Material properties of old steel bridges

Transcription

1 Material properties of old steel bridges NSCC2009 T. Larsson 1 & O. Lagerqvist 2 1 Vectura AB Consulting, Göteborg, Sweden 2 Division of Construction Engineering, Luleå University of Technology, Luleå, Sweden ABSTRACT: In order to provide a better knowledge of the material properties of steel bridges in Europe, a data base was established as a part of the European project Sustainable Bridges (2003). The data base was limited to steel bridges constructed before 1940 s. Information in the data base mainly comes from Swedish and German bridges that have had their material characteristics verified. The characteristic values from the bridges are compared to the recommended values in the Swedish Rail Administrations code BVS used in assessments of existing bridges. From the evaluation of the gathered material properties, the following characteristic values are proposed to be used in an assessment if the actual properties of the bridge are missing: f y = 220 MPa, f u = 350 MPa and f buk = 330 MPa. 1 INTRODUCTION In the beginning of the 20 th century a majority of the railroad and road bridges built in Europe where produced in steel and today, most of the steel railroad bridges in Europe have an age between 50 and 100 years, Sustainable Bridges (2004). The relative old age was achieved by designing the bridges with an over capacity to ensure that the bridge stock could bear predicted future alterations in axle loads. Concerning the road bridges the stock of old steel bridges remaining in service is not of the same extent. The difference in the number of old bridges still in use between railroad and road bridges is due to the simple fact that the early road bridges became too narrow. In a survey conducted in the European project Sustainable Bridges (2004) concerning the amount of railroad bridges produced in metal still in service, bridges were found. 40 % of these bridges have an age between 50 and 100 years old, the large amount of old bridges makes them an important part of the infrastructure in Europe. Due to their old age many of these bridges are reaching their design life. To replace these railroad bridges would be very expensive and not a realistic action, when there still is additional load and fatigue capacity left in many of the bridges. To perform assessments of the remaining load or fatigue capacity, the material properties are essential. The best ways of determining the actual properties are of course to take out samples from the investigated structure and analyze the steel. But lacking this information it is important to use as accurate values as possible in an assessment calculations. Information concerning which material properties that can be expected can come from calculations or blue prints used in the design. It is not always these documents are complete and guidance can in 120

2 these cases be found in national codes, however the recommended values are often estimation on the safe side. In order to provide a better knowledge of the material properties of steel bridges in Europe, a data base was established as a part of the European project Sustainable Bridges (2003). The data base was limited to steel bridges constructed before 1940 s, because steel produced after this period were better controlled and consisted of more homogenous quality due to the introduction of welding and the toughness demands it requires. The information in the data base mainly comes from Swedish and German bridges that have had their material characteristics verified. 2 GATHERING OF DATA The information about material properties for Swedish bridges were collected from tests performed by certified institutions and Swedish universities over time, and retrieved from different archives of the Swedish Road and Rail Administrations. The information covers bridges built in the late 19 th century to the 1940 s. The amount of material samples extracted from each bridge differs depending on the extent of the investigation originally performed. The creation of the German part of the data base was carried out with the help of Höhler (2005). The main part of the information about German bridges comes from literature surveys and tests performed at RWTH Aachen, Germany. Much of the information concerning German bridges cannot be linked to a specific bridge. The test samples analyzed were from bridges situated in and around Berlin and constructed in the beginning of the 20 th century. 3 STRUCTURE OF THE DATA BASE Material analyzed in the data base includes yield (f y ) and ultimate strength (f u ), Charpy-V (K v ) and fracture mechanic properties (J c ). Regarding f y the standard of evaluating the yield strength has changed from using the lower yield limit, R el, to the higher yield strength, R eh. When evaluating data for f y no difference has been made between R el and R eh. Evaluating f y in this manner provides characteristic values on the safe side. To be able to compare the material properties in the data base to a code, similar time periods as used in the Swedish Rail Administration code BVS (2005) was chosen. In BVS (2005) material properties is divided in to three time periods, steel produced before 1901, 1901 to 1919 and the final time period 1919 to The last interval stretches further than the information in the data base which only includes steel produced to the 1940 s. 4 EVALUATION OF THE DATA BASE Some of the tests of the yield strength were performed both at 0 C and -30 C. These tests were evaluated together since only a small difference in strength between the two temperatures was observed. The mechanical properties in the data base were determined as the 5 % fractile of a lognormal distribution. The mean values and the standard deviation are accounted for in each time period. Concerning the toughness properties, no characteristic values (the 5 % value) are presented due to the big scatter in the result. The mean value and the standard deviation of a lognormal distribution are however presented to illustrate the big scatter in the toughness results. 4.1 Mechanical properties of old steel The data from the Swedish and German bridges was combined to give as good basis as possible for defining the material characteristics of steel in old bridges, see Table 1. Besides the mechanical properties given by the data base, Table 1 also includes a comparison with the recommendations given in BVS (2005). If the material properties for the steel bridge in concern are unknown, BVS (2005) recommends to use the characteristic strengths for the steel SS 1311(f yk = 220 MPa, f uk = 360 MPa) reduced with a factor 0,55 for bridges constructed before 1901 and with a re- 121

3 duction factor 0,8 for bridges constructed 1901 to For bridges constructed after 1919 BVS (2005) gives a number of steel grades to choose from (it is apparently assumed that the steel grade is known), among them SS 1311, without reduction factor. For simplicity the values for SS 1311 are used for comparison in Table 1 for all three-time periods. The biggest difference in yield and ultimate strength between the data base and BVS (2005) is for the first time period. But one should keep in mind that the information regarding steel produced before 1901 comes from one bridge. Therefore it cannot be seen as a representative value for all bridges from this period. However it shows that the steel in these bridges can be considerably stronger than specified in codes, 200 %. The two remaining time periods in the data base, 1901 to 1919 and 1919 to 1940, has almost the same 5 % fractile for f y but for f u the 5 % fractile is higher for the time period 1901 to A comparison between the recommendations in BVS (2005) and the 5 % fractiles given by the data base for the time period 1901 to 1919 shows 40 % higher f y and 30 % higher f u in the data base. The statistics for the time period 1919 to 1940 shows 13 % higher f y for steel in the data base than recommended in BVS (2005). Concerning f u, a 4 % lower value was obtained in the data base compared to the value in the code. Table 1. Mechanical properties for German and Swedish bridges in the data base, Larsson (2009) Property Mean Stdv 5 % frac No. of Swedish samples / No. of bridges No. of German samples / No. of bridges Time period BVS (2005) Rec. char. values f y [MPa] f u [MPa] / 1 1 / x 0.55 = / 1 1 / x 0.55 = 198 Iron Iron f y [MPa] / 7 No recommendation f u [MPa] / 7 No recommendation f y [MPa] / / No record f u [MPa] / / No record Wrought iron f y [MPa] / No record f u [MPa] / No record x 0.8 = x 0.8 = 288 Wrought iron No recommendation No recommendation f y [MPa] / f u [MPa] / Mechanical properties of rivets Concerning the mechanical properties of rivets, material tests from two bridges were found. The first bridge was the Vindelälven Bridge built in 1896, where eight tests conducted on the rivet material were found, Åkesson (1994). The second bridge where properties of rivets were investigated was the Forsmo Bridge (built in 1912) for which five tests performed at the Royal Institute of Technology, Sweden, were found. To extend the content of the data base concerning rivet material, tests of mechanical properties were performed at Complab, Luleå University of Technology, Sweden, on 11 rivets extracted from parts from the Vindelälven Bridge previously investigated by Åkesson (1994), see Figure 1 and Figure

4 The two test series on rivets from the Vindelälven Bridge showed some differences in the results. Higher mean values were measured for f y, and f u at Luleå University of Technology compared to the results obtained by Åkesson (1994), however the standard deviations were in the same range. The deviation between the results can depend on the laboratory equipment, how it was calibrated and the test set up. Figure 1. Part of the web connected to the flanges, cut out to be able to extract rivets for the mechanical tests, Larsson (2009) Figure 2. On the left a machined rivet in the form of a tensile test specimen. To the right the shape of the rivet when extracted, with one of the rivet heads removed, Larsson (2009) A total of 24 tests on rivet material can be found in the data base, see Table 2. The material properties for the rivets from the two bridges show similar characteristics. As for the material properties of the steel taken from the bridge members the material properties of the rivets were compared to BVS (2005). Recommended values according to BVS (2005) for rivet material to use in an assessment is an ultimate strength (f buk ) equal to 330 MPa, but if the rivets are situated in a joint between two girders the value should be reduced with 15 %, giving f buk equal to 280 MPa. As can be seen in Table 2, the recommended values for f buk in BVS (2005) are almost as low as the yield strength of the rivets in the data base. Table 2. Mechanical properties for rivets from two steel bridges, Larsson (2009) Property Mean Stdv 5 % frac No. of samples BVS (2005) Rec char. values Period f y [MPa] No recommendation f u [MPa] / 280 Period f y [MPa] No recommendation f u [MPa] /

5 4.3 Toughness properties of old steel Toughness is the key factor to determine the type of failure that will follow due to cracking in steel. The toughness is highly dependent on the temperature. A test performed at a low temperature does not absorb the same amount of energy as an identical sample tested at room temperature. The temperature where the shift from brittle to ductile fracture occurs is called the transition temperature. A method to determine toughness properties was developed by Charpy in 1901, the Charpy-V method. The method includes a specimen with a sharp V-notch. The samples are then placed in the bottom of a stand equipped with a pendulum. The pendulum is released and strikes the sample. Due to that a certain amount of energy is needed to break the notched specimen the pendulum will not reach the same height as it had at the starting point. The difference in height of the pendulum is equal to the energy needed to break the sample, which is the notch value of for the material, called K v, see Figure 3. Figure 3. Charpy-V test used o determine the toughness, Larsson (2009) A disadvantage with the Charpy-V method is that the loading rate in structures often differs from the one in the test as do the geometry, the notches and the thickness of the material, all these factors contribute to the shift in transition temperature. Consequently Charpy-V tests and structures will not have the same transition temperature. The Charpy-V method is better used in validating homogenous newly produced steel. Fracture mechanic tests give a more realistic result of the toughness properties of old steel. The two most common tests are Compact test, CT-test, and the three point bending test, see Figure 4. A notch is machined in the tested sample which is then exposed to a fatigue loading to originate a crack in the notch. The test is then torn in two halves to determine its toughness. The toughness is either evaluated with non-linear fracture mechanics, the J c value [N/mm], or with linear fracture mechanics, the K c value [N/mm 3/2 ]. Structural steel is often too ductile to be evaluated by linear fracture mechanics, therefore the toughness should be evaluated with the non-linear approach. Figure 4. Three point bending and compact tension test, CT-test, Larsson (2009) As mentioned previously no characteristic values will be presented for the toughness properties of steel retrieved from bridges constructed before 1940 due to the big scatter in the information in the data base. An explanation for this big variation is that the toughness was not a controlled parameter in early steel production. In Table 3, measured toughness properties found for Swedish bridges are 124

6 presented. Graphical presentations of the divergence of the measured toughness s both between bridges and in one bridge are presented in Figures 5 to 7. In the code BVS (2003) Swedish Rail Administration, a lower limit is given concerning the toughness value J c for old steel in bridges equal to 50 [N/mm]. If the value for vital elements in the bridge is below 50 [N/mm] (evaluated at a temperature of -30 o C) special measures has to be done, from increased inspections to replacement of vital elements or the whole bridge. Table 3. Toughness properties for steel in bridges constructed 1901 to 1940, Larsson (2009) Property Mean Stdv 5 % frac No. of bridges No. of samples Period J 1 c [N/mm] Period K 2 y [J] J 2 c [N/mm] J 1 c [N/mm] Tested at 30 C 2 Tested at 20 C In Figure 5, the toughness measured on steel from bridges produced 1901 to 1919 is presented. The spread of the toughness is limited, but there are also steel where the results diverge. In Figure 6, samples are taken from the same bridge, Mora-Noret constructed 1921, and the different colors show where the samples are taken. The scatter of the result are depending on that different structural components where investigated, also two different contractors had delivered the steel. When these components where produced different types of steel qualities where obviously used. The biggest scatter of the toughness in the data base is in the period 1919 to 1940 with a standard deviation of the result 2.5 times larger than the mean value (Figure 7). Rec BVS Figure 5. The fracture toughness properties, J c, from Swedish bridges produced 1901 to The different colors of the bars represent different bridges tested at 30 C, Larsson (2009) 125

7 Rec BVS Figure 6. Fracture toughness properties, J c, for different structural elements in the bridge Mora-Noret, constructed 1921, Larsson (2009) Rec BVS Figure 7. Fracture toughness properties, J c, from Swedish bridges constructed , Larsson (2009) 126

8 5 CONCLUSIONS With the creation of the data base an increased knowledge concerning material properties of steel bridges constructed before the 1940 s have been attained. The evaluation of the data base indicates that, as a rule of thumb, f y = 220 MPa and f u = 350 MPa can be used as an initial assumption in an assessment of an old bridge. Investigations concerning rivet material are not as vast as for the plate material, however the recommended value f buk = 330 MPa from BVS (2005) can at least be used for both connection joints between girders as for rivets in girders. Concerning the toughness no recommendation can be provided since there is a large divergence in the collected material from the bridges. The recommendation is to validate the toughness of the investigated bridge to be sure that it has sufficient toughness properties. 6 REFERENCES BVS (2005). Bärighetsberäkningar av järnvägsbroar. Swedish Rail Administration BVS (2003). Brottseghet hos konstruktionstål I järnvägsbroar. Swedish Rail Administration Höhler, S. (2005). Material properties of Metal Railway Bridges. Technical Report: Draft. Sustainable Bridges. WP4-S-R-001 Draft Larsson, T (2009). Fatigue assessment of riveted bridges. Doctoral thesis, March ISBN Sustainable Bridges (2003). Assessment for Future Traffic Demands and Longer Lives. An Integrated Project during supported by the European Commission in the 6 th Framework Program, with 32 partners from 12 countries, Contract No TIP-CT Many reports and papers are listed on the homepage Sustainable Bridges (2004). European Railway Bridges Demography. Sustainable Bridges Assessment for future Traffic Demands and Longer Lives Åkesson, B. (1994). Fatigue Life of Riveted Railway Bridges. Doctoral Thesis, Division of and Timber Structures, Chalmers University of Technology, Publ. S94:6, Göteborg, Sweden 127