Structural Assessment: The Role of Large and Full-Scale Testing

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Structural Assessment: The Role of Large and Full-Scale Testing

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Daniel B. Due to these facts, shear tests on bridge girders from existing bridges are quite rare, 12 - 15 even though they provide important insights into the real structural behavior of bridges. The tests described in this paper will add to the knowledge in an area, which is not covered by databases with laboratory tests. The concept is based on the lower bound theorem of the theory of plasticity. Shear design according to Model Code 7 is divided into three analytical levels of approximation LoA 18 which are suitable to the various design phases. Since the focus of this paper is the assessment of existing structures, only the calculation procedure of the third level of approximation is described briefly in this paragraph.

This model for members with shear reinforcement is based on the simplified modified compression field theory. For the design of structural members, it can be chosen freely between the following limit values: 5. For members with a low amount of transverse reinforcement, the highest shear strength is achieved by using the lower limit of the strut angle.

The shear resistance of concrete V Rd,c is based on the ability of concrete to transmit stresses across cracks.


Thus, the contribution of the concrete decreases with increasing strain in the web, resulting in larger crack openings. Therefore, factor k v in Equation 4 is defined as 6. More background information regarding the theory and calculation examples can be found in Reference Therein, it is recommended that separate verifications of the shear strength for various zones of the beam be carried out.

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These areas are defined according to the presence of different crack types Figure 1 and thus several assessment approaches corresponding to the respective shear behavior need to be used. The mean tensile strength of concrete f ctm can be deduced from the characteristic compressive strength f ck according to Eurocode 2. As a simplification, the crack angle can be taken as a constant value of Arching action represents a beneficial shear transfer mechanism in prestressed members with a low number of stirrups.


In regions with bending cracks, this mechanism is able to transfer shear stresses until a critical biaxial stress state in the compression zone has been reached. A detailed description of the theory, the calculation procedure of the FSC model, and some design examples are given in Reference 8. For the tests, four Two destructive tests were performed to investigate the shear behavior 26 specimens SP2 and SP3 and one specimen was used to evaluate the prestressing forces still present in the beam after decades of use specimen SP1. Figure 2 shows the reinforcement and tendon layout for the girders.

The amount of prestressing still present in the beam was determined by cutting the bars and measuring the remaining strain with strain gauges. Figure 3 shows the cross section at the end of the beams and near the load introduction point, including reinforcement and tendons.

Two temporary steel bridge girders each consisting of three sections of type HEB from a railway bridge were used as the testing frame.

A structural equation modeling approach for examining position effects in large-scale assessments

To simulate different load positions, the distance between the load and the support was varied between 2. The mean cube compressive strength f cm,cube was Figure 4 a shows the load—deflection behavior of the bridge girders at the point of load introduction. All beams failed by yielding of the tendons and the longitudinal reinforcement. Figure 4 b illustrates the documented crack pattern at failure of both test specimens.

Although the measurements from the strain gauges confirmed yielding of the transverse reinforcement, failure of the stirrups was not observed in the tests. Flexural failure with rupture of the prestressing bars was not allowed to occur so as to avoid damage to the test stand due to large deformations and high test loads. Since no shear failure occurred, the maximum shear force values can be considered as a lower limit of the actual shear resistance.

It can be assumed that in these bridge girders, flexural failure occurs before shear failure for the tested load level and load positions. Since in situ shear tests would have been too expensive and time consuming, it was decided to remove two Since it would have been too heavy to lift the entire bridge deck slab, the slab next to the top flange was cut. Figure 6 shows the reinforcement and prestressing tendon layout as well as the cross section of the removed bridge girders.

Since the tendons needed to be cut during the removal of the girders, a new anchorage for the tendons had to be built on one side. Fortunately, the tendons had been grouted well during construction and thus the prestressing cables did not experience significant slip as they were cut. Nevertheless, a new anchorage, similar to the existing anchorage in the bridge, was added. The posttensioning system in the girder consisted of 12 tendons.

Each tendon was composed of 16 single wires with ribbed surfaces. The parabolic tendon profile of the tested specimens can be seen in Figure 6 , where the tendons in the beams have different inclinations, and the inclination of the resultant prestressing force is about 3. The residual amount of prestressing was determined by exposing the tendon and cutting through several wires. The strain in the wires was then measured with strain gauges. Figure 6 shows a schematic of the test setup.

The test setup was identical for both girders; therefore, the investigated parameter was the influence of the tendon profile straight vs.

Most of the difficulty comes from the fact that, since guidelines and common recommendations are lacking, there is a wide variety of approaches, techniques, models… which may lead to apparent contradictory results. A recent European standard EN opened the door towards a wider use of NDT, offering two ways for calibration namely by statistical correlation or by using a basic curve and fitting , but it requires a number of cores which often appears to be too large in practice.

In the same time, the increasing demand for efficient assessment of existing structures has induced the development of NDT use. For economical reasons, the number of cores used for calibration remains small, well below what is recommended in the Standards. It thus seems useful to prepare recommendations explaining what can be obtained in terms of accuracy, and defining what strategy can be followed, at what cost.

If one tries to look further, another issue is that of material variability. This issue is crucial because it impacts the quality of the assessment but also because variability can be considered as a basic information. Semi-probabilistic assessment like in Eurocodes is based on the use of Partial Safety Factors which are calibrated by accounting for material variability.

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NDT appears to be a very good tool for estimating the variability of concrete properties , among which concrete strength. The TC will aim at validating a methodology and writing guidelines defining how concrete strength of an existing structure can be assessed at best at a given scale local, component, storey, structure.

TC membership TC members will be recruited on the basis of their past experience in the field of non destructive assessment of concrete. A specific attention will be devoted such that all parts of the TC-program will be adequately covered. Regional balance will be looked not only for equilibrium but also because emerging countries are very active in this field. The TC members contribution may have various forms: participating to the State-of-the-Art analysis, taking part in the two round robin tests on real data and on synthetic data , sharing data and being involved in the building of a shared database… The time necessary for covering the program is estimated at four years.

State of the Art on practices: techniques and methodologies NDT techniques are used in many countries for concrete strength assessment. Various techniques can be used for gathering useful information either non destructive NDT - rebound, ultrasonic pulse velocity, resistivity, penetration… — or slightly destructive SD— pull-out, drilling…. First, these methods will be reviewed, highlighting how they can contribute to strength assessment, and to what influencing factors they are sensitive.

Second, the added-value of combining several techniques will be analyzed: what is the efficiency and what are the limits of such combination? Third, at a larger scale, strength assessment of a construction requires a strategic approach which defines how many tests are required, where they must be performed and what must be the sequence of NDT tests and coring. Comparison of models for data analysis and processing Model tells how the derived property strength is predicted from direct measurement. A large variety of models exist. There is agreement neither on what is the best model nor on how to evaluate the quality of a model for what purpose?

At what scale?

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The TC will review models and establish a comparison grid leading to criteria that can be used to compare their merits. Another important question is that of calibration.

All experts agree that NDT or SD always requires some calibration process, in order to derive strength values valid in each specific case. The issue of calibration induces many questions what is the reference strength — laboratory?