TRANSPORT AND ROAD RESEARCH LABORATORY Department of Transport

Con t rac to r Repor t 189

R R L

Load test to collapse on a full scale model six metre span brick arch bridge

by C Melbourne and P J Walker (Bolton Institute of Higher Education)

The work reported herein was carried out under a contract placed on Bolton Institute of Higher Education by the Transport and Road Research Laboratory. The research customer for this work is Bridges Engineering Division, DTp.

This report, like others in the series, is reproduced with the authors' own text and illustrations. No attempt has been made to prepare a standardised format or style of presentation.

Copyright Controller of HMSO 1990. The views expressed in this Report are not necessarily those of the Department of Transport. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.

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

1990

ISSN 0266-7045

CONTENTS

Introduction Description of test bridge 2.1 Constructional detaiis Test arrangements and instrumentation

3.1 Loading arrangements 3.1.1 Point load tests 3.1.2 Load test to fai}ure

3.2 Instrumentation 3.2.1 Deflection ,3.2.2 Backfill pressure 3.2.3 ,Brickwork strains

3.2.3,1 Embedment strain gauges 3.2.3.2 Vibrating wire strain gauges

3.2.4 Temperature 3.2.5 Load 3 . 2 . 6 Data l ogge rs 3 . 2 . 7 V i sua l o b s e r v a t i o n

Point load t e s t r e s u l t s 4.1 I n t r o d u c t i o n 4 .2 R e s u l t s

4. ~.1 B r i c k w o r k s u r f a c e s t r a i n s 4 . 2 . 2 Embedment s t r a i n s 4 . 2 . 3 B a c k f i l l p r e s s u r e s 4 . 2 . 4 Deflection

4.3 Summary Load test to failure 5.1 Introduction 5 .2 Deflection 5.3 Surface brickwork strain

5.3.1 Development of thrust line 5.3.2 Lateral separation of spandrel wall 5.3.3 Development of ring:wall separation

5.4 Brickwork strain:embedment 5.4.1 Ring separation 5 . 4 . 2 Spandre l w a i l : b a r r e l s e p a r a t i o n

5 .5 B a c k f i l l p r e s s u r e s 5 .6 Summary

6 A n a l y s i s of s t r u c t u r e 6.1 MEXE method 6.2 Mechanism analysis

7 Conclusions 8 Acknowledgements 9 References Appendix

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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.

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LOAD TEST TO COLLAPSE ON A FULL SCALE MODEL SIX METRE SPAN BRICK ARCH BRIDGE

I INTRODUCTION

The z e s t d e s c r i b e d i s one of a s e r i e s w h i c h a r e c u r r e n t l y b e i n g u n d e r t a k e n i n t h e UK to s t u d y t he b e h a v i o u r of masonry a r c h b r i d g e s , The main a im o f t h e r e s e a r c h is to i m p r o v e t he method of a s s e s s m e n t as p r e s e n t me thods (1 , 2) a r e g e n e r a i l y c o n s i d e r e d o v e r l y c o n s e r v a t i v e or i n a p p r o p r i a t e .

The TRRL programme of r e s e a r c h was to c o m p r i s e model t e s t s and t h e o r e t i c a l a n a l y s e s in a d d i t i o n to a s e r i e s o f f u l l - s c a l e t e s t s . T h i s r e p o r t r e l a t e s t o the s i x t h o f a p l a n n e d s e r i e s o f a b o u t t e n f u l l - s c a l e t e s t s ( 3 ) . The b r i c k w o r k b r i d g e had a span of 6 m e t r e s and a r i s e o f 1 m e t r e .

E a r l i e r t e s t s had shown t he r e l a t i v e i m p o r t a n c e o f t h e s o i l - s t r u c t u r e i n t e r a c t i o n and s t i f f e n i n { e f f e c t s o f t h e s p a n d r e l w a l l s . A s e r i e s o f e l a s t i c ' p o i n t ' l o a d i n g s were u n d e r t a k e n to s i m u l a t e whee l l o a d i n g s . V i b r a t i n g w i r e s t r a i n gauges were attached to the ring in an attempt to determine the pos i t i on of the th rus t l ine. Add i t iona l iy embedment s t ra in gauges were i n s t a l l e d to monitor r ing separation.

The construct ion and e l as t i c tests were undertaken during the summer of 1988 culminatin~ in the test ing to f a i l u r e on 16 September 1988. The work was car r ied out at the Boiton i n s t i t u t e of Higher Education in t he i r large scale t es t i ng f a c i l i t y .

2 DESCRIPTION OF TEST BRIDGE

2 .1 C o n s t r u c t i o n a l d e t a i l s

D e t a i l s o f t h e c o m p l e t e d b r i d g e a re g i v e n i n f i g u r e 2 . 1

The s e g m e n t a l a r c h b a r r e l b r i c k w o r k a c r o s s a 6m w i d t h . of lm.

( r a d i u s o f c i r c l e 5m) c o n s i s t e d o f two r i n g s o f The b a r r e l had an i n t e r n a l span o f 6m w i t h a r i s e

The b r i d g e was c o n s t r u c t e d and l o a d e d on a p u r p o s e b u i l t r e i n f o r c e d c o n c r e t e t e s t i n g s l a b , m e a s u r i n g 14m x 8m. R e i n f o r c e d c o n c r e t e a b u t m e n t s p r o v i d e d a span of 6m, t h e c i e a r h e i g h t f rom the base o f t h e s l a b t o t h e s p r i n g i n g was 500mm. L o a d i n g to f a i l u r e was a c h i e v e d t h r o u g h ' C C L ' p r e s t r e s s i n 8 t e n d o n a n c h o r a g e s c a s t i n t o t he s l a b a l o n g t he q u a r t e r span. A d d i t i o n a l l y , ' m a c a l t o y ' b a r and c o u p l e r t ype a n c h o r a g e s were p r o v i d e d a long t h e span o f t h e b r i d g e i n o r d e r t o a p p l y p o i n t loads a c r o s s t h e span and w i d t h of t h e b r i d g e .

The c e n t r i n g f o r t he b r i d g e c o n s t r u c t i o n was p r o v i d e d u s i n g a 'RMD' a r c h c e n t r i n g sys tem. T imber was used to fo rm t h e p r o f i l e r e q u i r e d f o r t h e a r c h . A r e l e a s e agen t was a p p l i e d to t h e t i m b e r p r i o r t o b r i c k l a y i n g t o a v o i d b o n d i n g o f t h e b r i c k s to t he c e n t r i n g . B r i c k l a y i n g was c a r r i e d o u t o v e r a f i v e week p e r i o d i n June and J u l y 1988.

The brickwork was built in a 'stretcher' bond with no bonding between rings.

other than through the mortar bed-joint. The tet~! thickness of the completed

arch barrel was 220mm. To accommodate 25 pressure celi~ into the arch barrel,

cavities were formed in the brickwork bending and the pressure cells were Eater

embedded in mortar. Additionally three ducts were buiit into the barrel to allow

the ioading tendons co pass through the bridge at the quarter span. The three

ducts were piaced at the mid-width and at 2m either side.

The spandrel, oarapet and wing walls were b u i l t in an 'Engl ish ' bend. cross- sections shown in f i gu re 2.1. Brickwork re ta in ing walls were provided at each end of the bridge to r e t a i n the b a c k f i l l , the ve r t i ca l cross-sect ion of the wails was the same as the wing wal ls. The width of the reta in ing walls, between the win~ w a l l s , was 4.46m. The v e r t i c a l faces between the r e ~ a i n i n g and wing w a l l s were de-bonded by i n c l u d i n g a shee t of PVC between the mor tar j o i n t and b r i c k w o r k . Cavi t ies were b u i l t in to the west spandrei wall to accommodate pressure ce l l s .

The proper t ies of the b a c k f i l l are given in the Appendix. F i l l i n g was achieved using a 10 tonne overhead crane f a c i l i t y and a concrete discharging skip. Compaction of the b a c k f i l l was car r ied out in 100mm layers using a v ib ra t ing compacting 'wacker' p la te . The bridge was f i l l e d to 200mm above the crown. A f te r f i l l i n g was complete a 100mm th ick road surface was provided by sub- cont ractors . Once the const ruc t ion process was complete the centr ing was removed. Throughout the f i l l i n g , road surfacing and centr ing removal operations the b a c k f i l l pressures, brickwork s t ra ins and de f lec t ions were monitored.

3 TEST ARRANGEMENTS AND INSTRUMENTATION

The details of the loading required and the instrumentation used to monitor the

structura} response are described below.

3.1 Loading arrangements

The br idge was subjected to two separate load tests. A series of ' p o i n t ' load tes ts was conducted, fol lowed by the app l i ca t i on of a ' k n i f e edge load' (KEL) at the quarter span point through to col lapse of the bridge.

3 . 1 . 1 P o i n t l o a d t e s t s

A s e r i e s of i00 kN p o i n t load t e s t s were to be a p p l i e d ac ross the span and w id th of the b r i d g e to s i m u l a t e wheel l o a d i n g . However, du r i ng a p p l i c a t i o n of the f i r s t load t e s t a t p o s i t i o n D7 ( f i g u r e 3 . 1 ) the b r i d g e cracked a long the r i n g / s p a n d r e l wa l l i n t e r f a c e and r i n g s e p a r a t i o n was a l s o reco rded . As a r e s u l t the maximum l o a d i n g was r e v i s e d to 50 kN. The loads were i n d i v i d u a l l y a p p l i e d us ing h y d r a u l i c j a c k s t h r o u g h a s t e e l r e a c t i o n beam and a p p l i e d to the road su r face th rough a c i r c u l a r 340mm d i a m e t e r m i l d s t e e l p l a t e . Load ing was a p p l i e d and re leased i n c r e m e n t a l l y and the response of the b r i d g e was recorded . The r e s u l t s are d i s c u s s e d in S e c t i o n 4.

3 . 1 . 2 Load t e s t t o f a i l u r e

L o a d i n g (KEL) was a p p l i e d , a t the q u a r t e r span across the f u l l w id th of the b r i d g e , i n c r e m e n t a l l y to f a i l u r e t h r o u g h a 750mm wide r e i n f o r c e d conc re te beam. The toad was p r o v i d e d u s i n g t h r e e 1000 kN, 600mm s t r o k e , ho l l ow c y l i n d e r h y d r a u l i c j a c k s and a p p l i e d u s i n g the p r e s t r e s s i n g tendons anchored in the t e s t s lab and

2

above the jac~:3.

3 .2 I n s t r u m e n t a t i o n

3 .2 .1 D e f l e c t i o n

A total of fifty-six linear dispiacement transducers were positioned around the bridge to monitor the movement of the arch barrel, spandrel walls, retaining walls, wing walls and road surface. The layout is shown in figures 3.2 - 3.4.

The directions of the arrows indicate a positive deflection on the graphs.

3 . 2 . 2 B a c k f i l l p r e s s u r e

The b a c k f i l l p ressures around the e x t r a d o s and spand re l wa l l i n t e r f a c e s were measured us ing t h i r t y "Gage Techn ique" BSP t ype v i b r a t i n g w i r e e a r t h p r e s s u r e ceZls i n c o r p o r a t e d i n t o the b r i d g e , P l a t e 3. t . The p o s i t i o n of each gauge is shown in f i g u r e s 3.5 - 3 .7 .

3 . 2 . 3 B r i c kwo rk strains

Interna! strains of the brickwork were measured using electrical resistance type strain gauges. The surface brickwork strains were measured using vibrating wire type gauges. The position of each type of gauge is shown in figures 3.8 - 3.12.

3 . 2 . 3 . 1 Embedment s t r a i n gauges

To monitor the development of ring separation and ring/spandrel wa l l separation

twenty electrical resistance type embedment strain gauges were placed in the mortar joints between the two rings of brickwork.

3 . 2 . 3 . 2 V i b r a t i n g w i r e s t r a i n gauSes

'Gage Technique' surface mounted vibrating wire strain gauges (with a gauge length of !40mm> were used to measure the brickwork surface strains around the arch

barrel and across the ring/spandrel interface.

3 . 2 . 4 Tempera ture

A total of twenty 'integrated circuits' were used to measure the temperature both inside the backfill and on the e×terna] surface of the brickwork. They were used to provide temperature compensation of the other instrumentation.

3 . 2 . 5 L o a d

The load app l i ed to the b r i dge was measured u s i n g two 'RDP' 1000 kN e l e c t r i c a l r e s i s t a n c e type load c e l l s .

3 . 2 . 6 Data loggers

Three data loggers were used to monitor the instrumentation: two Solartron Orions and a Soil Instruments Geoscan.

3 . 2 . 7 V i sua l o b s e r v a t i o n

Cracking of the brickwork was observed and reported throughout each of the tests.

To aid crack detection the arch soffit and the east spandrel wall were painted

white making it possible to observe cracks O. Imm wide with the naked eye. The

west spandrel wall was ieft unpainted.

4 POINT LOAD TEST RESULTS

4 .1 I n t r o d u c t i o n

A series of point load tests was carried out, the positions of load application are shown in figure 3.1. Initially, the tests were planned for incremental application of load up to I00 kN, as stipulated in the contract. However, during

the first load test at position D7, structural damage occurred at 80 kN and so the

loading was reduced to 50 kN for the subsequent positions.

R e f e r r i n g to f i g u r e 3 . 1 . t h e o r d e r of l o a d i n g was: " DT, D3, D~, [,5, D5A, C5A, C5, C7, C1, B1, B7, B5A, B5, H5A, H5. H7, H1, A1, A7,

A5, A5A, F5A, F5, F7, F1, E l , E7, E5 and E5A.

The time f o r one load cycle was approximately 1 hour.

4 . 2 R e s u l t s

4 . 2 . 1 B r i c k w o r k s u r f a c e s t r a i n

In f i g u r e s 4 . 1 t he t y p i c a l r e s u l t s f o r s u r f a c e b r i c k w o r k s t r a i n s r eco rded under p o i n t l o a d i n g a r e p r e s e n t e d . The l a y o u t o f t he s t r a i n gauges on the s t r u c t u r e is given i n figure 3.11.

D u r i n g load a p p l i c a t i o n a t t h e f i r s t p o s i t i o n , DT, c r a c k i n g a round the west s p a n d r e l w a l l / a r c h b a r r e l j o i n t was o b s e r v e d a t 80 kN a t the q u a r t e r p o i n t a d j a c e n t to t h e l oad . The c r a c k e x t e n d e d e q u i - d i s t a n t e i t h e r s i d e of the q u a r t e r point for a distance of approximately 500mm. The maximum crack width was estimated to be 0.2mm. In figure 4.1 the strains measured on the arch barrel during the application of the load, position DT, are presented. The results are presented for gauges G46 and G48, no other strain gauges demonstrated any significant change

in the leve} of strain. Both graphs illustrate an initial linear 'elastic' increase in the brickwork strain. Compressive strains were measured at the intrados at the north abutment, gauge G46, and tensile strains were recorded on

the intrados at the quarter point (adjacent to the load), gauge G48. The

orientation of the compressive and tensile strains measured was as expected, i.e. further loading would suggest the formation of hinges at the north abutment intrados and at the extrados at the quarter span. Figure 4.1 indicates that from

the outset of loading tensile strains were induced at the quarter point intrados.

As loading increased a change in slope of the graphs occurred at approximately 70 kN. This seems likely to have been due to the spandrel wail/arch barrel separation although cracking was not observed until loading reached 80 kN. Beyond 70 kN a rapid increase in the brickwork strains occurred, however the responses remained 'linear' and so the barre) may be considered to have remained 'elastic'. The

greater increases in strain,after cracking was due to the reduction in structural stiffness resulting from the wall/barrel separation. Increases in the tensile strain up to I00 kN at gauge G48, figure 4.1, indicates a shift of the thrust iine

further outside the middle third of the barrel. On reaching I00 kN the load was re{eased and residual strains of -27 microstrain (compression) and 41 microstrain (tension) were recorded by gauges G46 and G48 respectively. At a load of I00 kN

the crack opening was estimated to be 0.3 - 0.4mm, a f te r removai of the load the crack closed and was no longer v i s i b l e to the naked eye.

After load posi t ion D7 the maximum point load to be applied was reduced to 50 kN. To monitor changes in s t ra in across the wa l l / ba r re l separation a fu r the r v ibrat ing wire s t ra in gauge was placed perpendicular ly across the crack at the quarter point, gauge G71 ( f igure 3.11). All subsequent brickwork surface s t r a i n measurements corresponded to across the wa l l / ba r re l crack, some of the resu l ts are given in f igures 4.2 and 4.3. None of the other v ib ra t i ng wire s t ra in gauges demonstrated any s i g n i f i c a n t change in the level of s t ra in throughout the remaining point load tests.

The experimental load/s t ra in re la t ionsh ips ind icate a non- l inear increase in stra in; the rate of change in s t ra in increasing with load. For any p a r t i c u l a r load the strains were greatest for a loading pos i t ion nearest the west edge of the bridge, th is was due to the greater moments induced when loading was nearer to the edge of the arch barre l .

In f i g u r e s 4.2 and 4 .3 the s t r a i n s measured at gauge G71 are r e p o r t e d f o r l o a d i n g p o s i t i o n s C, B, A, F and E (no d e f o r m a t i o n o c c u r r e d f o r l oad ing a t p o s i t i o n H). With the e x c e p t i o n of p o s i t i o n A1 the s t r a i n changes were o n l y reco rded when the load was a t p o s i t i o n s 5 and 7. Other than when l o a d i n g was a t the crown the experimental re lat ionships showed an increase in s t r a i n with load; the rate of increase in s t r a in increasing with load. On load removal residual s t r a i n s were recorded du r i ng dec reas ing load. The leve l of s t r a i n was g r e a t e s t when the loading was app l i ed nea res t to the west spandre l w a l l , The s i g n i f i c a n c e of the load p o s i t i o n across the w id th in r e s p e c t to the spand re l wa l l was a l s o dependent upon load p o s i t i o n in r e l a t i o n to the span. For example, a t load p o s i t i o n s C, the strains for C7 at 50 kN were 1100% greater than when loading was at C5, a distance of only 530mm nearer to the spandrel wal l . However. a s imi lar movement of the load nearer the west wall caused an increase in s t r a i n of only 150%, movement from posit ion F5 to FT.

When the point load was applied at the crown, the crack at the greater span was recorded as closing (reduction in s t r a i n ) . This was due to the typ ica l act ion of a ! i re load applied at the crown of a symmetrical arch. However, the level of s t ra in induced in the r ing at other parts of the barrel was i n s u f f i c i e n t to i l l u s t r a t e a st ra in reversai thoughout the arch.

In f i g u r e s 4 .2 and 4 .3 the s t r a i n responses are compared as a f u n c t i o n of the load moving across the span of the bridge. Both graphs c l e a r l y i l l u s t r a t e the signi f icance of the load posi t ion on s t ra in development. The graph for the loading posi t ion nearest to the spandrel wail , f i gu re 4.2, shows the greatest deformation to have occurred when the load was nearest the cracked sect ion, posit ion C7. This is to be expected since the largest moment w i l l be produced at the cracked section when the load is at that sect ion. The e f f e c t of a 50 kN load moving across the span is demonstrated as an ' i n f l uence l ine diagram' in f igures 4.2 and 4.3.

The r e s u l t s f o r the d e f o r m a t i o n at the q u a r t e r span p o i n t have a l so been p resen ted in terms of an inc rease of the crack w i d t h . P r e d o m i n a n t l y , t h r o u g h o u t the l o a d i n g the crack w id ths induced by the p o i n t load were s i g n i f i c a n t l y less than O. lmm. However, c rack ing of t h i s magni tude may be c o n s i d e r e d u n a c c e p t a b l e fo r a l i m i t s t a t e in masonry arches and s u b j e c t to repeated l o a d i n g may lead to f a t i g u e problems f o r the b r i c k w o r k .

4.2.2 Embedment strains

During a F p i i c a t i o n of t h e p o i n t load a t p o s i t i o n D7 s p a n d r e l w a l l / a r c h b a r r e l s e p a r a t i o n o c c u r r e d a d j a c e n t t o t h e load p o i n t a t a l o a d i n g of a p p r o x i m a t e l y 70 kN. The cracking of the brickwork should have been detected by the embedment strain gauge 0R2/125 (figure 3.8) but due to an apparent gauge malfunction the

c r a c k i n g was not detected u n t i l a load of 80 kN.

Gauge 0R2/I05, placed at the quarter span of the barrel to monitor development of ring separation (figure 3.8), illustrated a large increase in tensile strain during load application at position D7. The compressive and flexural brickwork tests suggested that a strain of approximately 75 microstrain corresponded to tensile cracking (ring separation). In figure 4.4 the tensile strain response at 0R2/I05 for load stage D7 is illustrated. From zero to a load of 40 kN there was little change in the level of strain, however, by 70 kN a tensile strain sufficient to cause ring separation had been measured. At iO0 kN a tensile strain of 2250 microstrain was recorded, showing that a significant deformation had been caused to the arch barre[~ After removal of the load a residual strain of 850 microstrain was recorded, the arch barrel was, therefore, subject to a significant permanent defect. Ring separation was not reported at any other area of the arch

barrel during loading at DT.

Subsequent 50 kN loads positioned elsewhere on the bridge road surface caused the ring separation cracking to develop further. The results for ring separation at gauge 0R2/I05 are presented in figure 4.4. Loading placed at positions B, C, D, E and F caused the tensile ring separation strain to increase with load, the experimental load/strain responses were similar in format for all loading positions. The rate of increase of strain increased with loading up to a maximum value at 50 kN. On load removal significant residual strains were present, leading to a 'hysteresis' type envelope. Generally the ring separation strains were increased only when the loading was at positions 7 and 5. The behaviour was similar to the surface cracking of the bridge, section 4.2.1, in that the magnitude of the strains induced were subject to the loading position. The largest strains were recorded when the load was nearest to the deformation, at the

edge of the ring and adjacent to the quarter span.

The influence of the load position across the width is clearly illustrated in figure 4.2. A movement of only 630mm (from D3 to D5) caused the strain to increase by over 200%, from 140 to 300 microstrain. The increase in ring separation strain at the quarter span was zero when the loading was at the abutment and at the crown, the deformation reached a maximum of over 300

microstrain when loaded at C7.

F u r t h e r l o a d i n g , n e a r e r to t he c rown, caused r i n g s e p a r a t i o n to deve lop a t the c rown , gauge 0R2/109, d u r i n g load s t a g e B5. The g r e a t e s t s t r a i n s were i nduced by ioading at the crown, for load position A? a strain in excess of Ii00

microstrain was measured.

4 . 2 . 3 B a c k f i l l p r e s s u r e s

The backfill pressure measurements due to the point loads display a linear increase with increasing load. In all cases the pressures measured relate to cells in the immediate vicinity of the applied load. The pressure response was therefore local as no pressure measurements were recorded in the 'un-loaded' side of the bridge. As might be expected the backfill response was not truly elastic

6

due to compaction by the point load. Although the "Wacker" plate applied a vibration compaction, this was not equivalent to a point load of 50 to 80kN. Hence further local compaction took place resulting in a residual increase in backfill pressure. Current soil mechanics theory suggests that pressures in excess of "at rest" pressures, (K0~ h) are possible when dealing with shallow depths of fill. A figure closer to pressures on unloading may be more applicable

ie. Kr~ h where K r =I /K o.

For pressure cells at the crown, depth of material 300mm, assuming no distribution of the load shows very good agreement with the measured pressures. However, for the pressures measured away from the crown a better correlation was shown if a 450

distribution of the ]cad was assumed.

4.2.4 D e f l e c t i o n

Throughout the p o i n t load tests a comprehensive series of deflection measurements was taken. However, no deflection response was measured during any of the loading cycles. The resolution of the transducers along the spandrel walls and arch barrel edge was O. Imm, whereas, along the intrados centre line the accuracy was O.02mm. The crack widths recorded along the edge were, in general, significantly less than O. Imm and hence no deflection was recorded adjacent to the cracking. As the deflections of the arch were very small, it seems unlikely that deflection will provide an adequate serviceability limit state criteria for masonry arches. The resolution of the transducers indicated that the deflection throughout the

point load tests was less than (i/60 000) x span.

4.3 Summary

(i) A point load of only 70 kN was sufficient to cause separation of the spandrel wall/arch barrel leading to ring separation in the arch barrel. Subsequent load positions caused the cracking to open with increasing load

and ciose with decreasing load.

( i i ) During each load cycle increases in the backfill pressure were measured only in a position immediately underneath the load. The load/backfill pressure response was linear and the pressure was shown to be directly

related to the load.

( i i i ) No defiection response of the bridge was recorded throughout the point

load tests.

5 LOAD TEST TO FAILURE

5 . 1 I n t r o d u c t i o n

The load was applied through a steel loading beam via a system of three hydraulic jacks (Figure 2.1). Due to the defect on the west side of the bridge caused during the point load tests, it was necessary to ensure a balancing of the load across the full width of the bridge. The level of load was maintained constant

throughout each load increment.

The load was initially applied in increments of lOkN, increased to 20kN on reaching 280kN. and increased again to 60kN on reaching 530kN. including the dead

weight due to the loading beam and hydraulic jacks a total of 26 increments were applied, taking approximately 6.5 hours; an average loading rate of 180kN/hour.

The p r o g r e s s i v e deve lopment of c r a c k i n g on both s ides of the b r i dge is shown in f i g u r e s 5.1 - 5 .4 . P r i o r to l o a d i n g , c r a c k i n g a long the s p a n d r e l / b a r r e l i n t e r f a c e was p r e s e n t on the west s i de . On l oad ing the c r a c k i n g d e f e c t on the west s ide was observed to open and i n c r e a s e f u r t h e r around the arc of the b a r r e l . Sepa ra t i on of the s p a n d r e ] / b a r r e l i n t e r f a c e commenced on the east s ide at 360 kN, f i g u r e 5 .2 . F u r t h e r l oad ing caused the c r a c k i n g to spread around the arch b a r r e l . At 400 kN the f i r s t h inge in the b a r r e l was observed underneath the load l i n e . At 640 kN v e r t i c a l and shear c r a c k i n g of the spandre l w a l l s was observed.

Ring s e p a r a t i o n on both s i d e s , between the n o r t h abutment and crown, was a l so note~. At a l o a d i n g of 820 kN v e r t i c a l c r a c k i n g of the p a r a p e t / s p a n d r e l wal l was observed , t h i s was due to the l i f t i n g of the arch b a r r e l crown under

d e f o r m a t i o n .

F a i l u r e of the b r i d g e was due to the f o r m a t i o n of a fou r h inge mechanism at a t o t a l a p p l i e d l o a d i n g of 1173 kN. P r i o r to f a i l u r e , the on ly h inge which was v i s i b l y deve loped was undernea th the load p o i n t , f a i l u r e was very sudden. On f o r m a t i o n of the f ou r h inge mechanism, f i g u r e s 5.3 and 5 .4 , the s p a n d r e l / w i n g / p a r a p e t w a l l s were l i f t e d and r o t a t e d as shown. H o r i z o n t a l s p l i t t i n g of the wing w a l l s o c c u r r e d a long the b e d - j o i n t l eve l w i t h the abutments . Hinges formed at each abutment , unde rnea th the load and at the crown. E x t e n s i v e r i n g s e p a r a t i o n was c o i n c i d e n t a l w i t h f a i l u r e and the two b r i c k w o r k r i n g s were a lmost c o m p l e t e l y sepa ra ted around the f u l l a rc of the b a r r e l . I t was c l e a r from the f o r m a t i o n of the h inges t h a t two s e p a r a t e t h r u s t l i n e s e x i s t e d , w i t h two separa te compress ion zones, one in each r i n g . Immed ia te l y upon f o r m a t i o n of a mechanism the load reduced to 700 kN. A f t e r a f u l l set of read ings were taken the i n s t r u m e n t a t i o n was removed and the b r i d g e was s l o w l y loaded th rough to c o l l a p s e . I t is wor th n o t i n g t h a t the f a i l u r e mode of the f u l l - s c a l e b r i dge was very s i m i l a r to t h a t r e p o r t e d f o r 1.5m span model b r i d g e s t e s t e d a t Bo l t on I n s t i t u t e (4 ) .

5 . 2 D e f l e c t i o n

H o r i z o n t a l and v e r t i c a l d e f l e c t i o n s of the arch b a r r e l a d j a c e n t to the abutments , q u a r t e r span p o i n t s and crown were measured a t the west w a l l , east wa l l and a long the c e n t r e - l i n e of the i n t r a d o s ( f i g u r e 3 . 2 ) .

T h r o u g h o u t l o a d i n g t o f a i l u r e no m o v e m e n t , e i t h e r h o r i z o n t a l , o r v e r t i c a l , was

reco rded a t e i t h e r of the abu tments .

The deflection responses of the arch barrel at the quarter point underneath the load are p resen ted in f i g u r e s 5 .5 and 5 .6 . Both the h o r i z o n t a l and v e r t i c a l d e f l e c t i o n r e s p o n s e s show a s i m i l a r f o r m a t . An i n i t i a l l i n e a r ' e l a s t i c ' s t a g e u n t i l c r a c k i n g a n d h i n g e f o r m a t i o n o c c u r r e d may be c l e a r l y o b s e r v e d . T h i s was f o l l o w e d by r a p i d i n c r e a s e s o f d e f l e c t i o n w i t h l o a d l e a d i n g t o a l e v e l l i n g o f f o f t h e l o a d / d e f l e c t i o n r e l a t i o n s h i p s up t o u l t i m a t e . The d e f o r m a t i o n s r e c o r d e d a f t e r a c h i e v i n g u l t i m a t e l o a d a r e a l s o s h o w n in t h e g r a p h s . H o r i z o n t a l d e f l e c t i o n s a t the b a r r e l edges were in c l o s e agreement, however, h o r i z o n t a l movement a t the c e n t r e - l i n e was n o t i c e a b l y l ess . Downward d e f l e c t i o n s a t the q u a r t e r p o i n t were in ve ry c l o s e agreement ac ross the f u l l w id th of the b r i d g e . F o r m a t i o n of the f i r s t h inge was observed undernea th the l o a d - p o i n t a t 400 kN, the v e r t i c a l l o a d / d e f l e c t i o n response i n d i c a t e d an i nc rease in the d e f l e c t i o n c o i n c i d i n g w i t h the h inge f o r m a t i o n . Th roughou t l oad ing the v e r t i c a l (downward)

mevement exceeded the horizontal movement by a ratio of approximately 2:1. At ultimate load the total downward movement of the quarter point was estimated at

8mm, or (i/750) x span.

The horizontai and vertical (hogging) deflections at the crown are given in figures 5.7 and 5.8. The slope was similar to those at the quarter span and typical of load/deflection relationships for a masonry arch. in the horizontal

direction the deflection response was linear to 700 kN whereupon a rapid increase in deformation was observed, this coincided with separation of the spandrel wall/arch barrel at the crown and vertica} cracking of the spandrel. Deflection of the crown upwards shows three different curves across the width of the arch, figure 5.8. This may have been caused by ring separation, present on the west side (0R2/61), permitting greater movement at that side. The ratio of horizontal

to vertical movement was approximately one beyond hinge formation. The total upward movement of the crown at ultimate load was 2.5mm, (1/2400) x span.

The horizontal and vertical (hogging) deflections at the quarter span opposite the load point are given in figures 5.9 and 5.10. Prior to 800 kN little movement was recorded, however subsequently there was a rapid increase in deflection corresponding to the extensive cracking of the arch and hinge formation. The ratio of vertical to horizontal movement was approximately 2:1, total upward movement was approximately 4.5mm at ultimate, (I/1333 x span). A good indication of the overall deformation of the centre line of the arch at the load increment

prior to ultimate is given in figure 5.11.

The outward movements of the spandrel walls were measured around the arc of the bridge, both near to the wall/barrel joint and at the top of the parapet walls. The spandrel walls moved under loading; this effect at the loaded quarter point may be ascribed to deviatoric stresses induced in the backfill. At ultimate the walls had moved out by approximately l-2mm. Outward movement of the walls at the opposite quarter point may be explained by dilation of the backfill as it was pushed against by the deforming arch barrel. The magnitude of deformation was

significantly less than near the loaded area, only 0.25mm at ultimate.

Above 700-800kN the south retaining wall rotated and translated in response to the induced soil pressures. Additionally the spandrel walls rotated and translated in response to the interaction not only with the arch barrel but also

with the backfill frictional s%resses. /

I

Vertical deflection of the road surface was monitored throughout loading, figure 3.3. The results indicate a total downward movement at the load point of iOmm at ultimate, figure 5.12. However, taking into account the movement of the arch structure beneath, the net deflection is much reduced and indicate the fill to have been compacted by on}y 4mm and thus had probably not failed in bearing.

5.3 Sur face b r i c k w o r k s t r a i n

Surface mounted vibrating wire strain gauges were positioned on the bridge to monitor three aspects of strain behaviour of the structure during loading,

namely:-

( i ) P o s i t i o n of the t h r u s t - l i n e ( i i ) L a t e r a ] s e p a r a t i o n of the spandre l wa l l f rom the a rch b a r r e l (iii) Development of existing cracks.

The l ayou t of the s t r a i n gauges is g iven in f i g u r e s 3 . 1 0 - 3 . 1 2 . However the f o l l o w i n g m a l f u n c t i o n e d d u r i n g t e s t i n g : G35, G37, G39, G41, G43, G44, G50 and

G59.

5.3.1 Development of thrust line

Ten s t r a i n gauges were p laced around the a rc of the b a r r e l a long i t s west face, f i g u r e 3 .11 , and a f u r t h e r s i x t e e n gauges were p o s i t i o n e d around the i n t r a d o s and

e x t r a d o s of the a rch .

Gauges G45 and G46, p laced ac ross the s p r i n g i n g , show an i n i t i a l l i n e a r i nc rease in compress i ve s t r a i n u n t i l a p p r o x i m a t e l y 600 kN, c o i n c i d e n t a l w i t h e x t e n s i v e c r a c k i n g of the b r i d g e . Beyond 600 kN a r a p i d i n c r e a s e in s t r a i n occur red at the i n t r a d o s . Th roughou t t e s t i n g the t h r u s t - l i n e would seem to have been i n s i d e the m idd le t h i r d a t t h i s p o i n t as t e n s i l e s t r a i n s were not measured. Once the a p p l i e d load had exceeded 400 kN l a r g e t e n s i l e s t r a i n s were measured on the ex t rados of the b a r r e l , t h r e e courses above the n o r t h abutment (gauge G36). A rap id i nc rease in s t r a i n o c c u r r e d a f t e r deve lopment of the f i r s t h inge and the graph l e v e l l e d o f f app roach ing f a i l u r e i n d i c a t i n g the l i k e l i h o o d of a h inge a t t h i s p o s i t i o n .

T h i s was c o n f i r m e d by v i s u a l i n s p e c t i o n .

Under the load p o i n t t h e r e was a s teady i nc rease in t e n s i l e s t r a i n w i t h load a t the i n t r a d o s . At a l o a d i n g of 400 kN the f i r s t h inge was observed a t t h i s p o i n t , the rap id i n c r e a s e in t e n s i l e s t r a i n (G4B) suppor ted t h a t o b s e r v a t i o n . A t e n s i l e s t r a i n of 6000 m i c r o s t r a i n a t 1100 kN was over 100 t imes t h a t necessary f o r c r a c k i n g . Gauge G48 which i l l u s t r a t e d a deve lopment of a ve ry la rge t e n s i l e s t r a i n a t the q u a r t e r p o i n t i n t r a d o s was p o s i t i o n e d ac ross a h inge. I t seems gauges G56 and G63 were p laced ac ross j o i n t s a d j a c e n t to the h inge on the i n t r a d o s and hence they show l i t t l e change in s t r a i n , s u g g e s t i n g a s t r e s s r e l i e f in the b r i c k w o r k due

to the h inge .

At the crown a s teady i n c r e a s e in t e n s i l e s t r a i n a t the ex t rados face of the a r c h , gauges G49 and G40, show t h a t the t h r u s t - l i n e was o u t s i d e the midd le t h i r d . The magni tude of the s t r a i n app roach ing f a i l u r e , g r e a t e r than 2000 m i c r o s t r a i n , a l s o shows the deve lopment of a h inge at the crown. Th is was conf i rmed by o b s e r v a t i o n . L i k e w i s e the compress i ve s t r a i n development a t gauge G61 on the i n t r a d o s c o n f i r m s t h a t a h inge was f o rm ing a t the crown. Compressive s t r a i n s g r e a t e r than 2000 m i c r o s t r a i n would sugges t t h a t the t h r u s t l i n e was very c lose

to the o u t e r face of the i n t r a d o s .

At the s o u t h e r n q u a r t e r p o i n t , the compress i ve s t r a i n s at both gauges G51 and G52 i n d i c a t e t h a t the c r o s s - s e c t i o n was f u l l y compress ive t h roughou t and t h e r e f o r e the t h r u s t - l i n e remained w i t h i n the m idd le t h i r d . However, the s t r a i n g r a d i e n t d e r i v e d f rom measurements a t the i n t r a d o s and e x t r a d o s , w i t h i n the w id th of the b r i d g e deve loped a t e n s i l e s t r a i n at the i n t r a d o s i n d i c a t i n g t h a t the t h r u s t - l i n e was o u t s i d e the m idd le t h i r d . A l t hough the t e n s i l e s t r a i n s ( g r e a t e r than 100 m i c r o s t r a i n ) were s u f f i c i e n t to c rack the b r i c k w o r k , the magni tude of s t r a i n compared to o t h e r s e c t i o n s would sugges t t h a t a h inge d id not form a t t h i s p o i n t .

The south springing section remained in compression throughout loading. However, t e n s i l e s t r a i n s were measured on the e x t r a d o s of the arch b a r r e l a t gauge G44, a s e c t i o n t h r e e courses above the s p r i n g i n g where a h inge was observed to form. The p o s i t i o n of h inges and s t r a i n deve lopment was t h e r e f o r e s i m i l a r to t h a t a t the

n o r t h abutment .

10

The experimental strain measurements were used to determine the posi t ion of the thrust - l ine. Throughout loading the t h r u s t - l i n e under the KEL was close to the extrados. The effects of ring separation recorded at 300-400kN can be detected by a further sh i f t of the th rus t - l i ne posi t ion. The assumption of "plane sections remaining plane" can no longer be considered va l id when ring separation has occurred.

Also of interest is the experimental thrust-line positions for the south quarter span, figure 5.13: one represents that at the edge next to the spandrel wail and the other is for a section towards the centre of the arch barrel. A considerable difference in the location of the thrust-line is clearly apparent, the stiffness of the spandrel wall has, for most of the loading, ensured the thrust-line to remain in the middle third. However, away from the spandrel wall in the much less stiff section the thrust has remained near to the intrados throughout, if not for the stiffening effect of the spandrel walls a hinge would have formed across this section.

5 . 3 . 2 L a t e r a l s e p a r a t i o n o f s p a n d r e l w a l l

In a number of existing masonry arches the spandrel walls have become detached from the arch barrel. The separation of the spandrel wall has manifested itself in a number of bridges by a Iongitudina] crack on the intrados between the springings in line with the thickness of the spandrel wail. To monitor any possible development of such a crack ten gauges were placed around the intrados of the arch, underneath the line of the extent of the spandrel walls, figure 3.12.

Tensile strains were observed indicating that the backfill pressure was increasing and trying to push the spandrel wall off the arch. Under the load the barrel was spanning as a fixed ended beam. High transverse horizontal stresses were also being applied to the spandrel wall through the fill by the KEL. The combination of these effects was to cause the intrados locally to become compressive.

5 . 3 . 3 D e v e l o p m e n t o f r i n g : w a l l s e p a r a t i o n

During the point load tests spandrel wall/arch barrel separation was observed at the loaded quarter point. To monitor further development of this crack gauges G71-G73 figure 3.11) were placed perpendicularly across the crack around the arch.

The s t r a i n gauge p l a c e d a c r o s s the i n i t i a l c r a c k a t t h e l o a d e d q u a r t e r p o i n t , g a u g e G71, d e m o n s t r a t e d a s t e a d i l y i n c r e a s i n g t e n s i l e s t r a i n a c r o s s t h e c r a c k a s t h e l o a d was a p p l i e d , r e a c h i n g a v a l u e o f I 0 000 m i c r o s t r a i n ( e q u i v a l e n t c r a c k width 1.3mm) at only 800-850 kN. Beyond this loading the strain capacity o f the gauge had been exceeded. Further loading caused the crack to open wider, reaching approximately 5-10mm at ultimate load and between 50-75mm (observed) just before collapse.

At the r e m a i n i n g two l o c a t i o n s , c rown and u n l o a d e d q u a r t e r p o i n t , c o m p r e s s i v e s t r a i n s were measured a c r o s s t h e b a r r e l / s p a n d r e l w a l l i n t m r f a c e . The m e a s u r e m e n t s c o n f i r m e d t h a t t h e a r c h b a r r e l i n t h i s r e g i o n was p u s h i n g a g a i n s t t h e s p a n d r e l w a l l l e a d i n g t o r o t a t i o n o f t h e b r i c k w o r k w a l l s .

11

5.4 Brickwork strain: embedment

Twenty electrical resistance strain gauges were placed in the arch barrel to

monitor:- ([) r i n g separation (ii) spandrel wall/barrel separation.

The layout of the gauges is given in figure 3.8.

5.4.1 Ring separation

In the course of point load tests ring separation was detected at the loaded quarter point (0R2/I05) and later also at the crown (0R2/I09), both on the west side of the barrel. During the load test to failure it was of interest to consider the development of existing ring separation and also to monitor the

progression of further cracking.

Initially the ring separation was present only at gauge 0R2/I05, however, by a loading of 300kN ring separation was detected at gauge 0R2/I07. Ring separation therefore occurred across the width of the barrel at the north quarter point. Up to approximately 400 kN the curves followed very similar paths. Beyond 400 kN the load/strain relationships diverged. This was probably caused by the re- distribution of stresses due to hinge formation at this point. The gauges continued to monitor strains up to 2500 microstrain. Visible ring separation was observed at the edges of the barrel, with crack widths of about lOmm at failure. The ring separation at the crown, present before loading to failure, continued to develop with loading to failure. A discontinuity in the experimental relationship at 800 kN may have been caused by hinge formation. Unlike ring separation development at the quarter point the ring separation did not develop across the

full width of the arch ring until after ultimate load,

P r i o r t o l o a d i n g no r i n g s e p a r a t i o n was p r e s e n t in t h e r i n g a d j a c e n t t o e i t h e r a b u t m e n t . However i t o c c u r r e d a t t h e n o r t h a b u t m e n t a t 400kN, c o r r e s p o n d i n g t o h i n g i n g a t t h e q u a r t e r p o i n t and t h e i n h e r e n t g r o s s d e f o r m a t i o n s . S i m i l a r l y r i n g s e p a r a t i o n was r e p o r t e d a c r o s s t h e w i d t h of t h e load a t t h e s o u t h a b u t m e n t f rom a l o a d i n g o f 600kN. No r i n g s e p a r a t i o n was r e p o r t e d a t t h e u n l o a d e d q u a r t e r p o i n t u n t i l a f t e r r e a c h i n g t h e maximum l o a d .

5.4.2 Spandrel wa l l : b a r r e l s e p a r a t i o n

Vibrating wire strain gauges were installed across the arch barrel/spandrel wall interface. Separation was recorded beginning at 400kN and eventually extended around the entire span except at the south quarter span where, as might be expected, compressive strains occurred as the arch barrel pushed into the spandrel

wa i l .

5 . 5 B a c k f i l l p r e s s u r e s

A total of thirty-four earth pressure cells were incorporated into the bridge. Unfortunately, seven of the gauges malfunctioned during the test. Figures 5.14 - 5.22 shows the load/pressure response for the main locations on the extrados. In each case the initial response appeared to be linear. This is in keeping with the elastic deformation of brickwork. By the formation of the iirst hinge this

had ceased to be the case.

At the north abutment, pressure increases are directly attributable to the KEL

12

and the confined situation of the backfill.

The pressure cells beneath the KEL at the quarterspan showed a good agreement with a 45 o distribution. Pressures on the crown changed little during the test.

At the south quar}er point the pressure increased steadily with load reaching a maximum of 30kN/m" which represents approximately 60% of full passive pressure.

At the south abutment pressures decreased with negative pressures being recorded in the latter part of the test. This was probably caused by arching of the fill over the gauges as the arch barrel hinged just above them and additionally the

backfill displayed some adhesiveness.

Pressures on the spandrel walls followed the expected pattern. Adjacent to the KEL, a progressively increasing pressure was recorded peaking just prior to the spandrel wal] cracking. Additionally, the pressures adjacent to the south abutment increased steadily until hinges formed, at which stage significant increases were observed. These changes were commensurate with the stresses induced by the longitudinal pressures generated during hinge formation.

5.6 Summary

(i) The bridge fa i led due to the formation of a four hinge mechanism at an ultimate load of l173kN. The spandrel walls provided a significant restraint to the arch barrel. The failure mode was identical to that

observed in small scale model tests.

( i i ) The l o a d / d e f l e c t i o n r e s p o n s e f o r t h e a r c h b a r r e l was i n i t i a l l y l i n e a r until hinging of the barrel. This was followed by a rapid increase in

deflection with load up to eventual failure.

( i i i ) Passive pressure was not recorded, however, the backfill did provide a

significant lateral restraint to the arch.

(iv) The experimental thrust-line was successfully determined using measured brickwork surface strains and material properties for the brickwork.

(v) The surface strain measurements confirmed that the thrust was confined to the middle third at the three quarter span by the stiffness of the spandrel walls. Away from the influence of the wails the thrust-line

was outside the middle third.

6 ANALYSIS OF STRUCTURE

C o l l a p s e l oads f o r t he b r i c k w o r k a r c h were p r e d i c t e d u s i n g a MEXE a n a l y s i s (2 ) and a modified mechanism analysis. Both methods are outl ined below and the corresponding loadings are compared with the experimental collapse load of 1173kN.

13

6 . 1 MEXE method

The MEXE a n a l y s i s c a r r i e d o u t i n a c c o r d a n c e w i t h A d v i c e N o t e BA 1 6 / 8 ~ (2 ) i s s e t o u t b e l o w .

= 6.0m (L)

= l. Om (r c)

= 0.77m (rq)

= 0.22m (d)

= 0.3m (h)

= (740 (d + h)2)/61'3 = 1 9 . 4 8 t o n n e s .

Span

Rise a t c r o w n

R i s e a t q u a r t e r s p a n

A r c h t h i c k n e s s

Depth of fill at crown

Provisional axle load (PAL)

Modifying factors:

Fsr = O. 75

= 2.3(r c - rq)O'6/r c Fp = O. 95

F b = 1 . 2 ; Ff = 0 . 7

F= = ( ( F b. d ) + ( F f . h ) ) / ( d + h ) = ( ( 1 . 2 x 0 . 2 2 ) + ( 0 . 7 x 0 . 3 ) ) / 0 . 5 2 = 0 . 9 1

Fj = F w. F d. Fi0 = 0 . 9 x 1 x 1 = 0 . 9

F¢ = 1 . 0

M o d i f i e d a x l e l o a d (MAL) = 0 . 7 5 x 0 . 9 5 x 0 . 9 1 x 0 . 9 x 1 . 0 x 1 9 . 4 8

= 1 1 . 3 7 t o n n e s

( a x l e load for d o u b l e axle b o g i e )

A x l e l o a d f o r a s i n g l e a x l e = 1 4 . 5 5 t o n n e s .

C a l c u l a t i o n s d e r i v e a l o a d i n g f o r a s i n g l e a x l e o f 1 4 . 5 5 t o n n e s . As t h e b r i d g e was sufficient for two carriageways this provides a total loading in a similar

format to the KEL of 29.1 tonnes. Compared with the actual experimental failure load a factor of safety of 4.1 is arrived at:-

FS = 1 1 7 3 / 2 8 5 = 4 . 1

Although this factor of safety may seem more than adequate against failure a

single axle may indeed be sufficient to cause significant cracking of the bridge. A single axle load of 14.55 tonnes (143 kN) would produce two equal wheel loads of 71.5 kN. A load of 70 kN placed at 970mm from the outer edge was sufficient to cause spandrel wall/barrel separation leading to ring separation, clearly the single axie load would also be sufficient to cause structural damage to the bridge if applied nearer to the edge at the quarter span or, possibly, at the crown.

14

6 . 2 Mechan ism a n a l y s i s

A modified mechanism analysis was conducted to determine a collapse loading for the bridge. The method adopted was similar to that proposed originally by Pippard and Baker (5) and later by Heyman in the guise of 'Plastic' theory (6). However,

the method has been modified to account for lateral pressures and stiffening due to s p a n d r e l w a l l s . The t h e o r e t i c a l p h y s i c a l model i s g i v e n i n f i g u r e 6 . 1 .

Twelve separate analyses were carried out using a simple mechanism analysis with hinges assumed to be in the same position as those for the Dead Load only case. This is not strictly correct but it allows a relative 'feei' for the significance

of each restraining parameter.

I t can be seen from Table 6.1 that by al lowing for the se l f weight of the spandrel wall and the ef fects of f r i c t i on /cohes ion between the b a c k f i l l and the spandrel wall; a load at which a mechanism forms can be predicted.

Load Description Quarter Pt. Load Case at Failure (kN)

l V e r t i c a l DL s o i l & b a r r e l o n l y 280

2 Case i + (K0~ h) h o r i z o n t a l l y 307

3 C a s e 2 + DL s p a n d r e l w a l l ( sw) + a l l resistance (c~) 687

4 Case 1 + (K r ~ h) horizontally 554

5 Case 4 * (sw) 783

6 Case 4 + ( c~ ) 734

7 Case 4 ÷ (sw) * (c~) 1010

8 Case 1 + (Kp ~ h) h o r i z o n t a l l y 750

9 Case 8 + (sw) 978

10 Case 8 + (c#) 931

11 Case 8 + (sw) + (c#) 1159

12 Case 8 + (sw) + (c#) + (c~ @ KEL) i t 7 5

T a b l e 6 . 1 Mechanism A n a l y s i s

Load case 2 r e p r e s e n t s t he case of t h e a r c h b a r r e l u ~ r e s t r a i n e d by t h e e f f e c t s o f s p a n d r e l w a l l and w a l l f r i c t i o n . More r e a l i s t i c a l l y , b e c a u s e o f t h e c o m p a c t i o n of t he b a c k f i l l , h o r i z o n t a l s t r e s s e s w i l l be c l o s e to t h o s e g i v e n by K t ( c o e f f i c i e n t of p r e s s u r e f o r u n l o a d i n g and i s u s u a l l y t a k e n as K r = t / K 0 (K 0 = c o e f f i c i e n t of p r e s s u r e a t r e s t ) . T h i s s u g g e s t s t h a t a load o f 554 kN ( c a s e 4)

15

would result i n movement and a reliance on support from other parameters to prevent faiiure. Two cases are considered, firstly spandrel wall stiffening. The lower bound is that of dead load effects only which is considered in case 5. (If spandrel wall separation had existed then the cohesion/friction between the spandrel wall and the backfill would have resisted motion (case 6)). Either way at around 734 - 783 kN major deformation would be [ikely. The large rotations associated with the mechanism would thus result in the interaction between the spandrel walls and the backfill to be mobilised (in addition to the spandrel wall dead load). Thus suggesting a maximum load of I010 kN.

If spandrel wall separation had been present then a maximum load of 73a kN (case 6) would have been predicted.

Cases 5 to 12 ( T a b l e 6 . 2 ) f o l l o w a s i m i l a r l o g i c bu t f o r t h e f u l l p a s s i v e s o i l r e s i s t a n c e c a s e . A l t h o u g h t h e p r e d i c t e d l o a d s a r e c l o s e r t o t h e o b s e r v e d l o a d s , i t is important to note that the passive soil resistance was not recorded during the test.

Cases 5 and 6 predicted a change in stiffness of the bridge, which was observed in the graphs eg. figure 5.3. This may form a basis for determining a serviceability limit state, by applying a factor of safety of 3 to this load. For this bridge it would have given a safe working load of about 250 kN (cf. MEXE 285 kN). The arch ring is quite slender relative to 'normal' bridges which would normally have a four ring barrel for a 6 metre span. The thicker barrel would greatly increase the theoretical carrying capacity but would not affect the MEXE assessment. [n fact the test probably highlights one of the situations where MEXE should not be considered to be quite so conservative.

Additionally, the friction on the extrados will offer some resistance to movement. No measurements were taken to quantify this effect but a crude assessment would indicate a load carrying capacity enhancement of at least 100 kN.

7 CONCLUSIONS

The f o l l o w i n g c o n c l u s i o n s may be m a d e : -

(i) The bridge fa i led due to the formation of a four hinge mechanism. The f a i l u r e mode was i d e n t i c a l t o t h a t g i v e n by s m a l l - s c a l e model t e s t s .

(ii) A p o i n t l o a d i n g s i m i l a r t o t h a t g i v e n by a MEXE a n a l y s i s f o r a w h e e l l o a d was s u f f i c i e n t t o c a u s e s t r u c t u r a l d a m a g e , r i n g s e p a r a t i o n , t o t h e b r i d g e . F u r t h e r l o a d i n g l e d t o p r o p a g a t i o n o f t h e c r a c k i n g .

( i i i ) The b a c k f i l l p r o v i d e d a s i g n i f i c a n t l a t e r a l r e s t r a i n t t o t h e d e f o r m a t i o n o f t h e a r c h r i n g , h o w e v e r , p a s s i v e p r e s s u r e was n o t r e c o r d e d .

(iv) U s i n g a modified m e c h a n i s m a n a l y s i s , i n c o r p o r a t i n g the l a t e r a l b a c k f i l l p r e s s u r e s , s p a n d r e l w a l l s t i f f e n i n g a n d b a c k f i l l c o h e s i o n / f r i c t i o n s t r u c t u r a l i n t e r a c t i o n , t h e o n s e t o f m e c h a n i s m b e h a v i o u r and t h e c o l l a p s e l o a d c a n b e p r e d i c t e d . The p r e d i c t i o n o f t h e o n s e t o f m e c h a n i s m b e h a v i o u r c o u l d b e u s e d t o s e t a s e r v i c e a b i l i t y l i m i t s t a t e .

16

8 ACKNOWLEDGEMENTS

The authors wish to acknowiedge the support and encouragement g i v e n by John Page ~TRRL Project Off icer) in the execution of the contract . Add i t i ona l l y the financi~! support given by NAB and the Bolton I n s t i t u t e to provide the large scale tesing f a c i l i t y is recognised. The assistance of the support s ta f f is also acknowledged, pa r t i cu l a r l y Jim Briggs (Pr inc ipa l Technician), Phi] Owen, Nell McMiiian. Jim Burns and Len Nut ta l l for construct ing and tes t ing the bridge, and Mildred Jones for typing the report.

9 REFERENCES

'The Assessment of Highway Bridges and S t ruc tu res ' , Department of Transport Roads and Local Transport Di rectorate, Departmental Standard BD 21/84, Department of Transport, March 1984.

2 'The Assessment of Highway Bridges and S t ruc tu res ' , Department of Transport Roads and Local Transport Di rectorate, Advice Note BA 16/84, Department of Transport, March 1984.

Page, J . , 'Load t e s t s to c o l l a p s e on two a r c h b r i d g e s a t P r e s t o n , S h r o p s h i r e and P r e s t w o o d , S t a f f o r d s h i r e ' , TRRL R e s e a r c h R e p o r t 110, Department of T r a n s p o r t , 1987.

4 Melbourne , C. and g a l k e r , P . J . , ' L o a d t e s t s to c o l l a p s e of model Br iCkwork Masonry A r c h e s ' , P r o c e e d i n g s o f t he 8 t h I n t e r n a t i o n a l B r i c k / B l o c k masonry c o n f e r e n c e , pp 991-1002 , D u b l i n , 1988.

P i p p a r d , A . J . S . , and Baker , J . F . , ' The A n a l y s i s o f E n g i n e e r i n g S t r u c t u r e s ' , Second E d i t i o n , London, 1943, Edward A r n o l d .

Heyman. J . , 'The Masonry A r c h ' , F i r s t E d i t i o n . London , 1982, E l l i s Hopwood Ltd .

APPENDIX

A1 m a t e r i a l p r o p e r t i e s

23.500 sol id concrete engineering br icks, nominal dimensions 215 x 102 x 66mm, were used in the construct ion of the bridge. The average propert ies of the bricks, tested in accordance with BS6073, are out l ined in Table A1.1.

A1.2 Mortar

A 1 : 2 : 9 (cement:lime:sand) mortar mix by volume was used throughout for both the arch barrel and wails. The average propert ies of the mortar at 28 days are summarised in Table A1.1.

AI.3 Brickwork

A s e r i e s of s m a l l - s c a l e p r i sm t e s t s were c a r r i e d o u t to d e t e r m i n e t h e p r o p e r t i e s of the b r i c k w o r k . The average c o m p r e s s i v e , f l e x u r a l and s h e a r s t r e n g t h s a r e g i v e n in Tab le A I . I .

17

The initial tangent elastic modulus for the brickwork was determined by linear

regression of the experimental results, average E 1 = 6.8 kN/mm 2.

M a t e r i a i Compressive Tensile/

strength f l e x u r a ] Nlmm 2 strength

Nlmm ~

Brick 32.01

Shear Density 24 hr. strength kNlm 3 Absorption

N/mm 2 % wt.

22.2 6.1

Mortar 2.3 0.112

Brickwork 11.2 0.333

- 20.9

0.27 22.0

1 Bed-face direction 2 Splitting cylinder test

3 F ]exu ra l strength

T a b l e A1 .1 M a t e r i a l P r o p e r t i e s

A I . 4 B a c k f i l l

A 50mm graded limestone 'crusher run' was used for the backfilling of the arch.

The properties of the material are given in Tables AI.2 and AI.3 and in figures A. 1 and A.2. Shear testing of the backfill was carried out on a sample passing

a 20mm sieve in a 300 x 300mm shear box, the loading rate was 0.2mm/min. 2 Results show the backfill to have been a c-Q material: d = 540 and c = 6.5 kN/m (figure

A.I).

A 1 . 5 Bituminous road surface

A high density bituminous road surface using lOmm limestone aggregate was

provided. The average compacted density was 19.4 kN/m 3.

% PASSING

SIEVE SIZE TEST SAMPLE MOT TYPE I

75 mm i00 I00 37.5mm I00 83 - 100 I0 mm 4a 40 - 70 5 mm 28 25 - 45

600 m 7 8 - 22 75 m 2 0 - I0

T a b l e A 1 . 2 S i e v e a n a l y s i s o f b a c k f i l l

18

TEST PROPERTY TEST RESULT

AIV 12.3 BULK DENSITY 21.41 kN/m 3 MAX. DRY DENSITY 19.3 kN/m 3 OPTIMUM MOISTURE CONTENT 8.4

540 c B.5 kN/m 2

In-situ (sand replacement method) moisture content = 1.9%

T a b l e A1 .3 P r o p e r t i e s o f B a c k f i l l

19

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40