Piles can be classified according to the type of material forming the piles, the mode of load transfer, the degree of ground displacement during pile installation and the method of installation.
Pile classification in accordance with material type (e.g. steel and concrete) has drawbacks because composite piles are available. A classification system based on the mode of load transfer will be difficult to set up because the proportion of shaft resistance and endbearing resistance that occurs in practice usually cannot be reliably predicted. In the installation of piles, either displacement or replacement of the ground will predominate.
A classification system based on the degree of ground displacement during pile installation, such as that recommended in BS 8004 (BSI, 1986) encompasses all types of piles and reflects the fundamental effect of pile construction on the ground which in turn will have a pronounced influence on pile performance.
Such a classification system is therefore considered to be the most appropriate.
Piles are classified into the following four types :
(b) Small-displacement piles, which include rolled steel sections such as H-piles and open-ended tubular piles.
However, these piles will effectively become largedisplacement piles if a soil plug forms.
(c) Replacement piles, which are formed by machine boring, grabbing or hand-digging. The excavation may need to be supported by bentonite slurry, or lined with a casing that is either left in place or extracted during concreting for re-use.
(d) Special piles, which are particular pile types or variants of existing pile types introduced from time to time to improve efficiency or overcome problems related to special ground conditions.
During construction, temporary structures are required. Let us assume a simple task of painting a building. How could workers go up to the upper levels to paint? Typically, workers stand on a temporary structure known as a scaffold (Fig.1).
Fig.1. The painting of a building. A scaffold is built for painters to stand on while painting the building
1. SCAFFOLDS
Scaffolds are work platforms that enable workers to do their job at high elevations. The type of work can be brickwork, painting, steel work, concreting, or window installation. Most scaffolds are made of steel pipes. In some countries bamboo is stillused for scaffolds.
1.1 Pipe Scaffolds
Pipes are used to build scaffolds. Pipes are connected using special connectors. Platforms are provided for workers to stand on. Ladders are provided for workers to go from one platform to another one (Figs.2 and 3). Pipe scaffolds cannot be used for very tall buildings. Other methods such as outrigger scaffolds are used in such situations.
Fig.2. Pipe scaffold with one platform
Fig.3. Pipe scaffold with multiple platforms. Ladders are provided to climb to higher platforms
1.2 Outrigger Scaffolds
Consider this scenario: Brickwork has to be done on the 70th story of a 70-story building. How could the workers get to the 70th story? Should they build a pipe scaffold from the ground all the way to the 70th story? That may not be very feasible. In this type of situation, outrigger scaffolds can be used. Metal beams are attached to the building. These beams are used to build a work platform (Fig.4).
In the case of outrigger scaffolds, metal beams or a metal structure are attached to the newly constructed building. This has to be done with the authorization of design engineers. This metal structure is known as outriggers. The protruding metal structure is used to build a work platform.
Fig.4. Outrigger scaffold
1.3 Modular Scaffolds
Pre-made modules are becoming common in many construction projects (Figs.5 and 6).
Fig.5. Scaffolding modules
Fig.6. Scaffolding modules are fitted together to reach high elevations
Boards: Boards are made of metal or wooden planks attached to the scaffolding for people to stand and work. Uprights also known as standards and poles are used to carry the load to base. False uprights are mainly used near entrances to the work platform. False uprights do not transfer any vertical loads to the ground. Though it may provide lateral support handrails, it does not provide any lateral supports to the scaffold system.
2. SHORING
Scaffolds are built for workers to work. Scaffolds act as work platforms for workers. On the other hand, shoring is done to support wet concrete. Once the concrete is hardened, the shoring is removed. Other than supporting wet concrete, shoring can be used to support weak columns.Let us assume that an existing column in a building is deteriorated and has to be replaced. The procedure to remove an existing column and build a new column is shown in Fig.7.
Fig.7. Provide shoring prior to removal of the column
Once proper shoring is provided and has been approved by the relevant authorities, the contractor can remove the existing load bearing column and construct a new one.
3. BRACING
Bracing is a support element provided to strengthen an existing structural element (Figs.8 and 9).
Fig.8. Deteriorated element in a structure
Fig.9. Bracing is provided for the deteriorated element
3.1 Bracing Masonry Walls
Masonry walls need to be braced during and after construction. All masonry walls have to be supported laterally. In a building, masonry walls are tied to beams, columns, and other walls.
Until the masonry wall is laterally supported, it has to be braced. In practice, bracing of masonry walls has to follow OSHA guidelines. For the exam purposes, NCEES recommends “Standard Practice for Bracing Masonry Walls Under Construction” by Mason Contractors Association of America (MCAA).
Hence, you need to know both OSHA and MCAA guidelines (Fig.10). Masonry walls are made of bricks and mortar. Until a mortar brick joint is fully developed, masonry walls have little lateral stability.
Even after the mortar is hardened, a standing masonry wall has little resistance against over-turning. As per OSHA, any wall 8 ft or taller needs to be braced.
Fig.10. Schematic diagram of a masonry wall bracing. Bracing is required tomaintain the lateral stability of a masonry wall
4. COFFERDAMS
There are instances where construction has to take place near a river, lake, or ocean. Bridge piers, harbor structures, and flood control structures are some examples. In such situations water has to be kept away from the construction area.
How can you concrete when water is pouring in? A structure has to be built to keep the water away. In such situations a temporary structure needs to be constructed to keep the water away. These temporary structures are known as cofferdams.
Cofferdams are temporary structures constructed to keep water out ofthe construction area. The majority of cofferdams are constructed in riversmainly to build bridge piers. Thanks to improvement in caisson technology, in most cases cofferdams may not be necessary anymore.
Once a plant is in operation it is important to monitor the progress of any corrosion which might be taking place. The four approaches described below vary in sophistication and cost.
The most appropriate for any plant is determined by a number of factors, including the mechanisms of corrosion which are anticipated and the implications of catastrophic or unexpected failure. Key areas of the plant require closer monitoring than readily replaceable items.
The measures described below do not replace the mandatory inspections of pressure vessels, etc. for insurance purposes. The overall philosophy of corrosion monitoring is to improve the economics of the plant’s operation by allowing the use of cheaper materials and generally reducing the over-design that goes into plant to combat corrosion.
1. Physical examination
Full records of all constructional materials that are used in the plant should be maintained and updated when repairs are undertaken.
The exteriors of all parts of the plant should be subjected to frequent visual examination and the results reported and stored for future reference. This maximizes the warning time before corrosion failures occur, since the majority of failure mechanisms cause leaks before bursting.
Key items of plant, those in which some degree of corrosion is anticipated and those which might suffer catastrophic failure should be examined in greater detail. Internal visual inspection during shutdowns is sufficient to identify most corrosion effects.
Cracking can usually be seen with the naked eye but where cracking is considered to be a possible mechanism an appropriate non-destructive test method should be employed. In items of plant which are shut down only infrequently (relative to the timescale of possible corrosion or cracking failure) external non-destructive testing is often possible.
Candidate non-destructive test methods include:
a. Ultrasonic techniques:
Wall thickness can be mea- sured to monitor the progress of general corrosion, cracks can be detected and hydrogen blisters identified. Certain construction materials such as cast iron cannot be examined by ultrasound. Skilled operators and spe- cialist equipment is required. Plant can be examined in situ except when it is above 80°C.
b. Magnetic particle inspection:
Surface emergent and some sub-surface cracking can be detected in ferro- magnetic materials. The technique must be used on the side of the material in contact with the corrodent.
c. Dye penetration inspection:
This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks.
2. Exposure coupons and electrical resistance probes
If changes have been made to the process (e.g. if incoming water quality cannot be maintained or other uncertainties arise) concerning the corrosion behaviour of the construction materials, it is possible to incorporate coupons or probes of the material into the plant and monitor their corrosion behaviour.
This approach may be used to assist in the materials selection process for a replacement plant. Small coupons (typically, 25 × 50mm) of any material may be suspended in the process stream and removed at intervals for weight loss determination and visual inspection for localized corrosion.
Electrical resistance probes comprise short strands of the appropriate material electrically isolated from the item of plant. An electrical connection from each end of the probe is fed out of the plant to a control box. Instrumentation in the box senses the electrical resistance of the probe.
The probe’s resistance rises as its cross-sectional area is lost through corrosion. The materials should be in the appropriate form, i.e. cast/wrought/welded, heat treatment and surface condi- tion.
Metal coupons should be electrically isolated from any other metallic material in the system. They should be securely attached to prevent their being dislodged and causing damage downstream.
Simple coupons and probes cannot replicate the corrosion effects due to heat transfer but otherwise provide very useful information. It should be noted that any corrosion they have suffered represents the integrated corrosion rate over the exposure time.
Corrosion rates often diminish with time as scaling or filming takes place, thus short-term exposures can give values higher than the true corrosion rate.
3. Electrochemical corrosion monitoring
A number of corrosion-monitoring techniques, based on electrochemical principles, are available. These give an indication of the instantaneous corrosion rate, which is of use when changing process conditions create a variety of corrosion effects at different times in a plant. Some techniques monitor continuously, others take a finite time to make a measurement.
Polarization resistance: The current-potential behaviour of a metal, externally polarized around its corrosion potential, provides a good indication of its corrosion
The technique has the advantage of being well established and hence reliable when used within certain limitations.
This technique can only be used for certain metals, to give general corrosion rate date in electrolytes. It cannot be employed to monitor localized corrosion such as pitting, crevice corrosion or stress corrosion cracking, nor used in low-conductivity environments such as concrete, timber, soil and poor electrolytes (e.g. clean water and non-ionic solvents). Equipment is available commercially but professional advice should be sought for system design and location of probes.
Impedance spectroscopy: This technique is essentially the extension of polarization resistance measurements into low-conductivity environments, including those listed
The technique can also be used to monitor atmospheric corrosion, corrosion under thin films of condensed liquid and the breakdown of protective paint coatings. Additionally, the method provides mechanistic data concerning the corrosion processes which are taking place
Electrochemical noise: A variety of related techniques are now available to monitor localized No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analysed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking are obtained.
4. Thin-layer activation
This technique is based upon the detection of corrosion products, in the form of dissolved metal ions, in the process stream.
A thin layer of radioactive material is created on the process side of an item of plant. As corrosion occurs, radioactive isotopes of the elements in the construction material of the plant pass into the process stream and are detected.
The rate of metal loss is quantified and local rates of corrosion are inferred. This monitoring technique is not yet in widespread use but it has been proven in several industries.
The design of a plant has significant implications for its subsequent corrosion behaviour. Good design minimizes corrosion risks whereas bad design promotes or exacer- bates corrosion.
1. Shape
The shape of a vessel determines how well it drains (Figure 1). If the outlet is not at the very lowest point process liquid may be left inside. This will concentrate by evaporation unless cleaned out, and it will probably become more corrosive.
This also applies to horizontal pipe runs and steam or cooling coils attached to vessels. Steam heating coils that do not drain adequately collect condensate. This is very often contaminated by chloride ions, which are soon concentrated to high enough levels (10-100 ppm) to pose serious pitting and stress corrosion cracking risks for 300-series austenitic stainless steel vessels and steam coils.
Flat-bottomed storage tanks tend to suffer pitting corrosion beneath deposits or sediments which settle out. Storage tanks may be emptied infrequently and may not experience sufficient agitation or flow to remove such deposits.
Flange face areas experience stagnant conditions. Additionally, some gasket materials, such as asbestos fibre, contain leachable chloride ions.
This creates crevice and stress corrosion cracking problems on sealing surfaces. Where necessary, flange faces which are at risk can be overlaid with nickel-based alloys. Alternatively, compressed asbestos fibre gaskets shrouded in PTFE may be used.
Graphite gaskets can cause crevice corrosion of stainless steel flanges. Bends and tee-pieces in pipework often create locally turbulent flow. This enhances the corrosivity of the process liquid. These effects should be minimized by the use of flow straighteners, swept tees and gentle bends.
Flow- induced corrosion downstream of control valves, orifice plates, etc. is sometimes so serious that pipework requires lining with resistant material for some twelve pipe diameters beyond the valve.
Fig.1. Details of design creating corrosion problems
2. Stress
The presence of tensile stress in a metal surface ren- ders that surface more susceptible to many kinds of corrosion than the same material in a non-stressed condition.
Similarly, the presence of compressive stress in the surface layer can be beneficial for corrosion, and especially stress corrosion cracking, behaviour.
Tensile stresses can be residual, from a forming or welding operation, or operational from heating-cooling, filling-emptying or pressurizing-depressurizing cycles. The presence of a tensile stress from whatever origin places some materials at risk from stress corrosion cracking.
Some items of plant can be stress-relieved by suitable heat treatment, but this cannot prevent operational stress arising. Cyclic stresses can also give rise to fatigue or corrosion fatigue problems.
Information relating to the fatigue life of the material in the service environment is required, together with the anticipated number of stress cycles to be experienced by the item over its operational life. The fatigue life (the number of cycles to failure) or the fatigue strength (the stress level below which it does not exhibit fatigue problems) is then used in the design.
The presence of stress raisers, including sharp comers and imperfect welds, produces locally high stress levels. These should be avoided where possible or taken into account when designing the materials for use in environments in which they are susceptible to stress corrosion cracking or corrosion fatigue.
3. Fabrication techniques
Most fabricational techniques have implications for corrosion performance. Riveted and folded seam construction creates crevices as shown in Figure 2.
Those materials which are susceptible to crevice corrosion should be fabricated using alternative techniques (e.g. welding). Care should be taken to avoid lack of penetration or lack of fusion, since these are sites for crevice corrosion to initiate.
Welding should be continuous, employing fillets where possible, since tack welds create locally high stresses and leave crevice sites. Welding consumables should be chosen to create weld metals of similar corrosion resistance to the parent material.
This often requires the use of a slightly over-alloyed consumable, to allow for loss of volatile alloying elements during the welding process and to compensate for the inherently poorer corrosion resistance of the weld metal structure.
Strongly over-alloyed weld consumables can create galvanic corrosion problems if the weld metal is significantly more noble than the parent material.
In all welds the heat-affected zone is at risk. The new structure which forms as a consequence of the thermal cycle can be of lower corrosion resistance, in addition to the often poorer mechanical properties, than parent material.
Austenitic steels such as type 304 and 316 are also susceptible to sensitization effects in the heat-affected zone. In these materials carbide precipitation during the welding thermal cycle denudes the parent material of chromium.
This creates areas of significantly diminished corrosion resistance, resulting in knife-line attack in many corrosive environments. This is avoided by the use of the low-carbon equivalents (304L, 316L, etc.) or grades such as type 321 or 347 which are stabilized against sensitization.
With correct welding techniques, however, this should be necessary only with thick sections (5 mm for 304 and 8 mm for 316). Some materials, particularly certain aluminium alloys, duplex stainless steels in certain reducing environments and most steel plate, are susceptible to end-grain attack.
Penetration along the end grain can be very rapid, with corrosion exploiting the potential differences that exist between inclusions and ferrite crystals in steel and between austenitic and ferrite grains in duplex stainless steel.
Where end-grain attack is significant this should not be exposed to the corrosive environment. It can be covered by a fillet ‘buttering’ weld if necessary.
Fig.2. Details of jointing processes creating additional corrosion risks (crevices and stress concentrations)
4. Design for inspection
Unseen corrosion can be the most damaging type of attack. Items should be designed to permit periodic inspection.
This involves the provision of sufficiently large manways, the installation of inspection pits, the placing of fiat-bottomed vessels on beams instead of directly onto concrete bases and the facility for removal of thermal insulation from vessel walls.
Corrosion is generally taken to be the wastage of a metal by the action of corrosive agents. However, a wider definition is the degradation of a material through contact with its environment. Thus corrosion can include non-metallic materials such as concrete and plastics and mechanisms such as cracking in addition to wastage (i.e. loss of material).
In essence, the corrosion of metals is an electron transfer reaction. An uncharged metal atom loses one or more electrons and becomes a charged metal ion.
In an ionizing solvent the metal ion initially goes into solution but may then undergo a secondary reaction, combining with other ions present in the environment to form an insoluble molecular species such as rust or aluminium oxide. In high-temperature oxidation the metal ion becomes part of the lattice of the oxide formed.
Coastal structures, as their name implies, are structures situated on the coastline or inclose proximity to it, and they can comprise everything from seawalls to coastal bridges or even coastal buildings.
Coastal structures differ from port structures in that they have to deal with the effects of direct sea action from waves and sediment movement, as well as possible sea splash and spray. Wave action applies direct loads to sea walls and coastal jetties, and breakwater and revetment structures are required to absorb the wave energy. Sediment movement and wave action can cause abrasion of the structure.
1. Seawalls
Seawalls are constructed to protect land assets from sea action. They usually have a curved shape designed to redirect the flow from a breaking wave back in a sea ward direction and there by minimise overtopping. The wall therefore has to resist the full impact force of the breaking wave and the long-term effect of erosion and abrasion from water and water-borne sediment. A typical seawall structure is shown in Fig.1.
Fig.1. Typical seawall cross-section. When constructing on rock, a rough foundation isusually donefirst, and on top of this the formed wall profile is cast
Both in-situ and precast concrete elements can be used, depending on the particular configuration of the wall (Fig.2)
Fig.2. Sea Point promenade seawall, Cape Town
2. Breakwaters and revetments
The most common form of contemporary breakwater and revetment construction consists of an armoured rock rubble mound with a concrete capping. Breakwatersare normally freestanding and form the outer protection for a harbour. Revetments are constructed along a shoreline and protect it from wave attack.
The breakwater or revetment armour may consist of large-size rock or precast con-crete units depending on the design wave parameters. There are practical limits to the size and mass of armour rock that can be quarried due to the inherent geological joints, and therefore precast concrete units are used when a greater armour unit mass isrequired.
The concrete capping serves to reinforce the top of the mound and provide a roadway for in-service maintenance.
Fig. 3 shows a typical rubble-mound break-water cross-section with a cast in-situ plain concrete cap. The cap needs to be heavy enough to resist the wave loads that impact onto the splash wall on the seaward side and would typically be a minimum of 1m thick.
Sometimes a vertical wall is used as a breakwater instead of a rubble mound. KalkBay harbour on the Cape Peninsula is an example of an old vertical wall breakwater that was constructed from mass concrete blocks.
More common nowadays is to construct the wall from caissons, as shown in Fig.4.
Fig.3. Rubble-mound breakwater cross-section. Plain concrete is used for the armourunits and the cap, although some reinforcement may be needed to strengthen the splash wall
Fig.4. Typical caisson breakwater. The precast caisson used for breakwaters is usually ofsimilar form to those described in the quay wall section and is sand-filled and topped by a plainconcrete cap.
2.1. Breakwater armour units
A large variety of precast concrete armour units have been invented over the years, from a simple cube to complex shapes such as the cob and tetrahedron, and a selection of these are illustrated in Fig.5.
Fig.5. Breakwater armour units constructed from plain concrete
Breakwater armour units are typically plain, unreinforced concrete without any steel reinforcement. Some units are used in single layers, and some require two layers to pack and function correctly. When properly packed on a breakwater, armour units move very little, and there is no need for them to be reinforced to resist the hydraulic loads from the wave impact.
Attempts have been made to reinforce armour units to resist the forces that occur, should the units move if improperly packed, but this has never been successful due to the very high impact forces that occur.
The dolos armour unit originates from South Africa and has been used extensively around the world as it is economical, requiring less concrete per square metre of break-water surface area than most other units. It is a double-layer unit, and its shape has evolved as the design has been used and improved.
The current dolos shape has a relatively thicker waist and filleted corners in comparison to the original one.The largest dolosse that have been used in South Africa are the 30-tonne units on the Ngqura harbour breakwaters in the Eastern Cape (Fig.6).
Fig.6. 30-tonne dolos
As armour units are unreinforced, the tensile capacity of the concrete is an impor-tant functional requirement, as is good-quality casting. Any cracks or large surface defects can have a detrimental effect on the performance of the unit. Cracks cause a reduction in the cross-sectional area of the unit, and if this is at a critical section such as the intersection of the waist and fluke of a dolos, the section can be significantly weakened. Large surface defects may also cause structural weakness oran under weight unit and hence loss of stability.
2.2 Breakwater cap
The main function of the breakwater cap is to protect the top of the rock mound against scour from overtopping waves and to provide a roadway for in-service maintenance. The cap frequently has a wave wall or splash wall on the seaward side, and this serves to protect personnel and vehicles on the cap from overtopping waves.
The thickness of the cap and its splash wall facilitates the termination of the top of the rock mound ata lower level, thereby saving rock. Down-stands are sometimes incorporated to key into the top of the rock mound to mobilise additional weight for resistance against lateral wave loads.
2.3 Caisson breakwater
In deep water, it is sometimes economical and practical to construct the breakwater structure using reinforced concrete caissons instead of a rock rubble mound. The width and height of the caissons are sized to resist the hydrostatic pressures from the impact of the design wave. Wave conditions at the site have to be suitable for caisson placing, with sufficiently long periods of time when wave heights are less than 1 m, to positionand sink the caisson (Fig.7).
Fig.7. Port of Cape Town breakwater. The end section of the breakwater is constructedfrom seven caissons, each with four round cells. The caisson ends just above the layer ofmarine growth and is topped with a solid, plain concrete cap
3. Coastal jetties
Jetties are constructed in exposed coastal locations for a variety of functions, including cargo export/import, sand bypass facilities, seawater intakes, effluent outfalls and beach groynes. Coastal jetties are designed and constructed using similar principles to jetties located within protected harbours, except that the design of the jetty has to take into consideration the hydrodynamic loads resulting from the waves and ocean currents at the site.
Where possible the jetty superstructure is located above the maximum wave crest level to avoid the impact loads from the waves (see Fig. 2.8).
Fig.7. Coastal jetty wave impact. This is the groyne jetty at Hobie Beach in Port Elizabethand is subject to depth-limited waves.
How to choose the Layout of a Bridge Deck Surface?
The layout of the bridge deck surface should be determined according to the deck width, the design speed, and the hierarchy of roads. In general,there are following three types.
1. Undivided Carriageway
Undivided carriageway denotes that the traffic load located at the same surface, also uplink and downlink, was not divided. As the motor vehicles and nonmotor vehicles on the same road surface, the traffic can only in middle or low speed, it can easily has traffic jam on the bridge.
2. Divided Carriageway
To avoid the possible traffic jam on the carriageway, the carriageway can be divided by using the median strip, or sometimes the uplink and downlink located at two bridges. The separation between the uplink and downlink, or different transportation means such as the motor traffic and nonmotor traffic makes it become easy to control the traffic and improve the traffic capacity.
3. Double-Decked Bridges
Double-decked bridges denote the bridges that have two levels deck system.Double decks were generally used for different means of transportation, which are useful for improving the traffic capacity and traffic control. In addition, such bridge can be used for reducing the bridge deck width and make full use of the clearance. Such as the Nanjing Yangtze River Bridge in Fig.1, which is a double-decked road-rail truss bridge across the Yangtze River China. Its upper deck is part of China National Highway, and its lower deck carries a double-track railway.
A bridge deck (or road bed) is the roadway, or the pedestrian walkway, surface of a bridge. The deck may be of either cast-in-situ or precast concrete, wood which in turn may be covered with asphalt concrete or other pavement. The concrete deck may be an integral part of the bridge structure (e.g., T-section beam structure), or it may be supported with I-beams or steel girders, as so-called composite bridges. The deck may also be of other materials, such as wood or open steel grating.
Sometimes the deck system is called a floor system, such as for a bridge deck that installed in a through truss. A suspended bridge deck will be suspended from the main structural elements on a suspension or arch bridge.On some bridges, such as a tied arch or a cable stayed, the deck is a primary structural element, carrying tension or compression to support the span. But for girder beams, the bridge deck system is not the load carrying system .Despite this, they are important for the bridge service ability, safety as well as the aesthetics. Thus, deck system deserves special attention in all bridge design and construction.
Bridge Expansion Joint Functions and Dynamic Behavior
1. Functions of Expansion Joint
Bridge expansion joints are designed to adjust its length accommodating movement or deformation by external loads, shrinkage, or temperature variations, and allow for continuous traffic between bridge structures and interconnecting structures (another bridge or abutment).
The expansion joints can also be used for reducing internal forces in extreme conditions and allow enough vertical movement for bearing replacement. Steel expansion joints are most commonly used, though rubber joints are also often used to provide a smooth transition for modern bridge construction, or continuous girders (Toma et al., 2005).
It was suggested that expansion joints fall into three broad categories depending upon the amount of movement accommodated (Malla and Shaw, 2003), including:
(1) small movement joints capable of accommodating movement up to about 45 mm
(2) medium movement joints capable ofaccommodating total motion ranges between about 45 mm and about130 mm
(3) large movements joints include systems accommodating total motion ranges in excess of about 130 mm.
There are many different types of expansion joints according to JASBC (1984), such as blind type, slit plate type, angle joint type, post fitting butt type, rubber joint type, steel-covered plate type, and steel finger type. According to ICE (2008), there are buried joints, asphaltic plug joints, nosing joints, reinforced elastomeric joints, elastomeric in metal runners joints, and cantilever combor tooth joints. Some examples of expansion joints used in bridge structures are shown in Fig.1.
Fig.1. Different bridge expansion joints.
Expansion joints should be installed as late as possible in bridge constructions allow for shrinkage, creep, and settlement movements to have taken place. Similar to other semi permanent members, the expansion joints should be designed so as to be easily replaced or reset in service stage.
2. Dynamic Behavior of Bridge Expansion Joints
If a bridge is subjected to a dynamic moving load, the response varies withthe time. Until recently, the dynamic requirements of bridge expansion joints to be taken into consideration were relatively less important.The design methods of the expansion joints were meant to deal with the structural problem in statistical terms by using the so-called dynamic factoror impact factor.
The role of expansion joints is to carry loads and provide safety to the traffic over the gap between a bridge and an abutment or between two bridges. A further requirement is a low noise level especially in densely populated area.
Therefore, expansion joints should be robust and suitable for static and dynamic actions. Movements of expansion joints depend largely on the size of the bridges and the arrangement of the bearings. In design phases, expansion joints are required to have movement capacity, bearing capacity for static and dynamic loading, water-tightness, low noise emission, and traffic safety.
On the basis of the fact that the failure due to impact loading is the main reason for the observed damages, we shall focus our attention to the impact factor for vehicle load that is governed by traffic impact because it differs from the static loading.
The cantilever-toothed aluminum joint (finger joint) is one of the promising joints under impact loading to overcome this difficulty. From the viewpoint of design methodology, numerical studies for impact behavior were conducted for aluminum alloy expansion joints withperforated dowels. The design impact factor for the expansion joints with the perforated dowels against traffic impact loading was examined by using numerical simulations, in which the dynamic amplification factor defined as the ratio of dynamic to static response was compared at various input-load patterns to the factors for expansion joints (Figs.2. and 3).
Fig.2. Bridge expansion joint
Fig.3. Aluminum alloy expansion joints
The mechanical characteristics can be considered as indicators of the dynamic behavior so that the durability of an expansion joint is nothing more than the maintenance of performance in time. The value of frequencies and damping of the different modes is the main indicator. In laboratories, these characteristics can be more or less simulated, but the correlation with the situ behavior is not so easy. More precise methods of impact effectcontrol must be established to facilitate the appreciation of the stage of the cumulative damage.
