Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 1536 379 9

Structural analysis and rehabilitation of a nailed railway bridge in Genoa

A. 8rencich & L. Gambarotta University ofGenoa, Dept. ofStructural and Geotechnical Engineering, D/SEG, Genoa, /taly

ABSTRACT: The assessment procedure of a 90-years olel, nailed steel Italian railway bridge, the Campasso Bridge, is discussed in this paper. The characterization of materiais, the evaluation of the corrosion etfects on the structural members and the connections are taken into account by means ofboth in situ and laboratory tests . The identification and calibration of a structural model is then approached both by means of a load test and by comparison of the results given by 10, 20 and 3D models. On this basis, it is showed that the bridge can be re-opened to heavy commercial railway tratfic (04-type 4 axle carriages, 22.5 tons/axle) provided that the north-rail only is kept in service and few rehabilitation works are performed. The fatigue residuallife is estimated not less than 20 years , much longer than the expected service-time left, but it can be reached if the corrosion phenomena are stopped by adequate interventions.

INTROOUCTION

The Italian railway network was built essentially in the last decades of the 19th century, and only the spe­cific parts, such as connection lines and junctions, have been completed at the beginning ofthe 20th cen­tury, till World War r. The vast majority ofthe bridges and viaducts consist ofmasonry structures, except for some cases in which specific problems forced steel bridges to be useel, such as the one on which this paper is focused and for the new r.c . and steel bridges that are being built on the fast lines.

The loads and speeel, greatly increased since the building times, and the materiaIs, which are almost unknown either in their mechanical characteristics and in their uniformity throllghout the structure, are among the major problems related to old strllctures. The latter issue can be dealt with by means of chemical analyses and mechanical tests on specimens taken from the bridge in order to define the mechanical properties of the steel. In this paper, it is shown that old materi­ais may sometimes be comparable to the modern ones, which often happens in railway bridges, built accord­ing to very high technical standards. The material identification was carried out also by means of cou­pled in situ and laboratory chemical analyses, allowing an estimation of the steel inhomogeneity through the bridge.

For stee l bridges, the etfects of corrosion on the structural members need to be carefully investigated becallse also damages to limited areas, sllch as in the jllnctions, may result in a significant reduction

of the load carrying capacity of the structure or in a weakening of the stitfening systems, dangerolls with respect to bllckling.

The modern approach to strllctural analysis has been applied first to the original bridge, assuming uncorroded beams with their original size, and then to the actual bridge, i.e. taking into account the reduc­tion in the section thickness dlle to corrosion. Good agreement has been found between the numerical FEM model and the experimental data obtained from a static load test on the bridge allowing a complete identification of the structure.

Even though the designed rehabilitation works are in progress, the paper gives a general overview of the structural analysis performed and ofthe rehabilitation procedures.

2 THE CAMPASSO BRiDGE: PRESENT SITUAT10N ANO FUTURE NEEOS

The Campasso Bridge spans 243 m on the Polcevera river, close to Genoa (Italy), figures I and 2. Oesigned in 1906 by the technical otfice of one ofthe main Euro­pean contractors, the Società Nazionale delle OfJicine di Savigliano, this two-rails bridge was opened to traf­fic in 1915. Oamaged during World War II by plane bullets, it has been continuously operated since than, being now one ofthe oldest in-service Italian railway steel bridges. It can be considered a prototype of the steel nailed bridges lIsed by the Italian railway allthor­ity at the beginning of the 20th century; nevertheless

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Figure I. Campasso Bridge: a) general view of the area as it was at the bui lding times (nowadays the ri ver is na rrower); b) lateral and verti cal views of the bridge.

Figure 2. General view ofthe Campasso Bridge.

Figure 3. InternaI view of the Campasso Bridge.

it is not well known being shadowed by the more famous San Michele Bridge on the Adda River at Pademo, elose to Milan, built by the same company around 1885.

The structure is a three spans (76.5 m - 90 m-76.5 m) two-way continuous truss beam, liA m high and 9Am wide (8.0m is the width ofthe deck), 1800 tons in weight. It consists of two longitudinal beams at the sides of the deck, f igures 2 and 3, connected by transverse stiffeni ng frames . The longitudinal truss

Figure 4a. U-shape section of the spars.

I I I I I

- ~ -

Figure 4b. Spars at the intrados ofthe ma in beams.

beam consists of 28 sections, each 9.0 m long but fo r some sections of 8.25 m in the minor spans, and is approximately 45° skew to the piers, figure l a. The section of the lower beams is U-type (L-type profiles connecting severa I plates by nails), with the open part upwards for the lower beams, f igures 4; several plates at the bottom of the U -type sections, figure 4a, were used ifhigh values ofthe axial trust were expected. The upper spars of the beams present the same geometry but for the fact that the U-shaped section are reversed upside down, figure 4b.

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Corrosion of the L-type profiles

Figure 5. Corrosion ofthe internai L-type profiles connect­ing the plates and of the plates. South beam.

The structure, nowadays, is damaged by corrosion being the bridge located at 3 km only from the sea coagI. The U-shaped section at the intrados of the truss beams, along with rough maintenance proce­dures, made several parts of the beams to be reduced by corrosion of the plate thickness. A systematic inspection (IlS, 2003) showed that the bottom of the lower U-type beams is corroded, with some extreme cases in the south longitudinal truss beam where the internai L-type profiles, connecting the plates, and some plates are deeply corroded, figure 5, up to complete inconsistency.

The lack of serious maintenance allowed corrosion to penetrate deeply inside the nailed connections lead­ing, in some cases, to the corrosion of the nail itself, figure 6.

Some connections have been corroded for more than 50% of the original section, figures 6b and 6c, and the overall depth of corrosion is approximately 2 to 4 mm in the connections. Being the original plate thickness 12 to 16 mm, the heaviest corrosion accounts up to 25- 30% the original thickness.

The bridge is presently a C3-load bridge (20 tons per axle and 72 kN/m equivalent load) with speed limita­tion to 30 kmlh, connecting a large carriage deposit, the Campasso Area, to the Genoa-Milan line. The

Figure 6a. Deep corrosion ofthe nail ofthe upper spars.

Figure 6b. Severe corrosion of a junction.

- 0,0,,/ .1 '

Figure 6c. Corrosion in the junctions (hori zontal stiffening structures) below the bridge deck.

northern rail has been dismissed in 1999 when a seri­ous accident, outside the bridge, damaged severa I parts of the rail and some parts of a neighbouring minor bridge.

The assessment ofthe Campasso Bridge is strongly needed because the increase ofthe commercial traffic

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strongly relies on it, asking rec1assification to higher D4-type loads (22.5 tons/axle and 80 kN/m equivalent load) and because the corrosion of many structural elements, mainly in the nailed connections, has deeply affected the structure.

Besides, the present c1assification does not rely on any rational engineering assessment, but is simply due to the fact that ali the commerciallines in Italy are clas­sified C3 . For this reason, the railway authority asks a rational classification ofthe bridge: reliable estima­tion of the present load carrying capacity and of its residuallife.

3 STRUCTURAL ANALYSIS

According to the available original documentation and to the design procedures ofthe last century, the bridge has been designed referring to simplified structural schemes, i.e. to 2D truss beams.

A typical feature of the 19th century steel construc­tion technology, that can be found in the Campasso Bridge, is that the axes of the beams were assumed in the centre ofthe horizontal plates, not in the axis ofthe whole section, figure 7. This geometric characteristic

Figure 7. View ofa typical node .

shows that only the axial thrust had been considered in the design procedure. In spite ofthis simplified struc­tural model, the dimensions ofthe structural elements appear to be in an almost perfect agreement with the modem concepts of structural engineering.

In order to set a reliable mechanical model, a step­wise procedure is discussed in the following: a) testing of materiaIs (in situ and in laboratory), b) severa I structural models , with increasing leveI of detai l, are formulated in order to estimate their reliability by comparison, c) load test in order to corroborate the structural mode!.

3.1 Material characterization

The characterization of the materiaIs has been carried out by means ofboth direct mechanical tests on spec­imens taken from the bridge (stress- strain response, tensile strength, Charpy toughness, chemical analysis) and in situ tests (HV hardness and chemical analysis): Tables 1- 3 summarize the main data. According to modem standards (BS 4360 and EN 10025), the steel the bridge consists of can be classified as a weldable Fe 430 A stee!. This latter circumstance was rather unexpected since it shows that the material used for the Campasso Bridge was ofhigh levei at the building times.

Some nails have been extracted from the bridge and tested in direct shear. No sign of yielding was found ; the ultimate shear stress being 298 MPa on the average, with very little scattering of the data. According to the Von Mises criterion, this would lead to a tensile strength of 520 MPa in direct traction tests, higher than the value measured for the plates. Figure 8 shows the typical load-displacement response of the materiaIs for the plate steel and for the nail one.

3.2 Structural models

At first glance, the bridge appears a 3D truss beam; according to the design procedures of 19th century, it should have been designed assuming a 2D truss beam as structural model (Rossi, 1920 & Jorini , 1921).

For ali these reasons, in the following, severa I mod­eIs have been taken into account: 1) a lD continuous beam model equivalent to the truss beam, with both unbounded and finite shear stiffness; 2) 2D both truss and frame model; 3) 3D truss and a 3D frame scheme, figure 10.

Table I . Impurities in the steel [%] - average values on 32 tests.

C Mn Si S P Ni Cr Mo Cu Hardness - HV

Average 0.039 0.310 0.018 0.033 0.040 0.036 0.013 0.008 0.066 131 Minimum 0.016 0.175 0.000 0.056 0.015 0.030 0.001 0.007 0.137 118 Maximum 0.062 0.477 0.059 0.015 0.075 0.046 0.022 0.010 0.029 150

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Table 2. Main mechanical characteristics.

Sample group Average

No. Area [mm2] ay [MPa] a u [MPa] ê u [%] E [MPa] Notes

I 215 300 435 26 238000 Plate- thick. 15 nun 2 217 274 412 35 234000 Plate- thick. 15 rnm 3 160 306 401 33 213000 Plate- thick. 8 mm 4 153 311 433 32 210000 Plate- thick. 8 rnm 5 150 290 397 36 199500 Plate- thick. 8 mm 6 9453 314 435 34 205000 L-section- thick. 5mm 7 940 340 471 35 221000 L-section- thick. 5mm 8 941 325 453 23 226000 L-section-thick. 5rnm

Table 3. Toughness measurements - Charpy test at room temperature 20°C.

KV toughness (Charpy) [J]

Sample Section geometry Related to a group (mm) Measured 10 x 10 mm section Notes

IT 10 x 10 35.24 35.24 Plate thick. 15mm 2T 10 x 10 32.28 32.28 Plate thick. 15mm 3L 7 x 10 34.16 48 .55 Plate thick. 15 mm 3T 7 x 10 67.55 96.00 Plate thick. 8 rnm 6T 5 x 10 44.87 88.44 Plate thick. 8 mm 7T 5 x 10 39.32 77.47 L-section 60 x 40 x 5 8T 5 x 10 41.38 81.52 L-section 60 x 40 x 5

T = test performed orthogonal to the rolling direction; L = test performed along the rolling direction.

(J (a) - steel (b) - nail [MPa]

400

300 300

200 200

100 100

y O O

O 10 20 30 O 5 10 (Õ/<1>* IOO)

Figure 8. Load-displacement response of the materiaIs: a) plates; b) nails. Shear strain in figure 8.b is defined as: global displacement/nail diameter * 100.

In order to consider the effect of material degra­dation, the 3D trame model was considered also assuming the actual beams, obtained dropping from the resisting sections the corroded plates (corroded bridge), figure 11. The comparison of the data can

-~ \N/lI1I1I)~rnIWnDmmllim~~~ff\N\N/V

+ /1I1\N\Níillim~~W.wnDmfiPWVN\

Figure 9. Oecomposition of the statically indeterminate problem into two statically determinate problems.

(a)8 'º' 'º' ~

~X1~ (b)

Figure 10. Structural mo deIs for the bridge: a) 10, b) 20 and c) 30 model.

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(a) I (b) I i I I I I I

~I--_ .1..._.

--~~--T-

I I __ L __

Figure 11. a) Designed section (designed bridge); b) corre­spondent corroded section (corroded bridge).

be used to define the rel iability of the more compli­cate structural schemes. The corroded sections have been founel, and have been taken into account, in the lower chords of the south beam only. With the aim of assessing the bridge, ali the models re ly on an elastic material.

The outl ined procedure does not consider some aspects of the bridge response; the redundancy of the structure, the torsional stiffness of the bridge, which is typical of a 3D frames skew to the bearings, the eccentricity between the centers of the nodes and the centers of the chords, responsible of bending effects and deserving adequate attention. In the nodes of the bridge the connecting plates are 2.5 x 2.5 m wide and ali the web and the flanges are connected to the nodes, so that the bending moment can be transferreel, figures 2, 3 and 7: this suggests a 3D frame model for the bridge rather than a 3D truss scheme.

3.3 Load test on the bridge

In order to corroborate the mechanical model also by comparison with experimental data, several load tests were performed on the bridge using 2 to 4 carriages of approx. 60 tons on both the rails and in several positions. The displacements of the bridge have been measured by two systems: direct measurement (trans­ducers, prec. 1/ l00th mm) ofthe nodes in the middle of the central span, which were accessible from the public roael, and topographic measurements of the points at midspan and nearby for ali the three spans and on both the sides ofthe bridge (Giussani and Sguerso, 2003). The precision, considered at the end of the tests, includ­ing the errors coming from the experimental setup,

6

3

O

-3 E .s -6 c. -9

li)

Õ

-12 t:: Q)

> -15

-18 (a)

6

3

O

-3

-6

-9

-12

-15

-18

"*-- 45m

NORTH rail

SOUTH rail

-o-Transducers b. Topographic

-FEM model -O- Transducers b. Topographic

Figure 13 . Theoretical displacements of the nodes and measured values: triangles = topographic measurements; square = direct transducer measurements.

was ± 3/1 00 mm for the transducers and ± [li O mm for the topographic measurements, which account for an error, due to the measuring system only, ofO.2% and 0.6%, respectively, on the maximum displacements.

Figures 12 and 13 show the distribution of the carriages and the comparison between the analyti­cal previsions and the experimental measurements,

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Table 4. Typical code-type carriages for Italian Railway Authority - RFI-dimensions in m.

Load Total

Carriage laxle Im a b c length

C3 200kN 72 kN/m 1.80 1.50 4.50 11.10 D4 225 kN 80kN/m 1.80 1.50 4.65 11 .25

I

L 1 1 1 b a b c

showing that the theoretical model fits rather well the experimental data.

3.4 Loads Gnd dynamic response

The loads considered for the analysis are given 111

Table 4 and represent the actual 04-Type carriage; since this type of load is heavier than the heaviest loco motive used, only carriages have been taken into account. Single carriages and full trains have been con­siderecl, the latter being responsible ofthe most severe loading conditions. A preliminary study showed that, for this kind of bridge, no relevant difference arises when considering concentrated loads, as in table 4, or equivalent distributed loads.

According to the Italian building code, the loads have been amplified by e factor 1.12 to take into account the dynamic effect of the load. This facto r is given by means of an approximate formula:

rp = I + rp'+rp" , (I)

being:

rp'= K K=_v_ 1- K + K 4 ' 2Lrp nO ' (2.a, b)

rp"= ~r 56J ~~ r + 50(noLrp -1)J ~~ r 1 (3) 100 l 80 '

ex = v/22 if speed is v :s:. 22 m/s = 79.2 kmlh, ex = I oth­erwise; vis the train speed in mls; no stands for the first vertical eigen-frequency in Hz; L", is the characteristic span of the bridge in m. In this case it is assumed as the average span.

In order to verify this approach, a dynamic analysis has been performed on the bridge assuming a single

0,4

0,2

° -0,2

-0,4 x

'" E - - North Beam -Static

'" " -0,6 to Cl> >

North Beam -Dynamic South Beam -Dynamic

-0 ,8 L_---!... __ ~_.....:c:=======::f--'

Figure 14. Maximum displacements ofthe dynamic analy­sis compared to the corresponding static values.

Table 5.

Mode Freq [Hz]

First five natural frequencies.

I 2.20

2 2.49

3 2.65

4 3.02

5 3.75

carriage travelling on the rails at the speed of30 kmlh. For each position ofthe loacl, the vertical displacement has been computed in static conditions and during the dynamic analysis. Figure 14 shows the comparisons between the displacements due to the static load and the dynamic one for both the south and north beams. lt can be seen that the maximum difference between static and dynamic displacements is some 12%, almost exactly the dynamic amplification factor given by eqs (1 - 3). The first vertical frequency was calculated by means of modal analysis, Table 5.

3.5 Results

Table 6 shows a comparison between the response of the different structural models. In the I D continuous beam, the axial thrust in the upper and lower chords and in the braces have been deduced from the values ofthe bending moment and shear force, respectively. lt can be noted that 20 models, that do not take into account the transverse distribution of the loads, underestimat­ing the stress state calculated by a more detailed 30 mode!. This is due to the fact Ihat 2D models cannot take into account the torsional effects of the structural response. Besides, the 19th century approach to highly redundant truss bridges seems to evaluate the stress state in the load bearing members with a significant accuracy, but for the hangers.

Figure 15 refers to the most severe loading con­dition for the 3D model showing the reaction forces aI the bearings, figure 16a, the deformed shape, fig­ures 15b, c and d. A relevant feature of the structural response is activated by the skewness ofthe bridge and by a slight asymmetry of the main truss beams which

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Table 6. Extreme values of lhe stresses in lhe structura l members from the different models .

Stresses a [MPa]

Chord

Structural model Max. Min.

19th century design procedures - 2D - 177 178 I D continuous beam - designed 136 - 136 2D truss beam - designed 110 -96 2D frame beam - designed 114 - 104 3D truss beam - designed 81 - 84 3D frame beam - designed 102 - 124 3D frame beam - corroded 126 - 125

... o M

(a) ~ :li <ri

~§) q%jí ~\~ '" o N ;! ;j, ~ <O

(b)

(c)

Figure 15 . FuI! train on two adjacent spans: a) load di s­tribulion and reaction forces [tons]; deformed shape of: b) the bridge; c) lhe transversal frame, cfr. figure 15a; d) two adjacent units ; e) asymmetric load.

Figure 16. Buckling ofsome substructures of the bridge.

Brace Hanger Deflection

Max . Min. Max. Min. centra l span [mm]

180 -180 72 - 18 / 179 - 179 / / 67 59 - 16 1 19 1 - 35 88 79 -179 216 - 34 86 96 -105 138 - 175 74

16 1 - 160 127 - 161 67 165 - 160 165 - 160 69

Table 7. Local buckling: load multipliers for both the rai ls in use .

Loaded span Load mul tiplier À

2 5, 13

I 5, 28

1+2 5, 40

1+3 5, 02

Table 8. Global buckling: load multipliers for both the rails in use.

Loaded span Load multiplier À

2 19, 8

I 22, 2

1+2 18, 4

1+3 2 1,3

exhibit diffe rent deflections also for symmetrically distributed loads, figures 15b and 15c. The transverse frames, deformed by shear-type forces, distribute the load on both the beams. Figure l 5e shows the reac­tion fo rces fo r a train located on one rail only: the beam closer to the train bears 60% of the entire load, while the distribution due to the deck would account for approximately 72%. This fact underlines the impor­tance of the transverse frames and of their nodes for the refurbishment works.

3.6 Buckling analysis

Buckling deserves specific attention in any steel struc­ture and much more when corrosion of some members and of severa I junctions has been found.

In the case of the Campasso Bridge the buckling analys is needs to be performed on two leveis: i) local buckling of some substructures; ii) global buckl ing of the bridge or of large parts of the bridge.

In the f irst case, assuming the live load on both the rails, the criticalload is approximately 5 times the live load, figure 16 and Table 7; buckling is due to an hanger and some connected members.

In order to investigate global buckling, the degrees of freedom allowing the deformed shapes of figure 16,

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(a)

(b)

Figure 17. Buckling of the whole bridge: a) aerial view; b) upwards view ofthe buckling phenomenon.

have been locked. Figure 17 and Table 8 summarize the results obtained for global buckling, showing that criticai multiplier is around 20. For these reasons, the Campasso Bridge appears safe also with respect to buckling.

4 CONCLUSIONS

The assessment of the Campasso Bridge is being developed by means of several steps:

i) material characterization: steel and nails , showed that materiais are similar to modem medi um­quality steel;

ii) corrosion affected severa 1 parts of the structure: nails, plates and profiles, mainly in the lower chords of the south- beam, reducing the thick­nesses up to 50% of its original value in some cases;

iii) the mechanical model better fitting the experi­mental data is a 3D frame, different from the 3D truss model, often considered the natural model for these structures;

iv) the maximum loads (both rails in use) account for a stress state which is admissible for the material ifthe corrosion would not have reduced the struc­tural sections; considering the effects of corrosion,

the stress state appears to be beyond the admissible limit unless restricted to one rail only;

v) the buckling load appears to be far beyond the expected service loads;

vi) an evaluation ofthe fatigue life ofthe bridge would foresee, at least, a residuallife of not less than 20 years taking into account the modem loads, more than the required life of 10 years.

The assessment ofthe Campasso Bridge, therefore, asks for the closure to traffie of the southem rail, close to the most corroded beam, leaving in service the northern rail with no speed limit. Due to the high redundancy ofthe nails in the connections, and to their high quality, the re-opening ofthe bridge is subjected only to few repairs ofthe most severely damaged con­nections and to a global systematic inspection of the bridge.

ACKNOWLEDGMENTS

The authors acknowledge the contribution of Proff. A. Giussan i, Tech. Univ. of Milan and D. Sguerso, Univ. of Genoa, for the topographic measurements; they recognize the fundamental help in the experi­mental part of the work and in the logistic aspects of the research given by Ing. Gianfranco Pometto, Italian Railway Authority - RFI.

REFERENCES

BS EN 10025, 1993. Specification for hot rolled products of non-alloy structural steels and their technical delivery conditions, London: British Standards Institution.

BS 4360, 1990. Specification for weldable structural steels, London: British Standards Institution.

BS 7668, 1994. Specification for weldable structura l stee ls. Hot finished structural hollow sections in weather resis­tant steels, London: British Standards Institution.

Giussani, A. & Sguerso, D. 2003. Private communication. !.!.S. (ltalian Institute for Welding), 2003. Report ATC 1445L Cooper, S.E. 1985. Designing Steel Structures, Englewood

Cliffs: Prentice-Hal!. Eurocode 3: Design of Steel Structures. Jorini , A. 1921. Theory and practice in building bridges (in

Italian), Milan: Hoepli. Rossi, L.G. 1920. Steel bridges (in ltalian), Padova: Editrice

Universitaria. Steel Construction Institute, 1992. Steel designers manual ,

Oxford: Blackwell Scientific Publications.

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