Full Scale Test of a 60-Year Old Mass-Concrete Arch Bridge

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1 Full Scale Test of a 60-Year Old Mass-Concrete Arch Bridge M.S. Marefat, Assoc. Prof., Civil Eng. Dept., Tehran University; and Research Advisor, Iranian Railway Research Center, Tehran, IRAN. E. Ghahremani-Gargari, Civil Engr., Iranian Railway Research Center, Tehran, IRAN. R. Naderi, Civil Engr., Iranian Railway Research Center, Tehran, IRAN. Abstract In an experimental program, the remaining load carrying capacity of a 60-year old railway bridge, a five span mass concrete arch bridge, was studied. The program included both dynamic and static loading tests. In the static test, about 500 tons of weight, comprised of out-of-service rails, was gradually applied on the middle span and deflections at critical sections were recorded. In the dynamic test, a 120-ton locomotive passed over the bridge at different speeds and accelerations and deflections were measured. The experimental study showed that a relatively large strength reserve and a safe performance still existed under service conditions. Keywords: Railway bridge, Mass concrete arch bridge, Load test, Dynamic test, Static test. Introduction Determination of load carrying capacity of existing bridges is one of the most important and complicated issues in bridge engineering. The remaining load capacity cannot be determined only by using design drawings, numerical modeling, and conventional structural analyses because of many uncertainties involved in the bridge response. Change in material properties due to aging, the effects of corrosion and fatigue, the differences between as-built characteristics and design specifications, participating of non-structural elements in strength of the bridge and uncertainties involved in restrain conditions of the supports, all prevent theoretical analyses to produce reliable results. To overcome such shortcomings, load test provides a dependable solution [1,2,3]. In this document, loading test of Akbar-Abad Railway Bridge is described. The bridge is more than 60-year old, and has been subjected to environmental and structural damages, and a relatively wide and deep cracking at its key (Fig.1). Such indications have produced uncertainty about safety of the bridge under service conditions. To determine the remaining strength of the bridge, dynamic and static tests were carried out. The experiments indicated that there is still a relatively large strength reserve in the bridge under service conditions [1,2].

2 Instrumentation The bridge consisted of five identical spans; each has 6-meter length. Only the middle span was tested. The Instrumentation set-up is shown in Fig.2. As is seen, 9 LVDTs were used to measure displacements and 2 accelerometers were used to record accelerations. The 9 LVDTs were placed at critical locations, i.e., at the key (midspan), 1/4 th of the span, both ends of the arch, and above two foundations. The two accelerometers were mounted at the midspan and were used in the dynamic test. A 30-channel static data logger and a 10-channel dynamic data logger plus a PC computer were used for recording the measured data. All the instruments were of TML (Japan made) mark. Figure 3 illustrates the wide crack and the installed LVDTs and accelerometers at the midspan.

3 Dynamic Loading Test In the dynamic test, a locomotive passed above the bridge at speeds of 10,20,40,60,80 and 90 km/h. Recording of data were commenced before the locomotive reaching the bridge and continued for a while after passing over the bridge. The frequency of data sampling was 500 Hz. Figure 4 illustrates the site and the 6-axle (120 tons) locomotive used in the test [1,4,6]. In total, data were recorded from five channels for ten different speeds and 50 curves were obtained; from which, two are illustrated in Figures 5 and 6.

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5 Figure 5 shows acceleration variation for the speed of 90km/h, which was recorded at the key. There are two peaks in the curve that correspond to the instantaneous passages of two axles of front and rear bogies of the locomotive. The instantaneous speed of the locomotive was determined by dividing the distance between front and rear axles (12.33 m) by the recorded time period between two peaks. This curve shows a relatively fast fluctuation under dynamic load and a relatively rapid decay of amplitude after passing of the locomotive. These characteristics suggest relatively high natural frequencies, compared to the frequency of moving load, and a relatively big damping ratio for the bridge. These facts can be described by the existence of relatively large mass and high stiffness for these kinds of bridges, and the effects of fill material and ballast layers above the arch. To identify dynamic characteristics of the bridge, a method known as System Identification from Output Measurements was used [7]. To analyze the data, the Cross- Spectral Density Functions were calculated for the measured accelerations [7]. Analysis of the recorded data resulted in a value of 14.6 Hz for the first natural frequency [1]. Figure 6 exhibits variation of vertical deflection under dynamic load. This curve also involves two peaks, which correspond to instantaneous passages of two bogie-axles of the locomotive. Fast fluctuation of deflection amplitude under dynamic force and rapid decay of displacement immediately after passing of the moving load are seen. These characteristics are in harmony with those of previous curve for acceleration. In addition to previous conclusions, observation of the two curves indicates that no resonance effect is produced in the interaction between the locomotive and bridge. This means that the natural frequency of the bridge relative to the frequency of the moving load is large enough to avoid any kind of resonance. One important result of this experiment was determination of the impact factor. This factor depends on the length of free span, structural system, material specifications, support conditions, load patterns, and type of response. For this bridge, the recommended value, according to UIC is 1.67 (for bending) and 2.0 (for shear). In this recommendation, type of the bridge has not been included and may be different from the actual values. In this test, the impact factor has been calculated for vertical displacement and crack width variation at the key. The calculated values are seen in Figure 7. These values are deduced by dividing the maximum displacements at different speeds by the maximum displacement of 10km/h. This figure shows that the impact factor increases with speed and reaches a maximum value of 1.35 for 90 km/h [1].

6 Static Load Test In the static test, about 500 tons (metric ton) of out-of-service U-33 rails were used to make four-ton weights. The weights were laid down in parallel, perpendicular to the bridge axis, and at a distance from each other to ensure a uniform distribution of load on the middle span of the bridge (Fig.8). The load was increased gradually, and displacements were measured at the key, 1/4 th of the span, both ends of the arch and above the foundations, at every 20-ton increment in loading [1].

7 Figure 9 shows the measured vertical deflections, at the middle of the span, for different levels of loading. Two curves are observed in this figure that correspond to outputs of two vertical LVDTs on both sides of the crack at the key. The difference between the two curves is relatively small, and this indicates a symmetric response throughout the test. A symmetric behavior is expected because of symmetry in both loading and geometry. Figure 9 shows a relatively stiff behavior for this arch structure such that the maximum vertical deflection is 2.7 mm for a static load as large as 500 tons. The overall shape of the load-deflection curve is close to linear and suggests an elastic response for the arch. Field observations did not show any visible cracking or damage in the bridge during the loading procedure. In addition, analysis of the structure resulted in relatively small magnitudes of stresses in different critical sections of the bridge. These facts may confirm a linear response assumption. A drawback for this assumption is the existence of two teeth in the curves at load levels of 160 and 360 tons. But, these tooth correspond to two nights when loading had stopped. It should be noted that the test was carried out in three successive days and loading stopped over two nights. Despite the existence of such local non-linearities, the overall shape of the curves, field observations, and stress levels, all suggest a more or less linear behavior for the bridge in the static test.

8 The bridge, at the current service conditions, experiences a maximum load of 60 tons (3 axles of a 120-ton 6-axle locomotive on 6 meter span). If this magnitude of load is compared with 500 tons, which was applied in the test, and the relatively stiff and linear response is noted, it can be concluded that a large strength reserve and a safe performance is expected for the bridge under service conditions. This conclusion reveals that the wide and deep crack at the key and deterioration effects due to aging, fatigue, and corrosion have not lead to any significant problem for continued service of this railway bridge. Summery and Conclusion In this document, the static and dynamic loading test of Akbar-Abad Railway Bridge were explained. The aim of the study was to assess the remaining load capacity and real conditions of the bridge. There was concern about the safety and serviceability of this structure because of factors such as aging, corrosion, fatigue, and apparent structural damage. The dynamic test exhibited a safe performance as no indication of resonance or large dynamic magnification factors were observed. The static test indicated a relatively stiff and linear behavior and large strength reserve. Both tests confirmed a satisfactory performance under service conditions. Overall, the experimental program changed previous views about lack of strength in this aging bridge and provided a realistic assessment of its remaining load carrying capacity. The study demonstrated the efficiency of the arch system from the viewpoint of structural resistance and proper performance over a long period of time. Considering the

9 fact that full-scale test of mass concrete arch bridge has rarely been reported, this document may add to the knowledge about this type of structure. References [1] M.S. Marefat, 2000, Loading Test of Railway Concrete Arch Bridges, Research Center, Iranian Railway Company (in Farsi). [2] M.S. Marefat, and E. Ghahremani-Gargari, Apr. 30-May 2, 2001, Estimating Load Carrying Capacity of Akbar-Abad Bridge Using Loading Test Results, 1st International Conference on Concrete & Development, Tehran, Iran (in Farsi). [3] B. Bakht, L.G. Jeager and A. Mufti, 1994, Bridge Engineering, Recent Innovations. [4] L. Fryba, 1996, Dynamics of Railway Bridges, Thomas Telford, 1 st edition. [5] UIC776-1, 1994, Loads to Be Considered in Railway Bridge Design, 4 th edition. [6] O.S., Salwa, 1997, Assessment of Bridges: Use of Dynamic Testing, Canadian Journal of Civil Engineering, No.24. [7] J.S. Bendat, and A.G. Piersol, 1993, Engineering Applications of Correlation and Spectral Analysis, John Wiley & Sons, second edition,