Transcript Masonry Arch Bridges: Condition Appraisal and Remedial

Masonry Arch Bridges:
Condition Appraisal and Remedial
Treatment
CIRIA RP692
- Examples -
Assessment
Level of Analysis
Elements of Analysis
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Level 1
Basic Analysis
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Level 2
Detailed Analysis
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Level 3
Special Analysis
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Semi-empirical methods should only be considered as part of the appraisal by inspection
where the owner considers this procedure as appropriate.
Basic 2D limit analysis methods should be used to assess all structures except
significantly skewed bridges, long spans, bridges with unusual geometries and important
structures.
The bridges excluded from the previous group and those failing its assessment should be
analysed using solid mechanics methods.
These analyses should be adapted to the available bridge data and combined with site
investigations and monitoring as appropriate, if more refined analyses are required to
demonstrate the adequacy of the structure to fulfil its purpose.
The use of characteristic or worst credible strengths of materials may be used, based on
test results from samples taken from the structure.
The level of refinement achieved before it is decided that the structure is unfit for purpose
will depend on owner needs and constraints.
Where the use of refined solid mechanics methods cannot demonstrate structural
adequacy, it may be possible to demonstrate its adequacy and inherent safety by
comparison of its safety characteristics with other similar structures using stochastic
approaches and probability analysis
Actual live traffic loadings of the bridge might be determined statistically and used for
analysis
The safety criteria of the bridge in question might be assessed and specific relaxations
considered if this can be justified and the acceptability of risks clearly demonstrated with
adequate confidence
This level of assessment would require considerable specialist knowledge and research,
and the benefit is unlikely to be justifiable only in the most critical of cases.
Method
Main Parameters
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MEXE
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Applicability
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Span
Rise at crown and
quarter point
Arch thickness and
crown cover
Arch material
Backfill material
Mortar joint depth
and condition
General condition
factor
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HEYMAN’S
LIMIT
ANALYSIS
METHODS
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Arch geometry
Compressive
strength of masonry
(in some models)
Masonry and
backfill densities
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DISCRETE
AND
INDISCRETE
RIGID BLOCK
METHODS
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CASTIGLIANO’S
NON-LINEAR
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Arch geometry
Compressive
strength of masonry
(in some models)
Masonry and
backfill densities
Dilatancy
Angles of friction
(radial and
tangential)
Arch geometry
Compressive
strength of masonry
(in some models)
Masonry and
backfill densities
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Only applicable to
spans shorter than
18 m
Not applicable for
flat or appreciably
deformed arches
Not applicable for
multi-span bridges,
although the BR
version of MEXE
accounted for that
using an extra
factor
Advantages
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Can be applied to
multi-ring arches
Can be applied to
multi-span arches
Can produce
unsafe results in
shallow arches with
large spans
(bridges where
snap-through
failure is possible)
Some methods
might be able to
consider skewed
arches
Cannot be used
with skewed
If applied by an
experienced
engineer, the
condition factors
may allow to
account for effects
difficult to model
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Can be difficult to
apply to deep
arches
Can be difficult to
apply to shallow
arches with large
spans
Can be difficult to
apply to bridges
with complex
geometries
Disadvantages
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For simple
structures, it can
produce safe
results from very
limited input and at
a limited cost
These methods are
very effective when
the engineer has a
clear idea of the
mechanism by
which the structure
will fail
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Quick and reliable
for a significant
range of bridge
configurations
It is a very versatile
tool for an
experienced
engineer
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Simple and easy to
use
The only resisting mechanisms considered are the arch
and the weight of the backfill
The limiting load criterion is not realistic
Unnecessary assumptions on geometry and load
locations
When applied by inexperienced engineers, some
modifying factors can be dangerously subjective
Its results are assumed to be conservative, but can be
over-conservative as well as unsafe
Cannot consider the effect of strengthening measures
For Upper bound methods, if some failure mechanisms
are ignored, the method would provide an unsafe
prediction
For Lower Bound methods, if some kinematically
admissible equilibrium states are ignored, the method
would provide a conservative prediction. Similarly, if an
assumed equilibrium is not possible (because some
failure criterion has been ignored) the method will
produce unsafe results
Cannot consider ring separation
Cannot consider snap-through failures
Cannot consider the contribution of the spandrel walls.
Cannot consider snap-through failures
Cannot consider the contribution of the spandrel walls
The separation between rings during cannot be
reproduced. Instead, the used has to assume whether
ring separation will or will not take place
Consideration of masonry compressive failure might
increase the computational time
The prediction of the in-service behaviour can be quite
sensitive to the boundary conditions and the initial stress
state, which are very difficult to determine
Monitoring and Inspection
General Inspection
Techniques
Specialist Inspection
Techniques
Testing Techniques
Monitoring
Techniques
Bridge records
Visual observation
Hammer tapping
Surveying
Photography
Sonics
Conductivity
Radar
Ultrasonics
Infra-red thermography
Impact Echo
Tomography
GPR Techniques
Photogrammetry
Laser scanning
Strain measurement
Acoustic emission
Electrical conductivity
Endoscope
Scour detection
Coring
Petrological examination
Geotechnical investigation
Load testing
Flat-jack testing
Materials testing
Crack monitoring
Strain measurements
Displacement
measurement
Scour detection
Fibre-optic sensors
METHOD
COMMENTS
GOOD
MODERATE
POOR
VISUAL
Traditionally, visual
inspection has been
the first level of
inspection. Visual signs
of deterioration have
usually led to further
inspection and/or
repair.
Visual inspection has
the major disadvantage
of only recording that
which can be seen
which may of course be
a consequence of that
which cannot be seen.
All the basic
dimensions of the
structure should be
recorded.
The type of material
from which the
structure is
constructed, its general
condition and any
defects should be
mapped such as
cracks, settlements,
distortions etc
(including the location
of any slipped
voussoirs or bricks).
Any repairs or previous
works should also be
recorded.
The presence of water
should be recorded.
Material:
Hard stone
Engineering class bricks
Sealed surface
Well pointed
Shape:
Arch barrel defined shape
Walls, abutments and piers plumb
(or as built).
Fabric condition:
Units and mortar in good condition.
Cracks:
Longitudinal:
None present
Transverse:
None present
Diagonal:
None present
Abutment, Pier and Wall Cracks:
None present (Less than 1mm
measured over a 1m gauge length
in any direction)
Settlement:
Longitudinal and transverse relative
settlement less than
1 in 100.
Vehicular surface:
No significant defects
Wall Alignment:
No evidence of wall sliding, bulging
or tilting
Vegetation:
None present
Water:
None present and no evidence that
water has been present and caused
deterioration and/or damage
Material:
Medium stone
Building brick
Up to 20% not well pointed
Shape:
Arch barrel some movement over
up to 25% of the arch surface but
no areas flatter than a 0.1m offset
on a 2m straight edge set
longitudinally or 0.05m offset on a
2m straight edge set transversely.
Fabric condition:
Up to 5% of the mortar joints
displaying signs of deterioration ie
missing or crumbling
Up to 5% of the units displaying
signs of deterioration ie spalling,
crumbling, fissures.
Cracks:
Longitudinal:
Outside the middle third of the
arch, less than 1/10 of the span in
length
Transverse:
None present
Diagonal:
None present
Abutment, Pier and Wall Cracks:
Some present over 20% or more
of the surface such that they are
up to 6mm measured over a 1m
gauge length in any direction.
Settlement:
and surfacing.
Material:
Soft stone
Weak brick(fk less
than 20 N/mm2)
Up to 20% not well
pointed.
Shape:
Arch barrel has
general movement
over more than 25%
of the arch surface.
Areas identified
than are flatter than
a 0.1m offset on a
2m straight edge
set longitudinally or
0.05m offset on a
2m straight edge
set transversely.
Seriously misshaped arch barrels
and distorted walls
can be dangerous
and require
immediate
investigation.
Fabric condition:
More than 5% of the
mortar joints
displaying signs of
deterioration ie
missing or
crumbling.
More than 5%of the
the
Specialist
Inspection
Techniques
Comments
Sonics
This technique is dependent on measuring changes in the velocity of sonic pulses traveling in a solid
material, on the basis that velocity is dependent on the density and elastic properties of the
material. Internal discontinuities (e.g. cracks, voids, boundaries between material types) can
potentially be detected using this technique.
Conductivity
Electrodes are inserted into the structure or ground in order to determine its electromagnetic
conductivity and hence estimate the moisture content of the masonry or the presence of voids
etc.
Radar
Radar is an echo sounding technique that involves the transmission of short duration pulses of radio
energy into a structure and measurement of the reflected signals. The speed, strength and
frequency content of the reflected signals can be used to determine moisture content.
Ultrasonics
The velocity of ultrasonic pulses travelling in a solid material depends on the density and elastic
properties of the material. Pulses are not transmitted across voids, so by measuring apparent
speeds of pulses it is possible to determine the competence of the material and the location of
delaminations etc.
Infra-red
Thermography
Thermography involves the measurement of small variations in surface temperature (0.1 degree
Celsius) which are used to predict internal conditions.
Impact Echo
This technique can be used to determine the depth of delaminations (e.g. ring separation) and the
thickness of structural elements. It is based on the use of impact-generated stress waves that
propagate through the structure and are reflected by internal flaws and/or external surfaces.
Tomography
Involves the measurement of a net of stress waves (sonic) through the structure. This gives
information in 3-dimensions that enables an assessment of the location of possible defects to be
made.
GPR
Techniques
Ground Penetrating Radar (GPR) is an echo sounding technique where electromagnetic impulses are
transmitted into the bridge and a receiver detects reflections from material boundaries. It can be
used to determine construction details and conditions including delamination and voiding.
Photogrammet
Several digital images are recorded of the structure from different locations. Using the collected
information a 3-dimensional image of the structure can be created using specialist computer
Repair
Repair methods for consideration in the repair of arch barrels
(for single-span bridges)
Is it possible
to excavate
back to the
arch barrel
extrados?
YES
NO
It is assumed that the
spandrel wall is in good
condition. Any bulging,
tilting, sliding etc has
been appropriately
repaired using tie-bars
and patress plates or
other means.
START
NOTE:
The repairs identified in this
flowchart will change the nature of
the structural behaviour of the arch
and
consequently
appropriate
assessment techniques should be
used to determine the new carrying
capacity of the bridge.
YES
Is the
structural
integrity of
the arch
barrel very
poor?
NO
Does the
arch barrel
need
waterproofin
g?
NO
Is the
cracking of
the arch
barrel
extensive?
YES
Saddle, waterproof and
repoint arch barrel
Install relieving slab,
waterproof and repoint
arch barrel
Stitch cracks or
reconstruct
NO
Grout arch barrel
and/or retrofit
reinforcement
YES
Use spray/cast insitu
concrete repair to arch
barrel intrados
Reconstruct
YES
Is the
fault due
to active
subsidenc
e?
NO
Is the repair
required to
be
permanent?
YES
Is the
arch
heavily
distorted?
NO
Use sprayed concrete
repair to arch barrel
intrados
NO
YES
Install steel plate lining to
arch barrel intrados
NO
Underpin structure and/or
support arch with steel ribs
Is the
span of
the arch
>5m?
YES
Install corrugated steel
lining to arch barrel
intrados
Technique
Concrete
Saddle
Category
Structural Defect &
Location
Strengthening
Inadequate overall load
carrying capacity of arch
barrel in conjunction with
spandrel wall and
waterproofing failures
Engineering Aspects
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Parapet
Upgrading
Strengthening
Inadequate impact
resistance of parapet(s)
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Repointing
Remedial
Deterioration of mortar
joints
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Advantages /
Disadvantages
The adequacy of the
existing structure should be
checked to ensure that it is
capable of sustaining the
enhanced loading with the
saddle in place.
Decide whether the saddle
is to act compositely with
the existing structure or not.
Check the structural
interaction, bearing in mind
that the barrel is particulate
and heterogeneous whilst
the reinforced concrete
saddle is not.
Particular attention should
be given to the load paths
through the abutments,
piers and their foundations.
Advantages
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No change to
appearance as hidden
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Facilitates other
repairs/parapet
upgrades/waterproofin
g
•
Enhanced live load
capacity
Disadvantages
•
Traffic disruption
during construction
•
Relative cost
•
Increase in crown
depth possible.
The consequence of impact
should be given detailed
consideration particularly in
respect of vehicular
containment and falling
material.
Where the parapet is a new
build, its interaction with the
existing structure should be
checked.
Advantages –
- Enhanced vehicle
containment and/or
reduction in likelihood
of falling material
Disadvantages –
- Traffic disruption during
construction
- Provision of access
- Relative cost
Careful consideration
should be given to the
Advantages- Simple established
Concrete saddle
Cost Band
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Special
Considerations
Temporary Works design for all stages
Lightweight Concrete supply
Waterproofing
Heritage
Proposed remedial treatment to be assessed for impact on bridge’s heritage value
Durability
Structural concrete saddle to be designed to give a minimum design life of
120 years. However, the overall life of the structure will be governed by the current
level of dilapidation.
Inspection
Not applicable as the saddle will be buried.
Routine visual and tactile inspection of the structure in accordance with the asset
steward’s requirements.
Performance
Effective implementation and inspection / maintenance will enhance structural
performance in line with the strengthening or repair design life.
Further
Guidance
BA 16/97, BS 5400 Part 4, BS5628