repair cases paper

8/3/2019 Repair Cases Paper

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Fatigue and Repair Cases in Steel Bridges

by Chitoshi Miki, Yuichi Ito and Eiichi Sasaki

Department of Civil Engineering, Tokyo Institute of Technology

2-12-1 Oookayama, Meguro-ku, Tokyo 152-8552, Japan

Abstract

Steel bridge fatigue damage cases and their repair are summarized. They are organized by the causes of

fatigue damage such as the existence of weld defects, deformation, and so on. These fatigue damage cases and

repair methods were collected and stored in a database. This paper also introduces the database which is

accessible from the internet.

1. Introduction

Many steel bridges have been constructed with the development of highway and

railway networks. Welded girder bridges, which are sensitive to fatigue, have been positively

adopted since 1960s. As a result, many fatigue failures have been observed in the U.S.A, Japan,

and other countries since the 1960s[1-12]. Because the steels used will suffer damage year

after year, and both the traffic volume and the usage requirements will go on increasing from

now on, it is feared that the load for the structure will become too heavy. Therefore, fatigue

failures will increase, and bridge maintenance will gain importance in order to steel bridges

safe.

Because the causes of failures and repair measures for the cases are often referredfrom the repair cases reported in the past, it is very useful to gain knowledge from the past case

studies and it is expected that such accumulated information will be of use in the maintenance

technology. Therefore, repair cases for past fatigue failures must be collected so that anyone

can refer to them. As a result of the activities of IIW-XIII-WG5, many repair cases for fatigue

failures observed at various welded joints have been collected. Recently, networks of

information on the internet are very substantial throughout the world. We now consider

providing through the internet the information on many steel bridge repair cases for concerned

persons as well as bridge engineers who are engaged in bridge maintenance business.

This paper introduces the cases of fatigue damage, the repair and retrofit method

applied to these steel bridges, and the system of the database of repair cases. The evaluation ofthe repair methods is also discussed using this database.

2. Fatigue in Steel Bridges and Retrofit Works

The causes of fatigue of steel bridges may be classified as follows:

Welding defects were included at the time of fabrication.

An inappropriate structural detail of low fatigue strength had been adopted.

Stresses and deformations unforeseen in design occurred at joints of

members.

The structure behaved in a manner not expected, such as due to vibration.

There are two approaches to repairing and reinforcing:

To remove the cause of damage or to alleviate it.


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To increase the fatigue strength of the detail concerned.

A thorough explanation of the causes of damage is indispensable in selecting methods

of repair and reinforcement for these bridges. Depending on the damage, there are cases when

it is permissible to leave the component untouched. On the other hand, some cracks which willbe dangerous unless a remedial measure is immediately applied.

2.1 Existence of Welding Defect

There are cases of girders failing due to fatigue cracks occurring from welding defects

unintentionally left in the welds of the plate joints at the bottom flanges of plate girders.

Normally, such a joint is subjected to X-ray inspection after welding so that occurrences of

damage are rare. A fatigue crack, after propagating through the bottom flange, passes the fillet

weld between the flange and web and penetrates to the web. Brittle failure will occur in the

event the fracture toughness of the steel is low. Fig. 1 is a case of a bottom flange failing due to

insufficient penetration of a butt weld [13]. There are also cases reported of cracking of welds

being a cause besides lack of penetration [12].

Such structural details would be no problem at all if there were no defects, and it may

be said that it would suffice to perform welding so that a joint with sound quality is again

obtained. However, this welding would be done in the field, and in many cases it will be

difficult to secure working conditions under which sound welded joints can be obtained. There

are also many cases in which brittle cracking has gone through the web and has stopped

immediately under the top flange for problems such as removal of cracks, groove preparation,

or handling of the start-restart part of welding. Consequently, in such cases, distortion due to

cracking is removed by a method such as jack-up, after which repair is done by splicing using

high-strength bolts [12](Fig. 2).

Fig. 1 Fatigue crack propagated through girder flange

Fig. 2 Repairing by bolted splice


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It is close to impossible to predict the appearance of fatigue damage caused by a defect.

However, after a single case has occurred, it is possible to take measures such as intensive

inspection of joints made in the same factory around the same time using the same welding

details. In the case ofFig. 1, as a result of inspecting butt welds made in the same factory for

the same bridge, a large number of incomplete penetration defects were discovered.

2.2 Adoption of Joint Details of Low Fatigue Strength

This would include cases of bridges not designed against fatigue in the first place being

subjected to conditions of use where fatigue would be a problem, as often occurs with oldstructures, the allowable stresses used in fatigue design on subsequent review being found to

have been inappropriate. Fig. 3 shows a comparison of the allowable fatigue stress of a typical

joint in a steel bridge in 1964 and at present in the fatigue design codes in Japan [16]. As a

result of fatigue tests carried out since the 1960s on many full-scale and large models,

allowable fatigue stress ranges were reconsidered and made the present values. With regard to

such joint details, there are numerous cases in which fairly large-scale measures are necessary,

such as raising the fatigue strength of the detail or lowering the stress occurring in the zone.

2.2.1 Cover plate end

Fig. 4 shows fatigue cracking produced in weld toes of fillet welds used for

attaching cover plates[17]. This is a part where high stress concentrations occur, and the fatigue

strength, as classified by AASHTO and JSSC as the joint type of lowest fatigue strength, is

very low. Depending on the configuration at this part, there are cases of fatigue strengths being

even lower than assumed in design guides, and as seen in the King's Bridge accident [18], this

is also a detail in which welding defects are liable to occur.

In the case ofFig. 4, fatigue cracks occurred at many places, and the method of repair

differed according to the dimensions of the crack. With regard to cracks of surface lengths not

less than 38 mm, as shown in Fig.5, drillholes are provided immediately above the cracks and

splicing is done with high-strength bolts. For cracks smaller than this, TIG dressing or hammer

peening is applied [23](Fig.6).

Fig. 3 Change of allowable fatigue stresses from 60 code to 92 code


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2.2.2 Rigid frame bridge bent

Fig. 7 shows fatigue cracks in the bridge rigid bent in the Route 3 of Tokyo

Metropolitan Expressway. In the design of rigid frame bents, the stress concentration due toshear lag behavior is taken into account based on Okumuras studies [24]. However, the

measured stresses are much higher than calculated stresses in accordance with the standard

design procedures for these structures. Detailed inspection revealed fatigue cracks initiated

from the weld root of partially penetrated welds between the flange plate of the beam and the

flange plate of the column, which are defects of incomplete penetration.

Fig. 8 shows the variations of assembling method of rigid frame bent of box section

column and beam. Because of the complicated crossing of plate elements of beam and column,

welding work becomes almost impossible for some portions. These kinds of structural details

lead to the occurrence of inherent weld defects and become the causes of fatigue accidents. The

existence of these inherent defects and stress concentration due to the structural geometry are

the reasons for this fatigue accident.

All steel rigid bents in highway bridges in Japan were designed by applying the

Fig. 4 Fatigue crack initiated from the weld toe at the end of cover plate

Fig. 5 Retrofit details of bolted splice Fig. 6 Retrofit by TIG dressing and hammer peening

Peened weld toe

Gas tungsten arc remelted weld toe


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same design formula. Therefore, all rigid frame bents in Tokyo Metropolitan Expressway were

inspected and fatigue cracks were observed in more than 300 rigid frame bents. From the

viewpoint of preventing brittle fracture, the critical crack length of 30mm has been set and

measures have to be taken for cracks which exceed this length. In order to investigate the

suitable retrofit methods, wide research work has been performed, including large scale fatigue

tests [25] and field measurements. Four bents with long cracks have been fixed by applying

bolted splices as a temporary measure (Fig. 9).

Fig. 7 Fatigue Crack in Rigid Frame Bridge Bent

tw=26m

Column

Beam

tw=26mm

Fig. 9 Temporary Repairing by Bolted Splices

Fig. 8 Inherent defects due to assembling procedures

(a) Type A (b) Type B (c) Type C


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2.2.3 Gusset Plate Joint

As shown in Fig. 3, there is a large difference in the allowable stresses of gusset joints

made on webs or flanges between the 1960 and the present design standards for railwaybridges.

As a preventive measure, improving the fatigue strength of this detail was studied. Fig.

10 shows one of the methods being proposed to gusset plate on flange edge, in which a

specially developed circular cutting device (Fig. 11) is used to shape the radius of the gusset

end large, and the fatigue strength is increased by reducing local stress concentration at this

location which becomes the initiation point of fatigue cracking. Fatigue prevention measures

using this method are presently being applied on a trial basis.

Regarding the end of a gusset attached to a web, it is difficult to reduce stress

concentration by changing the configuration as in Fig. 10. The methods conceivable are to

raise fatigue strength by finishing the weld toe by grinding, TIG-dressing, or hammer peening.

However, gussets or attachments of this kind are attached by fillet welding in many cases, and

if the toe is finished, fatigue cracks will then occur from the root, and not much improvement

can be expected.

Fig. 10 Improving works of flange gusset detail

Fig. 11 Cutting works by applying

the newly developed tool

Improve the figure

(machine-cut or gas-cut)

Fig. 12 Improving works of flange gusset detail


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Fig. 12 shows a method which has been proposed and is being used on a trial basis as

a measure against fatigue cracks which have occurred in similar details in highway bridges,

aiming to reduce stresses at gusset ends by splicing with high-strength bolts. Two of the holes

for bolts have been drilled at the ends of a crack and serve as stop holes. Stresses near thegusset end can be reduced 20 to 30% by this method, and so this is an adequate

countermeasure.

2.3 Occurrence of Unexpected Stress and Deformation at Joints of Members

Various rules have been established in order to simplify calculations in the structural

design of a bridge. Particularly, with regard to connections between perpendicularly crossing

members such as main girders and cross beams or cross beams and stringers, simple supports

or pin connections are often specified, and deformations and stresses obtained from design

calculations differ considerably from those in actual structures, especially in the vicinities of

connections. Rules are on the conservative side when determining cross-sectional dimensions,

but fatigue is often caused by secondary stresses due to restraint moments.

2.3.1 Main girder-floor beam connection in through-type plate girder

Fig. 13 shows a through-type plate girder railway bridge in which fatigue cracking

occurred when the flanges of the floor beams were provided with cut-outs to facilitate joining

at the connections attaching the floor beams to the main girders [19]. In the design, floor beams

are provided simple support by main girders; thus only shear force, reactions of cross beams,

are transmitted at the joints, and there is no problem if cut-outs are made in the flanges.

Fig. 14 Retrofitting works at girder to cross beam connections

(a) Original detail (b) Improved detail

Fig. 13 Through-type plate girder railway bridge, Fatigue cracks

initiated at the connections between main girders


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However, since the floor beams are fixed to main girders in the actual structure, fixed-end

restraints moments are produced. As a result, fairly large direct stress components will occur in

the flanges. Because of the large decrease in cross section and the change in configuration due

to the cut-outs of the flanges, large stress concentrations occur at these locations.For repair, it will suffice to make a connection which is adequate against fixed-end

moment, and as Fig. 14 shows, the structural detail of the connection was changed so that the

force of a cross beam flange can be transmitted to a main girder flange. Also, holes were made

at the ends of the fatigue cracks, and a fairly large area of the web was spliced. Connections

attaching stringers to cross beams had fatigue cracks due to the same causes, and the repair

concept is the same.

2.3.2 Main truss-floor beam Connection

Fig. 15 shows fatigue cracking which occurred at the connection between floor beams

and panel points of trusses in a deck-type truss girder highway bridge. This damage occurred

because these detapass. Upon carrying out various examinations, it was decided that as the

reinforcing measure the top flange of a floor beam and the flange of the upper chord member

of a truss should be joined using a connection plate as shown in Fig. 16. In order to weld the

connection plates to the flange plate of the truss, concrete at the top surface of the truss was

blasted off by water jet.

Fig. 15 Fatigue crack at the connection

between floor beam and truss panel point

Fig. 16 Retrofitting works: Floor beam and the flange

of truss top chord was connected directly

using a connection plate


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2.3.3 Cross bracing connection in plate girder bridge[20]

Fig. 17 shows fatigue cracking which occurred at welds between a vertical stiffener

used attaching cross bracing of a plate girder bridge and an upper flange and web. Such fatigue

is due to fairly large forces being produced at the various members of the cross bracing from

the difference in deflection between the main girders caused by vehicle loads and the forcible

deformation of the upper flanges of plate girders by deflection of the concrete deck in a

direction perpendicular to the bridge axis.

Various methods are employed for repair of this damage considering the degree of

damage and ease of execution (Fig. 18). For the smallest cracks, in cases where there is no

problem about size with fillet welds, rewelding the crack is done by TIG dressing. In case of

small size of fillet weld with the possibility of crack occurrence from the root, after one to three

passes of fillet welding on top of the crack, the toe is finished by TIG or grinding. When the

Fig. 17 Fatigue cracks at the cross bracing connection details

Fig. 18 Various retrofitting methods for cross bracing connection details

Fig. 19 Cross bracings and stringers supplement work


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crack is large, it is removed by gouging after which rewelding is performed by full-penetration

welding, with toe finishing done by TIG or grinding. The results of follow-up investigations

indicated that most of the repaired spots remained sound, but fatigue cracks have occurred

again where the stresses produced were large and where cracks had been large.When it is possible to remove the concrete deck on top of the girders, vertical stiffeners

can be connected to upper flanges with high-strength bolts, and the forces at the various

members of the cross bracings can be transmitted smoothly to the main girders. Improving the

deformation behavior of the structure as a whole is also done by installing cross bracings and

stringers (Fig. 19). In this case, the plate thickness of vertical stiffeners for attaching new cross

bracings is changed from the 10 mm used in the past to about 16 mm, while their widths were

made as large as possible. Further, to lower stresses at damaged parts, horizontal members of

old cross bracings were removed.

2.3.4 Sole plate end

Support points on a bridge may be said to be points of the greatest load concentrations

on the bridge. Beam theory is normally employed in designing a bridge, but when sizes of

girders and locations of supporting points are considered, the stresses occurring near support

points differ considerably from those obtained by beam theory. Consequently, the regions

around of supporting points are susceptible to fatigue damage.

Fig. 20 shows fatigue cracking which occurred in a plate girder bridge where a sole

plate was attached to the underside of a bottom flange by fillet welding [21]. For sole plates,

when rotating function is lost, very high stresses occur at the front surface of the sole plate, and

this local stress is made even higher by the deformation of the flange when the flange of the

main girder and web are welded,

Fig. 21 Retrofitting method for sole plate detail

Fig. 20 Fatigue damage in sole plate connection detail at support


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An example of repair is shown in Fig. 21. The sole plate was changed to a longer one.

Joining with the bottom flange was done using high-strength bolts. As the cracks had

penetrated into the web, holes were drilled at the tips along with which splicing was done using

high-strength bolts. The holes at the tips of cracks were also stopped with high-strength bolts.

When cracks of webs are large, welding may also be done.

2.3.5 Cut-out web

Fig. 22 shows fatigue cracking which occurred where a cut-out had been made at the

end of a girder of a plate girder bridge. Regarding this part, bending stress is extremely small

according to the beam theory, but since a cut-out is provided, a high stress component is

produced in the normal-line direction at the corner of the curved part. This stress componentbecomes higher the smaller the curvature ratio of the curve. Since the flange and the web are

joined by fillet welding, when looked at locally, this fillet weld is supposed to transmit load.

Fatigue cracks often are initiated from the roots of welds because of this.

In repairing, in order to lower stress at the bottom flange as much as possible, and to

transfer stress smoothly to the supporting point, integrated reinforcing plates were attached to

the flange and web with high-strength bolts (Fig. 23). In bridges newly constructed, ribs are

attached in a manner to be continuous from the flange, the curvature ratios of cut-outs are

increased, and welds are all made full-penetration welds to reduce stresses at this part.

2.3.6 Orthotropic steel bridge decks

The orthotropic steel deck system is light-weight compared with concrete deck slabs

and is suited to long span bridges. Because decks support traffic loads directly, and the thinner

orthotropic steel bridge decks are flexible, actual stresses due to traffic loads in elements are

different from those in design calculations, and fatigue is very severe.

Fig. 24 shows the parts of orthotropic steel decks in which fatigue cracks developed.

Fatigue cracks have developed most frequently from field welded joints of trough ribs used

backing strips, and scallops at the intersections of the longitudinal and transverse ribs. Fatigue

cracks initiated from the root of welds between trough ribs and deck plate and penetrating into

the deck plate are one of the most serious examples of fatigue damage which have been

observed recently.

Fig. 22 Fatigue cracks at the corner

of reduced height of girder

Fig. 23 Retrofitting Works by Reinforcing Plates


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2.4 Vibration

2.4.1 Girders in the high speed train system

When a train travels at high speed, there may be a case of vibration of a kind

unforeseen occurring in a bridge. Fig. 25 is an example of fatigue damage due to vibration in

bridge structures of the Shinkansen Line of Japan. Since the start of operation in 1964, speedshave been increased, and from about the time that 200 km/h was exceeded, vibrations of

bottom flanges of stringers in plate girder bridges and truss girder bridges in directions

perpendicular to the bridge axes began to appear prominently when crossed by trains. Also,

vibrations in out-of-plane directions occurred in diaphragms of box-section girder bridges.

Fatigue cracks have occurred in vertical stiffeners and ribs restraining such vibrations.

Repair methods differ depending on the degree of damage. In case a crack has gone

through the thickness of web plate, holes are drilled at both ends of the crack, and splicing is

done using a plate provided with ribs (Fig. 26). When a crack is small with length at the

surface about 20 mm, one to three passes of fillet welding is done on top of the crack, followed

by TIG dressing (Fig. 27). Similar steps are taken as preventive measures for parts not yet

cracked. The results of surveys 10 years after implementing such measures indicated cracks

have not reoccurred, indicating that the measures have been appropriate.

Corner plateLongitudinal Rib

Deck plate

Transverse Rib

Vertical Stiffener

Fig. 24 Fatigue cracks in orthotoropic steel decksFatigue Cracks (Type )

Fatigue Crack

Fatigue Crack


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2.4.2 Sign poles on highway bridges

There are various types of poles on highway bridges such as sign poles, signal poles,

and light poles. Fig. 28 shows fatigue cracks developed at the end of rib plate of base plate of

sign pole on elevated highway structures. Severe vibration is induced by the passage of

vehicles, and combined with the low fatigue resistance of this detail, fatigue cracks penetrate

through the whole section of pole.

After this accident, a nationwide survey was carried out, and many similar fatigue

accidents have been reported. There are two reasons for such vibrations: one is due to traffic,

and another is due to wind, i.e. Karman vortex shedding.

Fig. 29 shows the newly developed rib detail with high fatigue resistance [26].

End of Vertical Stiffener

[Vibration]

Fig. 26 Adding plates with rib

Fig. 27 Retrofitting the end of longitudinal rib plates by applying TIG dressing method

Fig. 25 Fatigue damage due to the out of plane vibration of bottom flange of girder in bullet train systems


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3. Database of Repair Cases

3.1 Format of summarized repair cases

Repair cases of failed steel bridges which are included in this database were collected

from technical reports, proceeding, journals, and publications which have been reported in the

past [12, 27-60].

Repair cases were summarized using the same format as much as possible so that the

user could compare each case. Table 1 provides the format of the summary in these repair

cases.

Each summary provides for figures and photographs on the bridge design, crack,repair procedures, and so on. These figures and photographs were linked with descriptions in

the representative summaries. Pointing to and clicking on the underlined part such as Fig. 30

will automatically show the figure or photograph.

In addition to repair cases of failed steel bridges, this database also includes some

repair cases for fatigue failures in other structures. These repair cases are also summarized by

using the same format as that of steel bridges. After this, repair cases for structures such as

machines, ships, ocean structures, pipelines, and so on will be added in this database.

1 Field of application2 Circumstances of repair

3 Types of structure

4 Details of loading

5 Description of damage

6 Repair method applied

3.2 The composition of database system

The homepage address of this system is

http://iiw.wg5.cv.titech.ac.jp/ .

Fig. 29 Newly Developed Detail (U-shaped rib)Fig. 28 Fatigue cracks at the end of rib plate

of base plate of sign pole

Table 1 Style of Repair Cases

Fig. 30 Head of Homepage


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1 Welding defects were included at the time of

fabrication.

2 An inappropriate structural detail of low fatigue

strength had been adopted.

3 Stresses and deformations unforeseen in design

occurred at joints of members.

4 The structure behaved in a manner not expected such

as due to vibration.

A. Removal of crack

B. Re-weld

C. Surface treatments such as TIG dressing and Peening

D. Re-weld + post weld surface treatments

E. Bolted splice

F. Shape improving

G. Stop hole

H. Modification of connection detail

From the address, the user can access the homepage of this web-site (Fig. 30).

The causes of fatigue in the steel bridges were classified in four categories as shown in

Table 2 [23]. The repair measures are classified into eight types from A to H as shown in

Table 3. Repair F or shape improving means to repair the weld part and any nearby part in

order to improve the joint itself such as a full penetration weld. On the other hand, repair H

corresponds to modifying the connection details where spreads from the joint environment to

the whole structure. Therefore, repair H is different from repair F due to the detail of the

repair target and the degree of repairing scale. Generally, there are many cases where repair

was done by a combination of measures rather than only a single measure. Depending on the

repair cases included in this database, stop hole is commonly adopted as an emergency repairwith a view to arrest the crack propagation, and there are many cases where permanent repair

permits other methods. In such cases, the repair method adopted as a permanent measure is

recommended here.

At the bottom of this homepage, each repair method for each fatigue failure is

evaluated by the authors judgment. The detailed explanations are as follows.

a) Repair method for Cause 1

When the cause of failure is a weld defect unintentionally left in welds, the repairing

method first supposed is to have sound joints. Therefore, it is effective to repair locally such as

repair A -removal of crack, repair B -re-weld and repair D - re-weld + post weldsurface treatment. In case the crack propagates in a first member, it is recommended that

repair E - bolted splice should be used together with the above repair procedures. Because

there will be a higher possiblity that weld defects during fabrication and in the field are left in

many weld joints using the same details, it is very effective to modify the connection detail

(repair H) and improve the shape (repair F), although these are large-scale undertakings.

b) Repair method for Cause 2

There is a higher possiblity that a fatigue crack may re-initiate from the weld if repair

for the failure owing to this cause is done to restore the weld joint before cracking.

Consequently, two measures should be taken:

lower the stress occurring in the weld zone by adding to members.

Table 2 Cause of Fatigue in Steel Bridges Table 3 Repair Method


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increase the fatigue strength of the detail and lower the stress concentration

by dealing with the weld zone directly.

The former corresponds to repair E and H. On the other hand, the latter which

repairs the weld directly, corresponds to the repair method of lessening the stress concentration

by finishing the weld toe such as grinding or TIG dressing, i.e., repair C, of which the idea is

to strengthen in advance, and repair D, where the idea is to retrofit after cracking. The latter

also includes peening, which introduces the compressive residual stresses.

c) Repair method for Cause 3

This fatigue failure resulted from secondary stress due to fixed-end restraint moments

at the connection of the members, the deformation due to behavior of the structure as a whole

and the difference in deflection between the members. Therefore, there are two concepts

conceivable in repairing such a failure:

increasing stiffness to make possible adequate resistance of the joint against moments.

reducing moments.

From the viewpoint of the whole bridge structure, repair H, which is to modify the

structural detail, is the most effective method. However, repair E, which corresponds to the

stress reduction due to increasing the cross section, is recommended if there are structural

restrictions. There are a few cases where the localized repair such as repair C or D and the

releasing secondary stress such as repair F or G are taken, for example, the failure at the

bottom ends of vertical stiffeners due to out-of-plane deformation of the girder web.

d) Repair method for Cause 4For the fatigue failure resulting from the vibration due to vehicle or train passage or

strong wind, it is difficult in many cases to restrain such vibration by itself. Consequently, it is

effective to take a measure so that the stress occurring from vibration can be reduced. Because

only localized repair cannot lessen the stress occurring from vibration, the repaired part will

continue to have cyclic response to stress. Therefore, it is effective to reduce the stress

according to modification of structural detail (repair H) or in case of structural restriction

increasing the cross section (repair E), , or increasing the fatigue strength (repair C or

D).

From the above description, suggestions for selecting a method of repair andreinforcement are shown in Table 4. The grade of repair methods A to H as shown in

Table 4 is subjectively judged by the authors. This evaluation will be improved as we accept

many professionals opinions from the comment function of this system. Taking advantage of

Table 4, the users can see for each repair case, what action was actually carried out for each

cause of fatigue failure.

This web site also contains a web page to search for words and phrases within the site

contents.Byinputting of a keyword, it will be possible to search for the users expectations.

This database uses

Microsoft Index server

as its searching software.


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A B C D E F G H

1 G G G E G G

2 F F G E E E G E

3 F F G G E G G E

4 F F G G E G G E

E: Excellent G: Good F: Fair N:No good

Use of this function is explained as follows.

When searching the repair cases done involving TIG adopted for the Shinkansen

train, the user needs to input the appropriate keywords, for example, TIG and Shinkansen or

TIG & Shinkansen, or TIG & Shin*. After submitting the search, a list of appropriate

cases will be produced. In this function, users can search appropriate cases with differential

keywords similar to internet search engines.

3.3 Analysis of Repair Cases

It is possible to analyze the cases by making use of the search system. Regarding

Table 4 Applicability of Combination

Fig. 33 Relationship between years of service and causes.

Fig. 31 Distribution of the repair cases in Japan. Fig. 32 Distribution of the repair cases in other countries.


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reported repair cases of failed steel bridges, the distribution of all the combinations in the

Japanese cases in shown in Fig. 31 and all others are shown in Fig. 32. Depending on the

number of each case, the tendency of the distribution showed a differential relationship

between the cases in Japan and in other countries. Compared with the cases in other countries,

the cases in Japan included few cases for fatigue failures occurring from a weld defect. There

are many cases repaired by bolted splice for Cause 2 or Cause 4. No cases are repaired

only by a stop hole, which is well known as an emergency repair for cracking. However,

there are many cases that stop holes were used together with other permanent repair

methods.

The relationship between years of service until observation of fatigue failure and the

cause of fatigue failure is shown in Fig. 33. If the cause of fatigue failure is a weld defect

which is Cause-1 shown in Table 2, fatigue cracking is apt to occur in the early years of

operation [61]. On the other hand, if the cause of fatigue failure is an inappropriate structural

detail of low fatigue strength which corresponds to Cause-2, the occurrence of fatigue

cracking concentrates in the range over ten years of service [23]. Fatigue cracking caused by

stress and deformation unforeseen in design, which corresponds to Cause-3, the most frequent

cause in all the repair cases, has been found regardless of years of service. The occurrence of

fatigue failure due to Cause-4 showed the same tendency as Cause-3. However, fatigue failure

due to wind vibration occurred in the early years, and the fatigue failures occurring after ten

years of service were caused by vibration of the members due to traffic or train passage.

Fatigue failures due to this cause were observed at the bottom ends of the web verticalstiffeners in Shinkansen bridge structures and at the base joints of sign poles installed in

Japanese highway bridges. It is characteristic that Cause 4 as shown in Table 2 also includes

the loss of support function and the fixity of pin joints.

The relationship between the years of service and repair methods is shown in Fig. 34.

The cases that adopted Repair E(bolt splice) through Repair H(modification of connection

detail) are dealt with here. As years of service increase, the number of repair cases performed

by repair E - bolt splicing increases because of a permissible method. On the other hand,

repair H - modifying the connection detail and repair F - shape improving were

adopted extensively despite years of service. In general, modifications of connection detailbecome large-scale and more difficult. Repair is costly because of performing the measure in

advance for the same detail as crack location. In cases where this repair method is adopted to

steel bridges where cracking occurred soon after opening, it is suggested that it should be

examined if this measure will be appropriate or not and how much this measure will depend on

the Total Life Cost of the steel bridge. On the other hand, one needs to consider execution

and quality control such as trial testing in the case of repair F shape improving.

The relationship between years of service and detection year of fatigue cracking is

shown in Fig. 35. The occurrence of fatigue cracking has been rapidly increasing since the first

half of the 1970s when many welded girder bridges were opened. Supposing that the repair for


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fatigue failure was done at once after the detection of fatigue cracking, repair E bolt

splicing was frequently adopted until the first half of the 1980s. On the other hand, repair H

modifying the connection detail and repair D adding surface treatment such as TIG

dressing have become popular since the second half of the 1980s. This indicates that the

system of the structural analysis progresses and the improvement of fatigue strength for weld

joints such as filled weld toe become supported.

4. Conclusions

As mentioned at the beginning, in repair of fatigue damage, the basic principles in

repair and reinforcement, accurate investigation of causes is always an absolute condition, to

eliminate the cause, improve fatigue strength, lower overall stresses of members, lower local

stress concentrations, and increase or decrease stiffness at parts. An appropriate method must

be selected for each case. This is a difficult problem since design is predicated on structures of

the past.

Acknowledgement

The work described in this paper forms part of the study program of IIW-XIII-WG5.The members of IIW-XIII-WG5 contribute to this paper. We express whom it may concern

gratitude.

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Fig. 34 Relationship between years of service

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repair cases paper

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