j. herter, r. helmerich, k. brandes

Assessment of the safety and remaining fatigue

life of historical steel bridges

J. Herter, R. Helmerich, K. Brandes

Federal Institute for Materials Research and Testing (BAM),

D-12200 Berlin, GermanyEMail: [email protected], [email protected]

klaus. brandes@bam. de

Abstract

Thousands of old riveted steel bridges are getting older all over the world. They aresuffering from fatigue or other safety problems, caused by traffic loads, besidescorrosion. The owners have a great interest in Non- Destructive evaluation of theirtoday's remaining fatigue state.

A lot of investigations in that field have been done at BAM and are just underway. Field tests and full scale fatigue test series on original bridges improve thebasis for the fatigue assessment of existing steel structures. Some test results willbe presented.

The methodology of assessment is based on both, calculation and experimentalinvestigation. In situ measurement of strains give information about the actingstatic system. By calculation which include the results from measurements we willget an improved knowledge about stress distribution.

If using fracture mechanic theory for the assessment of the remaining fatiguelife it is very important to know whether there are already fatigue cracks in thestructure. Radiographic methods have been used in fatigue tests in our laboratoryto discover hidden cracks in sandwiched elements. Experiences from laboratorytests will be adopted to a subway railway viaduct in the near future.

In case the remaining fatigue life of a structure is small, a concept for monitoring,repairing or strengthening is under way. Some basics and aspects for furtherdevelopment will be discussed.

Transactions on the Built Environment vol 39 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509

86 Structural Studies, Repairs and Maintenance of Historical Buildings

1 Introduction

When developing the transportation systems of the industrial countries in the secondhalf of the last century and the following decades, steel bridges played a dominantrole. Many of them are still in service and there is no need to replace themimmediately. Quite on the contrary, after about hundred years of service, they arewell approved.

However, they suffer from corrosion and fatigue loading. Poor maintenancefor many decades has caused heavy deterioration of many bridges in easterncountries of Europe and elsewhere.

Consequently, comprehensive evaluation of the structures is necessary for ratingits further use, which is a demand of economy and ecology and a challenge forengineers dealing with structures of the cultural heritage.

2 Problems

2.1 Causes and Types of Damages

Bridges suffer from corrosion and from heavy loading. The weights of vehiclesgrow larger and larger overstraining some bridges built for lower loading manyyears ago.

The big number of repeated loading generates fatigue damage i.e. initiation ofsmall cracks which grow slowly during millions of loading cycles until they reacha critical length causing collapse of the respective structural element, Fig. 1. In thiscontribution, we only are dealing with this type of damage.

Fig. 1: Fatigue crack in a tie rod of a truss arch bridge in Berlin found in 1954

2.2 Structural Examination Engineering

In the design of new structures, structural safety can be ensured simply by followingcodes. In contrast, difficulties in disclosing the real conditions of an existing structureand the causes of damage make evaluation of its structural safety an extremely


Structural Studies, Repairs and Maintenance of Historical Buildings 87

demanding task. Structural examination requires comprehensive knowledge ofstructural engineering beyond the scope of codes[l].

3 Methodology of assessment

Valid rules and standards are focused on design of new structures. Only therailway companies developed a tool for the evaluation of existing buildings and

bridges during the last years. Some undispensable research work in this field isplaned by European institutes and railway companies. Non- Destructive-Evaluati-on and calculation methods based on different evaluation concepts are beingdeveloped.

The most important difference between design and evaluation is thepossibility to provide measurements to get information about the existing staticsystem and hot spots of strain concentration in the real structure. The measuredvalues should be used as input for calculation on the basis of fracture mechanics orS-N-curve method. The methodology is shown in Fig. 2 .

Evaluation of

Existing Bridges and Structures

Measurements Non Destructive Evaluationof Damage by:

Visual InspectionDesign calculation

Hot Spot Evalua-tion Static systemindentiflcation

- Ultrasonic inspection- Magnetic particle inspection- Radiographic inspection

Fatigue assessment withFracture Mechanics LFMor S-N-Curve-Concept

| Experiences^^fatigue cracks

Full scale testson dismantledstructures

i

_

no cracks |

Input inevaluation calculation- LFM based calculation- S-N-Curve-Assessment

Replacing • Monitoring• StrengtheningRehabilitation

• Repair

Unrestrictedfurther use

Fig. 2: Methodology of Assessment



4 Field tests on old Steel- Bridges

4.1 Stubenrauch Bridge

The Stubenrauch Bridge over the river Spree in Berlin was built in 1908 by KarlBernhard, who was a famous engineer in that time. The bridge consists of threearches, Fig. 3, the two arches at the banks are made of reinforced concrete, the archin the middle is a steel structure, Fig. 4.

Fig. 3: Stubenrauch Bridge in Berlin, built in 1908

Fig. 4: Steel construction in the midspan of the Stubenrauch Bridge

The bridge has an open cross section. The principal members consist of trussarches with two hinges and a tierod. The arches are connected with each other bytransverse girders, the roadway construction and one portal frame at midspan.

The bridge has been repaired several times in its life time and has been damagedduring the war. In 1994 the owner of the bridge, the highway administration ofBerlin had to decide once more whether it should be replaced or repaired. Due tothe fact that it was a historical monument all possibilities should be tried to preservethe bridge. On the other side very frequent and heavy traffic has to pass the bridgeand it should carry lorries of 30 tons. A first structural analysis showed, that thestability of the main steel arches could not be approved with sufficient safety andthat the remaining fatigue life has to be investigated.



Load tests under traffic load with strain measurements have been performed byBAM to investigate the remaining fatigue life. The measured strains were lowerthan the calculated ones and based on the measurements a sufficient safety regardingfatigue failure could be stated [2], [3].

The static stability of an arch depends on the deviation from the ideal positionin its plane. As to the rules, a deviation of 12.0 cm in the middle of the arch had tobe supposed in the static analysis. This demanded assumption caused such highstresses in the analysis that the safety could not be ensured. By measuring the realdeviation with a laser system we found that the deviation was not greater than3.5 cm, Fig. 5. Using the measured values for the static analysis, the calculatedstresses are lower than the design stresses and the bridge could be preserved.Nowadays the bridge is under reconstruction .

Figure 5: Measured horizontal deviation of one arch from its ideal plane

4. 2 Spree Railway Bridge

Unexpected transverse load distribution at the Spree-Bridge

Strain measurements are an useful tool for confirming an existing static system infield tests. Unexpected transverse load distribution had been found during strainand deflection measurements at the arch bridge crossing the river Spree (Fig.6) inthe center of Berlin. The bridge, consisting of three old pairs of twin arches, is thelast Berlin railway bridge built of wrought iron in 1882 during establishing theeast-west railway line.



Fig. 6: Arch bridge crossing the River Spree in the center of Berlin

First measurements with an 4-axle-601 engine were performed to approve the staticsystem. As an surprising effect, transverse distribution of load had been found notonly between the coupled twin arches, but also to the adjacent arches. The transversedistribution was caused at the level of the floor beams, arranged on columns.

-20 -20 Arch 2: 11.5% (2)

J3

CT)

IV)

10

0

-10

-20

%̂ number-/ \ i**g of

/ " " " ̂ x / nodes

X. 5 V 10 15

Arch 3: \x' (D

10

0

-10

-20

5 10 1'3»....,̂-2

Arch 4: 36.0%

0 20

"*?™

(?

25

[s]

)

calculated

Fig. 7: Measured distribution of the stresses at the quarter points of threeadjacent arches while an 60t engine is moving over the arch No. 3 ,compared with the calculated stresses for the loaded arch No. 3

Measured values were used as input values for calculation [4]. Measured maximumstress deviation for the 601 engine loading was about -1 IN/mm^, while the deviationfrom the calculated values was 30,7 N/mm^ . The calculated maximum stressdifference for the defined UIC load including all safety factors exceeded88 N/mm . Figure 7 shows measured and calculated distribution for the maximumloaded quarter point of the arch 3 and the adjacent arches.



Additional measurements with two engines of a weight of 120t each in the archquarter points verified that measured strains did not exceed an fatigue relevantlevel because of the unexpected transverse load distribution. The maximummeasured value in the arch was lower than 35 N/mm^ .More detailed evaluation had been done after dismantling two twin-arches by meansof Non-Destructive-Testings in the laboratory. No fatigue cracks have been foundin an 7t heavy bridge part from the maximum loaded quarter point of one arch afterthe bridge had been 115 years under traffic [5].

4. 3 Viaduct of the Subway Line- Ul

The line of the Berlin subway Route- Ul is designed in some parts as a viaductFig. 8. The viaduct is a steel construction opened to traffic in 1902. In 1996 someold riveted truss bridges were to be replaced by a new welded steel constructionwith web girders. A first expert's report based on calculations showed that manytruss elements of the old structure were overdue in terms of fatigue. The questionwas, whether further investigations could be done to improve the knowledge aboutthe remaining fatigue life of the construction or to its fatigue resistance.

Fig. 8: Viaduct of a subway line - Riveted truss Bridge opened in 1902

An expert's report by BAM stated that there is a great danger of fatigue failureof the gusset plates which is much higher than the risk of failure in the bars. Thisstatement is based on fatigue tests on truss girders in our laboratory. Also, therewas only a little chance to improve the knowledge by strain measurements on site.The only possibility seemed to be, to strengthen the structure which had causedadditional costs, but the authorities decided to replace the bridge.Some truss girders have been brought to the laboratory of BAM for further tests

(see Fig. 11 ). The intention is to get better knowledge about fatigue resistance ofreal structures and advanced non destructive test methods, which will assist to findmore sophisticated techniques to save old bridges in the future.

4. 4 Fatigue cracks in curved ballast steel sheets

During normal inspection cracks have been noticed at the connection of the ballaststeel sheets to the transverse beams in some parts of a viaduct. The sheets lay



between the transverse beams and carry the ballast and the full traffic load of thesubway trains, transmitted by the sleepers Fig.9.

SleeperBallast Rail \ 750 375_ 375 Traffic

load

StrainBallast

Direction Kottbusser Tor

Fig. 9: Longitudinal cross section of the viaduct

It was supposed that fatigue was the reason for the cracks. Therefore the structuralbehaviour was evaluated by strain measurements. The shape of the plates close tothe connection to the transverse beam seems to be not very suitable to transfer thetraffic load to the transverse beam Fig. 9. High additional bending moments withhigh tension stresses were expected. Load tests under normal traffic load havebeen performed to measure strains at critical points converted into stresses. Fig. 10shows the stress-time-diagram during the passage for a train for point No. 13.

Strain gauge No. 13

0 1 . 2 3Train enters 1. Bogie passesthe bridge the crossbeam

4 5 6

a = 210000 N/mm'xe

7 8 9

Time [seconds]

Fig. 10: Stress-time-diagram determined from strain measurements

The measurement shows a very high stress range ( about 78 N/mnf at pointNo. 13 ) and many cycles per passage Fig. 11. Every passage of a train with 8carriages induces 16 stress cycles, 1 cycle per bogie. During the life time since1930 up to now, the plate had to withstand about 85 millions cycles so that allsupposed fatigue strength curves will be exceeded. Taking this into considerationthe occurence of the cracks can be explained.



5 Laboratory Tests on Original Bridge Girders after

Dismantling

A special chance for completing the knowledge of old bridges in advance is to testgirders of the same type in the laboratory. BAM got this chance about two yearsago.The truss girder bridge of the Berlin underground Ul near Lausitzer Platz withparts of about 9m length was dismantled in 1996. By calculation on the basis ofvalid regulations, it was impossible to approve the demanded remaining fatigueresistance.Four truss girders have been tested under constant amplitude loading at BAM.Typical damage mechanisms had been found. The weakest points of truss girdersin terms of fatigue resistance are the gusset plates. Fig. 11 shows the truss girderduring the test and a cracked gusset plate after test [6].

Fig. 11: Original truss girder during the full scale test

Different types of loading have been applied out. During the first test series, thediagonals were the most stressed parts and during the following tests the lowerchord in the middle of the truss girder got maximum stress cycles. Basis for thediagnosis were strain measurements by means of strain gauge rosettes on the gussetplates. Change of strain values or direction of principal stresses in the gusset plateswere a sufficient indicator for crack initiation and crack propagation. Radiographictests have been done, if changes of the direction of principal stresses were registered.The failure mechanism was always nearly the same. In fig. 12 a comparison of aradiographic film and the separated node is given. For assessing the same type oftruss girders by field investigations, the results will be helpful.

Fig. 12: Radiographic film and truss joint after fatigue test



6 Repairing, strengthening, monitoring

Preservation of existing bridges is much more preferable to their replacing, thisstatement is now also accepted by the German government and the highwayadministration. It is especially aimed for bridges of historic value. Presumably,bridges will not be kept in service for many hundreds of years, because they havea function which requires their reliability and safety. And during the course oftime, they suffer from corrosion and heavy loading. However, their lifespan can beextended by strengthening or even by monitoring, i. e. steady measurement toevaluate their condition state. Recently developed techniques to meet this actualchallenge are just in their infancy, but have to be improved rapidly because ofurgent need. Some work has been launched in the field of repair and strengthening.A careful strengthening may be achieved by so-called "minimal invasive methods"adding more intelligence to the structure. Legal barriers are obviously hinderingachievements in this field.

7 Concluding remarks

Structural Engineering more and more is focussing on the conservation andextending the lifespan of structures. This demanding task is changing also the fieldof activity of engineering. Only interdisciplinary co-operation enables us to meetthat challenge as has been presented by this contribution.

References

[1] Briihwiler, E.: From Design to ExamineeringStructural Engineering International 5 (1995), S.69

[2] Professor Sedlacek & Partner (PSP)Gutachten Stubenrauchbrucke, 1994

[3] Brandes, K./ Knapp, J./ Herter, J.:Messungen an Stahlbrucken zur Ermudungsbeurteilung Stahlbau65,1996

[4] Hilbers, F.-J. " Untersuchungen zur Erhaltung von Stahlbrucken", Pots-darner Bauseminar, 11/97

[5] Helmerich, R./ Brandes, K./ Berner, K.: "Tragfahigkeit von alten Stahl-konstruktionen", Potsdamer Bauseminar 11/97

[6] Helmerich, R./ Brandes, K./ Herter,J.: "Full scale fatigue tests on rivetedsteel bridges", 1ABSE-Workshop Lausanne 1997


j. herter, r. helmerich, k. brandes

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