Turkish Bridge Rehab

A monumental bridge with a problem caused by oversights in design

A. B. MEHRABI*

Bridge Engineering Solutions, Inc., Lewiston, NY 14092, USA

(Revised version received 26 May 2006)

In January 2004, one of the lower hanger plates of the Bosporus Bridge in Istanbul,

Turkey, fractured. Emergency repairs were performed on some of the plates and a bridge

evaluation project was initiated to understand the cause of the problem and to devise

appropriate solutions. This paper summarizes the evaluation process and the results. The

scope included short- and long-term hanger force and plate strain measurements, finite

element analysis, non-destructive testing (NDT), probabilistic analysis, and remaining

service-life estimation. The investigation showed unusual behaviour of the bridge

superstructure that is attributed to the inclined configuration of the hanger cables

introducing additional stiffness and attracting additional forces from a variety of sources.

The evaluation has shown that many of the hanger plates are at the critical stage of their

service life. This paper describes an example of oversights in design that has left a

monumental bridge in a critical condition 30 years after its construction.

Keywords: Suspension bridges; Cables; Fatigue; Fracture; Service life analysis;

Instrumentation; Finite element; Non-destructive testing; Force measurement; Dynamic

response; Bridge evaluation; Temperature effects

1. Introduction

In January 2004, one of the lower hanger plates of the First

Bosporus Bridge in Istanbul, Turkey, fractured. Emergency

repairs were performed on some of the plates. The Turkish

Highway General Directorate (KGM) was concerned

about the fatigue performance and potential for cracking

of the remaining hanger plates. A bridge evaluation project

was initiated to understand the cause of the problem and to

devise appropriate solutions.

The main concern was the fatigue performance and

potential for cracking of the hanger (suspender) plates

connecting the hanger cables to the deck and main

suspension cable. For the fatigue analysis, existing stress

level and stress variation in time needed to be evaluated.

To obtain the stress level in the plates, existing forces in the

hanger cables, which connect to the suspect plates, were

measured. Finite element analysis was performed to

calculate the existing baseline stress contour on each plate,

and the critical stress location was identified. Inspection

and non-destructive testing (NDT) of the plate details

including the welds were also conducted to identify critical

locations and existing flaws in the plates. Sensors were

installed on cables and plates for longer-term monitoring

and to obtain the range of stress variation and the repeating

frequency in time. With these parameters and considering

the traffic trends, temperature effects, and probabilistic

extrapolation, the service life of the plates was estimated.

The scope of work for this project included:

(1) hanger cables force measurement;

(2) instrumentation of plates and cables for continuous

monitoring;

(3) probabilistic analysis of continuous monitoring

data;

(4) finite element analysis of plates;

*Email: [email protected]

Bridge Structures, Vol. 2, No. 2, June 2006, 79 – 95

Bridge StructuresISSN 1573-2487 print/ISSN 1744-8999 online ª 2006 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/15732480600852088

(5) inspection and NDT evaluation of hanger plates;

and

(6) remaining service-life analysis.

2. Description of the bridge

The First Bosporus Bridge carries six lanes of traffic over

the Bosporus Strait in Istanbul, Turkey. This bridge is the

older of two suspension bridges connecting the two

continents of Asia and Europe over the Bosporus Strait.

Figure 1 shows a view of this bridge from the Asian side.

The bridge construction was completed in 1974. It has a

main suspended span of 1074 m, and side spans of 231 m on

the European side and 255 m on the Asian side, spanning

approximately in a southeast – northwest direction. Two

closed H-shaped steel towers support the main suspension

cables. The deck structure is made of single steel box girder

with a total width of 28 m (from hanger to hanger). Two

2.5-m wide walkways, one on each side of the deck, are used

only for maintenance and access to the bridge.

There are 236 inclined hanger cables in the main

suspension span. These are divided symmetrically into four

groups, each group in one half of the main span on each

side of the deck. Inclined hanger cables are connected at the

top end to the main suspension cable via upper hanger

plates and to the deck via lower hanger plates. The first

connection point on the main suspension cable is located

18.05 m from the centreline of the tower and the remaining

connection points on the main cable are spaced at 17.9 m

(horizontal) toward the centre of the main span. The first

lower hanger plate is located 8.95 m from the centreline

of the tower with only one cable connected to it. The

remaining lower hanger plates are spaced at 17.9 m (along

the deck curve) for 292.4 m from the first hanger plate,

after which the hanger plates are positioned toward the

centre of the main span to accommodate a 658 angle for thehangers with respect to a horizontal line. There are a total

of 41 lower hanger plates in each quarter of the bridge

deck, 82 plates in each side of the deck and 164 in the whole

bridge. In each quarter of the bridge, lower hanger plates

No. 2 through No. 19 from each tower each connect

two hanger cables to the deck while the remaining lower

hanger plates each connect one hanger cable to the deck.

Fifty-nine upper hanger plates on each side of the bridge

(a total of 118 upper hanger plates in the main span of

the bridge) each connect two hanger cables to the main

suspension cable. Figure 2 shows a typical two-cable

lower hanger plate and figure 3 shows a typical one-cable

lower hanger plate. Figure 4 shows a typical upper hanger

plate. Hanger cable designation in one quarter of the bridge

is shown in figure 5.

According to information provided by Brunton Shaw,

Ltd, from manufacturing specifications issued by the

Brunton, Ltd, the original manufacturer of the cables, the

hanger cables consist of 58-mm nominal diameter galva-

nized steel wire single spiral strand connected to upper and

lower hanger plates via socket and pin connection. Per this

specification, the mass of the hanger cable is 16.62 kg/m,

cross-sectional area is 1960 mm2, and the nominal breaking

load is 288 tonnes. This information was used in conjunc-

tion with the field-recorded data for force estimation of the

hanger cables.

Figure 1. A view of the Bosporus Bridge.

80 A. B. Mehrabi

3. Hanger cable force measurement

Forces in all hanger cables were estimated using a laser-

based vibration technique. In this technique, a laser

vibrometer (shown in figure 6) targeted at each cable

records the vibration time-history, based on which, the

fundamental frequencies are calculated. The forces are

estimated using these frequencies, cables geometric and

mechanical properties, and a formulation developed spe-

cially for structural cables. Ambient excitation was used for

longer cables, where the shorter cables were impacted with

a rubber mallet. Details of this technique can be found in

Figure 2. A typical two-cable hanger plate.

Figure 3. A typical one-cable hanger plate.

A monumental bridge and design problems 81

Mehrabi and Tabatabai (1999) and Yen et al. (1997).

Figures 7 and 8 show forces in east and west hanger cables

of the main span, respectively.

The estimated hanger forces range from a minimum of

0.0 (loose hanger) to a maximum of 1377 kN. It should be

noted that the inclination angle of hanger cables with

respect to the horizontal plane increases toward to the

tower. Assuming that the dead load acting on panel points

remains the same, it should be expected that forces in cables

closer to the mid-span (smaller inclination angles) are

higher than those closer to the towers. The trend shown in

the results supports this expectation if forces in each

adjacent hanger pairs are added. The force patterns in

four quadrants of the bridge were in good agreement

with each other symmetrically. The sum of forces in the

south side hangers is about 2% higher than that in the

north side.

It is important to point out that, with very few excep-

tions, forces in hangers with odd-numbered designation

are consistently and significantly lower than forces in

even-numbered designation. It is likely that a portion of

the load has been transferred from odd-numbered hangers

to the even-numbered hangers. This occurrence may be

attributed to the thermal effects on the main suspension

span. The balanced condition would be that resulting in

almost equal forces in adjacent cables ascending from one

panel point on the deck. This condition has resulted in

stresses above those expected for many cables. This set of

measured forces was considered as the basis around which

the force variation in select cables due to live loads and

temperature effects are to be measured, as described next.

4. Instrumentation of select cables and plates

The objective of the instrumentation and monitoring was to

record force variation in the hanger cables for a duration

long enough to allow a probabilistic extrapolation of the

results. These forces could then be used to identify the

critical locations on the hanger plates and to obtain

the stress variation at those locations through analysis.

Figure 4. A typical upper hanger plate.

Figure 5. Hanger cables in one quarter of the main span.

82 A. B. Mehrabi

In the meantime, the strains on select locations on the

plates were to be measured for comparison and verifi-

cation. For this purpose, representative locations along the

south-east quarter of the main span of the bridge were

selected and instrumented for monitoring of strain in cables

and plates from traffic-induced live loads and gradual

temperature variation. Figure 9 shows the instrumentation

locations. A weather station recorded environmental con-

ditions concurrently.

The representative hanger plates were two two-cable

plates with the minimum and maximum cable inclinations,

one two-cable plate that had been repaired temporarily

Figure 7. Hanger force distribution for east cables.

Figure 6. Laser vibrometer targeting hanger cables.


after the incident, one one-cable symmetric plate, and two

one-cable non-symmetric plates.

The instrumentation system included wireless data

acquisition systems at six anchor plates. Each instrumented

location consisted of strain gages on the hanger plates, and

force transducers on the hanger cables. Figures 10 and 11

show typical detailed layout of instrumented location

corresponding to the plate and cable configuration shown

in figures 2 and 3, respectively.

Two types of data were collected: static and dynamic.

The static data were collected to capture the trend of force

variation in cables due to daily temperature variation and

contained data with 32 Hz rate per channel for 30 min

every 2 h, for a total duration of 2 days. Figure 12 shows

Figure 8. Hanger force distribution for west cables.

Figure 9. Locations of instrumented plates.

84 A. B. Mehrabi

Figure 10. Instrumentation details of a typical two-cable plate.

Figure 11. Instrumentation details of a typical one-cable plate.


a typical force variation trend due to temperature variation

for one of the hanger cables for 24-h duration resulting

from static data collection. The trends show clearly that

forces in cables are sensitive to temperature variation. The

directionality of cable force changes with temperature

variation depended on the location and inclination direc-

tion of the hanger cables.

The dynamic data were collected for probabilistic data

analysis intended for capturing the effects of fatigue cycles.

The dynamic data were collected with 128 Hz rate per chan-

nel for 5 min every 2 h, for a total duration of 4 days.

Environmental data were collected during the testing

period. Wind speed, wind direction, temperature, humidity,

and rainfall were recorded. Figures 13 and 14 represent the

temperature, humidity and wind speed recorded during this

phase of testing.

5. Probabilistic analysis of instrumentation data

Dynamic and static data were analysed separately. The

rain-flow method (Parker 1981) was used for counting half

cycles in the recorded dynamic data for stresses in hanger

cables. The numbers of half cycles collected for 5-min

periods were multiplied by 24 to estimate the number of

half cycles in each 2-h interval. The estimated numbers of

half cycles in each 2-h interval were summed up to find the

number of half cycles during the 3-day data recording

period. The results were tabulated and histogram charts

were plotted. Figure 15 shows a 3-day typical half-cycle

histogram for stresses in one of the instrumented cables.

The horizontal axis shows the stress ranges with an

increment of 3.45 MPa, starting from 3.45 MPa and ending

at 127.5 MPa.

From the static data, the strain variation in hanger cables

for a temperature variation of 11.28C was calculated and is

presented in table 1.

The strain variation from recorded data was converted to

stress variation for each cable using the following relation-

ship:

sT ¼ 159 000� ðT� 17Þ � ðms=1 000 000=11:2Þ ð1Þ

where sT is stress at temperature T in MPa, ms is the strain(microstrain) variation in that cable due to a temperature

variation equal to 11.28C. The elastic modulus of the cable

was assumed to be 159 000 MPa. The baseline cable force

measurement was conducted in an average temperature of

about 178C.The static data were analysed to estimate the force

variation in each instrumented cable with respect to

ambient temperature change (see figure 12). For a tempera-

ture variation range of 11.28C, for days when the data were

collected, stress variations of as low as 10.3 MPa and as

high as 89.6 MPa were estimated in different hanger cables.

Weather data were obtained from KGM for the year 2003.

These data were analysed to determine the daily range

of temperature variation shown in figure 16. With the data

from instrumentation and annual temperature variations,

annual stress variations were calculated. Hence, using

rain-flow analysis, annual half-cycle stress histograms in

Figure 12. Temperature and force variation.

86 A. B. Mehrabi

instrumented cables were constructed. Figure 17 shows one

such histogram corresponding to the cable for which the

live load effect histogram in figure 15 was presented. As

shown in figures 15 and 17, temperature variation induces

higher stresses with lower frequencies when compared with

the effects of live load (dynamic).

Figure 13. Temperature and humidity.

Figure 14. Wind speed.


6. Finite element analysis of hanger plates

Finite element modeling tool was used to identify the

critical locations of stress concentration on the hanger

plates and to calculate maximum stresses resulting from

cable forces. Three-dimensional finite element analyses

were conducted using the DIANA program (TNO Building

and Construction Research 2004) on three models covering

all configurations of instrumented plate – cable combina-

tions. The finite element model of a hanger connection

included a portion of the deck and the walkway and

spanned from one transverse diaphragm to another.

Figure 18 shows front and rear view of an example of a

typical complete finite element model used for stress

analyses. The deck stiffeners and the connection stiffeners

were also included in the models. Cable force was applied

at the centre of the hanger socket pin. Eight-node and six-

node flat shell elements were used throughout the mesh.

Figures 19 and 20 show partial finite element models

of two-cable and one-cable hanger plates, respectively.

Figures 21 and 22 show principal stress contours resulting

from application of unit cable force on two-cable and one-

cable plates, respectively. The stress contours shown in

these figures clearly show the stress concentration that

coincide with the termination of weld lines connecting

vertical and horizontal stiffener plates (see figures 19 and

20) to the hanger plate. This introduces a critical detail

concerning fatigue in many of the hanger plates. For

verification, the stresses in plates obtained from finite

element analysis were compared with those measured

directly (after conversion from strain measurements) on

the plates and a good agreement was observed.

7. NDT of plates

A comprehensive NDT program was carried out for the

hanger plates to detect the existing cracks and flaws. For

this purpose, all deck-level hanger plates, the welds around

the hanger plates, and the sockets were sandblasted to

allow a thorough visual inspection to be performed

followed by a dye-penetrant test. Additionally, ultrasonic

testing was conducted in areas found to contain anomalies

and cracks. It was determined that the welds on the

majority of plates and socket rings had fair to bad

workmanship with many defects, undercuts and/or flame-

cutting defects. Cracks were also detected on several plates,

many consistent with the results of the stress analysis

described above. Figure 23 shows one of the plates with this

type of crack.

Figure 15. Typical stress histogram (dynamic).

Table 1. Strain variation in cables for temperature variationof 11.28C.

Plate no.: 84 94 105 106 121

Cable no.: 2 3 22 23 41 42 57

Strain (ms) 250 64 310 135 583 677 383

88 A. B. Mehrabi

8. Remaining service-life analysis

The remaining service-life analysis for fatigue was carried

out based on the field-measured stresses and finite element

analysis results. Photographs of the fractured hanger plates

and NDT results were also utilized to gain insight into the

failure modes. The fatigue evaluation was performed in

accordance with the AASHTO LRFD Bridge Design

Specifications (AASHTO 1998).

In order to perform a fatigue evaluation, three types of

information are required. First, the applied stress range or

the applied effective stress range of a variable amplitude

Figure 16. Daily temperature variations in 2003.

Figure 17. Typical stress histogram (static).


spectrum must be determined through field measurements,

analysis, or a combination of two. Second, the number of

cycles produced at the effective stress range is also required.

Finally, the fatigue resistance of the detail must be known,

i.e. the detail must be classified. The applied stress range

and corresponding number of cycles were obtained through

the field measurements and extrapolations. Based on

previous experience with measurements made on other

long-span bridges supported by hangers, the response of

these members is more influenced by global behaviour or

the effect of multiple vehicles (e.g. several trucks side-by-

side), traffic jams, and other live load effects (wind, general

vibration, etc.) rather than individual trucks. It should

be noted that due to fatigue concerns, truck traffic

has not been allowed on the bridge for more than 20

years. Temperature changes can also produce substantial

variations in stress in individual hangers but with a very

low corresponding number of cycles.

As was discussed earlier and shown in figure 24, there is a

vertical weld that connects the hanger plates to stiffener

plates. At the termination of this weld near the horizontal

plate, a significant stress concentration exists due to the

abrupt change in the section at this location. The detail is

analogous to a transverse flange attachment termination in

a beam or the termination of a longitudinal stiffener on a

web, both of which are well known to have very poor fati-

gue resistance. Hence, for the hanger plates, the connection

has been classified as a Category E detail (per AASHTO

LRFD) for this investigation.

With the above information, the service life of repre-

sentative hanger plates was calculated to range from

about 20 years for two-cable plates to infinity for symmetric

Figure 18. Front and rear view of a complete finite element model of a two-cable hanger plate.

90 A. B. Mehrabi

one-cable plates. The estimated fatigue lives are reasonably

in agreement with the case of the fractured hanger plate

and cracked plates, taking into account typical scatter in

fatigue-life predictions. It is common to observe an order of

magnitude difference in fatigue life in full-scale fatigue tests,

and anticipated scatter in results is built into the resistance

curves used for this evaluation. Hence, the life predictions

that have been made should be viewed as lower bound

estimates. In any case, the results show that the hanger

plates with two cables attached are very susceptible to

fatigue cracking.

According to the findings of the investigation, the

fracture of the hanger plate in the Bosporus Bridge can

be explained as follows. The failure of the hanger plate is

Figure 19. A partial finite element model of a two-cable hanger plate.

Figure 20. A partial finite element model of a one-cable hanger plate.


believed to be the result of fatigue cracking which initiated

at the termination of the vertical weld attaching the vertical

hanger plate to the stiffener plates. The location where the

crack is believed to have initiated is illustrated in figure 25

using a photograph of a similar undamaged plate.

As with fatigue cracking in many other bridges, the crack

is speculated to have initiated and propagated in fatigue for

some time and remained undetected. The crack grew in

fatigue until eventually it reached a length where the

applied loads resulted in brittle fracture of the plate. Plate

Figure 21. Stress contour for a two-cable plate.

Figure 22. Stress contour for a one-cable plate.

92 A. B. Mehrabi

material of this thickness usually has substantial toughness

and can tolerate very large cracks prior to fracture. The size

of the crack that can be tolerated is a function of the

applied static and dynamic loads and material toughness.

Considering the typical factors of safety in most compo-

nents, the plate likely had considerable reserve strength that

permitted the crack to grow to a large length prior to

fracture. Because the fracture occurred during cold

temperatures, the decrease in material toughness may have

also been a contributing factor in the failure. However,

fracture at higher temperature would also have been

possible if the crack had grown to a sufficient length.

Figure 23. Crack detected by NDT (marked C).

Figure 24. Critical detail schematic.


9. Conclusions

The investigation described in this paper has been success-

ful in identifying possible damage sources and vulnerable

locations on the bridge, some requiring immediate atten-

tion. Specifically, the investigation has shown unusual

behaviour of the bridge superstructure that is mostly

attributed to the unique configuration of the hanger cables.

Inclined hanger cables introduce lateral and, in some

aspects, vertical stiffness in addition to that provided by the

main suspension cable and the bridge deck. Consequently,

the hanger cables attract forces from a variety of sources,

among which dynamic and static effects of live loading and

temperature variation are recognized to be of most

significance. Hanger cable force variation of about 50

tonnes was recorded during a relatively short monitoring

period for live load effects. A daily force variation of about

21 tonnes was recorded in some cables due to daily

temperature variation. These, if extrapolated linearly, can

generate force variations as large as 70 tonnes in hanger

cables. This phenomenon alone can explain looseness of

some hanger cables observed during this investigation.

Therefore, large-amplitude force variation in hanger

cables due to live and ambient loadings has created serious

potential for fatigue cracking in hanger plates, hanger

cables, and other bridge elements. This, in turn, was

exacerbated by introduction of fatigue-sensitive details in

the design of these elements. As an example, termination of

the weld lines attaching the hanger plate to the vertical and

horizontal stiffener plates present a high potential for

initiation of the fatigue cracking.

The inclined configuration of the hanger cables and

fatigue-sensitive details of the hanger plates, both features

left by oversights in design without recognizing the

consequences, seem to be responsible for the fatigue

problems in this bridge. In general, the service-life analysis

has shown that many of the hanger plates are at the critical

stage of their service life. Two-cable hanger plates have

been shown to be more susceptible to fatigue cracking.

NDT evaluation has already detected cracks with potential

for extension on some of the hanger plates consistent with

the findings of the analysis. Fatigue and service-life analysis

were able to explain the potential cause of the hanger plate

fracture incident in the bridge. Furthermore, it was shown

that for many plates, the stress at critical locations from a

combination of baseline forces and other effects could be

critically close to the yield strength of the plate materials.

This is influenced largely by differences between baseline

forces of odd- and even-numbered hanger cables that

are mostly attributed to sources such as temperature

effects, superstructure erection process, additional dead

load, and variation of the bridge geometry from the design

configuration.

10. Recommendations for future investigations

To reach more conclusive and comprehensive findings

necessary for design and application of repair and retrofit

Figure 25. How the failure occurred.

94 A. B. Mehrabi

schemes, further investigations are recommended. These

include:

. Inspection and fatigue analysis of hanger cables, upper

hanger plates, and deck structure. It is likely that these

elements are influenced by the significant force varia-

tions reported in the present investigation in a manner

similar to the lower hanger plates, therefore, requiring

immediate attention. A NDT inspection and fatigue

analysis is recommended for these elements.

. Hanger cables force measurement at a different ambient

temperature. The cable base force measurement in

the present study was performed at relatively warm

ambient temperature. Another set of force measure-

ment is recommended at a cold temperature to verify

the effects of temperature variation conclusively.

Instrumentation and force variation measurement in

cables in relation with solar radiation and temperature

variation for a longer period of time will also help to

quantify these effects.

. Detailed inspection of the bridge structure. A compre-

hensive detailed, hands-on inspection is recommended

for the bridge. This should cover the super- and sub-

structure including main suspension cables, deck and

tower structure, approaches, bearings and joints, and

piers. A survey and geometry measurement is also

recommended.

. Vibration study on hanger cables. Some of the hanger

cables experience excessive vibration. The vibration

characteristics of the cables and means for suppression

of excessive oscillation need to be studied.

. Finite element modeling of the bridge superstructure. It

is recommended that a finite element model of the

bridge be constructed for analysis of the effectiveness of

various repair schemes, dynamic effects of live loading,

seismic analysis, etc.

. It is recommended that after completion of the

inspection program and application of the repair

schemes, the bridge is installed with a continuous

health-monitoring system.

Acknowledgements

This project was funded by the Turkish General Directo-

rate of Highways (KGM) and was a product of a team

effort. Significant contributions by Mr Ertugrul Kasaci of

ERSE Industrial Installations and Dr Robert Connor are

appreciated. Special thanks to Mr Yakup Dost, Mr Ahmet

Akdeniz and Mr Erdogan Dedeoglu of KGM for

coordinating the process, and overseeing and guiding its

implementation, and to Mr Ozay Turnaoglu of Mak-Yol

for facilitating the contract and project implementation.

Sincere gratitude is expressed to Professor Dr Aydin

Dumanoglu for reviewing the process and providing

valuable comments and discussions. The opinions and

conclusions in this paper are of the author and do not

reflect necessarily those of the others.

References

AASHTO, Load and Resistance Factor Design (LRFD) Bridge Design

Specifications, 2nd edn, 1998 with 1999 and 2000 Interims (American Asso-

ciation of State Highway and Transportation Officials, Washington, DC).

Mehrabi, A.B. and Tabatabai, H., A unified finite difference formulation

for free vibration of cables. J. Struct. Engng ASCE, 1999, 124,

1313 – 1322.

Parker, A.P., The Mechanics of Fracture and Fatigue, 1981 (E. & F. N.

Spon Ltd, London).

TNO Building and Construction Research, DIANA Finite Element Analysis

Program, 2004 (TNO Building and Construction Research, Delft, The

Netherlands).

Yen, W.P., Mehrabi, A.B. and Tabatabai, H., Evaluation of stay cable

tension using a non-destructive vibration technique, in ASCE Structures

Congress Proceedings, vol. 1, 1997, pp. 503 – 507.


Documents

Turkish Bridge Rehab