PRESTRESSING UNDERSTRENGTH WALLS AND PARAPETS

Patrick Jansen and Dr Graham Tilly

 
PAPER PRESENTED TO 'STRUCTURAL FAULTS AND REPAIRS - 1999'
PRESTRESSING UNDERSTRENGTH WALLS AND PARAPETS

Patrick Jansen and Dr Graham Tilly
Gifford and Partners
Canton House, Ringwood Road
Woodlands
Southampton, S040 7HT

INTRODUCTION
There are many brick walls and parapets, supported on elevated structures and located at the sides of roads and railways, that are now over 100 years old, some over 150 years, and in need of maintenance. One of the main causes of deterioration is through ageing and degradation of the mortar which is invariably lime based in these older structures.

The walls must be able to withstand wind loading, and where they are located beside roads or railways, there are extra windage effects caused by passing traffic. Additionally, there are traffic induced vibrations which can exacerbate the live loading and accelerate deterioration of the walls by loosening the old mortar. ft is also not uncommon for walls, particularly those beside railway lines, to experience additional dead load effects due to utility pipes being bolted on, see Figure 1.

Figure 1  Parapet wall with bolted on utilities

Currently, old walls are failing strength assessments and it is necessary to undertake remedial measures by a suitable method. In addition many older walls are prone to inadequate stability. Differences in flexibility between the walls and their supporting structure, thermal movements, lateral loading effects, bed joint degradation and loss of adhesion can all lead to the walls becoming detached from their substrata. The supporting structure’s contribution towards stability is thus lost, resulting in a reduced factor of safety for stability.

Any strengthening proposal must satisfy the requirements for both strength and stability as necessary. This papers presents such a strengthening solution using Cintec Anchors, a system of post-tensioning that has been tailored to meet the requirements of brick walls. An in situ test to confirm the performance of a post-tensioned 100 year old wall in the field is described.

OPTIONS FOR STRENGTHENING
The appearances of old brick walls do not necessarily have intrinsic value on their own, but taken alongside other structures such as adjacent bridges and buildings of similar age, the collection often merits heritage status. It follows that methods of refurbishment and strengthening should be acceptable in a cosmetic as well as structural sense.

In the normal course of events, deep raking out and repointing of the mortar joints using a stronger cementitious material is the most straightforward refurbishment. The lateral bending strength of the wall can be raised some 70 per cent using this method. However, using conventional methods of analysis and assessment, it is difficult to justify that such repairs provide adequate strength and factors of safety. Furthermore, repointing is a time consuming activity made additionally expensive by costs of access and the need to have lane closures or track possessions to satisfy safety requirements. In any event, repointing is unlikely to satisfy the requirements for stability.

In situations where walls are located on top of retaining structures (a fairly common occurrence), strengthening is sometimes carried out by bolting vertical channel-section steel girders to the brickwork and retaining structure, see Figure 2. This is an unsightly method requiring regular maintenance painting and unpopular with heritage bodies. Furthermore, it is often impracticable to fit the girders when utilities are bolted to the walls.

Figure 2  Externally strengthened wall

Post-tensioning the brickwork into the substructure provides a mode of strengthening having none of the above objections. It is quick to carry out, leaves no external evidence on the brickwork and is economic. The post-tensioning can be designed to suit local conditions and strength and stability can be calculated with adequate accuracy.

POST-TENSIONING SYSTEM
The required level of post-tensioning, spacing of tendons and lengths of anchorages are calculated to meet the requirements of the local conditions. The main components of the Cintec system comprise stainless steel tendons, cementitious grout and a sock to contain the grout. Stainless steel tendons are required to provide long term durability in an environment that is akin to post-tensioned segmental bridges where serious corrosion can occur at the mortar joints. Cementitious grout is used in preference to an epoxy based material as it is considered important to have materials that are compatible with the wall. The sock has been designed to contain the grout and prevent any loose brickwork being displaced by the injection pressures of 3 to 4 bar. It also prevents unsightly leakage through cracks that may be present. The sock permits controlled leakage of grout to enable a structural connection to be formed with the surrounding brickwork.

The construction activities are similar to conventional post-tensioning:

• Remove capping pieces and bore vertical holes through the centre of the wall to the required depth in the substructure.
  
• Introduce the sock and stainless steel tendon into the anchorage length within the supporting structure and inject grout to form the anchorage.
  
• When the grout has fully hardened, post-tension and lock off against a plate fitted to the top of the wall.
  
• Inject grout into a second sock occupying the remaining space in the masonry wall.
  
• When the second injection of grout has hardened, remove the end piece and plate, replace the capping pieces to leave a cosmetically acceptable appearance to the wall.

STRENGTHENING DESIGN PARAMETERS
No specific standards are available for the assessment or refurbishment of existing masonry parapets. The following parameters have therefore been developed for this Cintec anchor strengthening system:

• The post-tensioned wall is designed in accordance with BS562:Part 2 assuming bonded tendons.
  
• Due account is to be given to the make-up and condition of the wall in deriving an appropriate characteristic compressive strength of the masonry.
  
• In accordance with BS5628:Part 2, at the Serviceability Limit State, no tension is to be permitted in the masonry.
  
• Wind loading to BS6399:Part 2 and additional loading due to dynamic pressure and suction from railway traffic derived from European Rail Research Institute Report ERRI D 189/RPI.
  
• Consideration of crowd loading.
  
• Bond stress between the Cintec anchor and masonry or other material is based on pull-out tests on similar materials.
  
• The prestress force must be sufficient to ensure a factor of safety against overturning greater than 2.

The governing criteria in the design of post-tensioned masonry structures is usually the restriction of no tension at the Serviceability Limit State. Accurate values for material properties are therefore not always necessary. This is fortunate since little data appear to be available for typical strengths and stiffnesses of old brick walls.

SUPPLEMENTARY LOAD TEST
A load test was carried out to confirm that the performance of the strengthened wall had been correctly calculated and provide assurance on the method. It was debated beforehand whether to do the test in the laboratory or in situ. A laboratory test has the merit that it can be accurately controlled and loaded to collapse so that the full non-linear load-deflection relationship is recorded. It is not affected by bad weather and a laboratory environment is conducive to good workmanship. On the other hand it was felt that a test on a freshly constructed model would be unlikely to reproduce the true conditions presented by a deteriorated wall containing weakened mortar and weathered bricks. It was therefore decided to test an existing wall in situ and accept the various shortfalls.

The load test was carried out according to the guidelines published by the National Steering Committee for the Load Testing of Bridges¹. Although written for bridge testing, the principles of the guidelines are fundamental and generally applicable to other structures. The guidelines define three types of load testing; supplementary, proof and proving tests. In this investigation the load tests were supplementary and, as the name implies, were planned to supplement the structural analysis. The level of loading was to be sufficient to produce measurable responses from the structure without causing any permanent damage.

The available space and access constrained the load test to being as simple as possible. The supplementary load test was undertaken on a section of wall identified as being understrength for wind loading. The brick wall was believed to be constructed from London stocks with a lime mortar and the strengthening scheme was therefore designed assuming a characteristic compressive strength of the masonry of 2.3N/mm². Built in English bond, the wall was supported on a mass concrete retaining wall. For the purposes of the test a 2m panel was separated from the rest of the wall by vertically saw cutting the parapet down to the top of the retaining wall.

Details of the wall together with the strengthening scheme using two 16mm diameter Cintec anchors are shown in Figure 3.

Figure 3 Details of Strengthening Scheme for Test Panel

The vertical Cintec anchors were each tensioned up to 57kN and, during jacking, the behaviour of the wall and anchors was monitored, see Figure 4. The elongation of the anchor between the jack and the anchored end in the retaining wall was as expected. However, the wall itself was also found to compress by about 4mm at the top which was significantly greater than the 0.2mm expected. The wall was also found to deflect by about 4mm towards the platform. 

Figure 4 Post-tensioning Anchor in Test Panel

For the supplementary load test, the applied wind load was simulated by the application of a lateral point load on a hothontal spreader beam positioned vertically at the centroid of the wind pressure. The lateral load was applied with a hydraulic jack pushing against a jacking frame anchored to the retaining wall supporting the parapet, see Figure 5.

Figure 5 Testing Arrangement

The behaviour of the test panel under lateral loading was monitored using twelve 5½ inch strain gauges located on both the tensile and compressive faces and six dial gauges on the tensile face. The test was undertaken with an incremental increase in applied lateral load up to 3.5kN/m, equivalent to 1.6 x nominal wind pressure. The maximum deflection at the top of the wall was 0.38mm while the maximum tensile strain at a position 500mm above the base of the wall was 48 micro strain.

The test results are plotted in Figure 6 against the predicted values which were based on a characteristic masonry strength f_k of 2.3N/mm2 and an elastic modulus E of 2.O7kN/mm² (=0.9f_k). The measured results identified that the loading was not uniform across the full panel so, for comparison with predicted values, weighted averages were determined in proportion to the tested area. The measured results demonstrate linear behaviour but give greater strains and deflections than those predicted. This suggests that the masonry strength and/or stiffness assumed for the strengthening calculations were too high. Nevertheless, considering the possible variation of material parameters for old masonry walls, the measured results match the predicted very well.

 Figure 6 Comparison of Test Results against Predicted

At the end of the test there was no evidence that the loading had caused any damage such as cracking or spalling.

CONCLUSIONS
The 2m long test panel of 100 year old parapet wall was post-tensioned against wind and dynamic pressure and suction loading, using two Cintec anchors. The supplementary load test with an applied load up to 3.6kN/m (1.6 x nominal loads) demonstrated a linear elastic response.

The predicted response of the strengthened wall, calculated beforehand and based on assumed values for the  material properties, were within 30% of the measured values. Bearing in mind the wide range of uncertainties in  relation to the wall stiffness and strength, this is surprisingly close. On completion of the test there was no damage such as cracking or spalling.

It is concluded that the supplementary load test was successful in demonstrating the efficacy of strengthening an old brick wall in a poor state of repair. The strengthening scheme presented is an economic and aesthetic solution to the refurbishment of understrength and unstable masonry walls and parapets.

ACKNOWLEDGEMENTS
The authors would like to thank Dr S Mehrkar-Asl for his assistance during the supplementary load tests and analysis of the results.

REFERENCES

1. ICE National Steering Committee for the Load Testing of Bridges ‘Supplementary Load Testing of Bridges’. Thomas Telford 1998.

LOAD TEST ON POST-TENSIONED PARAPET WALL

LUL EAST PUTNEY

Cintec International Limited

Factory Road

Newport

 
GWENT

NP9 5FA

Commercial-in-Confidence

CONTROLLED DOCUMENT

LOAD TEST ON POST-TENSIONED PARAPET WALL

LUL EAST PUTNEY

CONTENTS

1. INTRODUCTION.................................................................................................................. 1
2. ASSUMPTIONS USED IN DESIGN OF STRENGTHENED PANEL........................................... 2
3. PREPARATION OF STRENGTHENED TEST PANEL.............................................................. 3
4. SEQUENCE OF EVENTS FOR LOAD TESTING .................................................................... 4
5. RESULTS............................................................................................................................ 4
    5.1 Test One ....................................................................................................................... 5
    5.2 Test Two ....................................................................................................................... 6
6. CONCLUDING REMARKS ................................................................................................... 7
7. INTELLECTUAL PROPERTY RIGHTS................................................................................... 7

ABSTRACT

A 2m long section of a masonry panel wall at LUL East Putney Station was strengthened to resist wind forces and dynamic effects. It was post-tensioned using Cintec anchors.

In order to demonstrate the enhanced strength to the satisfaction of LUL, lateral load tests were carried out on 27 April 1999. The loads were taken to 30 per cent above the required level. The measured response of the wall was linear and values of deflection and strain were close to values calculated beforehand. There was no damage to the wall.

It was concluded that the strengthening was entirely satisfactory.

1.0       INTRODUCTION

Having undertaken assessments of many structures on their network London Underground Limited (LUL) have identified several as being under-strength but which could potentially be strengthened using a post-tensioned anchor system.

Cintec International Limited were approached by London Underground Limited (LUL) to demonstrate the adequacy of the Cintec Anchor System in strengthening existing LUL structures. Amongst these were the masonry parapet walls at East Putney Station which were found to be substandard under wind loading. The parapet strengthening is achieved by Stalling vertical Cintec anchors through the parapet wall and. anchoring them in a mass concrete retaining wall below.

As part of this demonstration, Clifford and Partners (Consulting Engineers to Cintec international Limited) designed a strengthening scheme for the parapets using posttensioned vertical Cintec anchors and developed a testing arrangement to verify the adequacy of the scheme.

A Method Statement was prepared in October 1998 and agreed with all parties.

The parapet walls at East Putney Station are 1.5 bricks thick at the top and vary in height from 1.5m to 1.8m approximately. For the purposes of the test, a panel has been selected with a height above substrate of 1.7m. The test panel is located on the disused platform on the outside of the westbound line (see Appendix A). At this location, the parapet is 330mm wide at the top (= 1.5 bricks thick) and 380mm wide at its base.

The structure is around 100 years old and the bricks are believed to be either London Stock or Staid Dean multi-stock clay bricks.

The test site was prepared by Nuttall, the loading was by Cintec and the gauges were fitted and supplied by Gifford.

2.       ASSUMPTIONS USED IN DESIGN OF STRENGTHENED PANEL

LUL provided Gifford with some calculations for strengthening the test panel at East Putney Station. These, however, did not acknowledge the varying width of the parapet which is critical in designing a strengthening scheme. Gifford therefore redesigned the strengthening scheme taking on board LUL's design requirements as agreed at the meeting between LUL, Cintec International Limited and Gifford on 15 October 1998. In the calculations of the strengthening design, the following have been assumed:

• Characteristic compressive strength of masonry = 2.3N/ram²
  Characteristic flexural strength of masonry = 0.35N/m²
  Elastic modulus -- 0.9f_k -- 2.07 kN/mm² (in accordance with BS 5628 : Part 2)
  
• Wind loading to BS 6399:Part 2. Conservative assumptions have been made with  regard to the required parameters.
  
• Loading due to dynamic pressure and suction from Railway Traffic provided by LUL  as 0.034kN/m² (derived from European Rail Research Institute Report ERR/ D 189/RPI "Loading due to dynamic pressure and suction from railway traffic"). This effect is dependent on the distance of the wall from the track and this value provided by LUL is believed to be appropriate for the wall at East Putney.
  
• A net pressure coefficient of 2.1, the nominal surface pressure due to wind load and  rail traffic is determined as 1.56kN/m². The height of wall exposed to this pressure is 1.4m.
  
• Partial load factors in accordance with BS6399:
  
  Wind Loading                  Y_f =  1.4 ULS    1.0 SLS
  Adverse Dead Load          Y_f =  1.4 ULS    1.0 SLS
  Relieving Dead Load         Y_f =  0.9 ULS    1.0 SLS
  
• Crowd loading not considered, as instructed by Dr Weng Lau of LUL.
  
• Capacity of masonry wall to BS5628:Part 2.  At Serviceability Limit State, no  tension is permitted in the masonry.
  
• No bond between the masonry parapet and the concrete retaining wail supporting it;  the factor of safety against overturning of the parapet to be greater than 2.
   
• Bond stress between Cintec anchor sock and concrete O.4N/mm² and between Cintec anchor sock and masonry 0.5 N/mm².

3.    PREPARATION OF STRENGTHENED TEST PANEL

The wall was in a poor state of repair having cracked joints, cracked bricks, bricks eroded by 10 to 15mm at ground level and joints damaged by ivy and other vegetation that had penetrated quite deeply. There had been repairs at different times, some successful, others not. Preparation of the wall was as follows:

3.1        Removal of vegetation from wall and temporary removal of metal railings in vicinity of parapet to be strengthened.

3.2        Removal of top course of bricks local to positions of Cintec Anchors.

3.3        Vertically saw cut 2m section of parapet to separate the testing panel from the adjacent length of wall.

3.4        Diamond drill 2 No. 56 mm diameter vertical holes at 1m centres through the centreline of the masonry parapet and through the supporting mass concrete retaining wall to provide an embedment length of 850mm in the concrete. Each  anchor to be located 500mm from the end of the panel (see details of strengthening panel in Appendix A).

3.5        Removal of all cores from the holes and depth checked.

3.6        Installation of 16mm diameter anchors in accordance with Cintec’s “Notes for Approved Installers Using the Cintec Designed Anchoring System”.

3.7        Anchors grouted into the mass concrete retaining wall.

3.8        Then the anchorage into the retaining wall had achieved adequate strength, tie bars tensioned to 57kN.

3.9        Anchors grouted into parapet walls.

3.10    When the anchorage into the parapet wall had achieved adequate strength,  tensioning jacks released and excess length of anchor removed.

3.11    Top course of brickwork to be replaced.

4.      SEQUENCE OF EVENTS FOR LOAD TESTING

4.1  Installation of Cintec anchor for reaction of load and jacking rig set up as shown  in Appendix A.

4.2  Installation of hydraulic jack and deflection gauges using scaffold frames.

4.3  Installation of vibrating wire gauges.

4.4  Application of lateral load on parapet through jack in increments. After each  increment deflection gauges and vibrating wire gauges were read. During testing deflections and loads were checked to ensure that elastic behaviour was encountered.

For the purposes of the lateral load test the required maximum applied load was  equivalent to 1.25 times the nominal load and was represented by a line load of 5.5kN (equivalent to 2.75kN/m). This load was applied with a single jack load on a spreader beam across the full width of the test panel. Its vertical location was intended to be at the centroid of the effective wind pressure, that is 700mm from the top of the wall. This would result in the same moment and shear force at the base of the wall as that due to the real wind pressure x 1.25.  Due to the site constraints, however, the load was actually applied at 635mm from the top of the wall.

5.   RESULTS

The load tests were carried out on 27 April 1999 starting at I 1.30 am. The weather was dry after fight overnight rain.  It was windy with sunshine and cloud.  Gauge temperature varied between 16.3°C and 23.4°C during the test. This was not ideal as the alternate cloud and sun caused variable temperature effects on the instrumentation.

The test was observed by people from LUL, Morrison Construction, Cintec and Gifford.

Two load tests were carried out.

5.1  Test One

All gauges were read at the start of the test, after each increment of load and after unloading on completion of the test. The maximum line load to be applied was 5.5kN to produce a value equivalent to the nominal wind load times a safety factor of 1.25 as required by LUL. The resulting values of deflection and strain are given in Appendix B.

Despite the application of the lateral load with a spreader beam the strain gauges identified that the distribution of load was not uniform. The strain gauge measurements wore therefore weighted in proportion to the wall area.

The maximum horizontal deflection (at the top of the wall) was 0.25mm and the maximum averaged tensile strain (at the bottom) was 41 microstrain. The relationships between deflection and load, and strain and load, were linear, see Figure 1.  The theoretical strain and deflection based on the load applied at 635mm from the top of the wall are also shown on Figure 1. These were calculated prior to the test and are based on assumed parameters for strength and stiffness, these being a characteristic masonry strength of 2.3N/mm2 and an elastic modulus of 2.07kN/mm2. Considering the possible variations of these parameters for old masonry walls, the test results match the predicted very well.

At the end of the test there was no evidence that the loading had caused any damage such as cracking or spalling.

5.2            Test Two

As some of the gauges malfunctioned in Test One, they were re-set and a second load test was carried out immediately after the first, starting at 12.30pm.

A higher maximum load of 7kN was applied, ie approximately 30 per cent higher than required by LUL. The resulting values of deflection and strain are given in Appendix B.

The maximum averaged deflection (at the top of the wall) was 0.38mm and the maximum averaged tensile strain (at the bottom of the wall) was 54.8 micro strain. The relationships with load were linear, see Figure 2. Again the theoretical strains and deflections are plotted for comparison with the test results, and again the correlation is surprisingly good.

As for the first test there was no evidence that the second and higher loading had caused any cracking or spalling to the wall.

6.    CONCLUDING REMARKS

The 2m long test-section of parapet wall at East Putney was successfully strengthened against wind and dynamic pressure and suction loading, using two Cintec anchors.

Two tests were carried out The first was to the required maximum lateral load of 5.5kN, the second to 7kN. Responses, as measured by deflection gauges and vibrating wire strain gauges were linear elastic with good correlation between the two tests.

Responses calculated beforehand, using assumed values of the material properties, were within 30 per cent of the measured values. Bearing in mind the wide range of uncertainties in relation to the wall stiffness and strength, this is surprisingly close. It should be noted that on a typical masonry wall, values of strength and stiffness can vary by ±15%. On completion of the tests there was no damage such as cracking or spalling.

It is concluded that the tests were successful in demonstrating the efficacy of strengthening an old brick wall in a poor state of repair.

7.    INTELLECTUAL PROPERTY RIGHTS

The Intellectual Property Rights for these strengthening and testing works are jointly owned by Gifford and Partners and Cintec International Limited.

APPENDIX A

LOCATION AND DETA1LS OF TEST PANEL

Location of Proposed Test Panel at LUL East Putney Station

View of Existing Parapet used for Test Panel

 
Details of Strengthening Scheme

 
Details of Testing Arrangement - Sheet 1

Details of Testing Arrangement - Sheet 2

Test 1 - Strain and Deflection Measurements for Lateral Load taken to 5.5kN

Test 2 - Strain and Deflection Measurements for Lateral Load taken to 7kN

NOTE: For location of strain gauges and trial gauges refer to Details of Testing Arrangement - Sheet 2 in Appendix A