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Table of Contents

Introduction ....................................................................................................................................... 2

Existing methods of strengthening .................................................................................................... 3

The Archtec Strengthening System .................................................................................................... 5

Numerical Simulation ......................................................................................................................... 6

Archtec Project Methodology .......................................................................................................... 12

Visual, Environmental and Heritage Benefits .................................................................................. 14

Generic Applications ........................................................................................................................ 15

Conclusion ........................................................................................................................................ 18

Acknowledgements .......................................................................................................................... 18

References ........................................................................................................................................ 19

Table of figures ................................................................................................................................. 20



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Assessment and repair of masonry buildings

Introduction

Around the world there are many masonry arch bridges in daily use on highways, railways

and canals. Most are over 100 years old, some are over 500 years old. Many of these pre-

engineered structures are inadequate for modern live loading requirements.

Fig. 1 shows Ambersham Bridge, a three span masonry arch bridge located in a rural area

of West Sussex, England.

Figure 1: Ambersham Bridge, West Sussex, England



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In 1999, under new European Union directives, the maximum allowable gross vehicle

weight for UK roads was increased from 38 tons to 44 tons, with the maximum axle load

increasing from 10.5 tons to 11.5 tons. This called into question the ability of many

existing bridges to carry the increased load requirement. The cost of replacing in-service

bridges is often prohibitively expensive and the cost of a temporary bridge and disruption

to traffic also needs to be taken into account. Many masonry arch bridges are listed

historical monuments, valued tourist attractions or local landmarks. Strengthening the

bridge is often the only viable solution.

Existing methods of strengthening

A variety of strengthening methods are available. These vary in effectiveness and each

has advantages and disadvantages.

i. Saddli ng

One of the simplest and most popular of the traditional methods is saddling. This involves

removal of the fill to expose the extrados of the barrel. A reinforced or mass concrete flat

or curved slab is subsequently cast in place over the original barrel.

Figure 2: Woolbeding Bridge, England Figure 3: A lined bridge in North Lincolnshire, England



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Fig. 2 shows Woolbeding Bridge with the fill excavated prior to installation of a reinforced

concrete saddle. Whilst saddling will undoubtedly increase the capacity of the bridge,

with minimal change to the external appearance of the bridge, it is expensive and will

cause considerable disruption to traffic and buried services which may be located within

the fill crossing the bridge. The bridge is also in a temporarily vulnerable state once the fill

has been removed, unless the barrel is supported from underneath which can be a costly

process.

ii . Spra yed Co ncrete

Another traditional method is the use of sprayed concrete applied to the intrados of the

arch. This may be used in conjunction with a reinforcing mesh. Whilst this negates the

need for drilling, the intrados is the part of the barrel exposed to weathering, resulting in

friability. Applying sprayed concrete can cause moisture to be locked into the barrel.

Other problems include poor composite behavior and incompatible materials. Fig. 3

shows a photograph of a lined bridge in North Lincolnshire, England. Sprayed concrete

has been applied in conjunction with a corrugated metal lining.

iii. Surface Reinf orcement

There are proprietary systems available using a network of steel bars located in slots cut

into the intrados and bonded using special adhesives. Such systems have been shown to

increase the strength of the bridge. [1] However, access to the arch intrados is not always

easy or possible.

More importantly, the application of sprayed concrete and/or externally bonded or

slotted reinforcement will have a detrimental effect on the appearance of the intrados.

This is un-desirable in many situations and unacceptable for many structures of historic

importance.



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The Archtec Strengthening System

The Archtec strengthening system involves drilling through the road surface to install

internal reinforcement into the arch, without causing any change to the existing

appearance of the bridge and with minimal environmental disruption.

i. Retrofitted Reinforcement

A proprietary system known as the Cintec anchor is used to enable reinforcement to be

accurately retrofitted. The reinforcement anchor consists of three main components, a

stainless steel bar, a fabric sock and a cementations grout. The stainless steel bar provides

increased tensile and compressive capacity. The woven polyester sock permits sufficient

leakage of grout during inflation to develop a chemical and mechanical bond with the

surrounding masonry whilst protecting the masonry from being displaced or otherwise

damaged by the pressurized grouting and limits the volume of grout escaping into the

surrounding masonry. The grout used is similar to Portland cement based products. It is

easily pumpable with good strength characteristics and no shrinkage. The use of stainless

steel and a high performance grout give enhanced durability and as the anchors are

installed internally in the arch barrel, no maintenance is required. Fig. 4 shows

Typical anchor positions for a single span masonry arch bridge and a view of the typical

method of installation, in this photograph of drilling being carried out at Ceredigion

Bridge, Wales.



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Figure 4 Typical anchor positions and installation of reinforcement from the road

surface

ii . Stru ctural Analysi s

The performance of the existing structure and the strength enhancement provided by

reinforcement is predicted by numerical simulation using the DE method. This calculates

the strength deficiency and targets the internal reinforcement where required.

Numerical Simulation

i. DE Technique

Conventional assessment of strength is either based on semi-empirical methods such as

the MEXE method [2] [3] or mechanism analysis. Whilst these methods are useful for

initial assessments and quick to use, they can only be applied to certain arch

configurations, for example single spans and give no information on displacements.

Numerical simulation by the DE method permits a more generic approach to predicting

the behavior of masonry.

The accurate representation of masonry for structural analysis is a complex problem. In

addition to the materially and geometrically non-linear behavior of the masonry blocks



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themselves, contact gap-friction effects along joints and the evolution of further joints

due to fracture are important. The ability to model post failure behavior in order to verify

analysis against physical tests is also useful. Many of these features can be modeled to a

degree using continuum finite element methods with sophisticated material models and

arrays of gap elements. Such methods require pre-defined contact definition and post

failure behavior cannot be observed. More natural approach to the problem is to use DE.

The Distinct method was first developed in the late 1960’s to address geotechnical and

granular flow applications. [4] The basic concept is that separate parts are modeled,

possibly connected at boundaries, with friction, cohesive and adhesive contact. Initially

the interaction of rigid parts was considered but later developments led to what is now

termed the DE method where deformation and fracture can be included. [5] The

computer program, Elfen, by Rockfield Software Ltd. based at the University of Wales,

Swansea, is used for the modeling of masonry for Archtec.[6]

Retrofitted reinforcement is modeled using the finite element technique, modeling the

reinforcement independently of the masonry using a partially constrained spar

formulation. [7] Non-linear bond elements are used to connect the reinforcement and

masonry models. Modeling of reinforcement is completely automated with no

requirement for consistent mesh topology. This allows for rapid modification to the

reinforcement geometry. Non-linear material characteristics of the steel can be

considered as well as the non-linear shear coupling of the bond between anchor and

masonry.



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For application to masonry arch bridges, typically discrete parts using non-linear material

properties where appropriate and allowing non-linear geometric deformations are used

to model the abutments, piers, fill, any backing, and representative vehicle axles. The

masonry of the arch barrel is modeled by a number of individual blocks, each normally

representing a number of real masonry parts. Multiple rings can be modeled where

appropriate, allowing the effect of features such as existing ring separation to be taken

into account. Multi-span structures can also be modeled and existing defects such as

mortar loss and weathering can be included. The forces between all components parts

are automatically calculated, both under initial dead loads and then required traversing

live load conditions. In this manner the complex non-linear behavior of the masonry is

Accurately represented at a fundamental level. Fig. 5 shows DE and FE meshes used for

the analysis of a typical two span, multiple ring arch bridge.

Figure 5 Discrete and finite element meshes for a two span, multiple ring bridge



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ii . Verification

Both the analysis technique and the efficacy of the strengthening system have been

verified by full-scale tests.

a. Un-strengthened

The analysis technique has been used to model both the TRL collapse test on Torksey

Bridge in Lincolnshire [8] and also a further TRL test conducted on a full scale model

bridge, built and tested under laboratory conditions. [1] The TRL laboratory test

considered a 5 meter span, 3 ring brick arch. A line load was applied above the quarter

point of the bridge and a test to failure carried out under displacement control. Fig. 6

shows the progressive collapse predicted by the computer analysis of the un-

strengthened bridge.

Figure 6 Simulation of the TRL laboratory test of an un-strengthened bridge

The calculated failure load using the DE method was 18.6 tons which compares well with

the actual failure load of 20 tons.



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b. Stre ng then ed

Two further identical TRL bridges, built in the same manner were strengthened using the

Archtec method and tested to destruction in the same way. Both tests considered

specified arrangements of anchors installed from the barrel extrados.

Figure 7 Anchor arrangement installed in the 1st strengthened TRL test

Fig. 7 shows a diagrammatic cross section of the model bridge used in the 1st

strengthened TRL test including the anchor positions. The second test considered a

modified arrangement of anchors. Fig. 8 shows the collapse sequence for the first of the

strengthened bridges.

Figure 8 Simulation of the TRL laboratory test of the 1st Archtec strengthened TRL

bridge

The calculated failure load using the DE method was 42 tons which again compares well

with the actual failure load of 41.5 tons.



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c. Conclusions

In all cases the calculated failure load using the DE method was within 10% of the actual

failure load. Bearing in mind normal variations in material properties and assumptions

made in the numerical modeling, this degree of accuracy represents an excellent level of

correlation, especially when compared with alternative methods of masonry arch bridge

assessment.

Comparison of the un-strengthened TRL test and the first strengthened TRL test showed

a 105% increase in strength provided by the Archtec system. Fig. 9 shows graphs of

results obtained from the tests and simulations of the verification tests. The graph on the

left shows a comparison between the un-strengthened test and two Archtec

strengthened arch tests. The variation in vertical intrados displacement at the quarter

point of the intrados, with vertical load applied (per meter width of the bridge) is plotted.

The graph on the right shows a comparison of the first strengthened TRL test and the

corresponding computer simulation results. This shows the close correlation

obtained between the computer simulation and the actual test.

Figure 9 Graphs of results from verification work



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Figure 10 Thurstaston Bridge, un-strengthened and strengthened analysis results

A significant number of bridges have been found to be strong enough when using this

advanced analysis technique and certification to 40/44 tons gross vehicle weight has been

possible without the need for strengthening.

iii. Design Process

It is widely recognized that most masonry arch bridges fail in a manner that shows distinct

hinge points. Targeting the reinforcement to where the hinges are predicted to form

reduces the material used and thus the material cost of the system. Reinforcement is

placed in a longitudinal direction in the arch barrel, approximately tangential to the

curvature of the arch, generally around the quarter point, although the exact positioning

to make optimal use of the strengthening is determined using the DE analysis method

described. Depending on other specific defects observed during the visual inspection,

reinforcement may also be installed in a transverse direction through the elevation of the

arch barrel. In bridges where ring separation is a problem, radial anchors may also be

appropriate.



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iv . Installation

The main longitudinal anchors are most often installed from the road surface, although

installation from below or from adjacent arches in a multi-span structure can also be

used. Precise setting out data is produced from the same three-dimensional CAD model

derived from the survey data used previously to determine the analysis section geometry.

Diamond tipped coring technology is used to drill the precisely placed holes.

Visual, Environmental and Heritage Benefits

Archtec has many visual, environmental and heritage benefits. The Archtec system

involves minimal structural intervention. Strengthening is targeted only where it is

required, utilizing the existing structure to its maximum potential, thus reducing the

amount of material resources required. The system needs no maintenance once installed.

Disruption to traffic is minimized and no heavy construction traffic is required. The use of

the fabric sock prevents grout leakage, especially important in installation over water

courses. The system leaves no external visual trace.

Figure 11 Examples of bridges assessed or strengthened using the Archtec system

Fig. 11 shows pictures of three bridge which have been assessed or strengthened using

the Archtec system.



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Generic Applications

The engineering principles applied to the masonry arch bridge in the preceding part of

this paper can be equally applied to other classes of structure subject to other loading

effects. Numerical analysis using the DE technique is a generic tool and the Cintec anchor

can be used in a wide range of masonry repair and strengthening applications.

i. Seismic As sessmen t of Buildings

The performance of masonry structures to seismic loading has also been considered. The

same numerical analysis technique has been employed to investigate the performance of

masonry shear walls, both with and without masonry reinforcement. [9]

Seismic loading was modeled by the application of a sinusoidal ground movement. The

behavior of the wall was investigated and the benefits of strengthening the wall using

passively stressed reinforcement were assessed. The reinforcement is again represented

explicitly in the analysis allowing direct assessment of damage and identifying potential

failure mechanisms. Work so far has shown that unless carefully placed, reinforcement

may actually reduce seismic resistivity. As this technique develops it is hoped that the

performance of whole buildings can be checked before strengthening systems have been

installed. Fig. 12 shows diagrams of un-strengthened and strengthened masonry wall

panels, used to investigate the efficacy of various geometries of strengthening.



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Un-strengthened Wall- Maximum acceleration 0.15g

Strengthened Wall using diagonal, horizontal and vertical anchors

Maximum acceleration 0.15g Maximum acceleration 0.3g

Figure 12 Diagrams of the un-strengthened and strengthened masonry walls and their

behavior under seismic loading

Analyses of more complicated building configurations have also been investigated. Fig. 13

shows a diagram from an analysis of the Al-Ghuri mosque in Egypt. The analysis

considered a two dimensional model of the mosque subjected to a sinusoidal horizontal

ground movement. The actual mosque had shown signs of distress from previous ground

movements, mainly a large crack between the main building and the bell tower. This is

also clearly shown in the results of the DE analysis.



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Figure 13 Al-Ghuri mosque, Egypt

Figure 14 Discrete Element Analyses of a masonry wall without retrofitted

reinforcement



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ii . Blast Assessment of Building Facades

The discrete element formulation can also be employed to investigate fracture of

materials and the explicit method is suited to simulation of highly dynamic phenomena.

The analysis technique is therefore ideal for use in assessing masonry structures

subjected to blast loading. Analyses have been carried out to investigate the effect of

blast event loading on masonry walls both with and without retrofitted reinforcement.

Verification against full scale tests is ongoing for this application. Fig. 14 shows the

deformation of a masonry wall, without retrofitted reinforcement, following a blast

event.

Conclusion

Archtec is a rigorously engineered and thoroughly verified method of strengthening

masonry arch bridges that has been proved to be cost effective and environmentally

sympathetic. To date, over 70 bridges in the UK, United States of America and Australia

have been successfully strengthened using this technique. The accurate simulation of the

complex behavior of masonry arch bridges used for assessment has enabled a number of

existing structures to be given improved load ratings without the need for strengthening.

The innovative anchor design and DE method can also be applied to other classes of

structure including seismic and blast assessments and research is ongoing.

Acknowledgements

The Archtec reinforcement system is supplied by Cintec International Limited. ELFEN

software is by Rockfield Software Limited.



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References

[1] Sumon S.K., New Reinforcing Systems for Masonry Arch Rail Bridges, Paper presented

at Railway Engineering 1999, Second International Conferenc.

[2] Department of Transport, The Assessment of highway bridges and structures,

Departmental Standard BD21/97, London, HMSO, London, 1997.

[3] Department of Transport, the Assessment of highway bridges and structures, Advice

Note BA21/97, London, HMSO, London, 1997.

[4] Cundall P.A., A computer model for simulating progressive, large scale movement in

blocky systems, Proc. Symp. ISRM, Nancy, France, Vol. 2, 129-136 1971.

[5] Munjiza A., Owen, D.R.J., Bicanic, N., 1995, A combined finite/discrete element

method in transient dynamics of fracturing solids. Engineering computations, 1, 145-175.

[6] Owen DRJ, Peric D, Petrinic N, Brookes CL and James PJ, Finite/Discrete Element

Models for Assessment and Repair of Masonry Structures, Second International Arch

Bridge Conference, Venice, Italy, October 1998.

[7] Roberts, D.P., 1999. Finite element modelling of rockbolts and reinforcing elements,

PhD Thesis, University of Wales, Swansea.

[8] Page J., (editor), Masonry Arch Bridges – State of the art review, TRLReport, HMSO

Publications, 1993.

[9] Brookes CL and Mehrkar-Asl, Numerical Modeling of Masonry Using Discrete Elements

-Seismic Design Practice into the Next Century, Society for Earthquake and Civil

Engineering Dynamics, London, March 1998, pp 131-138.



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Table of figures

FIGURE 1: AMBERSHAM BRIDGE , W EST SUSSEX , E NGLAND................................................................. 2

FIGURE 2: W OOLBEDING BRIDGE , E NGLAND…………………………………………………………………………….....3

FIGURE 3: A LINED BRIDGE IN NORTH LINCOLNSHIRE , E NGLAND........................................................... 3

FIGURE 4: T YPICAL ANCHOR POSITIONS AND INSTALLATION OF REINFORCEMENT FROM THE ROAD SURFACE ... 6

FIGURE 5: DISCRETE AND FINITE ELEMENT MESHES FOR A TWO SPAN , MULTIPLE RING BRIDGE ..................... 8

FIGURE 6: SIMULATION OF THE TRL LABORATORY TEST OF AN UN-STRENGTHENED BRIDGE ........................ 9

FIGURE 7: ANCHOR ARRANGEMENT INSTALLED IN THE 1ST STRENGTHENED TRL TEST ............................. 10

FIGURE 8: SIMULATION OF THE TRL LABORATORY TEST OF THE 1ST ARCHTEC STRENGTHENED TRL BRIDGE . 10

FIGURE 9: GRAPHS OF RESULTS FROM VERIFICATION WORK ............................................................... 11

FIGURE 10: T HURSTASTON BRIDGE , UN-STRENGTHENED AND STRENGTHENED ANALYSIS RESULTS ............. 13

FIGURE 11: E XAMPLES OF BRIDGES ASSESSED OR STRENGTHENED USING THE ARCHTEC SYSTEM ................ 14

FIGURE 12: DIAGRAMS OF THE UN-STRENGTHENED AND STRENGTHENED MASONRY WALLS AND THEIR

BEHAVIOR UNDER SEISMIC LOADING....................................................................................... 16

FIGURE 13: AL-GHURI MOSQUE , E GYPT ........................................................................................ 17

FIGURE 14: DISCRETE E LEMENT ANALYSES OF A MASONRY WALL WITHOUT RETROFITTED REINFORCEMENT . 17

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