Masonry Arch Bridges Flashcards
Why are masonry arches difficult to monitor and maintain?
- Majority are over 100 years old
- They carry loads that are different to when constructed
- Have complex geometries
- Built using different construction methods
Describe the different parts of masonry arch bridges
Backfill, spandrel wall not commonly used on other bridge types*
What is masonry made of?
- Made of blocks that are bonded together, with or without mortar
- A bridge can contain a range of different masonry materials (bricks, stone masonry, concrete blockwork)
Is masonry strong/weak in tension/compression
Are masonry units in lateral tension or lateral compression
Strong in compression
- very weak in tension
- it is a brittle material (no significant deformation before failure)
- mechanical properties may degrade under persistant high loading due to creep; leading to propagation of micro cracks
Masonry units are in lateral tension, mortar is in lateral compression
When may creep occur in masonry bridges?
Creep may occur in a very large masonry viaduct, if:
- stresses are high in relation to the strength of the ‘constituent material’
- a thin external skin of stiff masonry surrounds the rubble infill
What is the difference between fatigue and creep?
Fatigue:
- leads to (micro) crack propagation
- can occur in masonry arches due to repeated short-term loading from vehicles (cyclic loading)
- how cracks or fractures are likely to develop due to fluctuating stresses and strains
Creep:
- results in gradual deformation
- focuses on the deformation under constant load and temperature conditions over extended periods
What is another way masonry bridges can degrade?
Due to weathering
- e.g. degradation of parapet due to freeze-thaw action
What are the three masonry unit types?
What does stone have?
Clay brick
Stone:
- higher compressive strength
- properties can be anisotropic (weathering resistance dependent upon orientation)
Concrete
What are the three mortar types?
Lime:
- slow setting and relatively weak
- more plastic (able to accommodate movement)
Roman cement:
- slow setting but higher strength than lime mortars
Portland cement:
- typically comprise cement, lime and sand
- stronger and stiffer mortars
What is the function of backfill?
- Better stress distribution through to the arch
- Keeps the arch in compression
- Keeps the surface level
- Restrains the movement of arch, prevents lateral thrust
What is backfill usually made of?
- Granular and/or cohesive (clay) material, heterogeneous (opposite of homogeneous)
- Top layer highly compacted because of repeated loading
- Granular backfill shear strength reduced by pwps reducing the normal stress
- Clay backfill, strength very dependant on moisture content
Name two types of arch barrel shapes
Pointed arches:
- two circle segments, with segments located on the intrados base line
- provide great headroom (e.g. for boats), and greater CSA for flood discharge
Segmental arches:
- less than a semi-circle
What are arch barrels made of?
Stone or concrete voussoirs
What can piers, abutments and foundations be made of for masonry arch bridges?
- Solid masonry
- Foundations may have timber piles; these timber piles may rot and lead to foundation settlement (or even failure)
NB. filling voids in piers or abutments with foamed concrete is bad and may lead to the bearing capacity being exceeded
What is the spandrel zone above the arch barrel commonly made of
Commonly filled with soil backfill
In long-span bridges and viaducts longitudinal or transverse voids, spanned by vaults or stone slabs, are used
How do masonry gravity structures resist loads?
When might the load-carrying capacity be reduced?
They resists applied load through their inherent self-weight and thickness
Capacity of a masonry gravity structure reduces during a flood event
What are different failure modes of masonry arch bridges?
Frictional sliding resistance:
- can disappear abruptly when deformations reach a tipping point
Delamination/ring separation
Is a masonry arch statically indeterminate or statically determinate?
Statically indeterminate, but can crack and result to a statically determinate structure:
- e.g. the arch can change to SD form when abutments move, leading to a set of hinges through which the thrust must pass
- if cracked, even relatively small loads can cause ‘large rigid body displacement’
- there are many possible load paths in an untracked arch; not possible to ascertain where the line of thrust passes & when areas are most highly stressed
Why is masonry prone to cracking?
What else can reduce the load-carrying capacity of a masonry arch?
Because it is a quasi-brittle material:
- there is little tensile or shear strength
- tensile or shear cracks (e.g. delamination/ring separation) may form unpredictably
Delamination of rings in a masonry arch
Are long-span or short-span masonry arches more prone to overloading?
Likewise, which experiences higher stresses?
Overloading - short span:
- vehicle loads are higher in relation to the bridge self-weight
Stresses - long-span:
- internal stresses scale linearly with the size of the gravity structure
What is the most onerous (troublesome) loading location in a multi-span bridge?
The crown of one span, when an adjacent span is unloaded
What is the most onerous (troublesome) loading location in a single-span bridge?
Near the quarter or third point
What can have a significant impact on bridge response?
A change in the wheelbase of applied traffic load:
- change in pattern of loading can even be more important than increase in magnitude of axle loading
What can significantly influence the behaviour of a multi-span bridge above the piers?
Using a strong/stiff infill material above the piers can make it act as a single block (skewback)
What are skew arches, and what types of failure mechanisms can form?
Skew arches span obstacle with an angle
Failure mechanisms typically involve the formation of diagonal cracks
- for multi-span bridges, intermediate piers are likely to be subjected to overturning and twisting actions
Describe the pressures on a masonry arch, where stresses develop and the movement
Stresses develop due to friction between the soil and arch ring
Why does having soil infill in the arch barrel improve the design?
The soil infill material provides vertical loading which pre-stresses the arch barrel; enhancing its resistance to external applied loads
Soil infill material behind the abutments provides a passive restraint to the arch barrel; enhancing the ULS load-carrying capacity
How can the distribution of load modify the failure mode?
- Can change the failure mode from from a conventional four-hinge mechanism to a three-hinge plus sliding mechanism, which is more ductile
- Load distribution can mean that a bridge with a large depth of soil fill over the crown (relative to its span) may not be significantly affected by live load
What caused the gross movement of the pier shown in the image?
Scour effects
What is this common defect seen in masonry arch bridges, and what caused it?
Distorted arch profile
Caused by movements and/or overloading
What is this common defect seen in masonry arch bridges, and what caused it?
Ring separation (in a multi-ring brickwork arch barrel)
Caused by presence of weak inter-ring mortar joint
Ring separation significantly reduces the load-carrying capacity
What is this common defect seen in masonry arch bridges, and what caused it?
Longitudinal cracks in the arch barrel
Caused by either:
- high relative stiffness of the spandrel walls, or
- high stresses near the centre of a bridge carrying two-way traffic
The outward movement of spandrel wall causes the crack
What is this common defect seen in masonry arch bridges, and what caused it?
Transverse cracks in the arch barrel
Caused by translational or rotational movements of the supports in the plane of the bridge
What is this common defect seen in masonry arch bridges, and what caused it?
Diagonal cracks in the arch barrel
Caused by differential transverse movements of the support
What is this common defect seen in masonry arch bridges?
Loss of masonry units or mortar
What is the biggest maintenance problem in masonry arch bridges?
Spandrel/wing walls:
- frequently affected by dead and live forces generated in the fill
- no possibility of closing cracks when vehicles can travel close to the spandrel wall
What are the four types of common spandrel/wing wall defects?
- Tilting
- Bulging
- Sliding
- Cracked arch ring
What is a measuring device that can be used for measuring crack width?
Dial calipers
What is a measuring device that could be used to measure plugging, tilting and loss of verticality?
String line and plumb bob
What must be remembered when estimating the strength of old masonry using a whole brick removed from the structure?
The strength will vary widely over the structure
What is a non-intrusive method that can be used for detecting cracks?
Acoustic emission:
- as crack develops, elastic strain energy is released (in the form of elastic waves)
- piezo-electric accelerometers can detect these elastic waves
Name different methods of access for masonry arch bridge inspection?
- Hydraulic access platforms (beneath bridge)
- Under bridge inspection equipment (on top of bridge)
- Scaffolding
- Boat
- Abseiling
- Divers (underwater inspection of piers, abutments, foundations)
For repointing and masonry repairs, what is the minimum depth of raking?
Minimum depth of raking is 15mm
- the raking out should be to a uniform depth and square
Give an overview of the saddling repair method
- Used to ensure that existing and defective spandrel walls are redundant
- Normally implemented on multi-span structures
- Involves removal of the fill and casting an in-situ arch, which may be reinforced on top of the existing arch
- Minimum saddle thickness is 150mm, and concrete should be poured symmetrically
- The new concrete provides support to the ballast/fill material
Give an overview of the sprayed concrete repair method
- Used to increase arch thickness
- Raises load capacity and stability of badly-weathered masonry
- Original intrados ring can be removed and replaced with sprayed concrete lining to minimise loss of clearance
- Layer between 150 and 300mm thick
- Quick method; does not involve disruption or require extensive formwork
Give an overview of the stitching repair method
- Used in repairing ring separation and the detachment of a spandrel wall facing from its backing
- Dowels inserted into pre-drilled holes in the arch barrel
- The dowels are grouted to restore shear transfer
Give an overview of the reinforced fill repair method
- Used to provide a stronger fill and relieve pressure on the spandrel wall
- As well as being stronger, the new fill remains flexible
Give an overview of the relieving slabs repair method
- Used to help distribute the load on the arch; transferring some of it on to the abutment
- Flat reinforced concrete slab installed on top of the fill, supported on two edges
- Waterproofing (at top of slab) and drainage (at end of slab) also required
Give an overview of the tie bar repair method
- Used to restrain the movement of spandrel walls
- Fixed to each end of the arch, installed by drilling through the arch
- Bedded against the spandrel wall with mortar to provide max. bearing area
- Need to be “as close to the geometric centroid of the area of masonry” to be retained; otherwise risk producing stress concentration
(NAQ) summary of repair and strengthening methods
