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UNIVERSITY OF GREENWICH

DEPARTMENT OF CIVIL ENGINEERING

BRIDGE DESIGN AND ASSESSMENT

BRIGDE ASSESSMENT ACCORDING TO BA 16/97 AND BD 21/01

BY

BELLO IDRIS

MSC CIVIL


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CONTENTS LIST

y Introductiony MEXE Methody Mechanism Methody Pippard-MEXE Methody Conclusiony References


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Introduction

Masonry bridges have been for a long time, in the U.K a choice option for Engineers settingout to achieve the task of building a bridge. Some 40,000

(1)masonry arch bridges are in

continual use by highways, railways and waterways, most of which are over 100 years old.

Due to the availability of steel, masonry bridges have been replaced by metal bridges for

obvious reasons of better load bearing capacity and aesthetics.

From January 1999 the European Commission Directive 96/58/EEC requires (2) all trunk road

bridges to be capable of 40 tonne axle loadings. This is a prerequisite so as to ensure that a

minimum safety zone is achieved to avoid any unforeseen circumstances.

The purpose of this project is to investigate the procedures of bridge assessment by methods

following the guidelines set out in the BA 16/97 and BD 21/01 The Assessment of

Highways Bridges and Structures.

MEXE Method

The most widely used method for masonry arch assessment is the Military Engineering

Experimental Establishment (MEXE) method that was evolved from Pippards 1930s(3)

work

during the 1930s. Pippard started off from his observation that a slight spread of the

abutments would cause the formation of hinges, or pins, at the abutments. His analysis was an

elastic one of the parabolic two-pin arch with live loads acting at the centre. He derived two

expressions for the safe live load on a bridge, W1 and W2.

During the war Engineers found(4)

that Pippards expression ofW2 can be simplified into anomogram so that a provisional axle loading can be read off immediately. It is then

multiplied by a number of modifying factors, to give the final axle loading. However slight

alterations in the depth of fill alter the provisional axle loading greatly; thus it is the problem

with the MEXE method that requires a need for a better method of assessment.

During the course of this project we will examine the MEXE method as presented by the

ASSARC software made available via the web.

The results, etc obtained are displayed below.

Comment [B1]: ref

Comment [B2]: Ref

Comment [B3]: ref

Comment [B4]: ref


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--------------------------------------------------------------------------

* ASSARC 10.0 *

Assessment of Masonry Arch Bridges (c) MSW SoftwareMEXE Assessment to BA16/97 Date: 29/11/2010

--------------------------------------------------------------------------

Bridge Name - 12.75 MEXE

Bridge Reference No - BELLO IDRIS

ARCH DIMENSIONS (Segmental):

Clear span parallel to the axis of the arch - 12.75 m

Rise of the arch barrel at the crown - 4.250 m

Rise of the arch barrel at the quarter points - 3.470 m

Barrel thickness (adjacent to the keystone) - 0.280 m

Average depth of fill over the crown (incl surfacing)-

0.200 m

Fig 1 Arch Barrel Properties

MEXE AXLE LOADS:

Provisional - 6.231 tonnes

Modified - 2.996 tonnes

CENTRIFUGAL EFFECTS: non-applicable


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Fig 2 Arch Barrel Shape

MODIFYING FACTORS:

Span/Rise Factor - 1.00

Profile Factor - 0.83

Barrel Factor - 1.20

Fill Factor - 0.70

Material Factor - 0.99

Joint Width Factor - 0.90

Joint Mortar Factor - 0.90

Fig 3 MEXE Results


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Joint Depth Factor - 0.90

Joint Factor - 0.73

Condition Factor-

0.80

ASSARC 10.0 Date: 29/11/2010

RESULTS OF MEXE ASSESSMENT to BA16/97

--------------------------------------------------------------------------

MAXIMUM PERMITTED GROSS AXLE AND BOGIE WEIGHTS:

< assuming no axle lift-off >

per Axle per Bogie

(tonnes) (tonnes)

Single axle 5.10 5.10

Double axle bogie 3.00 5.99Triple axle bogie - 2.6m spread 2.25 6.74

less than 40 tonne requirements

WEIGHT RESTRICTIONS:

A 3 TONNE WEIGHT RESTRICTION IS REQUIRED

Bridge must be CLOSED to Fire Engines

Fig 4 MEXE Weight Restrictions


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The Mechanism Analysis

The method depends on the formation of four hinges to cause failure of the arch. The shapeof masonry arch is a statically indeterminate one which turns determinate when the hinges

form in arch ring due to failure loads(5)

. The arch is assumed to be rigid and loads are moved

across the span to find the collapse load for that load position. Once the locations have been

determined the set of equations from below can be used to determine the thickness of arch

ring (d) required for the given loading. The failure mode of the arch is the set of hinge

positions that require the lowest collapse load.

B

C

A

D

Fig 5 Profile of Beam Mechanism

The equations are formed merely by taking moments and are shown below:-

Again the assessment for this method was carried out using the ASSARC software with the

results presented below.


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--------------------------------------------------------------------------

* ASSARC 10.0 *

Assessment of Masonry Arch Bridges (c) MSW Software

Modified Mechanism Analysis to BD21/01 Date: 29/11/2010

--------------------------------------------------------------------------

Bridge Name - MECHANISM 12.75

Bridge Reference No - BELLO

BARREL PROFILE - Segmental (Single Span)

Clear Span - 12.75 m

Rise at Crown - 4.250 m

Barrel Thickness-

Crown-

0.280 m- Springings - 0.280 m

Depth to mortar in Barrel - 0.012 m

Height of effective haunches - 0.000 m

Width of carriageway - 7.200 m

Min distance from carriageway

to edge of barrel - 0.800 m

No of Barrel Segments used in Analysis - 30

Characteristic Strength of Masonry of Arch Barrel - 8.60 N/sq mm

Fig 6 Arch Barrel with Surface Profile


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ROAD SURFACE PROFILE: Horizontal.

Depth of Fill (including surfacing) at the Crown - 0.200 m

Depth of surfacing-

0.100 m

CENTRIFUGAL EFFECTS: non-applicable

DEAD LOAD PARAMETERS:

Assumed Unit Weights x partial load factors:

Barrel - 21.00 x 1.15 kN/cu m

Fill - 19.00 x 1.20 kN/cu m

Surfacing - 23.00 x 1.75 kN/cu m

Fig 7 Material Properties

LATERAL EARTH PRESSURE ON BARREL:

A Rankine-Bell Lateral Pressure Distribution is assumed

Angle of internal friction - 35.00 deg

Cohesion - 0.00 kN/sq m

Coefficient of Active Pressure, Ka - 0.27

Coefficient of Passive Pressure, Kp-

3.00

Coefficient of Pressure at Rest, Ko - 0.43

Partial factor on active pressure effects - 1.50

Partial factor on passive pressure effects - 1.50


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ASSARC 10.0 Date: 29/11/2010

RESULTS OF ANALYSIS FOR AW BOGIES to BD21/01

--------------------------------------------------------------------------

Assumptions:

Partial Factor for Live Load (crit. axle) - 3.40

(other axles) - 1.90

Partial Factor for Material Strength - 1.00

Condition Factor - 0.80

Centrifugal Effects Factor - 1.00

Assessment Load Effects Factor - 1.00

Vehicle width - 2.50 m

Width of Load Contact Area - 300.0 mm

Load Dispersion (Horizontal: Vertical) - 1 : 2

Fig 8 Lateral Pressure Distribution


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MAXIMUM PERMITTED GROSS AXLE AND BOGIE WEIGHTS:

(incorporating axle weight conversion factors in accordance with

BD21/01 Table 6.2)

< assuming no axle lift-off >

per Axle per Bogie

(tonnes) (tonnes)

Single axle 6.17 6.17

Double axle bogie - 1.0m spread 4.28 8.55



Triple axle bogie - 2.6m spread 4.20 12.60

Triple axle bogie - 2.8m spread 4.39 13.16

* less than 40 tonne requirements

Fig 9 Mechanism Results

WEIGHT RESTRICTIONS:

A 7.5 TONNE WEIGHT RESTRICTION IS REQUIRED

Fire Engines are restricted to Group 2


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Fig 10 Mechanism Weight Restriction


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Pippard-MEXE

This method utilises the sample load bearing capacity calculations inserted in the Annex F

(6)

of the BD 16/97. The method allows for calculation of single axle and double axle allowable

loads using information obtained from a plane frame computer program. Such programs

analyse structural dimensions in an elastic non-linear manner to obtain values for bending

moments and axial forces as well as shear forces if the need be. In this case, the Structural

Analysis and Design (S.A.N.D) software was used for analysis of the frame. A load of

1000kg was applied across the span in order to gain a behaviour pattern of the arch. The co-

ordinates used were those obtained from the ASSARC software in order to exemplify the

similarities in physical and geometric properties of the arch.

These barrel co-ordinates are shown below along with the results of this method as calculated

by hand.

Comment [DHL5]:ref


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Single

Double

1.8m

Triple

2.6m

Per Axle

(tonnes)

Per

Bogie

(tonnes)

Per Axle

(tonnes)

Per

Bogie

(tonnes)

Per Axle

(tonnes)

Per

Bogie

(tonnes)

MEXE 5.1 5.1 3 5.99 2.25 6.74

Mechanism 6.17 6.17 4.91 9.81 4.2 12.6

Pippard-

MEXE 133.5 78.5

As seen from the results, errors from the S.A.N.D program calculations for the live load were

incorrect. Therefore as a result the values obtained for Pippard-MEXE calculations were

wrong. This notwithstanding, I was able to verify that the results for the method in question

are less than values of the ASSARC based methods which goes to show that the values in

terms of real work calculations have to be regarded closely.

Conclusion

The condition of highway structures is determined by visual inspection. There are five main

types of inspection which are undertaken at different frequencies. These inspections cover a

range of detail, from a cursory check for obvious defects, through to a close examination of

particular areas or defects causing concern. The quality of data collected depends on theability of inspectors to observe and accurately record details on visible defects.

This could be affected by many factors, such as the environmental conditions, and the

knowledge and experience of the inspectors.

Strengthening methods that have been applied successfully and found to be useful include

using(7)

plastic based fibres as a reinforcement tool on the intrados of the arch. Also, increasing the width of concrete beams by placing additional concrete and reinforcement can

be undertaken to enhance the load bearing capacity of bridges.


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REFERENCES

1.

Ashurst, D. AnAssessmentof RepairandStrengtheningTechniquesContractor Report, 19922. BA 16/97, TheAssessmentof Highway BridgesandStructures Highways Agency, 19973. Thorne, A.Assessmentofa Masonry Bridge University of Exeter, 20034. Wang, J.MEXE MethodForMasonryArch BridgeAssessment, University of the West

England, 2010

5. Thorne, A.Assessmentofa Masonry Bridge University of Exeter, 20036. 16/97, TheAssessmentof Highway BridgesandStructures Highways Agency, 19977. Task Group Report,Enhancingthe Capacity of Concrete Bridges , Cement & Concrete

Industry Publication, 2008

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