991 •

lOAD TESTS TO COlLAPSE Of HODEl BRICKWORK MASONRY ARCHES

DR. C. MELBOURNE and DR. P.J. WALKER, Department of Civil Engineering and Building,

Bolton Institute of Higher Education, Bolton, England.

ABSTRACT

A number of model brickwork masonry arches have recently been tested at B.I.H.E. as part of a programme of research currently being undertaken in Britain to study the behaviour under load of masonry arch bridges. The paper summarises the results of load tests to failure of three semi-circular brickwork arches spanning 1500 mm. The results are presented of a series of backfill pressure measurements taken to study the effect on the behaviour of arches of soil/structure interaction. The influence of spandrel wall stiffening and backfill properties on the failure load and mechanism are also discussed briefly.

INTRODUCTION

Presently in Britain there is a renaissance of interest in the behaviour of masonry arch bridges due to the age of the stock and the ever increasing loads which they are expected to carry. Current assessment techniques (1) have been shown to be very conservative when compared with experimental and field test data (2-4). The effects on the strength of arches of soil/structure interaction and spandrel wall stiffening have been neglected. This has necessitated an experimental investigation into the fundamental behaviour of brickwork arches in an attempt to ascertain the effects of the backfill and spandrel walls. Therefore, a number of model semi-circular arches have been tested at B.I.H.E.. This pape r reports in detail upon the behaviour of three of those arches. Further to the experimental work a number of recommendations are made for the analysis of masonry arches at the ultimate limit state using a modified mechanism type analysis (5).

MATERIAL PROPERTIES

The arches were built using full-scale concrete common bricks having an average compressive strength of 9.5 N/mm 2

, A 1:1:6 (cement:lime:sand) mortar mix by volume was specified, the results are reported in Table 1 for the average compressive strength and density of the mortar at the time of testing of each arch.

992

Previously tests of model masonry arches have used sand as a backfill material, but failure of the sand under loading prior to the arch has been a problem (2). In an attempt to overcome this 10 mm graded limestone aggregate was used throughout for the backfill; the angle of internaI friction of the limestone was 55°.

EXPERIMENTS ANO INSTRUMENTATION

The dimensions of the arches are outlined in figure 1. The backfill was placed to a depth of 150 mm above the crown and the arches were 1000 mm wide. A 'K E L' loading was applied at the third point across the full width of the arch.

r 4800 ~

SPANOREL WALL LIAO

F lU SURFACE I - - -------------r------ - - ------·-- - --- -ISO

~

t--' 900 t ARCH RING THICKNESS =/OOmm

SPANDREL =100iTlm

ARCH WIDTH = 1000 mm

Figure 1

1 1C0 T

i18UTt1ENT

1500 - - --.....j--- l liOO --1

ALL OfHENSIONS mm

Test arch details

a a ~

1

The tensile or flexural bond strength of typical masonry will be similar for both full-scale and mo deI arches. However, due to scaling effects the bond strength will have a more significant effect upon the behaviour of the model arches. In an attempt to minimise this scaling effect and also improve the uniformity of s trength it was decided to coat the bed faces of the concrete bricks with 'mould' oil prior to laying . This reduced the average flexural tensile strength to 0.07 N/mm 2

, instead of a more typical value for this type of masonry of 0. 5 N/ mm 2

•

•

993 • Arches 1 and 2 consisted of the arch ring only and limestone fill.

As there were no spandrel walls the fill was contained by perspex and wood sides to the test rig. The backfill in Arch 2 was vibrated for improved compaction and therefore density. Arch 3 was built with spandrel walls, the dimensions of which are outlined in figure 1. The individual details of each arch are summarised in Table 1.

The radial deflections were measured using linear voltage displacement transducers at five positions around the intrados of each arch. These positions were coincident with the springings, third points and crown.

A total of nine 'Kulite' earth pressure cells were used to monitor the backfill pressures around the extrados of the arch ring. In arder to accommodate the pressure cells into the arch ring, such that the active diaphragm was flush with the surface of the extrados of the arch ring, recesses were formed in the brickwork at the required positions. These were formed by leaving part of the brick unit out of the bonding pattern and the pressure cells were fixed in place using either a weak mortar ar plaster mixo

The arches were loaded using a hydraulic manually operated jacking system. The applied load was measured using two 'ROP' 67 kN capacity load cells. The abutments were horizontal concrete filled 100 mm steel channels bolted to the base of the rig and so were prevented from separation. The loading was applied in approximately 50 increments up to failure.

EXPERIMENTAL RESULTS ANO DISCUSSION

AlI three of the model arches reported in this paper failed due to the formation of a four hinge mechanism. Although some localised crushing of the mortar joints was observed at a number of the hinge points a material failure of the brick unit voussoirs did not occur in any of the tests. The failure modes of Arches 1 - 3 are given in figures 2 4, the hinge positions are defined by the number of courses of brickwork. In alI three tests the first hinge to form was that underneath the loading point, which was followed by a hinge developing in a similar position in the opposite half span of the arch ring, figures 2 - 4. Finally hinges formed at between 3 - 7 courses of brickwork above each springing. Once alI four hinges had fully developed the ultimate load was recorded, this was followed by a decrease in load carrying capacity and eventual collapse.

Figure 2 Failure mode of Arch I

994

N0. DENOTES COURSES tJF BRICKWO F?K

Figure 3 Failure mo de of Arch 2

lhe spandrel walls did not significantly alter the pattern of the hinges at failure, figure 4. On the side remote from the load the spandrel walls acted as a resistance to the deformation of the ring. Once the upward thrust on the wall was sufficient a bond failure occurred along a horizontal bedding plane leveI with the hinge, figure 4. lhe wall continued to act as a dead weight restraint to the arch ring and with further deformation of the arch ring the wall rotated about the end extreme from the loading point.

6

Figure 4 Failure mode of Arch 3

lhe failure load for each arch is given in lable 1. lhe effect of improving the method of compaction of the fill (Arch 2) was to increase the failure load by 6%. lhe increase in load may be directly ascribed to the increase in stiffness of the structure resulting from the enhanced density of the fill . lhe stiffening effect of the spandrel walls is also clearly apparent as an increase in the failure load of 72% was recorded. lhe spandrel walls provided a significant restraint and consequently a strengthening of the semi-circular arch •

•

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995 • TABLE 1

Summary of Test Resu1ts

MORTAR BACKFILLIFAILURE 1ST HINGE FORMATION MODIFIED DENSITY LOAD PIPPARD &

ARCH 1- ----

KN/m J kN BAKER - .. _- '- . .. _ .......... _._-._.-

.,.~ .-.... - .. _-_._._- ._-I

NO· ISTRENGTH DENSITY LOAD FAILURE LOAD THEORY

1

2

3

N/mm 2

4.8

3.8

5.1

~ .:: C.)

"' C) -..J

l..) Lq ...., -..J ()

rt: -:. r:

Kg/m J kN 1ST HINGE KN

2101 15.3 19.8 8 2.5 23.7

2070 16.6 21.0 8 2.6 24.2

2122 16.1 36.2 14 2. 7 35.6

o 40 r-----~~------+-------+-------~----~ 40

2 4 6 8

/

30 /

30

/ I

20 ~===~~:::=':'::.::::-.l1:.::::.fJJs1L__ ._._ .. f-! ----------.-=-=~ 20

MAX.LOADI9·akN .~.

10 --ARCHl 10

- ' -ARCH 2 - - -ARCH 3

O ~----~~----~------~-------+------~ O O 2 4 6 8 10

DfFL E[TlON (mm)

Figure 5 Comparison of load/def1ection responses

EXPT.

1.20

1.15

0.98

996

A fill failure at the point of loading was observed in alI three tests and therefore the limestone aggregate did not solve the problem of premature failure as previously expected.

lhe loading at which first hinging was observed is given for each arch in lable 1. lhe load was the same for Arches 1 and 2 but the stiffness of the spandrel walls in Arch 3 were sufficient to delay hinge formation until the applied load was 14 kN. An interesting comparison is made if the load to cause first hinging is compared with the failure load of each arch. lhe average ratio of failure load to load at first hinging, lable 1, was 2. 6 and the lack of variation in the ratio may partly be ascribed to the uniform weakn~ss of the mortar joints due to application of the 'mould' Gil to the bricks.

lhe various stiffening processes the arch ring undergoes with increasing fill density and spandrel walls may also be appreciated by studying the load/deflection responses up to failure of the arch ring . In figure 5 the load/radial deflection responses underneath the load point for Arches 1 - 3 are compared. lhe load/ deflection responses show a similar format from zero load up to failure. Initially the relationship was linear until formation of the hinges, this was followed by a post hinging non-linear relationship up to failure. lhe load at which the hinges formed and failure occurred as well as the initial slope of the curve was dependent upon the relative st i ffness of each arch, varying between Arch 1 having least stiffness and Arch 3 having the largest .

O 5 l a 15 20 40 , I I I 140

f\ / + 30 30

/ ~ / .::: C-J

/ "'l: <::l ,/

+20

/ -...J 20

/ C)

'"-' ...,

/ / -...J Q Q "'l:

~ --ARCHl / - ' - ARCH2 t 70 10 +

/ /' - - - ARCH 3 / .

/ .1/ o' Ql o

20 O

Figure 6

5 10 15

TOTA L PRESSURE (kNlm 2)

Relationship between laad and backfill pressure at springing af the laaded side af the Arch

•

997 • The backfill pressures were measured normal to the surface around

the extrados of each arch ring. For the purposes of this paper the discussion has been limited to a consideration of the pressures measured near the springings, underneath the load point and at the opposite third point. The results are presented for each position in figures 6 - 9 .

In figure 6 the total pressures measured near the loaded side springing are compared for Arches I - 3. AlI three relationships show a steady increase in pressure as the load was applied up to failure. Prior to hinge formation the elastic deformation of the arch ring outwards acted against the fill and the pressure therefore increased with load. The pressures in Arch 2 were larger than Arch I due to the greater stiffness of the fill as a result of enhanced compaction. The pressures measured in Arch 3 were less than Arch 2 due to the stiffness of the spandrel walls which relieved the backfill of the loading from the arch ring. The pressures were measured below the position of the hinges in this region (figures 2 - 4) and therefore the increase in pressure after hinge formation was most likely due to the reaction thrust at the hinge causing this part of the ring to act against the fill.

;:: ~ r:;, "<:: C) -.J

Cl l<.i ......

& "'C

o 50 100 150 200 250 40 r-------+-------~------~------_r------~ 'O

30 n 20

10

50

,/

/ /

,/

I

/

I

/

--ARCH1

--ARU/2 - - - ARCH3

/

/

100 150 200

TOTA L PRESSURE (k/','/ m2 J

]0

20

10

o 250

Figure 7 Relationship bet.een load and backfill pressure underneath load point

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Figure 7 illustrated the fill pressure measurements taken underneath the loading point. lhe pressure was directly proportional to the applied load and, consequently, alI three curves follow a similar path up to failure. lhere was some re-settlement of the pressure response with fill failure in Arch 1.

lhe pressures on the side remate from the load were the arch ring acted against the fill and where the passive pressure might have been expected to occur are presented in figure 8 . Although alI three responses increased with load, as expected, the curves differed considerably . Firstly, the pressures in Arch 2 were greater than Arch 1 due to the greater stiffness of the fill. Whereas the pressures in Arch 3 were less than those in Arch 2, this probably resulted from the stiffness of the spandrel walls . Although the pressures measured represent a significant restraint to the movement of the arch ring in nane of the tests was the passive pressure reached .

~ .::s

2~ -.J

Cl "-' ..... -...J Q.. Q 'o::

D 10 20

3i 40 4 ) I I I

I 1\+30 30 + I (

I I

20 / /- + 2{'

I / I / 10 + I --ARCH 1 +':')

I / - -- ARU/2

- --ARCH3

o I a / I I ' O O 10 2a 30

Figure 8

TOTAL PRESSURE (kN/ 111 2 )

Relationship between load and backfill pressure at t hird point

lhe backfill pressures measured at the springing on the side remate from the side are shown in figure 9 . Arches 1 and 2 showed an initial steady increase in pressure with the elastic deformation of the arch ring . Subsequently, a decrease in the pressure was recorded once the hinges had developed . lhe pressures in Arch 3 decreased almost

•

999 •

immediately from first loading, the spandrel walls resisted the applied load from the arch ring and therefore relieved the pressures in the backfill .

o 40

\

\

30 \

;::: .:.:

Q

" Co - -.J

Q 20 ~I '-., -.J

8:: "

10

O O

figure 9

:! 4- 6 8 10 40

\ n 'O

\

\

"-"- .",j

\

\

\

\

\

\ --ARCH7 v

\ - ·- ARCH2

\ -- - ARCH3

C 2 4- 6 B 10

TOTA L PRESSURE (kNl rr,2 )

Relationship between load and backfill pressure at springing remote from the load

ANALYS IS Of HASONRY ARCHES

A mechanism type analysis for masonry arches , as first proposed by Pippard and 8aker (5 ) , has been widely adopted as a satisfactory method of analysing the ultimate limit state of the arch ring (6 - 9). The backfill provides a significant lateral restraint to the arch ring and in doing so, enhances its load carrying capacity. This phenomenon may be simply incorporated into the mechanism analysis by assuming a

1000

lateral pressure has developed. However, it is also clear that the spandrel walls make a significant contribution to load cgrrying capacity of the arch.

It has been observed from experimental tests under third and quarter point loading that the hinge positions at failure in a semi-circular arch are generally unchanged by geometrical characteristics and the loading position (11). The first hinge to form is fixed by the load point and the second generally forms at a similar position in the opposite half span of the arch ring. Throughout the testing of semi-circular arches the remaining two hinges have formed at similar positions away from the springings. Thus using the test results for semi-circular arches under both third (11) and quarter (10) point loading the following recommendations may be made regarding the hinge positions nearest the springings. The positions of the hinges may be defined by the angles~and ~ (figure 10). Average values from nine tests for~and ~ are 40° and 25° respectively, each angle showing a variation of approximately ! 5°. For the analysis carried out in this paper the hinges were assumed to form at a , ~ and underneath the load point. The remaining hinge is fixed by mathematical iteration to give a minimum collapse load as originally proposed by Pippard and 8aker (5).

A

D

e,~e,

figure 10 Hinge positions at failure

The analysis for spandrel walls proposed here may be considered to be a lower bound solution as it relates to an unrestrained spandrel wall, as with Arch 3. From observed behaviour the spandrel wall, once hinging has occurred. acts as dead weight resistance to the deformation of arch ring

8 ----'-- r.

'fi

A

o

figure 11 Idealisation of spandrel .all

1001 •

and therefore in the analysis the spandrel walls are assumed to rotate about their extreme edge as a dead weight, figure 11. The proportion of the spandrel wall resisting the arch ring and the lever arm will be determined by the size of the wall and the tensile strength of the brickwork.

The fill provides a significant restraint to the arch ring which in turn increases the load carrying capacity of the arch. Analysis assuming a lateral pressure between the at rest and passive pressure acting on the arch ring as a point load has proved to a satisfactory model (10, 11). For the analysis carried out on these arches a passive pressure was assumed to develop at the fourth hinge at failure, acting on the arch ring between the third and fourth hinge points.

Table 1 gives the failure loads predicted using the above modifications of the Pippard and Baker analysis. A very satisfactory comparison was shown for alI three arches as the ratio of theory to experiment varied between 0.98 and 1.20.

In conventional limit state design the structure needs to satisfy both the ultimate (ULS) and serviceability limit states (SLS). For a masonry arch the ULS in design may be taken as the load given by the modified Pippard and Baker analysis. However, a SLS for masonry arches is not as straightforward as for other structural forms where variables such as deflection and cracking are used to define the SLS. Deflection is unlikely to be a factor in the design of a masonry arch, however, cracking may be of more significance. This may be taken as the formation of hinges or separation of the spandrel wall/arch ring. Earlier it was shown that the hinge formation occurred at approximately 38% of the ultimate load. A preoccupation with ultimate limit state performance overlooks the possibility that an adequate factor of safety for this limit state may give a working load which would possess an inadequate factor of safety with respect to a serviceability limit state. A factor of safety of 3 with respect to ULS will not necessarily give an adequate factor of safety against first hinge formation compatible with current design philosophy, a factor of safety of possibly 4 or 5 applied to the ultimate load may be more appropriate.

CONCLUSIONS

(1) Using a mathematical model in which the variables are represented by a lower bound performance the mechanism analysis yields a satisfactory correlation between theory and experiment for ULS.

(2) If the SLS for the arch is defined as the formation of the first hinge, then a factor of safety of 3 with respect to the ULS may be inadequate. A factor of safety of 4 or 5 with respect to the ULS may be more appropriate.

1002

REFERENCES

1 Oepartment of Transport Roads and Local Transport Oirectorate; 'The assessment of highway bridges and structures' , Oepartmental standarc B021/84.

2 Hendry (A.W), Oavies (S.R), Royles (R), Ponniah (O.A), Forde (M.C) and Komeyli-Birjandi (F); 'Load test to collapse of a masonry arch bridge at Bargower, Strathclyde'. TRRL contract report 26, 1986.

3 Page (J) and Grainger (J.W); 'Load test to collapse on a masonry arch bridge at Preston-upon-the-Wealdmoors, Shropshire', TRRL working pape r WP/B/117/86.

4 Page (J) and Grainger (J.W); 'Load test to collapse on a brick arch bridge at Prestwood, West Midlands', TRRL working paper WP/B/118/86.

5 Pippard (A.J.S) and Baker (J.F); 'The analysis of engineering structures' , 2nd edition, London, 1943, Edward Arnold.

6 Heyman (J); 'The masonry arch', Ellis Horwood, Chichester, 1982.

7 Crisfield (M.A) and Packham (A.J); 'A mechanism program for masonry arches', TRRL working paper, WP/SA/l/87.

8 Harvey (W.J) and Smith (F.W); 'Semicircular arches', Technical note 495, Proc. Inst. Civil Eng., Part 2, 1987, pp 845-849.

9 Oavies (S.R); 'The influence of certain parameters on the assess masonry arch bridges', Proceedings of International Conference on Structural faults and repair, Vol 2, pp 305-309 London, 1987.

10 Walker (P.J) and Melbourne (C); 'Report on the load tests to collapse of four model brickwork masonry arches', Report to TRRL, BIHE, March 1987.

11 Walker (P.J) and Melbourne (C); semi-circular brickwork arches' ; December 1987.

'A study of the behaviour of five Oraft report to TRRL, BIHE,

ACKNOWLEDGEHENTS

The authors wish to acknowledge the support of SERC, TRRL and the Oepartment of Civil Engineering and Building, Bolton Institute of Higher Education.