of Existing RC Structures

The conditional probability of failure of a structure, given that it has already survived for T years, shows that structural reliability increases for service proven structures. Further, construction loads act essentially as proof loads, and so provide a means to detect the presence of gross construction errors. The present paper investigates the effect of construction error, proof load tests prior to occupation, realistic construction loads, and dead loads and office floor live loads on the probability of failure of service proven structures. The human reliability analysis (HRA) approach is used to simulate the effects of construction error. Results obtained from a Monte-Carlo computer simulation analysis showed that up to 50% of failures will occur during construction for reinforced-concrete beams constructed without engineering inspections. This suggests that construction loads act essentially as proof loads; hence, for structures with low live-to-dead load ratios the adverse consequences of construction error will most likely become evident during construction. Nevertheless, engineering inspections is a more effective error control measure.

Field Evaluation of Steel Girder Bridge

The objective of the study was to verify the load-carrying capacity of an existing steel girder bridge. The selected structure was a 70-year-old deteriorated bridge in Michigan. The load-carrying capacity of the bridge was in question because of extensive corrosion of the steel girders. An initial rating indicated that the bridge had a marginal operating rating factor for 11-axle two-unit trucks, which are the heaviest vehicles allowed in Michigan. To avoid the load limit posting, it was decided that investigators would verify by nondestructive testing whether the bridge is safe to carry normal truck traffic. The test procedures used on the selected bridge included tests for obtaining stress histogram measurements and weigh-in-motion measurements and a proof load test. The methodology and the results are described.