TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

RESEARCH REPORT 259

FATIGUE CLASSIFICATION OF WELDED JOINTS IN

ORTHOTROPIC STEEL BRIDGE DECKS

by J R Cuninghame

The views expressed in this report are not necessarily those of the Department of Transport

Bridges Division Structures Group Transport and Road Research Laboratory Crowthorne, Berkshire, RG11 6AU 1990

ISSN 0226-5247

Ownership of the Transport Research Laboratory was transferred from the Department of Transport to a subsidiary of the Transport Research Foundation on I st April 1996.

This report has been reproduced by permission of the Controller of HMSO. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

CONTENTS

Abstract

1 Introduction

2 Fatigue assessment method

3 Test methods and loading

3.1 The deck panel

3.2 Loading

3.3 Definit ion of stress

3.4 Residual stresses

3.5 Weld classif ication

4 The fatigue tests

4.1 Longitudinal st i f fener to deck joints

4.1.1 Cracking in the weld throat

4.1.1.1 6 r a m fi l let welds

4 .1 .1 .2 9 r a m fil let welds

4 .1o l .3 Partial penetrat ion welds

4 .1 .2 Cracking at the weld toes

4.1.2.1 Weld toe on the trough

4 .1 .2 .2 Weld toe on the deck plate

4 .1 .3 Weld toe t reatment

4.2 Trough splice joints

4.2.1 Butt welded joints

4.2.2 Fillet welded joints

4.3 Trough to crossbeam joints

4.3.1 Troughs f i t ted between crossbeams

4.3.2 Troughs passing through crossbea ms

4.4 Crossbeam to deck joints

Page

1

1

1

1

2

2

2

2

4

4

4

8

11

11

11

11

11

11

11

11

14

16

20

Page

4 .5 Web to deck joints 20

4 .6 Deck plate jo ints 24

4.6.1 Longitudinal but t jo ints 24

4 .6 .2 Transverse but t jo ints 24

Discussion and Conclus ions 24

A c k n o w l e d g e m e n t s 28

References 28

© CROWN COPYRIGHT 1990 Extracts from the text may be reproduced,

except for commercial purposes, provided the source is acknowledged

FATIGUE CLASSIFICATION OF WELDED JOINTS IN ORTHOTROPIC STEEL BRIDGE DECKS

ABSTRACT

The TRRL has carried out a number of investigations to assess the fatigue performance of welded joints in orthotropic bridge decks. The assessment method is based on the UK design code, BS 5400 part 10. Stresses due to traffic loading were obtained from static load tests on trial deck panels in the laboratory under a single wheel load, and on bridges using a two axle test vehicle.

This report describes constant amplitude fatigue tests carried out on some of the main types of welded joint in two designs of deck panel used on bridges in the UK and elsewhere. The test results were used to assign the joints to the appropriate weld class in the design code. The joints tested were; the longitudinal stiffener (trough) to deck; alternative types of trough to crossbeam; trough splice; crossbeam to deck; longitudinal web to deck; and deck plate butt welds.

For most joints the test specimen and loading were designed to simulate the passage of single wheels over the joint and fatigue strength can be expressed in terms of stress in the plate adjacent to the weld. It was found that several of the joints could be upgraded compared to the BS 5400 classifications, but inspection and quality assurance requirements are likely to limit the weld class which can be used for joints on bridges.

1 INTRODUCTION

The UK has three very long span steel bridges. These plus five cable stayed and box girder bridges and a handful of moving bridges make up the total use of orthotropic steel bridge decks. Although small in number, these bridges represent a considerable capital investment and form vital links in the national road network.

Orthotropic steel decks are used in long span and moving bridges because of their high strength to weight ratio. However the decks are subjected to high dynamic stresses due to wheel loading by heavy goods vehicles so that they have to be assessed for fatigue cracking.

The fatigue assessment of orthotropic decks is excluded from most bridge design codes because of the complexity of the stress distribution and the fact that bending stress may predominate whereas weld classification tables are based on axially

loaded joints. The approach adopted by the TRRL is to use the methods of calculation and the traff ic loading given in the UK bridge design code, BS 5400 part 10 [British Standards Institution, 1980] wi th stresses and fat igue classif ication of welded joints determined by experiment.

This report draws together the results of fat igue testing of the main types of joint in orthotropic decks done by or for TRRL.

2 FATIGUE A S S E S S M E N T M E T H O D

Stresses at welded joints due to traff ic loading were obtained from load tests, either on a full size deck panel in the laboratory under a single wheel load, or on a bridg_e using a test vehicle wi th known wheel loads. In both cases the load was applied at many dif ferent positions on the deck, and strains were measured by ERS gauges at each welded joint. Stress influence lines were calculated from the measured strains.

By assuming that stress is proportional to wheel load it was possible to build up influence lines for the vehicles defined in table 11 of BS 5400 part 10 by superposit ion, for the various transverse positions in figure 17 of the code. A stress spectrum was then calculated for each joint by performing a reservoir count (as defined in appendix B of the code) on each' vehicle inf luence line, and assigning the specified number of vehicles to each transverse position.

Fatigue life was calculated using the Palmgren- Miner cumulative damage rule. This is based on the constant amplitude fatigue strength of the joint obtained from the fatigue tests. The aim was to carry out suff icient tests to assign the joint being assessed to one of the weld classes in the design code.

3 TEST M E T H O D S A N D LOADING

Both specimens and deck panels were fabricated using welding procedures modelled on those used in bridge construct ion. All welds except the deck plate butt welds were made by manual metal arc (MMA) welding. Deck panels are normally fabricated upside down, so all joints can be

welded downhand. On the other hand, any weld ing on an ex is t ing bridge wi l l be in the overhead posi t ion. Therefore, for tests aimed at developing repair or s t rengthening procedures, we ld ing was carried out in the overhead posit ion. Except where noted, the joints were free of s ign i f icant weld defects.

3.1 THE DECK PANEL A typ ica l deck panel is shown in f igure 1 i l lustrat ing the d i f ferent types of welded joint. Stress inf luence surfaces for single wheel loads have been obtained for each jo int and these provided the basis both for the calculat ion of stress spectra and for select ion of the fat igue tes t loading.

The great major i ty of decks have closed longi tudinal s t i f feners. Open st i f feners such as bulb f lats, angle or channel sect ions are general ly less ef f ic ient , and are used where the t ra f f ic loading does not jus t i f y a more elaborate design. St i f feners of var ious cross sect ions have been used, the most common are trapezoidal or V- shaped. The cross section of the decks used in the work at TRRL are shown in f igure 2. Al l the tes ts were on decks wi th trapezoidal s t i f feners, except for the t rough splice jo ints and the trough passing through the crossbeam, which were on decks w i th V-shaped st i f feners.

3 .2 LOADING Tests were carried out on sect ions containing ful l scale joints, and on a full size deck panel. A detai led descr ipt ion of each specimen is g iven in the re levant sect ion.

The aim of the fa t igue tests was to represent as near ly as possible the dynamic stresses applied to the jo int in service. Al l the tes ts were run under constant ampl i tude loading to obtain points on an S-N curve. For some joints sinusoidal loading was suf f ic ient , but in others more complex wave fo rms were applied to s imulate the passage of a wheel over the joint.

In all the tests loading was appl ied by servo hydraul ic actuators. The load was adjusted to obtain the required strain at the joint under test . Load and strain were recorded per iodical ly and e lectronic sw i t ches were set to stop the tes t if load or d isp lacement dr i f ted outs ide preset l imits. The strain at gauges near the crack locat ion general ly began to change (mean level f i rst , then range) at about three quarters of the tota l l i fe, and before the crack was visible.

3.3 DEFINITION OF STRESS For the purpose of fat igue assessment it was decided to use the stress at r ight angles to the

weld at a distance of 15 mm from the weld root for f i l let welds. Gauges were attached at this position both for stat ic tests under wheel loading, and for fat igue test ing. Using this gauge position has a number of advantages:

(i) It al lows for variat ions in weld size, and avoids the local influence of weld toe defects.

(ii) The results of di f ferent sets of experiments can be compared.

(iii) The stress obtained from the same gauge position is used to define both the fatigue strength of the joint, and the stress spectrum due to traf f ic loading, so that comparable values are used throughout the assessment.

(iv) This stress can be measured direct ly, whereas stress at the weld toe can only be obtained by extrapolat ion.

It is interesting to note that in BS 5400 Part 10, the design stress for weld toe failure is the plate stress at the weld toe, and for weld throat failure i t is the stress in the weld. The draft Eurocode 'Design of steel structures' (EC3) al lows either stress in the plate adjacent to the potential crack location, or the geometr ic stress to be used. The latter is the 'hot spot stress' at the weld toe, defined as the extrapolat ion of the maximum principal stress to the weld toe. It takes into account the overall geometry of the joint, but excludes local ef fects due to the weld profile and toe defects.

Some of th'e welded joints in orthotropic deck panels contain more than one weld. The stress usually varies along the length of each weld, and there may be several potential failure modes. The fat igue strength of the joint can only be defined by a single stress value if the specimen design and loading reproduce the stress distr ibution in the full size deck. Also any change in joint design could change the fat igue strength.

3.4 RESIDUAL STRESSES Welded joints in large structures generally contain high residual stresses. The distr ibution and magnitude of these stresses is highly variable and depends on the joint geometry, f i t-up of the plates and the welding procedure.

For some joints the stress influence line is partly compressive and so the applied stress cycle wil l be more or less damaging at di f ferent locations around the joint, depending on whether the residual stress is tensile or compressive. Most of the fat igue specimens were large enough to develop full residual stresses, and weld procedures were modelled on those used in bridges.

Deck plate

Web of box

Longitudinal trough stiffener

S-:::"

Alternative connections Crossbeam

Fig.1 Main welded connections in a typical orthotropic bridge deck

305 305 305 305 305

I l I ! I i I I

i 11.5 ,

(a) Trapezoidal troughs

1 °

X x ~ Web of box

Al l dimensions in mm

305 305 12.7

1 I = '_V 257

Crossbeam

6.4

(b) Vee shaped troughs passing through crossbeams

Fig.2 Cross section of decks used in tests

3

There is a potent ial d i f f i cu l ty if remedial weld ing becomes necessary. A weld added after a jo int is o therw ise complete wi l l be highly restrained and may reverse a favourable pat tern of residual stresses, for example a short length of weld on the apex of a t rough splice.

3.5 WELD CLASSIFICATION The object ive of the fat igue test ing was not to define the fat igue strength of each joint in the rigorous w a y set out in the design code, but to obtain su f f i c ien t data to assign the joint to the appropr iate weld class in the design code.

However , for design purposes there is a basic problem for classes above D. Classes A and B are not appropr iate as they apply to plain steel plate. The inspect ion requirements and permit ted defect levels for class C welded jo ints would make them uneconomic for the major i ty of bridge construct ion.

4 T H E F A T I G U E T E S T S

The tests on each of the jo int t ypes are described in the fo l low ing sect ions.

4.1 LONGITUDINAL STIFFENER TO DECK JOINTS

The closed longi tudinal s t i f feners on the underside of the deck plate are referred to as troughs. There are three possible modes of fa t igue cracking for the t rough to deck plate joint:

(i) through the weld throat .

(ii) through the t rough web at the weld toe.

(iii) through the deck plate at the weld toe.

Stress inf luence line measurements under a stat ic wheel load were reported by Nunn and Cuninghame (1974a) for s t resses in the deck plate and t rough web. For a g iven load, the stress adjacent to the weld on the deck plate and the t rough are quite d i f ferent , t ransverse inf luence l ines are shown in f igure 3. Stress in the underside of the deck is predominant ly compress ive, w i th a magni tude wh ich varies wi th t ransverse posit ion. The corresponding stress in the t rough web var ies in both magni tude and sign.

Fatigue tes ts on specimens compr is ing a 305 mm long sect ion of t rough and deck plate as shown in f igure 4, were reported by Maddox (1974a). Addi t ional tests have since been carried out, main ly a imed at f inding a method of st rengthening ex is t ing jo ints [Gurney and Maddox, 1987] . The specimen geometry was the same in all the tests, but we ld size and penetrat ion were varied. The

test results were analysed in terms of stress at the weld toe and in the weld throat. Stress at the weld toe was obtained by extrapolating from three gauge posit ions at various distances from the toe. The weld throat stress was calculated from the stress at the toe and the measured throat thickness, using a formula derived in earlier work [Maddox (1974a)].

Defining fat igue strength in terms of stress in the trough web or the deck at 15 mm from the weld root as adopted in this report is simpler and avoids the 'correct ion factors' required to calculate the weld stress.

The specimens were loaded through the deck plate as shown in f igure 4. Loading in one direction (equivalent to a wheel load between troughs) produced tensi le stress at the weld root and compressive stress at both weld toes. This was expected to be the worst case for weld throat cracks. Loading in the other direction (wheel load over a trough) produced tensi le stress at both weld toes and compression at the weld root so toe cracks were expected. This did not represent the wheel load entirely, it wil l be seen f rom figure 3 that stress in the deck plate should be compressive irrespective of the posit ion of the load. Some tests were also carried out under alternating loading to invest igate the ef fect of stress ratio.

The welds were either f i l let, 6 mm or 9 mm leg length, or partial penetration, wi th 6 mm or 9 mm leg length. The 6 mm fi l lets, representing the as- built joint, were welded downhand, the rest were welded in the overhead position to simulate repairs to exist ing joints.

Fatigue tests have also been carried out on a full size deck panel wi th 6 mm fi l let trough to deck welds. Constant amplitude loading was applied through a 25 mm thick rubber pad shaped to represent a tyre contact patch. The load was posit ioned midway between troughs to apply tensi le stress at the weld root.

Residual stresses were measured in the trough web and deck plate on the panel. Strain gauge rosettes were installed adjacent to each weld toe and a small section containing the gauge removed by trepanning. The principal stresses calculated f rom the change at each gauge element are given in table 1 together wi th residual stresses obtained by similar methods, reported by Maddox (1974a) for the small specimens.

High tensi le residual stresses were present in the panel but not in the small specimens, except where they were spot heated. The measurements were made on the surface adjacent to the weld. It is not possible to measure residual stress at the weld root. The implication of this is that tests

4

E E Z

o "o

E o

E E

Lt3

40

20

0

-20

- 4 0

- 6 0

- 8 0

I (a) Stress in underside of deck plate

kGauge

Transverse position of load

150mm I I

40

20

E E z 0

o

"o

E -20 £

E E

L~

- 4 0

- 6 0

- 8 0

(b) Stress in trough web

Gauge

I I

Transverse position of load

150mm I I

Fig.3 Transverse in f luence lines o f stress at t rough t o deck j o i n t f o r a 3 0 k N whee l l oad

l I\

I o . i i i i i / i i

\1 4

l _ _

LOADING

@ Q

760 ~ I

305 =1 11

All dimensions in mm Specimen length = 305

O

4[Approx. 150

Fig.4 Test specimen for trough to deck joint

E E Z v

300

200

E o

E E 1 0 0

~ 6O

.E

N 4 0

20

O Zero- tension at weld root • Alternating loading z~ Alternating loading (spot heated)

BS 5400 class F (95% confidence limits)

•

O oGb

I i

O

o ~

105 106 107 108 Endurance (cycles)

Fig.5 Test results for trough to deck plate joints with 6mm fillet welds cracking in the weld throat

6

TABLE 1

Trough to deck jo int - - res idual stress measurements

Deck plate stress (N/ram 2) Trough web stress (N/mm 2)

Location S1 S2 e S1 S1 e

Deck panel

1 2 3 4

Specimens

As welded side 1 side 2

+ 1 7 9 + 1 2 0

+ 1 1 0 + 3 0

28 60

+ 2 3 8 gauge lost

+ 9 0

+ 2 0 2 + 1 1 0

+ 3 7 + 100

+ 1 0 0 - 1 5

+ 2 0 + 3 3

46 89

46 41

+ 234 + 2 0 5

- 3 0 - 2 6

+51 - 8

+11 + 5 6

Spot heated at weld root

side 1 gauge lost side 2 + 5 7 + 8 . 5

Spot heated on weld surface

side 1 + 9 9 + 326 side 2 + 8 2 + 333

6 10

+ 2 5 + 6 0

+ 5 3 6 + 1 3 7

+ 140 + 1 3 5

+ 2 1 2 + 4 1 0

89

89 86

40 1

2 2

89 7

Deck plate

Sl

4 $2 9.5

mrn

Weld

carried out on as-welded specimens under compressive or alternating stress are less damaging than similar stresses on a structure.

4 .1 .1 Cracking in the weld throat

4.1.1.1 6 mm fillet welds

The test results for joints wi th a 6 mm fi l let weld (manual metal arc welding) are shown in figure 5. Results from Maddox (1974a) are included. All the specimens failed by cracking from the weld root, through the weld throat. No cracks were obtained in the deck panel tests. The reason for this is not known, but some weld penetration would have prevented cracking at the stress levels used in the tests.

Comparing the data shown in figure 5 with the BS 5400 part 10 design curves [British Standard Institute, 1980] suggests that the trough to deck

Trough web

$2 /

9.5 m m

line

jo int w i th a 6 mm f i l let weld can be t reated as class F for stress in the trough web 15 mm from the deck plate. As expected the l owes t fat igue strength was for tensi le stress at the weld root, and some results f rom t w o of the tes t series fall below the lower bound of class F. However the best f i t line for the data has a sha l lower slope than the class F line, and the lower bound of the data is above the class F line for st resses below about 70 N/mm 2.

As a check on whether i t is safe to use class F, the lower bound curve to the data points (drawn by eye) was used to calculate fa t igue l ife under BS 5400 part 10 t ra f f ic loading. The result ing l i fe was 4.3 years, compared to 4.2 years calculated using class F. Thus class F would only be unsafe if the stress spectrum contained more high stresses, ie above 70 N/mm 2. The max imum stress range for this deck under BS 5400 loading

7

is 90 N/mm 2 for normal 'Cons t ruc t i on and Use' vehic les. Given the shor t l i fe under th is loading it is un l ike ly tha t a jo in t w i t h a 6 mm f i l le t we ld w o u l d be used on a deck carry ing heav ier t ra f f ic .

An inves t iga t ion of t rough to deck plate we lds made on an unprepared t rough edge, ie the edge w a s square and not mach ined to f i t the deck plate, [Janss, 1988 ] f ound fat igue s t rength cons i s ten t w i t h class F for stress in the t rough web . The mean we ld th roa t th ickness was 3 .8 mm, and 1 . 6 - 2 mm penet ra t ion was obta ined because of the gap at the outer edge of the t rough .

4 . 1 . 1 . 2 9 m m fillet we lds

The resul ts for spec imens w i t h 9 mm f i l let we lds are s h o w n in f igure 6. Al l these spec imens w h i c h fa i led did so th rough the we ld th roat .

There are on ly six resul ts for spec imens w i t h 9 mm f i l le t we lds but all are above the class D des ign curve in te rms of stress in the t rough 15 mm f rom the deck plate. It is there fore

suggested that class D can be used for we ld th roa t fai lure of 9 mm f i l let welds.

4 . 1 . 1 . 3 Partial penetration welds

Test results for specimens w i th partial penetrat ion we lds loaded to produce tensi le stress at the weld root are shown in Figure 6. They are all wel l above class D, w i t h the except ion of those where a jo int preparation was hand ground using a disc grinder.

The results are also given in Table 2 together w i th the weld throat th ickness and penetrat ion. While it is clear that fat igue strength increases w i t h throat th ickness and penetrat ion, there is a lot of scatter in the data.

The specimens were made by manual metal arc weld ing. With some semi automat ic processes it is possible to obtain partial penetrat ion welds w i t h o u t a machined preparation. Therefore it seems likely tha t high fat igue strength could be obtained w i t hou t the cost of edge preparat ion.

T A B L E 2

Test resul ts for t rough to deck plate jo ints w i t h 9 mm fi l let and

Descr ip t ion

no prep 9 mm f i l le t as we lded

2 mm prep 9 mm f i l le t as we lded

3 mm prep 6 mm f i l le t as we lded

4 mm prep 9 mm f i l le t as we lded

2 mm prep (hand ground) 6 mm f i l le t as we lded

)artial penetrat ion welds

Throa t th ickness

(mm)

D

m

B

7.9 8 .0 8.3 8 .0

5.7 5.8 5.5

8 .4 6 .4 7 .4

4.7 5.3 4 .5 6 .4

Penetrat ion (mm)

0 .8 1.0 2.7 1.4 0 .8 1.4 1.3 1.4

2 .2 1.7 2.1 2 .5

1.4 2.1 2.1

1.8 1.2 2 .9

1.6 1.0 1.7 2.1

Stress 15 mm f rom weld root

t rough N/mm 2

192 189 168 172 176 167 143 112

227 183 183 141

204 164 140

201 151 132

1 80 132 102

89

deck N/mm 2

167 174 147 155 139 141 125

91

endurance x 10 6

0 .49 0 .49 1.0 1.0 5.2 0 .75 1.01

10.0

cracking

weld throat weld throat weld throat weld throat weld throat weld throat weld throat weld throat

191 145 145 174

160 127

2 .94 7 .63 7.49 6.87

1.29 10.0

both toes no cracks no cracks no cracks

weld throat no cracks

113

133 119 114

161 106

84 70

10.8

10.9 5.4

10.0

0.31 2.3

35 .0 10.0

no cracks

no cracks weld throat no cracks

weld throat weld throat no cracks no cracks

E Z

~ 2 0 0 E O

E E ~ 1 0 0

80 O

60 c

~ 40

20,

_ _ BS 5400 class D design curve 95% confidence l imits

O 9mm fi l let weld

• Partial penetration +9mm fi l let I z~ Partial penetration +6mm f i l let Loading zero-tension at weld roo t

• Partial penetration (hand ground) +6mm f i l let

o o ~ o ~,

~ z

105 ! I

10 6 10 7 Endurance (cycles)

Fig.6 Tes t results f o r t r o u g h t o deck p la te j o in t s w i t h 9 m m f i l l e t we lds o r p a r t i a l p e n e t r a t i o n w e l d s c rack ing in the w e l d t h r o a t

108

E Z

t~

E o

E E

LO

.E

E

300

200

IO0

80

6O

40

20

• 0 0 0 ~ ' ~ • o

o [ ]

[3

I

• 6mm f i l let ~ Zero-tension at weld toe O 9mm f i l let

n 9mm f i l let a l ternat ing loading

BS 5400 class D (95% conf idence l imits)

• o

I I 10 5 10 6 10 7 0 8

Edurance (cycles)

F ig.7 Test results f o r t r ough t o deck p la te j o i n t s c r a c k i n g f r o m the w e l d t o e in t h e t r o u g h

E E z

300

200

E o

E E 100 r

~ 80 f ~ 60

~ 40

g

20

105

o

Fig.8

O 9mm penetration welds-- Zero-tension at weld root

_ _ BS 5400 class O (95% confidence limits)

- ° o

o ~

| I 106 107

Endurance (cycles)

Test results for t rough to deck plate joints cracking at the weld toe in the deck plate

10 8

E z 300

200

E 0

E 100 E m 8 0

~ 6O

.E 40

E

20

105

O Weld toes shot peened

• Weld toes plasma dressed t Zero-tension at weld toe

- - BS 5400 class D (95% confidence limits)

o" •

I I 10 6 10 7

Endurance (cycles)

Fig.9 Test results for t rough to deck plate joints, welds with toe t reatment

10 8

1 0

4 .1 .2 Cracking at the weld toes 4 . 1 . 2 . 1 Weld toe on the trough

Test results for specimens fail ing from the weld toe on the trough are shown in Figure 7. All the results are above the minimum for class D, wi th 6 mm fi l let welds towards the bottom of the scatterband but stil l above the class D design curve. There is some evidence of a cut-of f at around 90 N/ram 2.

4 . 1 . 2 . 2 Weld toe on the deck plate

Very few specimens failed from the weld toe in the deck plate. This is not surprising as the stress range in the underside of the deck plate was lower than the stress in the trough. The test results are shown in Figure 8. The specimens all had 9 mm penetration welds and three of the four which cracked in the deck plate were loaded to produce tensile stress at the weld toes, and the toe on the trough was treated by plasma dressing. The fourth was untreated and loaded to produce compressive stress at the weld toes.

The run-outs in Figure 8 refer to specimens which did not crack at the weld toe on the deck plate (though some fai led from the weld toe in the trough). These results confirm that the joint can be regarded as class D for toe failure in the deck plate, for stress in the deck 15 mm from the weld root.

4 .1 .3 Weld toe treatment Some specimens wi th 9 mm penetration welds had both weld toes treated by shot peening or plasma dressing. They were tested wi th tensi le stress at the weld toe, see figure 9. Only one plasma dressed specimen failed and the results suggest that failure should not occur at stress ranges less than 180 N/ram 2.

4 . 2 T R O U G H SPLICE J O I N T S 4.2.1 Butt welded joints An assessment of the fatigue performance of butt welded splice joints including fat igue tests was reported by Cuninghame (1982). The test results were consistent w i th BS 5400 class C, but other workers had obtained lower fat igue strength as a result of weld defects [Tromp, 1974 and Kondo et al, 1982]. As splice welds have to be made on site in the overhead position, and f i t-up depends on the alignment of adjacent sections of the bridge, Cuninghame (1982) suggested that class D would be appropriate.

4 .2 .2 Fillet welded joints Additional tests were subsequently carried out on three types of f i l let welded splice joint. There

were four series of tests. For series 1 and 2 the specimens were 2 m long wi th a single t rough loaded in four point bending as in Cuninghame (1982). In series 1 both inner and outer splice plates were formed to f i t the trough. In series 2 the inner spl ice plates were f lat rectangular plates attached to each trough web. Detai ls of both types are shown in Figure 10. Series 3 and 4 were tested in a separate invest igat ion wi th a shorter spec imen loaded in three point bending, see Figure 11. Series 3 had a single splice plate f i t ted over the trough. Series 4 had inner and outer spl ice plates formed to f i t the trough, s imi lar to series 1.

The splice plates were cold formed to f i t the troughs and welded in place in the overhead posit ion by manual metal arc weld ing. Electrodes were low hydrogen type, 2.5 mm (root run) and 3.2 mm diameter , to BS 639 [Bri t ish Standards Inst i tut ion, 1976] Class E51 33 B 120 28H.

The results of the fat igue tests are given in Table 3 and plot ted in Figure 12.

The results for the double f i l let welded splice joints (series 1 and 4) are very close to the mean line of class E, w i th a tendency to longer l ives at lower stress ranges.

There are too f e w results for the simpl i f ied spl ice joints in series 2 and 3 to draw f i rm conclusions, but the fat igue strength is c lear ly lower than the double spl ice jo in t (series 1 and 4). The single splice jo int in part icular is close to the mean of class G and the addit ion of f lat inner plates does not appear to increase the fat igue strength s igni f icant ly . Since the specimens were made w i th good a l ignment between trough sect ions and good f i t -up of spl ice plates, the s impl i f ied joints are l ikely to be sui table only for l ight ly t ra f f icked bridges.

The service l i fe of all t ypes of spl ice joint is af fected by the posit ion of the jo in t relat ive to the crossbeams. The fat igue life for a jo in t 400 mm from a crossbeam is tw ice that of a simi lar jo int at 1000 mm f rom the crossbeam [Cuninghame, 1982] because the stresses generated by wheel loads are lower.

4 . 3 T R O U G H T O C R O S S B E A M J O I N T S This joint can be made in t w o ways . Unti l about t w e n t y years ago it was d i f f icu l t to form t rough sect ions in long lengths, so sect ions of t rough were made to f i t be tween cont inuous crossbeams. The troughs were butted up to the crossbeams and at tached w i th a f i l let weld around the end of the trough. In more recent designs cont inuous troughs pass through cut-outs in the crossbeams. Both types of jo in t have been invest igated at TRRL.

11

>.

Splice p la te - -

(8ram)

. . . I

~ ' ~ 130

170

#

s, J

, \

t t

~ T w o run fu l l penet ra t ion weld

8 m m t w o run f i l le t we ld

l ~ 305 ~ [

I I

(a) S imp le lap splice j o in t , series 3

splice p late Inner (6ram)

Outer spl ice plate (Smm)

|

- 8mm t w o run f i l le t we ld

• 6mm single

run f i l le t we ld

(b) D o u b l e lap splice j o i n t , series 1 and 4

L

I I I

1 1

- - I . . . . . . . . . . . . . . . . . . . 1 . . . . I I I I

- ! , ' . . . . . . . . . -~.- . . . . . . . ! - - -

' "X, , , ' I I

- - I I - - \

f

..a

splice plate Inner

Oute r spl ice p la te

F i l le t we ld

(c) D o u b l e lap spl ice j o i n t w i th f l a t inner plates, series 2

Fig.10 Details of trough splice joints

12

1 4 0 0 f~

E e l I J ' J ~ d

I I I I I I I I I

I

I ~ r

I I

i I I I

' 1 I

I I I I I I I I

J

I

~ 1 ~ .

279

I

12

279

Load

Fig.1 1 Specimen for series 3 and 4 tests on trough splice jo in t

E

z

200

~ loo

~ 8 0

g c 6 0

~ 4 0 f f l

2 0

_ z ~ . ~ ~ A ~ o O "~"~ "~

o Series 1 l ,~ Double lap splice

• Series 4 )

z~ Series 2 Double lap w i th f lat inner plates

• Series 3 Single lap splice

d"

BS5400 Class E (95% con f i dence l imi ts)

8S5400 Class G (95% con f i dence l imi ts )

, I I 105 106 107

Endurance (cycles) 108

Fig.12 Test results for fi l let welded trough splice joints

13

TABLE 3

Test resul ts for f i l le t welded trough splice joints

Specimen type

series 1 double lap 2 m specimen

series 2: f la t plate inner spl ice plate; 2 m specimen

series 3 single lap splice 1.4 m specimen

series 4 double lap spl ice 1.4 m specimen

stress range

(N/ram 2)

80 1 O0 110 120 155 180 200

100 130 160

46 60 77 77

124 154 183

endurance x l 0 6

cycles

15 5.4 2 .25 1.8 1.03 0 .55 0 .43

1.92 0.47 0 .22

10 2.5 1.45 1.01

7.8 0 .77 0 .92

crack length (mm)

104 106 105

78

180

100 113

136 90 76

comments

no cracks weld toe in trough weld toe in trough weld toe in trough weld toe in trough weld toe in trough weld toe in trough

weld throat crack weld throat crack weld toe in trough (0 .5 -1 .0 mm undercut)

no cracks weld throat crack weld throat crack weld throat crack

4 . 3 . 1 Troughs fitted between crossbeams An invest iga t ion of the older type of jo int was reported by Nunn (1974) including tr ia ls on a deck panel set in the A 4 0 t runk road at Denham, fa t igue tes ts on single s t i f fener spec imens and tes ts on a deck panel in the laboratory.

Since then a number of fa t igue tes ts have been carr ied out as part of a project to devise a method of repair ing and s t rengthening jo ints which had fai led in serv ice [Cuninghame, 1987] . In tes ts on a deck panel, t w o ac tuators posi t ioned over a t rough on ei ther side of a crossbeam were loaded a l te rna te ly to s imula te the passage of a wheel over the joint . Specimens simi lar to those used by Nunn were also tested, but a new loading rig was designed to produce bending st resses in the c rossbeam as wel l as the longi tudinal stress in the t rough. Figure 13 g ives detai ls of the loading ar rangements for both the panel and specimen.

Consider ing the panel f i rs t , for each load cycle there are t w o stress cyc les in the t rough and one in the crossbeam ( test endurance is for stress cyc les) . There were large var ia t ions in stress for a g iven load be tween indiv idual t rough to crossbeam jo ints . Loading each actuator resul ted in a higher s t ress in the t rough on the same side of the c rossbeam, than on the other side. The stress at each gauge posi t ion was taken as the root mean cube of the t w o stress ranges, and the stress at the jo in t as the mean of the t w o gauge posi t ions.

Details of the small specimen are given in Figure 14, and Figure 15 shows a specimen under test. For each load cycle there is one cycle of stress in the trough and in the crossbeam. The stress in the trough was higher on the side of the crossbeam nearer to the actuator, but cracking did not always occur on that side. The results are given in terms of the mean of the two stress ranges. Before the actuator load was applied, the specimen was preloaded to induce bending stress in the crossbeam, to obtain the required alternating stress cycle in the crossbeam.

The differences in stress between individual joints is thought to be due to differences in geometry and fit-up between the trough and crossbeam, in particular whether a gap exists between the end of the trough and the crossbeam.

Fatigue tests were carried out on joints wi th three types of weld, single pass 6 mm fi l let, 9 mm fi l let welds made in three passes, and 9 mm partial penetration welds. As the objective was to test repairs to the joint, the 9 mm welds were made in the overhead position.

Three modes of failure are possible.

(i) Through the weld throat from the root.

(ii) At the weld toe on the trough.

(iii) At the weld toe on the crossbeam.

14

Jack 1 Jack 2

] ~ Gauges

J-'L-~

0

I_ 1 load cycle =j

;~X,.ok, ~--,,:cy~ ~0~,~ ,,A ^ , .,,I C % ~w

E + '

~ 0

• _.. ~ , .....~_J (a) Deck panel

Jack React ion

i i

I 1 load cycle 71]i-~ .....

& I E

i11

I

o

b .E

o ~,,, A X - - , , _ V V V V , V V \

A.-%/',,_.,kJ (b) Specimen

Fig.13 Load and stress for trough to crossbeam joint

L - Deck plate Crossbeam I 600 oad / 1500 / Reac t ion "= ~-

_ I t ~/ . . . . . . ". °\. / 2 (31 ij " ! ~ ~"-~

/ l , J I _ 1 o

mp T r gh l |....t..... + ....... %...t.....2J

All dimensions in mm * Sequence o f we ld ing (all 6mm f i l le t we lds )

Fig.14 Test specimen and loading arrangement for trough to crossbeam joint

15

Fig. 15 Fatigue test on trough to crossbeam joint

B 1 7 3 7 8

All of the 6 mm fi l let welded joints fai led through the weld throat. As the longi tudinal stress in the trough is compressive, the fai lure is inf luenced by the residual stresses. Figure 16 shows the pattern of residual stress and the mean posi t ion of cracks on test specimens. The cracks are on the trough web where there are tensi le residual stresses, rather than on the sof f i t at the posit ion of maximum stress.

This pattern applied to joints where the weld across the sof f i t was made f irst, fo l lowed by the web welds. If the weld ing sequence was reversed, thus reversing the pattern of residual stresses, cracking occurred in the sof f i t weld. Both types of crack have been observed on a bridge, consistent w i th the sequence of weld ing. Cracks in the web were from the weld root, but sof f i t cracks tended to be at the toe on the crossbeam. These are more di f f icul t to repair.

A 6 mm fi l let weld w i th no visible cracking was broken open after test ing to reveal a large sub- surface crack as shown in Figure 17. Thus quite large cracks can appear suddenly and this must be al lowed for in sett ing inspect ion intervals.

The 9 mm fi l let and the partial penetrat ion welds were made in a sequence, on the web of the trough first, then across the soff i t . Cracking

occur red at the we ld toe in the crossbeam, excep t for one bu t t we lded jo in t w h i c h also cracked in the we ld th roat .

The tes t resul ts are s h o w n in Figure 18. Resul ts f rom the deck panel tests we re at the upper end of the scat te r band and those f rom Nunn (1974 ) were l owe r than the more recent tests . H o w e v e r the serv ice life of we lds on a bridge was s h o w n to be cons i s ten t w i t h class G fa t igue s t reng th [St reams, 1987 ] and as the l ower bound of the tes t data is c lose to the c lass G design curve it is recommended tha t th is c lass i f i ca t ion be used.

For th is jo in t there is l i t t le to be gained f rom a larger we ld w i t h or w i t h o u t penet ra t ion . Cun inghame (1987) s h o w e d tha t class D per fo rmance can be obta ined by the add i t ion of a steel strap over the end of the t rough on each side of the crossbeam.

4 . 3 . 2 Troughs passing through c r o s s b e a m s

As an ex tens i ve inves t iga t ion of th is jo in t has been under taken by Beales (1990) , on ly a br ie f summary w i l l be g iven here. Three des igns w e r e inc luded:

16

E 360

280

o 200

120 .E

~- 40 0

,~ --4O

c -120

-- -200 "O

-280 rr

Weld 7 -~ ~ /~ /20~ ' " Meancrack ~ ' ~ / ~ / V ' '

Deck plate

Cracked weld section

1 4 5 m ~ / fener

Section through the weld

Fig.16 Residual stress pattern at trough to crossbeam joint

Fig.17 Section through trough to crossbeam weld showing subsurface crack'

Type A - - w i t h a U-shaped cut-out around the apex.

Type B - - w i t h crossbeam shaped to f i t closely to the trough.

Type C - - w i t h a circular cut-out around the apex of the trough.

The three types of joint are shown in Figure 19. The work comprised static stress inf luence line measurement on a deck panel, fat igue test ing, calculation of service lives and stress monitor ing on a bridge under normal traff ic.

Fatigue tests were carried out on types A and B wi th a similar specimen and loading arrangement to that described in section 4.3.1 above. The results are given in Table 4. The stress ranges given in the table were obtained from strain gauges placed at right angles to the direct ion of crack growth, and at 15 mm from the crossbeam (for gauges on the trough), or 15 mm from the trough (for gauges on the crossbeam), see Figure 19.

Fatigue strength equivalent to class E was obtained for the type B joint, based on longitudinal

stress in the apex of the t rough at 15 mm f rom the crossbeam. Cracks occurred at the we ld toe on the t rough around the apex of the t rough.

On the type A jo ints cracks deve loped at f ive locat ions though not all on the same spec imen, see Figure 19. The cracks we re at the ends of the weld in t rough and crossbeam, and along the we ld toe in the t rough. The stress range was d i f fe rent at each of these locat ions so def in ing the fa t igue s t rength of this t ype of jo int in te rms of a single stress, say at the apex of the t rough, is unsat is factory . Changes in d imens ions or we ld procedure could change st resses at the we lds w i thou t a f fec t ing the stress at the apex of the t rough. Beales deal t w i th th is prob lem by calculat ing st resses due to t raf f ic , f rom measurements on a deck panel w i t h the same geomet ry as the fa t igue test spec imens. It was then possible to compare jo ints in te rms of life under BS 5 4 0 0 part 10 t ra f f ic loading.

Fatigue s t rength for crack ing at the upper we ld end in the t rough w a s equiva lent to class G. It is therefore recommended that th is detai l be avoided by f i t t ing the c rossbeam to the t rough r ight up to

17

z v

E 100

N 8O 0 u

~ 6o

E E ~ 40

& £

.E & 20

(a) Joints with 6ram fil let welds

o o.O ~ ~ ~ ° °o

" " ' ~ ' , , ~ % ~ o ° o,,,

o • o •

O Small specimen

• Deck panel

/% Small specimens [From Nunn (1974)]

BS 5400 class G design curve (--2 s.d.)

I 106

Endurance (cycles)

107

E z

E 100

.Q

£ 80

E £ 60 E E

L¢3

-~- 40

&

c

& 20

(b) Joints with large fi l let or penetration welds

" o Mean line for 6ram fillet w e b d ~ ~ , ~

O Small specimen 9mm fillet weld

• Small specimen penetration weld

I ! 105 106 107

Endurance (cycles)

Fig.18 Test results fo r t rough t o crossbeam jo in ts w i th troughs f i t ted between crossbeams

1 8

T A B L E 4

Test results for trough to crossbeam joints

Specimen type

Type A with cut-out around apex of trough

Type B welded all round

No.

1A IA

2A 2A 2A 2A

3A 3A 3A 3A

5A 5A 5A 5A 5A 5A

8A 8A 8A

2B 3B 4B 6B 7B 8B

stresss range

(N/mm 2

39 50

47 59

150 156

73 63

200 219

56 54

144 175 158 192

63 63

160

95 150 125 115 100 200

endurance cycles (x 106)

6.3 8.2

3.0 4.0 5.2 5.8

1.8 2.4 1.8 2.3

Crack location (1)

upper weld end, trough, LHS upper weld end, trough, RHS

upper weld upper weld weld toe in lower weld

end, trough, LHS end, trough, RHS trough, LHS end, crossbeam, LHS

upper weld end, trough, LHS upper weld end, trough, RHS weld toe in trough, LHS bottom of weld, crossbeam, LHS

2.7 2.9 1.4 2.1 2.1 1.8

2.9 5.2 6.3

11.7 0.9 2.8

upper weld upper weld weld toe in weld toe in lower weld lower weld

end, trough, LHS end, trough, RHS trough, LHS trough, RHS end, crossbeam, LHS end, crossbeam, RHS

upper weld upper weld weld toe in

end, trough, LSH end, trough, RHS trough, LHS

1.67 3.2 0.47

No cracks Weld toe in trough Weld toe in trough Weld toe in trough No cracks Weld toe in trough

(1) see Figure 19 for strain gauge and crack locations.

Type 'A" Type 'B ' T y p e ' C '

Deck plate

oftrou h Welded c o n n e c t i o n u n d e r t e s t

• S t ra in g a u g e l o c a t i o n s \ C r a c k l o c a t i o n s

Fig.19 Trough to crossbeam joint with trough passing through cut-out in crossbeam

19

the deck plate, ie w i thou t a cope hole over the t rough to deck plate weld. Some exist ing decks have cope holes so measurements were made on a bridge under t ra f f ic loading to assess the risk of cracking (it was expected that stresses would be reduced by composi te action of the surfacing). Stresses were recorded for two weeks under the s low lane of a mo to rway bridge. The deck was surfaced w i th 38 mm of mast ic asphalt and the temperature averaged 7 .6°C (this is less than the year ly average). The recording period was too short to draw f i rm conclusions, but the highest recorded stress range was half of the max imum for an unsurfaced deck under design t ra f f ic loading, so it seems that substant ial stress reduct ions are possible.

For cracks at the weld toe on the trough and at the lower weld end in the crossbeam, the fat igue st rength was very high, corresponding to class C for longitudinal stress in the trough web adjacent to the lower end of the weld. It is suggested that class D be used for design because of the inspect ion requirements and the possibi l i ty of defec ts at the weld end.

4 . 4 CROSSBEAM TO DECK JOINTS Transverse st i f feners (crossbeams) are general ly connected to the deck plate by double f i l let welds. Inf luence lines for stress in the deck and crossbeam obtained f rom stat ic tests on a deck panel are shown in Figure 20. As a wheel passes over the joint the weld toe on the crossbeam is subjected to a single al ternat ing stress cycle. The toe on the deck plate sees a compress ive cycle, w i th a reduction in stress when the wheel is d i rect ly over the crossbeam.

The loading required to reproduce these stresses for fa t igue test ing on a ful l size deck panel is also shown in Figure 20. Two servo-hydraul ic actuators simulated the repeated passage of a single wheel over the joint. The actuators were posi t ioned m idway between troughs at 150 mm ei ther side of the crossbeam and the load was appl ied through 25 mm thick rubber pads shaped to represent the contact area of an HGV tyre.

A loading rig was bui l t to reproduce the stresses due to th is loading on a small specimen, and a series of fat igue tes ts carried out. Figure 21 shows the test specimen and loading arrangement and the results are given in Table 5. They are conserva t ive in that , for s impl ic i ty , the e f fec t of the small extra cyc le of stress in the deck plate is omi t ted . A separate series of tests using the same spec imen and loading was carried out by Maddox, see Table 5.

The type FS16 specimens fai led f rom the weld toe in the deck plate except for one which also cracked at the toe in the crossbeam, suggest ing

that there may be l itt le difference in the fatigue life for the two modes of failure. This was confirmed by the results for series 2000 specimens which had a 2 mm gap between the crossbeam and deck plate. These specimens failed from the weld toe on the crossbeam but at similar endurances.

Neither of the tests on the deck panel produced cracking. Af ter 21.6 mil l ion cycles in the second test, the load was increased by 30% and the test continued for a further 5.5 mill ion cycles. However no cracking occurred and the test was then stopped.

The test results are compared wi th class D in Figure 22, all exceeded the required fat igue strength. Maddox (1974b) reported similar fatigue strength for double f i l let welds tested under sinusoidal loading to produce bending stress in the 'deck plate' and axial stress in the 'crossbeam'.

4.5 WEB TO DECK JOINTS Longitudinal web to deck joints occur at the top of the vertical web of a box section, or girder. The joint is usually made wi th a double f i l let weld and so is geometr ical ly similar to the crossbeam to deck, but the stress influence lines due to wheel loading are di f ferent, see Figure 23.

There are two main differences between the applied stress on the web to deck and the crossbeam to deck joints.

(i) The minor stress cycle in the deck plate is not present in the web to deck. In this respect the weld toe on the deck is less" severely stressed than the crossbeam weld toe.

(ii) The stress in the web is entirely tensile or compressive (depending on the transverse position of the load) rather than alternating as in the crossbeam. However, when the effect of residual stresses in a joint on a bridge is added the resulting weld stress cycle is l ikely to be similar in both joints.

The web to deck joint has not been fatigue tested but it is suggested that it is suff ic ient ly similar to the crossbeam to deck joint to be given the same weld classif ication, ie class D.

A caut ionary note must be added regarding this joint. It is well established that stresses at welded joints close to the deck plate can be signif icantly reduced by a suitable surfacing system. Although this ef fect cannot be included in design wi thout special tests, many joints in existing decks are free of fat igue cracks only because of the effect of surfacing.

20

A

z v

-10

J

-20

|

i

1 **4.° ° Actuator # * ~ 1 ~ Actuator 2

o l , ' ~

(a) Dynamic loading for fatigue test

Time

- 1 0

~- -20 E z

-30

-40

- 5 0

% 1 ~ Gauge / Gauge 1

' , \ / L " , , ' / ~ Crossbeam / l ~

\ \ ,x , , ~ • , , \ ~ / ~

',V ~-,'~ ',,/ v

(b) Stress influence line for single wheel (deck plate)

300mm I I

40

30

20

IO-- E

Z

0

-10

-20 -

-30

-40

-50

A /s ~\

/ \ / ', l \; -...

"X/~Gaug e

(c) Stress influence line for single wheel (crossbeam)

300mm

i = r

Fig.20 Crossbeam to deck load and stress w a v e f o r m s

21

Load A

I

o - - -x j - - - v

0

Load waveform

~ ' ~ Y ~ 1 Load B

Deck plate

+

0

Crossbeam

+

0 ~ = ~ "~Stress r

Stress waveform at gauge position

range

Fig.21 Specimen and loading for tests on crossbeam to deck joint

Specimen No

FS16.1 FS16.2 FS16.3 FS16.5 FS16.7 FS16.8

21 53 2155 21 38 2137 2 1 3 9

DP1 DP2 DP2A

T A B L E 5

Test results for crossbeam to deck plate

Stress range in deck (N/mm 2

150 200 250 220 240 180

227 170 208 185 154

72 106 153

Stress range in crossbeam

(N/mm 2)

184 263 322 276 303 235

282 271 268 223 184

99 1 54 209

Endurance (cycles x 106)

1.86 1.4 0.53 0.98 0.82 1.45

1.2 1.1 0.76 2.06

14.9

20.0 21.6

5.5

oint

Cracking

weld toe in deck plate weld toe in deck plate weld toe in deck plate weld toe in deck plate weld toe in deck plate weld toe in deck plate

and toe in crossbeam

weld toe in crossbeam weld toe in crossbeam weld toe in crossbeam weld toe in crossbeam unbroken

no cracking no cracking no cracking

Notes: (i) Type FS16 (TRRL)--endurance is to 100 mm long crack. (ii) Series 2000 (Maddox, Welding Instutute)--endurance is to through thickness crack. (iii) Series DP tests were on a full size deck panel.

22

(a) For cracking at toe in deck plate

300

E 200 z

" o

c

g

~ 100

50 105

O o j z~ °

O d

o A-"

O Specimens with good fit-up

A Specimens with gap

• Deck panel

\ ~ B S 5400 class D design curve (--2 s.d.)

I 106

Endurance (cycles)

107

A

J

(b) For cracking at toe in crossbeam

300

E --- 200 z

v

E

e~

g

~ 100

50

J o"

O

O Specimens with good fit-up

A Specimens with gap

• Deck panel

BS 5400 class D design curve (--2 s.d.)

I

A

o I

A" Z~

=4

=d"

105 106

Endurance (cycles)

10 7

Fig.22 Test results for crossbeam to deck joints

23

20

--2O

E Z

--40

--60

--80

ZT "-[ X rough Gauge Web Trough

D

Transverse position of load

(a) Stress in deck plate

0f, .0 1 mr°ugh A Tr°ugh ~

Gauge

Transverse position of load

(b) Stress in web

Fig.23 Transverse influence lines of stress at web to deck joint for a 30kN wheel load

The web of a box sect ion is very s t i f f ver t ica l ly compared to the deck on either side. Def lect ion of the deck over the web may be suf f ic ient to cause cracking of the surfacing along the line of the jo int d i rect ly over the web. This dest roys the beneficial e f fec t of the surfacing at this joint.

4 . 6 DECK PLATE J O I N T S Orthotropic deck plates forming the top f lange of a box sect ion are general ly made in sect ions which are then welded together by automat ic weld ing using a permanent backing strip. During erect ion of the bridge the box sect ions are joined in a simi lar way . Butt welds on the exper imental panel were made by the Fusarc process (similar to submerged arc) which has been used on several UK bridges. A longitudinal but t weld was placed m idway between t roughs, and a t ransverse weld at 25 mm from a crossbeam.

Four fat igue tests were carried out on these joints, t w o each on the t ransverse and longitudinal welds. Loading was applied through a 25 mm th ick rubber pad shaped to represent a tyre contact patch. The loading ar rangement is shown in Figures 24 and 25. The tes t loads and stresses in the deck adjacent to the but t we lds are g iven in Table 6.

4.6.1 Longitudinal butt joints An ul t rasonic inspect ion before the tes t showed a general ly sound weld w i th smal l buried defects, main ly porosi ty , and very small lack of penetrat ion defects. The f i rs t tes t was stopped when a crack

was found in the underside of the deck plate at the toe of a tack weld attaching the backing strip to the deck. In the second test cracks occurred in the throat of a tack weld, and subsequently in the butt weld start ing at the weld root.

4.6 .2 Transverse butt joints Radiographic examination before the tests showed no signif icant weld defects. The load was posit ioned over a trough for the f i rst test , and midway between troughs in the second. Both tests were stopped when longitudinal cracks developed in the deck plate under the actuator. No cracks were found in the transverse butt weld.

No conclusions can be drawn from so few results but it is encouraging that the fatigue strength of the test welds was well above class D.

5 DISCUSSION AND CONCLUSIONS Constant ampli tude fat igue tests have been carried out on the main types of welded connection in orthotropic steel bridge decks. Tests were carried out on laboratory specimens and a full size deck panel. Loading was applied to reproduce stress in the joint due to a wheel load on the deck panel.

The test results reported here have been used to assign the joints to weld classes in the UK bridge design code BS 5400 Part 10 [British Standards

24

Trough 5 Trough 4

Loading pad first test j

s

.11 -i , /

Tack welds

I I I I

Weld

Backing bar

950mm to crossbeam

toe crack length 40mm

Loading pad Second test

Weld throat Crack length 46mm

• / . - Deck pilate

--~I /11 I I ~.11-1-1-,- ', ! . / / 1 ( \ \ i I

Tack welds

View of underside of deck panel

crack

850mm to crossbeam

Fig.24 Details of tests on longitudinal deck plate butt weld

25

/ / / / r

300 d i

15 ~ T r o u g h ~ ~ - 4 0 - - ~ T a c k w e ds T r o u g h

Cross0eam I I I I

. . . . I . . . . . . . . . . . . . . . . L . . . . . . . . . . . . . . . . . . . l . . . . . . . . . . . . . . . J . . . .

. . . . ..[ . . . . . . . . . . . . . . . [ - - - ~ ~ r ~ i ~ - ' ~ ; . . . . . I . . . . . . . . . . . . . . . 1 . . . . B a c k i n g bar 4 0 r a m x 6 m m y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Nos . in b r a c k e t s re fe r to gauges on

u n d e r s i d e o f pane l

f

A l l d i m e n s i o n s in m m

I I 4 (8)- -13 (7) 215mmtol /

c rossbeam

I ,o.,o~°°° I , [ , I L . . . . . . . . - I

Fig.25 Detai ls of tests on transverse deck plate butt weld

T A B L E 6

T e s t results for deck plate butt welds

Stress range at gauge no (N/mm 2) Tes t no

Load (kN) 1

Longitudinal but t weld (se{

12

13

50

6 0

+ 1 8 4

+ 1 7 9

Transverse butt weld (see

18

19

15

15

+ 1 7 4

+ 124 ( - 1 2 0 )

2

Fig 24)

+ 2 7 + 1 6 7

+ 4 0

g 25)

- 8 9

- 4 9 ( + 52)

+ 1 6 0

4

+ 3 1

+ 3 7

+ 2 5 + 3 8

+ 1 3 0 - 7 2 ( - 1 1 9 ) ( + 1 9 )

(brackets denote gauges on unders ide of deck)

Endurance x 106 cycles

3 .24

,1.87

3 .54

1.66

0 .52

comment

crack in deck plate at toe of tack weld. length = 40 mm.

weld throat crack in tack weld, 45 mm long crack in deck plate butt weld 350 mm long

no cracks in weld. crack in deck plate under loading pad. length 360 mm.

no cracks in weld. crack in deck plate under loading pad. length 280 mm.

26

TABLE 7

Recommended joint c lassi f icat ions

Recommended Joint Weld BS 5400 class

Longitudinal st i f fener to deck plate

Longitudinal st i f fener splice

Longitudinal st i f fener to crossbeam joint

(i) st i f feners between crossbeams

6 mm leg length f i l let 9 mm leg length f i l let Partial penetrat ion

Butt weld Fillet (double splice) Fillet (single splice)

6 ram, 9 mm f i l let Partial penetrat ion

(ii) stiffeners passed through cut-outs in crossbeams

Crossbeam to deck joint

Longitudinal web to deck weld

Deck plate butt welds

6 mm f i l let all round 6 mm f i l let on web only Cope hole over trough

to deck plate weld

Double 6 mm f i l let

Double 6 mm f i l let

Automat ic f rom one side

F D D

D E G

G G

E D

G

D

D

Institution, 1980]. A summary of the recommended joint classif ications is given in Table 7. These wi l l apply to deck panels of similar geometry and plate thickness to those tested, care should be taken in applying them to designs which dif fer markedly from the test panel.

It should also be borne in mind that the weld class. depends on how the stress is defined. Ideally a single 'nominal ' stress would be used to define fatigue strength for each joint but this is not always possible. More work is required in this area. The classif icat ions in Table 7 are generally for stress at right angles to the weld at 15 mm from the root.

The weld class for most of the joints tested is higher than the classif icat ion in BS 5400. This is consistent with the evidence that the fat igue strength of welded joints in thin plate ( 6 - 1 2 mm) is higher under bending stress (as occurs on a deck panel), than under axial stress (as in much of the laboratory test ing used as the basis for the BS 5400 classif ications).

There is also a tendency towards less steep S-N curves under bending stress. This is beneficial for joints in orthotropic steel bridge decks where fatigue life is usually governed by the behaviour of the joint at high endurance and low stress range.

The tests on the ful l size deck panel tended to give longer endurance than the specimens. The reason for this is not known. More work needs to be done on compar ing stresses on small specimens and large structures. Jo in t f i t -up, restraint and residual stress, and edge e f fec ts on the specimens might account for some of the di f ference.

The tests reported here were carr ied out on joints which were free of serious defec ts and lack of f i t . Defining to lerances for joint f i t -up and defect levels, and their e f fec t on fa t igue st rength, was not part of the programme.

For most jo ints the t ransverse inf luence line is short so that only jo in ts d i rect ly under vehic le wheel t racks are at risk. Therefore wherever possible jo ints should be placed clear of the wheel t racks.

For jo ints which are repeated across the deck the idea of local s t rengthening under the wheel t racks is a t t ract ive, for example by spec i fy ing a larger f i l let , or a penetrat ion weld for t rough to deck joints. However , a f e w cracks have occurred in service in jo ints not under the whee l t racks, and changes in the posi t ion of t ra f f ic lanes are not unknown.

27

Some joints are best avoided except on lightly traff icked bridges, for example the cope hole in the crossbeam over the trough to deck plate weld and the single fillet welded trough splice joint.

Stresses in deck plate butt welds are higher than at most other welded joints. Particular attention should be paid to avoiding defects which reduce fatigue strength.

For the same reason temporary attachments and 'construct ion details' on the deck plate should be avoided, or at least kept clear of the traffic wheel tracks.

6 A C K N O W L E D G E M E N T S

The work described in this report was carried out in the Bridges Division of the Structures Group of the TRRL. Many people have contributed to the testing reported here, in particular the leadership and guidance of Mr D E Nunn is gratefully acknowledged.

Some of the fatigue testing (attributed to Dr S J Maddox in the text) was carried out by the Welding Institute. The contribution of Dr T R Gurney (Wl) is also acknowledged. Fatigue tests on the deck panel were carried out by the National Engineering Laboratory under contract to TRRL.

7 REFERENCES

Beales C (1990). Assessment of trough to crossbeam connections of orthotropic steel bridge decks. Department of Transport, TRRL Research Report RR276, Transport and Road Research Laboratory, Crowthorne.

British Standards Institution (1 976). Covered electrodes for the manual metal-arc welding of carbon and carbon manganese steels. British Standard BS 639, British Standards Institution, London.

British Standards Institution (1 980) Steel, concrete and composite bridges. Part 10: Code of practice for fatigue. British Standard BS 5400, British Standards Institution, London.

Cuninghame, J R (1982). Steel bridge decks: Fatigue performance of joints between longitudinal stiffeners. Department o f the Environment, TRRL Report LR1066, Transport and Road Research Laboratory, Crowthorne.

Cuninghame J R (1987). Strengthening fatigue prone details in a steel bridge deck. Proceedings of International conference on Fatigue of Welded Structures, The Welding Institute, Abington.

Gurney T R and Maddox S J (1987). Fatigue tests on joints in orthotropic decks. Proceedings of International conference on Fatigue of Welded Structures,The Welding Institute, Abington.

Janss J, (1988). Fatigue of welds in orthotropic bridge deck panels with trapezoidal stiffeners. Journal of Constructional Steel Research, vol 9, pp147-154.

Kondo A, Yamada K, Kikuchi Y, Miyagawa K and Aoki H (1982). Fatigue strength of field welded rib joints of orthotropic steel decks. Proceedings of IABSE colloquium, 'Fatigue of steel and concrete structures', IABSE, Lausanne. ISBN 3 85748 030 0.

Maddox, S J (1974a). The fatigue behaviour of trapezoidal stiffener to deck plate welds in orthotropic bridge decks. Department of the Environment, TRRL Report SR96UC, Transport and Road Research Laboratory, Crowthorne.

Maddox, S J (1974b). Fatigue of welded joints loaded in bending. Department of the Environment, TRRL Report SR84UC, Transport and Road Research Laboratory, Crowthorne.

Nunn, D E (1974). An investigation into the fatigue of welds in an experimental orthotropic bridge deck panel. Department of the Environment, TRRL Report LR629, Transport and Road Research Laboratory, Crowthorne.

Nunn, D E and J R Cuninghame (1974a). Stresses under wheel loading in steel orthotropic decks with trapezoidal stiffeners. Department of the Environment, TRRL Report SR53UC, Transport and Road Research Laboratory, Crowthorne.

Nunn, D E and J R Cuninghame (1974b). Stresses under wheel loading in a steel orthotropic deck with V-stiffeners. Department of the Environment, TRRL Report SR59UC, Transport and Road Research Laboratory, Crowthorne.

Streams, B (1987). Prediction of future rates of fatigue failure from observed incidences on a structure. Proceedings of International conference on Fatigue of Welded Structures. The Welding Institute, Abington.

Tromp, W A J (1974). Fatigue of field splices in ribs of orthotropic steel bridge decks. Stevin Laboratory, Department of Civil Engineering Report No 6 - 7 4 - 1 5 , Delft University of Technology.

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