11th INTERNAT10NAL BRICKJBLOCK MASONRY CONFERENCE

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

SHEAR FAILURE MODES OF PRESTRESSED BRlCKWORK RETAINING WALLS

Faridah Shafii1

1. ABSTRACT

The shear strength of prestressed walls are significantly oifferent for cracked and uncracked section. The mode of cracking, namely joint eracking and brick cracking, occuring in prestresssed wa1ls affect the ultimate strength At first diagonal cracking the shear strength of prestresssed brickwork is more ihan twice the value specified by the design code.

2. INTRODUCTION

The potential of prestressed briekwork for applications in retammg walls, bridge abutments, diaphragm and fin wa1ls for ta1l buildings particularly in resisting flexural tensile stresses developed by !ateralload, was highlighted in previous publications [1 , 2]. In these structures , partieularly for geometric wa1ls, it is often shear rather than bending which govems the load carrying capacity. Shear is an important aspect of design in prestressed brickwork that requires detailed consideration however, researeh in this field ean be regarded as limited particularly in comparison to studies in flexure. Previous research by Daou [3] and Roumani and Phipps [4] provide a general knowledge of shear behaviour in geometric prestressed wa1ls.

Keywords: Prestressing, Brickwork, Retaining Walls, Shear, Diagonal cracking •

1. Lecturer and Researcher, Faculty of Civil Engineering, Universiti Teknologi Malaysia, Loeked Bag 791 , 80990, Johor Bahru, Johor , Malaysia.

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The current provisions for shear design based on BS 5628: part 2 [5] is Iimited to the characteristic design vaIues recommended for plain masonry. ConsequentIy, conservative designs are produced for prestressed brickwork since a comparatively low design vaIue for shear has been considered for designing materiaIs with relatively higher strength.

This paper describes a part of research conducted at Sheffield University, United Kingdom, in collaboration with MarshaIls Clay Product, a leading brick manufacturer in United Kingdom, to investigate the shear strength of post-tensioned brickwork retaining waIls. The observations on the type of shear failure modes occuring in prestressed brickwork walls are discussed and the corresponding shear strength examined and compared to the design vaIues specified by BS 5628: part 2: 1985

3. DEVELOPMENT OF PRESTRESSED BRICKWORK MODEL

3.1 MateriaIs:

The bricks used in the construction of test walls were 0.3 scaIe cIay bricks manufactured to the specifications of high quality solid, engineering bricks and cut to the nominaI dimensions of a British Standard brick. The mean dimensions based on a representative sample of ten bricks selected at random from the batch were 64 x 30 x 19 mm. The mean initiaI rate of suction and water absorption determined in accordance to BS 3921 were 0.378 kg!m2/min and 3.71% respectively. The mean compressive strength ofthe model bricks tested on bed surfaces was 127 N/mm2

.

Mortar designation (i), corresponding to one part Ordinary Portland cement, 1/4 part of hydrated lime and three parts of sand by volume, conforming to BS 5628 : part 1: 1978 [6] was used in the construction ofthe test walls. The mortar mixture in the proportion by weight ofcement, l: lime, 0.134: sand, 3.887 with a water to cement ratio of 0.97 produced a mean compressive strength of approximately 18 N/mm2 at 28 days when cured in dry conditions.

The prestressing tendons used in the test walls were 22 mm diameter bars made of high tensile grade EN26 steel with a mean ultimate tensile strength 1030 N/mm2

.

3.2 Test walls

The test waIls were T-sections and designed in accordance to BS 5628: part 2 [Fig. 1] where flexuraI cracks are prevented from developing during prestressing operation .. The used of T -sections, particularly in prestressed structures, results in an efficicent use of brickwork, where a relatively large increased in radius of gyration decreases slenderness and permits an increased prestress force to be applied [2]. The section used, designated section 5, was prestressed eccentrically with the tendon positioned within the void at 89 mm from the neutraI axis, Fig. 1.

A reduced scaIe brick model which represents a criticaI portion of prestressed brickwork under shear failure was adopted. In this model only the portion under the shear span was constructed and the forces simulated to represent that of a full scaIe

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wall, Fig. 2. The development of a reduced height prestressed brickwork retaining wall was described in detailed in previous publications [7, 8, 9]. The walls were designed to fail in shear under the applied test conditions.

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Fig. 1. Cross section of wall

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Fig.2 Reduced height model ofprestressed brickwork

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The test walls were constructed on a reinforced concrete base by laying bricks over mortar spread in between steel jigs. The steel jigs ensured consistent vertical alignments ando mortar thickness throughout the model. The completed walls were cured in dry conditions for at least 28 days before testing. In this programme, walls were prestressed to 50 kN, lOO k:N and 150 kN designated S5A150, S5AlOO and S5A150 respectively and their behaviour assessed.

4. TEST OPERATIONS ANO MEASUREMENT TECHNIQUES

In the reduced height model a capping beam was bedded on a 3 mm joint of rich mortar spread on top of the briclrn'ork at the end of the curing period. The steel tendon was then placed at the required location.within the web and anchored. [7,8,9].

Prestressing was applied to the walls in the vertical direction from the top of the capping beam using a hydraulic jack to introduce the prestress force and a steel block arrangement capable of withstanding the forces in the tendon generated during testing. The lateral load applied to the wall was introduced by a 500 k:N capacity computer controlled jack mounted on to an upright structure. The force was applied by displacement controlled and logged into the computer throughout the test period.

The prestressing force and strain in the steel was monitored using sttain gauges installed along the tendon. Transducers were used to measure the lateral deformation of the walls. Brickwork strains were monitored by using potentiometers fixed to the faces of the walls.

In order to facilitate detection of cracks during testing and to ensure reasonably accurate results, a crack detection system was assembled prior to prestressing operation. Eleétrically conductive paints were drawn on the rear and faces of web to detect the occurrence of first tlexural crack and first diagonal crack in the wall. The walls were painted white in order to highlight the occurrence of cracks

5. DISCUSSION

5.1 Failure modes

Walls S5A50 and S5A100 failed in shear by tlexural cracking at the base and followed by diagonal cracking ofthe web. An initial tlexural crack started at the base ofthe wall when the tlexural tensile strength of brickwork was reached. As the lateral load increased the crack length propagated through the web towards the tlange. Diagonal cracking occurred at some ppoint in the web when the critical value of the principal tensile stress, fpt, was reached. The first independent crack occurred independentIy and away from the tlexural at the perpend ofthe brick-mortar interface. For these walls the first diagonal crack was formed in the region of the section adjacent to the void and at a levei in the brickwork which remained uncracked before this evento The diagonal crack was followed by bed joint cracking and there onwards the crack propagated in a step­wise manner. A typical shear cracking pattem in the web of prestressed brickwork models is shown in Fig. 3

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Fig. 3. Typical shear cracking pattem of prestressed brickwork models

Increasing the effective prestress levei to ISO kN caused a transition in shear failure from that cracked in tlexure to that uncracked in tlexure. In this particular case the first diagonal crack was formed in wall S5AI50 at the perpend joint before the tlexural tensile strength ofthe brickwork was reached. Ultimate failure of wall S5AI50 was by diagonal cracking ofthe web alone. The transition in faiIure mode shown by these tests is an indication that the load required to produce tensile cracking at the base had been increased by the application of larger prestress force.

5.2 Wall behaviour and detlected profiles

The load-detlection behaviour of the walls are shown in Fig. 4. At the early stage of testing the walls the deformation of the walls are more or less linearly proportional to the applied moment. After the occurrence of either tlexural cracking or the first diagonal crack, there was a non-linear increased in detlection with applied load due to rapid developments of cracks in the web.

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Fig. 4 Load-detlection response of prestressed brickwork models

The lateral detlections of the walls were measured at heights of 280 mm and 580 mm above the base of the walls. After prestressing and at low leveis of applied load, an almost linear deflected shape was obtained for low prestressed wall. In comparison these deflected shapes become non-linear in a high1y prestressed wall. This is attributed to the negative curvatures caused by the application of high prestress force. For walls tested in this research the non-linear detlected profile became more pronounced at high leveis of applied load for both low or high1y prestressed walls. The increased in deflection was caused by the brickwork losing its integrity due to rapid development of cracks in the web.

5.3 Load and stresses in brickwork

The improvements in flexural cracking, diagonal cracking and ultimate load with higher leveis of prestress was evident from the tests, Table 1. Increasing the effective prestress force from 50 kN to 100 kN increased the estimated flexural cracking load by 48% and enhanced the diagonal cracking load by approximately 42% .. The ultimate load was increased by 65%. Higher leveis of effective prestress also reduced the detlection at which failure occur. At ultimate the lateral deflections for wall S5A150 were approximately 50010 ofthe values measured for wall S5A50.

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Walls Flexural cracking Diagonal cracking Ultimate

Pc (kN) Vc (kN) Pdc (kN) Vdc (kN) Pu (kN) Vu (kN)

S5A50 51.3 14.0 67.5 29.5 132.5 49.3 S5AI00 101.8 32.0 105.6 42.0 170.0 69.4 S5AI50 - - 153.9 46.0 207.8 81.05

Table 1: Load at the criticai stages of testing

An altemative calculation for shear strength at first diagonal cracking at local position of failure was proposed by Shafii [10]. The theory assumed that the first diagonal crack occurred at a section of the brickwork uncracked before this event and therefore validity of the elastic analysis. The calculation, therefore, is independent of the state of the wall base , whether it is cracked or uncracked at flexure. The calculations take into account of the position across the section and height on the wall at which the diagonal cracking took place. Both compressive stress and shear stress due to the applied load was assurned to be linearly distributed across the section. The principal tensile stress and principal compressive stress corresponding to these stresses were calculated using the Mohr-Circle equation relationship.

The shear strength of the test walls based on the proposed method of calculation is indicated in Table 2. The shear strengths produced from these calculations are unsurprisingly higher from the recommended design limits specified in BS 5628 : part 2 because the tabulated values were derived at positions of local failure in the wall. From these test results it is not unreasonable to suggest that the shear strength of prestressed brickwork made of high quality materiais could exceed the design limits recommended by the Britsh Standard codes of design considering the fact that the recommended limit is a nominal value set to achieve a conservative designo As evident from these tests higher leveis of effective prestress force increased theshear strength of prestressed brickwork. For the range of walls tested, the shear strength appears to increase linearly with the effective prestress force.

WaIls Compressive Shear Principal Principal stress stress tensile compressive

stress stress N/mm2 N/mm2 N/mm2 N/mm2

S5A50 1.2 1.96 1.44 2.6 S5AI00 2.15 2.57 1.71 3.87 S5A150 3.37 3.06 1.8 5.19

Table 2 : Stresses m bnckwork at shear failure

Also for the range of walls tested, increasing the prestress force appears to increase the principal tensile strength. These tests clearly indicate the trend of increasing principal tensile stress, fpt, with higher leveis of prestress force similar to the observations in Daou's test [3]. The principal tensile strength ofthe walls as shown in Table 2 exceed the limit ofhalfthe nominal shear strength recommended in BS 5628 : part 2

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There appears to be a significant reserved in strength afier the occurrence of the first diagonal crack. In these tests the ratio of u1timate shear load over diagonal cracking for ali walls appears to be constant at approximately 1.7.

6. CONCLUSIONS

(i). At low prestress leveIs shear failure in prestressed brickwork occurred in the form of diagonal cracking in the web which was preceded with a flexural crack.

(ü). The increased of etfective prestress force enhanced the flexural tensile resistance of brickwork causing transition of shear failure mode to diagonal cracking of the web only.

(iü) The development of the first diagonal crack in prestressed walls by cracking of a perpend joint was immediately followed by cracking of the bed joint.

(iv) Increasing the effective prestress improved the shear strength and principal tensile strength ofbrickwork.

(v) The shear strength and principal tensile strength of prestressed brickwork exceed the design limits recommended in BS 5628: pa.rt 2.

7. REFERENCES

1. Beck, lK, Shaw, G. and Curtin, W.G., The Design and Construction of 3m High Post-tensioned Concrete Blockwork Diaphragm Earth Retaining Wall in a Residential Landscaping Scheme, Practical Design of Masonry Structures, Thomas Telford, London 1987, pp 225-236

2. Curtin, W.G., Shaw, G., Beck,. lK. and Howard, l, Structural Testing of a Post­Tensioned Brick Fin Wall, 9th International BrickIBlock Masonry Conference, Berlin 1991, Vol. 1, pp 333-343

3. Daou, Y.A., Behaviour of Post-tensioned Brickwork Cantilever Fin WalI, Phd Thesis, University of Sheffiel~, 1989.

4. Roumani, N. and Phipps, M.E., The U1timate Shear Strength of Unbonded Prestressed I and T Section Simply Supported Beams, Proc. British Ceramic Society, Masonry (2), Stoke-on Trent, No. 2, Apri11988, pp 82-84.

5.British Standard, BS 5628: part 2, Reinforced and Prestressed Masonry, London, 1985.

6. British Standard, BS 5628 : part 1, Structural Use ofUnreinforced Masonry, London 1978

7. Shafii, F and Hobbs, B., Structural Modelling of Post-Tensioned Brickwork Cantilever Fin WalIs, 4th International Masonry Conference, London, 1995

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8. Shafii, F., Experimental Modelling for the Investigation of Shear in Post-tensioned Brickwork Cantilever Fin WaIls, Confereoce 00 Computatiooal Mechanics and Testing, Wessex University, United Kingdom, 1996

9. Shafii, F., The Performance ofBrickwork Retaining Walls Subjected to Lateral Load, cm Iotemational Confereoce 00 Education and Modernisation, Beijing, China, October 1996.

10. Shafii, F. Shear Behaviour of Post-Tensioned Brickwork Cantilever Fin Walls, PhD Thesis, University ofSheffield, United Kingdom, 1994.

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