NSCC2009

1 INTRODUCTION

In the beginning of the 20th 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, 47 000 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 import-ant 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 essen-tial. 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 ac-curate 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

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 Sus-tainable 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 583.11 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: fy = 220 MPa, fu = 350 MPa and fbuk = 330 MPa.

Material properties of old steel bridges

T. Larsson1 & O. Lagerqvist2 1Vectura AB Consulting, Göteborg, Sweden

2Division of Construction Engineering, Luleå University of Technology, Luleå, Sweden

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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 19th 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 per-formed 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 20th century.

3 STRUCTURE OF THE DATA BASE

Material analyzed in the data base includes yield (fy) and ultimate strength (fu), Charpy-V (Kv) and fracture mechanic properties (Jc). Regarding fy the standard of evaluating the yield strength has changed from using the lower yield limit, Rel, to the higher yield strength, Reh. When evaluating data for fy no difference has been made between Rel and Reh. Evaluating fy in this manner provides char-acteristic 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 583.11 (2005) was chosen. In BVS 583.11 (2005) material properties is divided in to three time periods, steel produced before 1901, 1901 to 1919 and the final time period 1919 to 1955. The last interval stretches further than the information in the da-ta 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 ev-aluated together since only a small difference in strength between the two temperatures was ob-served. The mechanical properties in the data base were determined as the 5 % fractile of a lognor-mal 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 583.11 (2005). If the material properties for the steel bridge in concern are unknown, BVS 583.11 (2005) recommends to use the characteristic strengths for the steel SS 1311(fyk = 220 MPa, fuk = 360 MPa) reduced with a factor 0,55 for bridges constructed before 1901 and with a re-

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duction factor 0,8 for bridges constructed 1901 to 1919. For bridges constructed after 1919 BVS 583.11 (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 583.11 (2005) is for the first time period. But one should keep in mind that the information regarding steel pro-duced 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 fy but for fu the 5 % fractile is higher for the time period 1901 to 1919.

A comparison between the recommendations in BVS 583.11 (2005) and the 5 % fractiles given by the data base for the time period 1901 to 1919 shows 40 % higher fy and 30 % higher fu in the data base. The statistics for the time period 1919 to 1940 shows 13 % higher fy for steel in the data base than recommended in BVS 583.11 (2005). Concerning fu, 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 583.11 (2005) Rec. char. values

Steel Steel

fy [MPa] 295 31 243 32 / 1 1 / 1 220 x 0.55 = 121 fu [MPa] 454 31 402 32 / 1 1 / 1 360 x 0.55 = 198

Iron Iron

fy [MPa] 259 20 218 - 7 / 7 No recommenda-tion

fu [MPa] 333 45 249 - 7 / 7

-1901

No recommenda-tion

Steel Steel

fy [MPa] 300 35 246 84 / 11 468 / No record

220 x 0.8 = 176

fu [MPa] 435 38 375 66 /11 471 / No record

360 x 0.8 = 288

Wrought iron Wrought iron

fy [MPa] 266 29 219 - 26 / No record

No recommenda-tion

fu [MPa] 334 38 273 - 26 / No record

1901 - 1919

No recommenda-tion

Steel Steel

fy [MPa] 297 32 248 92 / 12 - 220 fu [MPa] 444 64 347 63 / 10 -

1919 -1940

360

4.2 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 ma-terial 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 Tech-nology, 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 2.

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The two test series on rivets from the Vindelälven Bridge showed some differences in the results. Higher mean values were measured for fy, and fu 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 proper-ties 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 583.11 (2005). Recommended values according to BVS 583.11 (2005) for rivet material to use in an assessment is an ultimate strength (fbuk) equal to 330 MPa, but if the rivets are situated in a joint be-tween two girders the value should be reduced with 15 %, giving fbuk equal to 280 MPa. As can be seen in Table 2, the recommended values for fbuk in BVS 583.11 (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 583.11 (2005) Rec char. values

Period - 1901

fy [MPa] 362 28 315 19 No recommenda-tion

fu [MPa] 496 40 429 19 330 / 280

Period 1901 - 1919

fy [MPa] 348 13 319 5 No recommenda-tion

fu [MPa] 477 10 454 5 330 / 280

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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 tem-perature 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 me-thod. 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 Kv, 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 homoge-nous 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 Jc value [N/mm], or with linear fracture mechanics, the Kc value [N/mm3/2]. 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

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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 583.12 (2003) Swedish Rail Administration, a lower limit is given concerning the toughness value Jc 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 oC) 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 1901 - 1919

Jc1 [N/mm] 34 25 - 8 30

Period 1919 - 1940

Ky2 [J] 135 289 - 2 17

Jc2 [N/mm] 293 499 - 1 37

Jc1 [N/mm] 272 657 - 12 67

1Tested at – 30 °C 2Tested 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 struc-tural 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 big-gest 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 583.12

Figure 5. The fracture toughness properties, Jc, from Swedish bridges produced 1901 to 1919. The different colors of the bars represent different bridges tested at – 30 °C, Larsson (2009)

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Rec BVS 583.12

Figure 6. Fracture toughness properties, Jc, for different structural elements in the bridge Mora-Noret, con-structed 1921, Larsson (2009)

Rec BVS 583.12

Figure 7. Fracture toughness properties, Jc, from Swedish bridges constructed 1919-1940, Larsson (2009)

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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, fy = 220 MPa and fu = 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 rec-ommended value fbuk = 330 MPa from BVS 583.11 (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 in-vestigated bridge to be sure that it has sufficient toughness properties.

6 REFERENCES

BVS 583.11 (2005). Bärighetsberäkningar av järnvägsbroar. Swedish Rail Administration

BVS 583.12 (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 2009. ISBN978-91-86233-13-6

Sustainable Bridges (2003). Assessment for Future Traffic Demands and Longer Lives. An Integrated Pro-ject during 2003-2007 supported by the European Commission in the 6th Framework Program, with 32 part-ners from 12 countries, Contract No TIP-CT-2003-001653. Many reports and papers are listed on the home-page www.sustainablebridges.net

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 Steel and Timber Structures, Chalmers University of Technology, Publ. S94:6, Göteborg, Sweden

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