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THE ASSESSMENT OF MASONRY ARCH BRIDGES

Dr C Melbourne Principal Lecturer School of Civil Engineering and Building Bolton Institute of Higher Education UK

ABSTRACT

Masonry arch bridges are statically indeterminate structures which possess a reserve of strength due to their ability to re-distribute load and the diversity of potential load patterns through them. The presence of defects, in shape, material condition, cracks, etc., limit the opportunity for re-distribution and influence the interaction between the constituent parts. This will have a profound effect on the assessment of load carrying capacity, repair strategy and designo The paper suggests a theoretical model which is discussed in relation to recent large scale laboratory tests.

INTRODUCTION

In the UK, the existing stock of over 40 000 masonry arch bridges continue to give good service, carrying loadings well in excess of those envisaged by their designers. They have shown themselves to be durable structures, tolerant of neglect.

In view of the age of the UK arch bridge stock and changes in the loading regime; a nationwide research programme was instigated in 1985 to improve the understanding of the behaviour of these structures and thus improve the analysis methods available and the methods of assessment.

METHODS OF ANALYSlS

F.E. methods have been developed by Crisfield(t), Hughes(2) and Choo(3). Non-linear elements are used to model the arch which can be studied under various load conditions and modes of failure predicted. One problem with the F.E. methods is that certain assumptions have to be made regarding the material properties and the initial stressed state of the arch - each, with the passage of time, will change from those assumed: thus rendering the original analysis incorrect.

An alternative approach equates the eccentricity of the thrust line to a bending moment, the upper l{mit qf which occurs when the thrust line is just inside the arch barreI and is equated to the "plastic" moment. At this point a "hinge" is assumed to formo Thus the

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plastic theory may be applied to the masonry arch. The lower bound theorem of the plastic theory states: "If a thrust line can be found, for the complete arch, which is in equilibrium with the externalloading (including self weight), and which lies everywhere within the masonry of the arch ring, then the arch is safe"(4).

This has been confirmed by recent research(5,6,7,8) which suggests that, if free to do so, a single span masonry arch bridge will fail by the formation of four hinges, thus transforming the arch barrei into a mechanism. This has led to the development of a theoretical physical model, figure 1, which draws the attention of the designe r to the significance of the contribution by the soil and spandrel walls to the stability of the arch barreI. The model allows the stiffening effects of the spandrel walls to be considered -previously, these have been ignored by other researchers.

Reference is frequently made to the possibility of local failure in the form of "snap through" or "punching" shear failure. This confirms the complexity of arch bridge behaviour and the need to consider the three-dimensional nature of the structure.

A more generalised approach considers the arch barrei as a three-dimensional structural element rather than a two-dimensional one - ie as a curved slab rather than a curved beam. In this idealisation the thrust line is replaced by a "thrust surface" and the "hinge" by a "fracture line", figure 2. This allows the effects of different loading cases to be considered. It may be that the simpler two dimensional model (figure 1) is adequate to study the behaviour. But this will be a function of the loading conditions and extent and nature of the arch barrei restraint.

Notwithstanding the above, the stability of the barrei will be assured provided it can be shown that there is at least one way in which the applied load can be transrnitted to the foundations by a thrust surface which lies wholly within the masonry. The importance of this philosophy is that it relates the equilibrium and geometry of the structure to its carrying capacity; thus small changes in shape are not significant as they only have a small effect on the basic equilibrium equations. This means that the problem of deterrnining both the stress history of the structure in order to carry out an analysis and quantifying the effects of secondary stresses associated with creep, temperature etc. are eliminated.

6 METRE SPAN CONCRETE BRICK ARCH BRIDGE

The purpose of this section is to study the behaviour of a masonry arch bridge and relate the observations to the proposed theoretical physical model. Details of the completed bridge and the KEL set-up are shown in figure 3 the segmental arch barrei (radius of circle 5 m) consisted of two rings of brickwork across a width of 6 m. The barrei had a span of 6 m with a rise of 1 m. The arch barrei brickwork was built in a "stretcher" bond with no bonding between rings, other than the mortar bed-joint. The total thickness of the completed arch barrei was 220 mm.

The spandrel, parapet and wing walls were built in English bond; the cross sections are shown in figure 3. The width of the retaining walls, between the wing walls, was 4.46 m.

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" 1----7 soi! pre4surE' = I

}---t f (soil proper fies and I

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movemenl

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ARCH

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Figure 1

thrvs t. Sut'faee

Figure 2

Dead load spandrel walls plus

c ohesive;fric fional inferac fion

on lhe back Df lhe walls

I~:~~J 1" .... ;:-. ...

I~-', ,

Isoil pressure _,

( \ I f (soil proper fies and

c ondilion ) : ( I

I /

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The concrete brickwork had a compressive strength of 11.2 N/mm2 with an initial tangent modulus of 6.8 kN/mm2. A 50 mm graded limestone "crusher run" (cp = 54°, C = 6.5 kN/mm2) was used for the backfilling of the arch.

A full presentation of the results is given elsewhere(8). The bridge was subjected to two separate load tests. A series of 'point' load tests was conducted, followed by the monotonic application of a KEL at the quarter span through to collapse of the bridge.

The series of point load tests simulated a single wheelload and was applied to the road surface through a 380 mm diameter steel plate. During the application of the first load test at the quarter span, adjacent to the spandrel wall, the bridge cracked along the ring/spandrel wall interface and ring separation was also recorded (ring separation occurs in multi-ring brickwork arches where the adhesion between adjacent rings is lost). This occurred at a maximum applied load of 80 kN. Subsequently, point loads were reduced to a maximum of 50 kN; even so, it was found that the original cracks opened and c10sed during the application and removal of the load respectively. Additionally, only the soil pressure cells immediately under the point load showed any change. These tests indicated that:

* *

the load transfer through the backfill was a local effect; point loads can cause local damage to the bridge fabric; cracks within an arch barreI respond to loading remote to them.

The load test to failure comprised the application of a KEL at the quarter span across the full width of the bridge.

Separation of the barrel/spandrel wall interface commenced at 360 kN. Further loading caused the cracking to spread around the arch barreI. At 400 kN the first hinge in the barreI was observed underneath the load line. At 640 kN vertical and shear cracking of the spandrel walls was observed. Ring separation was also recorded between the abutment adjacent to the load and the crown.

Failure of the bridge was due to the formation of a four hinge mechanism at a total applied load of 1173 kN. On formation of the four hinge mechanism the spandreljwingjparapet walls were lifted and rotated by the barreI as it "swayed" under the applied load. Horizontal splitting of the wing walls occurred along the bed joint leveI with the abutments. Hinges formed in the barreI at each abutment, underneath the load and at the crown. Extensive ring separation was coincidental with reaching the maximum load; at this stage the two brickwork rings were almost completed separated around the fuH arc of the barreI.

The hinge positions and typical graphs are shown in figure 4. Initially, an analysis of the bridge can be undertaken using the modified mechanism idealization given in figure l. The analysis comprises setting up equilibrium equations for each of the assumed hinge positions and solving for the unknowns.

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For this bridge assuming only vertical dead load contributions from the fil! and barrei the failure load was 280 kN; if on the other hand fuI! passive soil resistance and spandrel wal! stiffening were included a figure of 1175 kN was calculated. As fuI! passive soi! resistance was not observed this latter figure is an over-estimation (although fortuitously dose to the observed maximum load). Use of earth pressure at rest (Ko = 1 - Sin<1» or compaction (K,. ~ l/Ka) bear a closer resemblance to the actual soi! state. These gave figures (induding spandrel wal! dead load) of 690 kn and 1070 kN respectively. Although the KEL forced the barrei to behave uniformly across the width of the bridge; the same was not true of the barrei remote from the loading. Here the spandrel wal!s restricted the movement of the barreI as they were much stiffer than the soi! and thus attracted more load. Figure 4 shows how the spandrel walls confined the thrust line within the middle third for much of the test, whilst at a corresponding position mid-width the thrust line can be seen to be outside the middle third ie probably cracking. This suggests that even in this special loading case a thrust surface approach may be more appropriate. The loaded part of the span moved away from the spandrel walls -thus spanning as a 'curved slab' between the crown and abutment. Because of the transverse stiffness of the loading beam the 'curved slab' cracks under the load and at the abutment and crown forming 'fracture lines' (hinges) parallel to the KEL. Considering the other half of the barrei; there is a thrust surface applied along the crown fracture line, figure 5. The 'curved- slab' can then be considered as simply supported along its edges and subjected to a variable distributed load on the extrados which is a function of the applied load and 'plate' kinematics. If the spandrel wall support loads are determined in terms of a lower bound ie self weight onIy from a compatible fracture pattern; then the thrust surface contribution can be fed into the set of equilibrium equations which appertain to the barrei equilibrium. Abutment movement may be allowed for in as much as they affect the mode of behaviour and contribution resulting from the interaction between the soil and spandrel walls. If the expected movements are small in comparison with the overall geometry of the structure, then this wil! not sensibly affect the equilibrium equations.

The thrust surface approach, thus allows an investigation of the effects of a more generalised Ioading pattern.

THE EFFECTS OF DEFECTS

Subsequent tests investigated the effect of two specific defects on the behaviour of arch bridges namely spandrel wal! separation and ring separation(9).

Three 3 m span bridges have been built and tested to failure. The segmental arch barrei (radius 1875 mm) had a span/rise ratio of 4 : 1 and consisted of 2 rings of brickwork (solid CIass A Engineering bricks). The brickwork was built in stretcher bond with no bonding between the rings ot~er than through the mortar in the "bonded" case and damp sand in the "ring separation" case. The arch barrei was 2835 between spandrel walls. In two of the bridges a gap of 10 mm was provided between the spandrel walls and the arch ring (thus creating spandrel wall separation). The third arch was the same in every respect except the ring was extended to allow the spandrel wall to rest upon it. In each case, graded 50 mm Iimestone backfill was compacted in 100 mm layers. Each bridge was fil!ed to 300 mm above the crown.

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11 0 0

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14000

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Figure 5

A similar loading sequence was followed for each test; this comprised a series of "elastic" tests followed by the application of a KEL at quarter span which was increased monotonically through to collapse.

In each case, failure occurred by the formation of a four hinge mechanism. AlI the arches started to crack at comparable loads indicating that the subsequent behaviour was indicative of the significance of each of the parameters studied. The following observations can be made:

* * *

* *

ring separation reduced the maximum load by 30% spandrel wall separation had little effect on the ultimate carrying capacity the barreI stiffness was reduced by spandrel wall separation for the "bonded" cases ring separation was induced in the vicinity of the load the stiffening effect of the spandrel walls was confirmed by strain readings in the barreI which indicated that longitudinal stresses was higher adjacent to the walls.

CONCLUSIONS

The tests have shown that a generalised theoretical model which studies the equilibrium of the barrei in terms of a thrust surface may be used to predict global and local behaviour under different loading conditions.

Additionally, the significance of ring separation and spandrel wall separation have been studied. The former has a profound effect on the ultimate carrying capacity whilst the latter appear to only affect barrei stiffness not strength. Good quality workmanship was achieved in the construction of each of the bridges which ensured that the properties of each element were known in so far as they can be determined by standard tests.

REFERENCES

1

2

3

4

5

6

7

8

9

Crisfield, M.A.

Bridle, RJ., Hughes, T.G.

Choo, B.S. Contie, M.G. Gong, N.G.

Heyman, J.

Hendry, A.W. Davies, S.R Royles, R

Page, J.

Page, J.

Melbourne, C. Walker, P.J.

Melbourne, C.

ACKNOWLEDGEMENTS

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'Finite Element and Mechanism Methods for the Anlysis of Masonry and Brickwork Arches' TRRL Research Report 19 1985.

'An Energy method for Arch Bridge Analysis' Proc Inst Civil Engrs Pt 2 Vol 89 Sept 1990.

'Analysis of Masonry Arch Bridges by a Finite Element Method' Developments in Structural Engineering Spon 1990.

'The Masonry Arch' Ellis Horwood 1982.

'Load Test to Collapse on a Masonry Arch Bridge at Bargower, Strathclyde' TRRL Contractors Report 26, 1986.

'Load Tests to Collapse on Two Arch Bridges at Torksey and Shinafoot' TRRL Research Report RR 159, 1988.

'Load Tests to Collapse on Two Arch Bridges at Strathmastie and Barlae' TRRL Research Report RR 201, 1989.

'Load Test to Collapse on a Full Scale Model Six Metre Span Brick Arch Bridge' TRRL Contractors Report 189, 1990.

'The Behaviour of Brick Arch Bridges' British Masonry Society Proceedings No. 4, 1990

The author wishes to acknowledge the financiai support given to his research by SERC, TRRL, British Rail and also the encouragement and support of the staff af the Schoal of Civil Engineering and Building at the Bolton Institute of Higher Educatian, UK.