Numerical Modelling of Masonry to Explore the Performance of Anchor Based Repair Systems and the Repair of Monuments in Cairo

C L Brookes and R J R Swift
Gifford and Partners, UK

ABSTRACT: This paper is in two parts; firstly a method of numerical modelling is described that is being used to investigate the performance of masonry strengthening; secondly how more pragmatic approaches are being used for localised masonry repairs with examples taken from the Mosque of al Ghuri, Cairo.

Numerical modelling based on a discrete element formulation has been employed to simulate the response of masonry to seismic loading. Using dynamic non-linear numerical analysis the performance of shear walls with and without retrofitted strengthening has been compared. Models representative of ashlar masonry laid with a weak lime-based mortar have been investigated. The benefits of strengthening by the introduction of passively stressed reinforcement are predicted for various arrangements when subjected to seismic loading. The reinforcement is represented explicitly in the analysis allowing direct assessment of damage and potential failure mechanisms. It is concluded that, although more work is required to verify simulations against tests and field experiences, the discrete element technique is ideally suited to dynamic masonry simulation and overcomes many difficulties experienced with traditional finite element analysis. The overall performance of masonry acting compositely with retrofitted reinforcement has been predicted and comparisons made between different reinforcement dispositions. It has also been shown that unless carefully placed additional reinforcement may actually reduce seismic resistivity.

Gifford and Partners have been assisting with the conservation of the monuments of Cairo since the earthquake (5.9 on the Richter scale) in 1992. Recently repairs have been completed to the mosque of Al Ghuri. These works are described.

INTRODUCTION

The use of continuum based numerical models to simulate discontinuous structures, such as masonry, is fraught with difficulty. The introduction of discontinuities such as cracks during the loading event, or as a result of loading history, has to be wholly or partly predetermined. The use of gap elements allows cracks to open and maintain normal and shear force connection when closed but the crack locations have to be known in advance. Another approach is to avoid explicit representation of discontinuities but instead smear their effect by using a brittle non-linear material model. However, these models fail to predict mechanisms where, for example, initially isolated parts react dynamically together. Continuum methods can give satisfactory results but generally fail to provide a practical method of analysis for masonry.

As an alternative to the traditional finite element continuum approach, a discrete element (DE) formulation has been employed to simulate masonry with and without strengthening. So far the results of the analyses have been applied to parts of buildings and used to help develop remedial design philosophies by providing simulations under specific ground excitations. A separate project where the engineering analysis has been based on the DE technique has involved the successful strengthening of over thirty masonry arches. Predictive verification of full-scale tests underlies this work and has involved calculated collapse loads of masonry arch bridges as well as supplementary load tests on in-service bridges. Results have been shown to correlate very closely with tests (Brookes, Tilly 1999). As the technique is developed it is hoped that the performance of whole buildings can be checked before strengthening systems have been installed.

1 ANALYTICAL REQUIREMENTS

In order to represent masonry with or without retrofitted reinforcement, particularly in seismic engineering where non-linear structural performance defines how ductility and energy absorption characteristics are exhibited, the following types of fundamental behaviour need to be included in the model.

i) Material and geometric properties of the masonry blocks themselves.

ii) Contact-gap-friction effects along joints between the masonry blocks.

iii) Depending on block and joint properties, the ability to evolve further joints by fracturing which in turn depends on limiting tensile strength and fracture energy.

iv) Full account of stiffness and derived inertia loads which may occur over very short time intervals.

v) The capability to model post-failure behaviour to help verify simulations against the evidence collected after observed seismic damage and collapse.

vi) To allow stress and initial damage from previous seismic events to be included.

vii) The ability to represent retrofitted reinforcement including materially non-linear behaviour of the steel and the non-linear shear coupling behaviour of the bond with the surrounding masonry.

To date, most numerical simulations of masonry have been based on finite element continuum methods in which sophisticated and often-complex material models in conjunction with arrays of gap elements are used to account for the requirements listed above. A more intrinsically satisfactory approach for masonry is to base the analysis on a series of discrete elements. This more natural approach can be used to represent ranges of masonry from completely intact buildings to piles of random rubble.

2 DISCRETE ELEMENT TECHNIQUE

The technique used to perform all the analysis in this study is the discrete element (DE) method. This is a development of the distinct element method (Cundall, 1971) in which the concept of individual elements being separate and reacting with their neighbours by contact through friction/adhesion was first successfully applied to geotechnical and granular flow problems. Here elements were considered rigid but later developments (Munjiza et al, 1995) included the addition of element deformations and fracturing, with some overlap with traditional finite element theory.

In the current investigation the DE formulation available in the explicit dynamic version of ELFEN (Rockfield Software Limited, 1998) has been used. Essentially three different approaches have been used for the non-linear analysis of masonry each requiring different modelling approaches.

Macro Blocks. The category where the joints between blocks have predominantly no strength and models the construction generally found in historic structures.

Brittle Material. This is where the masonry blocks and joints have predominantly similar strengths, as is more likely in modern forms of construction or where masonry is weak and random.

Brittle Macro Blocks. Here the macro block approach is used with brittle materials thus permitting a blocky representation to fracture into further parts.

The first two approaches have been investigated for shear wall applications to investigate the sensitivity of seismic resistivity to mortar properties (Brookes, Mehrkar-Asl, 1998).

2.1 Macro block

The macro block approach has been achieved by separately modelling each block or group of blocks in the structure and applying permanent static loads and seismic excitation to the base. Individual blocks of elements have defined elastic and plastic materials and are arranged to the required bond. All joints and therefore potential discontinuities are predefined and have friction parameters assigned. It is assumed that failure at joints always develops before blocks fail. However, the introduction of a von-Mises non-linear material model without hardening has been used to approximately represent block crushing thus giving a compressive stress cap. Material properties have been based on characteristic values determined for the masonry as a whole.

2.2 Brittle material

Where masonry includes high strength mortar or where the strength of blocks is low, a brittle non-linear material model has previously been used. Here the continuum becomes discretised due to evolving fractures in the blocks and possibly through joints. This is achieved in the analysis automatically using adaptive mesh algorithms. Using a Rankine material model, including fracture energy, newly generated cracks become contact surfaces requiring friction parameters to be assigned as for the macro block approach. However, for ancient ashlar walls with little or no mortar the macro block approach is preferred.

3 REINFORCEMENT REPRESENTATION

The finite element technique is used to model the reinforcement independently of the masonry using a partially constrained spar formulation (Roberts, 1999). Connection between the reinforcement and masonry models is achieved through non-linear bond elements. Modelling of reinforcement arrangements is completely automated without the need for topologically consistent element meshes thus accelerating the modelling process and permitting rapid comparison of designs. Currently, the capacity for reinforcement elements crossing masonry joints to generate transverse shearing strength or dowel effects is ignored. This is believed to be conservative.

4 SHEAR WALL INVESTIGATION

As part of the continuing development of Cintec anchor applications and the expansion of joint venture historic structure remedial projects, Gifford and Partners with Cintec International Limited are undertaking limited studies to investigate how the seismic resistivity of low-rise masonry buildings might be improved (Cintec International Limited, 2000).

Cintec anchors are comprised of stainless steel bar(s), a grouting sock and an engineered grout. Installation is by precisely drilled holes using wet or dry diamond coring technology. The sock consists of a specially woven polyester fabric shaped into a tubular sleeve to fit the required hole diameter. The sock controls the volume of grout used and ensures good contact is achieved with the surrounding masonry. Presstec grout is used having similar characteristics to Portland Cement based products, contains graded aggregates and other constituents which, when mixed with water, produce a pumpable grout that exhibits good strength with no shrinkage. The size of the steel anchor, strength of grout and diameter of hole can be varied to provide the required design parameters and to provide good stiffness compatibility with the masonry. Design parameters such as the bond strength between the grout and the masonry, which is often critical, are normally derived from static pullout tests. Figure 1 shows a diagrammatic Cintec anchor.

To date effort has concentrated on the detailed analysis of masonry shear walls, the primary structural element in masonry buildings, it being recognised that the out of plane behaviour of masonry panels has been the subject of much previous work (Key, 1998). These shear walls are described below. The main objectives of the analyses were to provide some comparison between the performance of the walls with and without strengthening and to continue to explore the potential of the DE technique including modelled reinforcement applied to masonry under dynamic loading. An on-going programme of strengthening projects using the DE method to predict the ultimate strength of masonry arches, including comparisons with full scale tests, has shown the technique to be very accurate and better than alternative analyses for the case of static loading. Further work involving the out of plane prediction of masonry wall behaviour under high-speed dynamic loads arising from blast is also encouraging.



Figure 1 Typical Installed Cintec Anchor

4.1 Description

The masonry shear wall under investigation is shown in Figure 2. The wall is 6.15m wide, 6.76m high, has shallow foundations over rock and has been considered with and without a large opening in each storey. It forms the shorter side of the rectangular building shown in Figure 3 and supports two floors capable of behaving as diaphragms. The longer side walls (not explicitly modelled) partly support the floors, and have little out-of-plane shear resistance. Vertical body forces and imposed loads are supported uniformly by all of the external walls. Loads developed by horizontal seismic ground accelerations in the transverse direction of the building are resisted by in-plane forces in the side walls. One of these walls is the subject of the current investigation.

4.2 DE Model

Several plane stress DE models of a single side wall were developed incorporating the masonry blocks and slabs. The vertical loads and masses attributed to the slabs were modified to reflect mass and load transfer from the rest of the building. The wall is constructed from ashlar blocks, bedded on narrow and relatively weak lime-mortar.



Figure 2 Stone Masonry Shear Wall Details
(all dimensions in mm)


Without full scale testing of part of the wall, uncertainties exist for most material parameters and indeed the modelling. Previously both macro block and brittle material masonry models were used to help characterise the walls behaviour (Brookes, Mehrkar-Asl, 1998). The focus of the work was the sensitivity of structural behaviour to equivalent friction of the mortar joints rather than the relative performance of different strengthening arrangements. In the current investigation masonry material parameters have been fixed and based on those most representative of the form of construction. Similarly macro block models have been used throughout so that the focus of the investigation is on the effectiveness of strengthening.

Although it is feasible to include all of the blocks in the macro block representation, previous work on masonry arches had shown that there was little advantage in terms of accuracy and computational efficiency. Hence, each block may in reality include several squared stones. The floor slabs and foundation were defined as separate continuums with similar perimeter frictional properties to the blocks. It has been assumed that the floors and foundation are constructed such that their global behaviour is linear elastic. For example reinforced concrete slabs.

All models were developed within a solid modelling environment using DISPLAY3 (EMRC, 1999), translated using in-house software and imported from within the ELFEN pre-processor.
 

4.3 Material Properties

Masonry material properties were based on those typical of well-built ashlar construction and using a weak calcareous sandstone laid with a lime based mortar. The strength and stiffness of the modelled blocks have been based on average composite values for the masonry treated as a whole. The contact and frictional behaviour of the mortar is modelled explicitly at the joints. Hence, it is not necessary to individually include the stiffness and strength of the joints or the stiffness and strength of the stone blocks. Table 1 summarises the material parameters used.

Table 1 Masonry Material Parameters

4.4 Loading

Hypothetical horizontal seismic loading based on a circular frequency of approximately 0.6 Hz and containing six shocks was derived and applied to all of the models as displacement functions at foundation level (Figure 4). Two magnitudes of this simplified motion have been used with peak accelerations of 0.15g and 0.3g.

Vertical accelerations were not considered due to the inherent inconsistencies in the distribution of mass that were required to simplify the problem to one of two dimensions. Horizontal motion results in a greater proportion of the effective mass being distributed to the two shear walls than that corresponding to vertical motion. All load-bearing walls resist vertical motion. However, whilst concurrent vertical motion has an influence on the overall behaviour of masonry shear walls, it is generally accepted that horizontal motion is critical.


Figure 4 Time History of Input Motions

4.5 Strengthening

Both the plain wall and wall with openings were modified to include various arrangements of strengthening. Proposed dispositions of reinforcement included horizontal through individual block courses, vertical at the ends of the wall and end diagonal. Combinations of these patterns have also been considered. The reinforcement is fully bonded along its length.

The reinforcement is introduced into the wall using the Cintec anchor system. All dispositions of reinforcement investigated used single 20mm diameter ribbed bars installed in 50mm diameter holes. The anchors are designed not to be deliberately stressed but attract load during a seismic event. The modelled anchors permit recovery of bond stresses, axial stresses and slippage along the length of the anchor at any time during loading.

Table 2 shows the various arrangements of wall and reinforcement that were the subject of the investigation. Figure 5 shows a typical model including DE boundaries (bold), finite element subdivisions (fine) and modelled reinforcement (bold).

Table 2 Wall and Strengthening Arrangements

Arrangement	No.	Description (friction with cohesion and 0.15g loading used unless otherwise indicated)
	2,12 14	Plain wall. Also plain friction and 0.3g loading investigated.
	3,11	Horizontal reinforcement at the centre of the first storey. Also plain friction investigated.
	4	Horizontal reinforcement at the centre of the second storey
	5	Horizontal reinforcement at the centre of the first storey and the centre of the second storey
	7	Diagonal reinforcement at the ends of the first storey
	8	Diagonal reinforcement at the ends of the second storey
	6	Vertical reinforcement at the ends of the wall
	13	Combination of diagonal and horizontal reinforcement. Bottom reinforcement in second layer of blocks
	19	Combination of diagonal and horizontal reinforcement. Bottom reinforcement in first layer of blocks
	20 21	Combination of diagonal, vertical and horizontal reinforcement. Bottom reinforcement in first layer of blocks. Also 0.3g loading investigated
	22	Wall with openings in each storey
	23 24	Combination of diagonal, vertical and horizontal reinforcement. Bottom reinforcement in first layer of blocks. Also 0.3g loading investigated

Figure 5 Typical Wall DE and Reinforcement Model

5 DISCUSSION OF RESULTS

Chiefly, DE simulations were carried out to show how the strength and ductility of the walls varied with reinforcement arrangement. However, for the plain wall and the wall with reinforcement arrangement No. 3 mortar with cohesion, which is considered to be most probable, and without cohesion has been considered. This limited investigation into the influence of mortar properties was carried out to allow comparisons to be made with simpler earlier work. Where walls have exhibited a high degree of seismic resistivity an additional ground motion with peak accelerations of 0.3g have been applied. The results have been illustrated by arrays of contour diagrams in which, the friction behaviour in the joints, the magnitude of ground motion and the position of reinforcement have been varied.

5.1 Unstrengthened simulations

Figure 6 illustrates compressive stresses and deformed geometry half way through the seismic event (time 5.2s) and after ground motion has ceased (time 12.5s). The shaded contours range between 2.2 N/mm2 (dark) and -0.2 N/mm2.

It has been theoretically shown that the predicted ductility of the walls is highly sensitive to the properties of the joints and the duration of the event (Brookes, Mehrkar-Asl, 1998). Furthermore, the inherited damage history from preceding shocks increases the seismic vulnerability of the wall. As part of the current investigation the influence of openings is also considered with the aim of developing an arrangement of reinforcement that works equally well for plain walls as well as those with openings. At the level of the idealised openings, representing smeared windows and doors, approximately 36% of the wall is removed. A lintel supports the wall remaining above each opening.

Halfway through event	After event
Plain wall ( No.2) - Maximum acceleration 0.15g
Plain wall (No.14) - Maximum acceleration 0.3g
Wall with openings (No.22) - Maximum acceleration 0.15g

Figure 6 Unstrengthened Walls
(Contours of principal compressive stresses)

Figure 6 shows deformed principal compressive stress contours halfway through the event and after the event for the plain wall with (No.22) and without openings (No.2). The plain wall is also loaded with a 0.3g ground motion. After the 0.15g events the plain wall remains relatively undamaged with cracking in both storeys. The cracks in the second storey result in significant dilation across the wall whereas the cracks in the first storey are less dilated and are associated with locked in stresses. Doubling the acceleration results in massive damage. With much less masonry capable of resisting shear, the wall with openings is severely damaged after three shocks and collapses before the end of the event.

The results show the general behaviour as well as the initiation of cracking. Cracking is marked by sudden stress discontinuities as well as the relative movement of blocks. This movement develops rapidly into local failure mechanisms when subjected to continued shocks in the hypothetical seismic event. The predicted failure and local collapse, reflecting modelled ductility and energy absorption, is similar to damage frequently sustained in seismic regions. Hence, these three models have been used as the benchmarks to compare the performance of various retrofitted reinforcement schemes.

5.2 Strengthened simulations

Dispositions of reinforcement that have been investigated including horizontal, vertical, diagonal at the ends of the wall and combined patterns.

Figure 7 shows stress and reinforcement slippage results obtained for horizontal reinforcement with (No.3) and without (No.11) cohesion effects included in the mortar. In the more realistic case with cohesion the introduction of reinforcement has resulted in more damage. Without cohesion but with µ=0.6 remaining, less damage occurs and good correlation is obtained with earlier simulations (Brookes, Mehrkar-Asl, 1998). The following reasons are likely for this behaviour.

i) With cohesion, less energy is dissipated across the joints resulting in less structural damping.

ii) Increased shear capacity across joints provided by cohesion helps a rocking mechanism develop in the wall. This mechanism causes sudden vertical oscillations of the wall above the reinforcement level resulting in additional damage.

Axial slippage of reinforcement in excess of 5mm indicates that bond with the masonry has failed. The graphs show progressive bond failure at both ends of the reinforcement. As with the masonry, less damage occurs to the reinforcement when no cohesion is considered.

Plain wall ( No.3) - Friction with cohesion
Halfway through event	After event
Axial slippage of reinforcement (Ordinate - slippage [mm], Abscissa - relative position)
Plain wall ( No.11) - Simple friction
Axial slippage of reinforcement (Ordinate - slippage [mm], Abscissa - relative position)

Figure 7 Horizontal reinforcement
(Contours of principal compressive stresses)

In Figure 8 the results at the end of the event are given for different reinforcement arrangements. All schemes except for No.19 and No.8 (not shown) cause either more damage to the masonry or produce higher locked in stresses compared with the similar unstrengthened wall. Arrangement No.19 has marginally improved resistance and prevented any blocks from falling away from the wall. In the case of No.13 an oscillatory mechanism develops above the first storey horizontal reinforcement causing increased damage by vertical movement.

Maximum acceleration 0.15g
Diagonal (No.7)	Vertical (No 6)
Diagonal and horizontal (No.13)	Diagonal and horizontal (No.19)

Figure 8 Diagonal, Vertical, Horizontal Reinforcement
(Contours of principal compressive stresses)

Results obtained from schemes with the most developed dispositions of reinforcement combining diagonal, vertical and horizontal bars are shown in Figure 9. Here similar schemes have been applied to both the plain wall and the wall with openings. It is similar to No.19 except that the bottom horizontal reinforcement lies in the bottom course of blocks. This helps eliminate vertical motion caused by rocking of the blocks below the reinforcement level.

The plain wall remains intact throughout the seismic event with the reinforcement controlling the development of cracking. Even under 0.3g loading, although higher locked in stresses are predicted little damage is evident. Without reinforcement, blocks left unarrested rapidly propagate failure mechanisms leading to collapse.

The wall with openings is similarly improved except that considerable cracking occurs to the first storey with appreciable dilation on one side. Although the improvement compared with the unstrengthened wall is dramatic debonding of some reinforcement has marked the beginning of a failure mechanism and continued shocks would be very damaging.

In both walls the reinforcement, apart from directly supporting the ends of the walls, has encouraged shearing at the foundation level creating an energy absorbing process as well as offering some degree of base isolation.

Maximum acceleration 0.15g	Maximum acceleration 0.3g
Diagonal, horizontal and vertical (No.20 and 21)
Diagonal, horizontal and vertical (No.23 and 24)

Figure 9 Combined reinforcement arrangements
(Contours of principal compressive stresses)

6 CONCLUSIONS

By combining the discrete element technique with a finite element formulation for reinforcement numerical models have been developed that have allowed rapid evaluation of the relative performance of reinforcement based retrofitted strengthening. This process has been illustrated using a plane shear wall subjected to simplified hypothetical ground movements. The following conclusions have been drawn.

i) The overall performance of masonry acting compositely with retrofitted reinforcement can be predicted.

ii) The sensitivity of seismic resistivity to the disposition of reinforcement has been determined thereby allowing the comparison of practically viable schemes.

iii) A disposition of reinforcement has been investigated that improves seismic resistance and that works equally well for walls with and without openings.

iv) The results have also shown that retrofitted reinforcement unless carefully placed may actually reduce seismic resistivity.

The development of strengthening schemes would have been extremely difficult and costly using conventional analysis or testing. However, to be completely confident that the results obtained by these numerical simulations are correct further work is required to verify the simulations against observed behaviour of masonry structures subjected to seismic loading.

7 MONUMENTS IN CAIRO

Although numerical modelling of masonry is being used to develop retrofitted strengthening systems often the representation of these systems and respective building parts are necessarily idealised. In practice a degree of pragmatism and engineering judgment is required especially where localised repairs are involved. The recent experiences in Egypt described below are a case in point.

7.1 Background

Egypt has a long and relatively well-documented earthquake history. The country was home to some of the earliest known civilisations, as a consequence of which records of earthquake damage extend back at least as far as 2800BC. In recent years significant earthquake events have occurred once every 30-50 years.

There are over six hundred monuments in Cairo which have earned the city its title of a "City of Human Heritage" by UNESCO. Many of these monuments have endured at least seven earthquakes exceeding five on the Richter scale.

Cairo has not only suffered from the effects of an earthquake but has also suffered from the problems of a leaking drainage system and the contaminated perched water table has caused the rapid degradation of the masonry structures. Thus before the 1992 earthquake, the monuments of Cairo had been suffering from serious neglect and decay.

According to initial evaluations, the 1992 earthquake occurred approximately 30km to the south-west of Cairo, was of only medium magnitude 5.5-5.9 on the Richter scale and was of shallow focal depth. The city stands on very deep alluvial sands, silts and clays forming either side of the Nile delta. The peak ground acceleration was estimated at 10% gravity. The nature of the quake with its relatively high frequency motion meant that damage to low structures of up to 5 storeys was more likely.

The Egyptian Antiquities Organisation reported that 150 of the 600 monuments had been damaged by the earthquake. Typical of the damage incurred was the damage to the Mosque of al Ghuri.

7.2 The Mosque of al Ghuri

The Mosque of al Ghuri, monument number 189 dates from 909AH or1504AD and is shown in Figure 10.


Figure 10 The Mosque of al Ghuri


The funerary complex of Sultan al Ghuri is situated in the Fahhamin quarter of old Cairo in al Muizz Street. On the west side of the street there is a kanqah and mausoleum as well as a Sabil Kuttab. The minaret is a four storied rectangular structure approximately 50 metres high.

An inspection of the madrasa reveals some very severe long-standing problems. The floor of the mosque undulates dramatically, evidence of very significant foundation problems of the masonry vaults supporting the floor. Attempts have been made in the past to underpin the sleeper walls supporting the vaults. Those have failed. All of the walls of the mosque exhibit very severe fractures. These are a function of the problems brought about by the rising contaminated ground water table as well as the shaking due to the earthquake.

The next result of the above is that the mosque of al Ghuri is now in a very delicate state of equilibrium. Despite having survived for nearly 500 years, the toll of a rising water table, earthquakes and neglect had brought this structure to the point of collapse. Urgent measures were required to reintroduce some structural strengths and stiffness into the building.

In summary, typical damage evident at al Ghuri included:-

• Lack of adequate anchorage between the walls resulting in vertical cracks and separation in the corners at the connection between walls at right angles - 'Out of plane bending'.
   
• Spreading of the sahn arches of the court and dropping of voussoirs.
   
• Settlement of the floors.
   
• "Pounding" adjacent to the minaret.
   
• Failure of roof to wall connections.
   
• Loss of integrity of the wall construction.

7.3 Repair

Several monuments did not survive the earthquake because of their decayed conditions. It was important therefore to use appropriate repair techniques to ensure continued survival.

The Cintec masonry anchor, described above, was introduced extensively at al Ghuri and used to restore structural integrity and provide additional ductility to the building. Figure 11 illustrates a typical anchor application in an arched facade. The anchors, which were installed, were up to 12 metres long and served to stiffen each individual wall. The walls of al Ghuri are generally of two facing skins infilled with a core of rubble. The large arched openings in the mosque are particular points of weakness in the structure. Longitudinal ties in each of the stone facings of the wall above the arch would serve to resist the thrusts naturally produced by the arch as well as serving to assist the walls to resist the next earthquake. In addition to longitudinal ties, transverse ties of length equal to the thickness of the wall acted as consolidation anchors. The Cintec ties were also intended to be used to tie the roof structure to the perimeter walls and create a diaphragm action.

The intention of the designs for the anchor is:-

• Restore integrity of construction of wall thickness.
   
• Resist "lateral toppling" out of plane bending by tying walls at right angles to each other.
   
• Connection of roofs and floors to walls.
   
• Repair of arches.
   
• Pinning of individual decorative voussoir stones.

Figure 11 Anchor Arrangement in Arched Facade

Of particular concern to the Egyptian Antiquities Organisation was the possible damage which might be caused to the decoration of the soffits of the voussoirs stones of the arches to the court. This concern was overcome by using a thin walled core drill to extract a cover pellet first and then the anchor was installed and the pellet re-fixed.

The repair works were recently completed and the mosque re-opened.

8 REFERENCES

Brookes, C.L. Mehrkar-Asl, S. 1998. Numerical modelling of masonry using discrete elements. Proceedings of the 6th SECED Conference on Seismic Design Practice into the Next Century, Oxford.
Brookes, C.L. Tilly G P. 1999. Novell method of strengthening masonry arch bridges. Structural Faults and Repair - 99, 8th International Conference, London.
Cintec International Limited. 2000. The Cintec anchor system. Newport, Wales, UK. 
Cundall, P.A. 1971. A computer model for simulating progressive, large scale movement in blocky systems. Proceedings: Symp. ISRM, Nancy, France, Vol. 2, 129-136.
Engineering and Mechanics Research Corporation 1999. DISPLAY3 version 9.0 production. Troy, Michigan, USA.
Key, D. E. 1998. Parameter influences on the out of plane seismic collapse of unreinforced masonry panels. Proceedings of the 6th SECED Conference on Seismic Design Practice into the Next Century, Oxford.
Munjiza A., Owen, D.R.J., Bicanic, N. 1995. A combined finite/discrete element method in transient dynamics of fracturing solids. Engineering computations, 12, 145-174.
Rockfield Software Limited 1998. ELFEN version 2.8.0a_MT Archtec version. University of Wales Swansea.
Roberts, D.P. 1999. Finite element modelling of rockbolts and reinforcing elements. PhD Thesis, University of Wales Swansea.


Mr Carl Lester Brookes
Gifford and Partners, Carlton House, Ringwood Road, Woodlands, Southampton, SO40 7HT

Tel : 023 8081 7529
Fax: 023 8081 7600
Email : carl.brookes@gifford-consulting.co.uk




CURRICULUM VITAE CARL BROOKES 


Full Name
Carl Lester Brookes

Qualifications
BSc(Hons)

Profession
Engineering Analyst

Present
Position
Associate with Gifford and Partners

Outline
Carl Brookes has over 15 years' experience in the analysis of Civil, Building and Marine structures and is responsible for the Analysis Team within Engineering Studies. Work has included design, assessment and simulation which has often involved structural analysis and support to major projects. Considerable experience has been gained in the analysis of bridges ranging from ancient masonry arches to highly technical lightweight military structures. Much expertise has been gained using industry standard software including NISA/DISPLAY, LUSAS/MYSTRO and ELFEN.





Mr Richard John Rupert Swift
Gifford and Partners, Carlton House, Ringwood Road, Woodlands, Southampton, SO40 7HT

Tel: 023 8081 7523
Fax: 023 8081 7600
Email: richard.swift@gifford-consulting.co.uk




CURRICULUM VITAE RICHARD SWIFT 


Full Name
Richard John Rupert Swift

Qualifications
CEng MICE Grad Dipl Cons AA

Profession
Civil and Structural Engineer

Present
Position
Technical Director with Gifford and Partners

Outline
Richard Swift joined Harris and Sutherland after graduation and then undertook an MSc in Advanced Structural Engineering at Southampton University. In 1976 he joined Gifford and Partners and pursued an interest in the conservation of historic buildings. This led to gaining a diploma from the Architectural Association in 1992 after two years study. Richard is the Technical Director responsible for the historic buildings structure section at Gifford and Partners. He has been a consultant for UNESCO and has presented several papers at conferences in the UK and abroad. He is also an external reviewer for the Getty Grant Project.

Major recent projects have included the repair of the fire damage at Windsor Castle, the Great Courtyard project at Somerset House and the Queen's new gallery at Buckingham Palace. Other works have included the repairs to the curtain wall at Rochester Castle and structural repairs to the chapel and west stables at Petworth House for the National Trust. Overseas projects include the seismic retro-filling of some of the mosques of Cairo seismic strengthening to masonry buildings in the USA.


C L Brookes and R J R Swift: Numerical Modelling of Masonry to Explore the Performance of Anchor Based Repair Systems and the Repair of Monuments in Cairo