11th INTERNAT10NAL BRICKlBLOCK MA::iONRY CONFERENCE

TONG11 UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

THE COLLAPSE BEHA VIOUR OF A MUL TI-SP AN SKEWED BRICKWORK ARCHBRIDGE

C. Melbourne I

1. ABSTRACT

There are many thousands ofrnaso:uy arch bridges in the UK, the majority ofwhich have a varying amount of skew. The paper describes a large scale laboratory test on a two­span 45 degree skewed brickwork arch bridge. Observations are presented and conc1usions drawn which will be helpful to both the assessing engineer and the design engineer.

2. INTRODUCTION

There are many thousands ofrnasonry arch bridges in the UK, the majority ofwhich have a valying amount of skew. Bridges with very small degrees of skew can be constructed using bedding planes parallel to the abutments; however, larger degrees of skew present the engineering with significant constructional difficulties. Figure 1 shows the developed intrados of a 450 skewed segmental arch with three possible methods of construction: the French (or orthogonal rnethod); the English (or helicoidal method); the bedding plane parallel to the abutment, respectively.

Keywords: Masonry, Arch Bridge, Multi-span, Skew.

I Professor, Depattment of Civil & Environrnental Engineering, Telford Building, University ofSalford, M5 4WT, England.

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The French method is not applicable to the skewed brickwork arch bridges since it requires the use of varying sized masonry blocks with almost every block in the barreI being a unique shape. The English method provides a saving in the cost of construction since each voussoir is similar to alI other voussoirs. However, the English method produces an untidy edge detail in multi-ring skewed brickwork arch bridges. In this method, the bedding joints are only perpendicular to the int~nded line of the faseia at the crown of the arch; therefore, unless special bricks are used, or normal bricks are cut to shape in situ, a saw-toothed effect is produced. An a1temative solution is to provide a stone voussoir edge which is "bonded" into the brickwork barreI. It is important to be aware of the different methods of construction and materiaIs used in these bridges as they can fundamentally influence their stiftbess and strength.

Springing Springing

Springing

a) Jolnts parallel to sprlnglng

Springing

Springing

b) Engllsh or helicoidal method

Figure 1

Springing

c) French or orthogonal method

This paper describes the load testing of a two-span 450 skewed brickwork arch bridge. The test was the last in a series of square(1) and skew(2,3) single span bridge tests and three square multi-span bridge tests(1,3). It was clear from these tests that a global mechanism involving the piers and abutments was possible. It was a1so clear from the single span skewed arch bridge tests that the load transfer at the springings was not uniform even though the KEL was applied uniformIy across the full clear width of the barreI; additionally, the final collapse mechanism involved more than the traditional four hinge mechanism associated with a two-dimensional system. The test was therefore designed to investigate these phenomena.

3. CONSTRUCTION

The 450 skewed brickwork arch bridge comprised two spans each nominally 3000 mm (square) with a nominal square span/rise ratio of 4 i.e. a rise of 750 mm. The square barreI profiles were segmental, each with an intrados radius of 1875 mm. The width of each barreI, measured parallel to the abutments, was 3500 mm with an average thickness of 220 mm. The overall length of the bridge was about 14 metres and was constructed

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(including backfilling) over an eight week period (Figure 2). Class A solid engineering bricks (MarshaI\'s "Nori" bricks) were used throughout. A 1 :2:9:0.045 (cement : lime: sand : plasticizer) was used throughout the bridge; the average cube compressive strength was 2.00 N/mm2 at the time ofthe bridge testo The average crushing strengths of the bricks and brickwork (5 brick panel test) were 154 N/mm2 and 22.7 N/mm2

respectively. The backfill was a 50 mm graded limestone with a bulk density of 2304 kg/m3 and an internaI friction angle of 61 o .

N0I1 \;11\ 113 1$ ] 1\

EAST ELE V ATION

Figure 2

The two ring arch barreIs are constructed on centering which comprised curved steel beams supporting timber formwork. The brickwork was set out using the helicoidal or English method, that is, the bedding joints were perpendicular to the spandrel walls. Headers were provided every fourth course to reduce the risk of ring separation occurring. The centraI brickwork pier was 440 mm thick and 1500 mm high and was construction on the test bed. The pier elevations were not skewed to line up with the spandrel waIls but were kept square and were extended to accommodate the skew. Precast concrete skewbacks were bedded onto the top of the pier and each of the abutments.

The surrounding waIls were constructed in English bond using Class A Engineering bricks with aI : 2 : 9 : 0.045 mortar. There was no connection between the spandrel waIls and the arch barrei other than that created by the mortar bond. The roll over of the barreI elevations meant., that the extrados edge was not a straight line in plan.

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ConsequentIy, only at the crown did the spandrel wall and barreI face line up; elsewhere the wall either overhung the barreI or encroached.

A 50 mm gap between the end retaining wall and the wing walls was provided to ensure structural independence. Backfilling was carried out four weeks after the construction of the arch barreI; a graded 50 mm limestone was used. It was compacted in 150 mm layers with six passes of a vibrating "wacker" plate and took three days to complete. The centering was removed as soon as the backfilling was completed. Testing of the bridge commenced two weeks later.

INSTRUMENT ATION

The bridge was extensively instrumented to measure surface strains, deflections and soil pressures.

The intrados and extrados strain gauges were installed only in the longitudinal direction.

The boundary surface pressure (BSP) cells (which had been previously cast into a 150 mm cube of no-fines lean mix concrete) were installed directIy into the backfill. Where appropriate, pressure cells were positioned facing away from nearby masonry surfaces so that movement of the masonry would not affect the recorded pressure. AlI the pressure cells were covered in a layer of fines to prevent the sharp edges of larger partic1es causing pressure concentrations and thus render the data inaccurate.

NON-DESTRUCTIVE TESTING

As with previous single span bridge tests, the bridge was subjected to a series of point loads (plan area 325 mm x 324 mm) in nine separate locations on the North span Figure 3. A triple load-unload procedure was followed as with earlier similar single span bridge tests, to facilitate comparison and to reduce hysteresis effects. There was no visible evidence of separation occurring during the point load tests, aIthough the barreVspandrel interface mortar did crack locally when the point load was adjacent to the spandrel wall . This was caused by local elastic shortening of the barreI with consequential induced

PLAN

Figure 3

','P',.p ".

'. ' .

tensile stresses resulting in the observed cracking. The cracks c10sed up when the Joad was removed. Direct comparison with the single span 45° skew brickwork arch bridge tests is difficult because single and muIti-span arches are vulnerable to 1/4 span and

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crown loading respectively. Nevertheless, the maximum deflections recorded under the point loaJing of 125kN for the single span were of the order of 0.5 mm. (measured under the load) whilst for the multi-span bridge the maximum deflection was 2.3 mm which occurred when loaded at point 4 (see figure 3). It is significant to note that a 0.2 mm spread between the springings will produced a rigid body vertical displacement of 2 mm at the crown. It follows, therefore, that much of the perceived loss of stiffness is attributed to the "yielding" ofthe pier springing.

The strain measurements on the barreis suggested that cracking of the intrados under the point load had occurred. Also, the intrados strains under the spandrel walJ and adjacent to the load were much smaller, implying that the brickwork remained uncracked. This meant that cracking initiated rnid-width and moved towards the barrei edges where it was observed under greater loading in later tests. Fine cracks in the intrados (as recorded by the vibrating wire strain gauges) remained unobserved because for safety reasons, close inspection was not possible. It follows that the spandrel walls played a significant role in stiffening the barreI. Notwithstanding the small deflection of the pier, there was some load transfer into the "unloaded" arch barreI.

The horizontal soil pressures in the backfill immediately above the North abutment reduced when the point load was applied - indicative of the arch barrei moving away from the backfill . Whilst the horizontal soil pressures over the central pier increased as the barrei pushed on the pier and rotated about the springing into the backfill. It is interesting to note that at this levei of loading the soil pressure changes above the springings were uniform across the width ofthe bridge.

COLLAPSE TEST

The final test was carried out by applying a line load at the crown of the North Span, figure 4, Initially, the load was increased in 10kN increments. The test took approximately six hours to reach failure. The first crack appeared between the barrei and the spandrel wall in the vicinity of the KEL and was due to elastic shortening of the barrei - this was a reopening ofthe cracks which had formed during the point load series

Figure 4

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oftests. At 140 kN the East spandrel wall cracked 500 mm south ofthe crown, this was caused by the tensile strains induced by the lateral backfill pressure under the KEL, lateral movement of the springing (pier movement), rotation caused by the barrei movement and release ofbond between the spandrel wall and the barreI.

The first crack in the barrei occurred at 170 kN, under the load on the East elevation i.e. at 58% ofthe failure load. This was followed by a similar crack in the barrei on the West elevation at a 10M of 210 kN. This was further evidence that the barrei was tending to span between obtuse comers and facilitated by pier movement which was permitted by flexing of the South span. At 230 kN, a second crack formed in the East spandrel wall, 500 mm South of the crown. This was caused by further movement of the barrei and pier.

At 240 kJ.'l, there was almost complete separation between the barrei and the spandrel wall on the North span and the North East part of the South span.

At 280 kN, more general cracking occurred, the result of which was to create sufficient release for a three -dimensional mechanism to start to form o The West spandrel wall cracked at the crown. The pier underwent a rigid body rotation with a crack developing along the North side of the base, at the East springing of the North span, and the underside of the pier head skew block at the West end. A1so there was complete separation between the North barrei and the West spandrel wall.

At 290 kN, the South wing walls separated along their bases being lifted by the South barrei which formed a rising hinge at its crown. A crack had formed along the entire length ofthe wall side ofthe pier. The crack at the South West comer ofthe North span extended over the pier head skew block and up to the sixth course of barrei brickwork. The South West spandrel wall eracked at the crown. The latter was necessary to alIow rigid body rotation ofthe South West spandreVwing wall.

At 295 kN the bridge failed. Extensive cracking developed allowing the bridge to form a three-dimensional mechanism. The central pier formed hinges at both top and bottom and twisted (East relative to West) by up to 2.5 mm; the East end pier top moved southwards more than the West end pier topo This was further confirmation that the loadedbarrel was trying to span between obtuse comers. To accommodate the barrei hinge formation the spandrels cracked over the crowns thus releasing the barreis.

Having reached a failure load of295 kN, subsequent behaviour was monitored in relation to deformation against a falling carrying capacity.

The loaded North Span developed hinges under the load and an intrados in-span hinge 16 courses from the North Springing. This latter hinge was particularly interesting because the pier experienced torsional deformation. The East springing spread more than the West springing; with the result that torsional (in plan) deflections had to be accommodated by twisting of the in-span "plates" which form within the arch barreI. At gross deformation the pier developed torsional (diagonal) cracking mid-width rising from east to west at approximately 45°.

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Figure 5 shows the bridge close to collapse. The unloaded South span forrned hinges at each springing and the crown, lifting the entire ha1f spandrel and wing wall, which rotated about its extremity. The spandrel walls over the pier rotated, resting on the South span and completely separating along the interface with the barreis and pier - the points of contact being determined by geometrical compatibility.

Figure 5

CONCLUSIONS

1.

2.

The two span 45° skew two ring brickwork arch bridge (with headers every fourth course to prevent the spread of any local ring separation) was weaker than the equivalent three span square two ring brickwork arch bridge. The failure load was 295 kN compared with 325 kN for the l quivalent square span bridge.

Collapse occurred by the forrnation of a hree dimensional mechanism which involved the unloaded span, pier and spandreVwing walls as well as the loaded span.

3. Strains commensurate with cracking were induced in the intrados below the point load of 125 kN (42% failure 10ad, 295 kN).

4. During the point load tests up to 125 kN, there was an interaction between the arches, with highest intrados compressive strain occurring under the East crown of the unloaded span. This demonstrated that a pier of slendemess, h/d = 3.4, was insufficient1y stiff to provide a "rigid" support. The spandrel walls stiffened the barreI.

5. The flexibility of the pier support to the arches resulted in greater flexibility of the loaded barrei compared with the "fixed" abutment comparable tests. This is significant when considering serviceability criteria.

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6. The loaded span tried to carry the load square (i.e. between the obtuse corners); this created torsion in the pier and resulted in the unload span' s East crown trying to lift. Eventually a torsional (diagonal) crack forrned in the pier.

7. No where was full passive soil pressure induced. Mid-width longitudinal pressures were greater than those adjacent to the walls. Additionally, the pressure distribution was not symmetrical about the centreline, greater pressures were recorded in the obtuse corners.

8. A1though the strain and detlection contours initially indicated that non-parallel hinges might forrn, it was noted that once the pier became "released" within the mechanism forming process, then the arch barreI articulation became more a1igned to parallel hinge forrnation .

9. The spandrel and wing walls played a significant and interactive role in the forrnation of the collapse mechanism.

ACKNOWLEDGEMENTS

The support ofthe UNK programme is gratefully acknowledged in particular the valued input by the industrial partner L G Mouchel and Partners.

REFERENCES

1. Gilbert, M., "The Behaviour ofMasonry Arch Bridges Containing Defects" PhD Thesis Univ Manchester, UI< 1993 .

2. Hodgson, lA., "The Behaviour of Skewed Masonry Arch Bridges" PhD Thesis Univ. Salford UI< 1996

3. Melbourne, C "Arch Bridges" Thomas Telford Ltd UI< 1995

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