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.
The pavement is the important portion of the bridge deck that vehicles come in direct touch, and a structurally sound, smooth riding, and long lasting pavement is very important for bridge users.
The bridge pavement is used for protecting the slab deck from the impact due to traffic load, rain-water, and other meteorological conditions, and providing durable and comfortable traffic conditions.
A rough pavement is uncomfortable to the drivers, and a quality pavement should be designed and constructed according to appreciate design specifications for the pavement.
1. Functions and Requirement of Bridge Pavement
The main functions of bridge pavement include:
(1) prevent the vehicle tie or the caterpillar track directly wear the bridge deck
(2) protect the bridge deck and main girder from water erosion
(3) dispersion of the concentrated truck load
Bridge pavement quality is important for the survivability and durability of the bridge structures. Nonetheless, bridge deck pavements must meet a large number of requirements related to strength, wear-resisting, crack-resisting, antiskid, and good integral with bridge deck.
The bridge pavement shall have adequate resistance to permanent deformation, vehicle sliding without cracking, etc. It also must protect and seal the underlying supporting structure as this determines to the durability of bridge superstructure.
The pavement should also be able to absorb traffic loads and transfer them tothe deck and supporting structures but remain even within allowable deformation and provide good antiskid conditions for vehicles. Besides, they must protect the bridge structure from surface water.
2. Classifications of Bridge Pavement
Cement concrete pavement and asphalt pavement are most often used pavement method in bridge structures. Cement concrete pavement has advantages like less expensive, wear-resisting, suitable for high-traffic bridge,environmental sustainability, durability, and requires less repair and maintenance over time, but requires longer curing time. On the other hand, the asphalt pavement requires less curing time and has lighter weight, easy repair or replacement, but easy to get aging and deform.
A cement concrete pavement shall be constructed simultaneously with the slab concrete in order to form an integral structure. If casted separately with slab concrete, the pavement concrete will be vulnerable to drying shrinkage crack because of their relatively thin thickness.
There is a concern of stripping due to bridge vibration, impact of vehicles, and rain water permeation. For this reason, the pavement concrete and deck concrete should be constructed at the same time but not separately. If rainwater permeates the concrete slab, notonly causing the corrosion of the reinforcements or any other structural steel in the concrete, but also accelerate the concrete deterioration, particular for the deck under repeated load in the service condition.
These have remarkable effect on the durability and load carrying capacity of the bridge. Because of this, adequate sealing measures should be taken not only for the concrete deck but also for the members near to expansion joint and other accessory devices.
In general, the asphalt bridge pavement system consists four different layers: a sealing layer, a waterproofing layer, a protecting layer, and surface layer (asphalt). For an asphalt pavement, a waterproofing layer should be used to present rainwater permeation. Although different application techniques and materials can be used on steel and concrete bridge decks, the general construction steps on a bridge deck starts by surfacing of thedeck, followed by sealing layer, a waterproofing layer, a protecting layer, and the surface layer on top.
The surface and subsurface drainage system should be applied on both steel and concrete decks. The sealing layer can be made from various materials, including bituminous materials. The bridge pavement surface is generally built as a parabola curve with the cross slope of 1.5%–2%. The sidewalk pavement surface is usually built as straight line with a cross slope of 1%.
Cladding is prefabricated panels that are attached to the structural frame of the building. The main function of cladding is to prevent the transmission of sound, provide thermal insulation, create an external facade, and prevent the spread of fire.
There are different cladding systems, such as curtain wall, metal curtain, stone cladding, brick claddings, precast concrete, and timber cladding.
Cladding systems are non structural elements. However, cladding can play a structural role in transferring wind loads, impact loads, and self-weight back to the structural framework. In particular, wind causes positive and negative pressure on the surface of buildings, so cladding must be designed to have adequate strength and stiffness to resist this load, both in terms of the type of cladding selected and its connections back to the structure.
Particularly in tall buildings, the wind pressure on the glazing is one of the important design considerations, this is because if one glazing fails, when it is fallings off, it will also hit the glazing on the below floors, which will cause a continues failure of the glazing.
Fig.1 shows the typical connection between the facade to structural members. On tall buildings, access systems must be provided to cladding system allowing regular inspection, maintenance, cleaning, and replacement (in particular, replacement of external seals).
Fig.1.Connection details of cladding to structural members
Curtain wall are used widely for tall buildings. Typically, curtain wall systems comprise a light weight frame onto which glazed or opaque infill panels can be fixed. These infill panels are often described as ‘glazing’ whether or not they are made of glass as shown in Figs.2 and 3.
Fig.2.A typical cladding of a tall building in Hague, Netherland
Fig.3. A glass curtain wall example of a building in Delft, Netherland
The frames play an important role in transferring loads back to the primary structure of the building and accommodating differential movement and deflection. Therefore, needs to be designed in detail. In some companies, therefore special fac ade design team to handle it.
As the OWTs are located on the sea, therefore, support structure and its foundation are important for design considerations.
There are several conventional bottom-mounted support structures, which can be categorized into five basic types:
monopile structures (using pile foundation),
tripod structures (using pile foundation),
lattice structures (using pile foundation),
jacket foundations,
gravity structures,
floating structures.
There are also hybrid support structures which use the combined features of the above categorized structures. For the first three type of supporting structures (monopole, tripod, lattice structures) pile as the foundation is usually used. Pile foundations are one of the most common forms of offshore structures; they are widely used for both offshore oil platform and OWT.
The standard method of piling method is to lift or float the structure into position and then drive the piles into the seabed using either steam or hydraulic powered hammers.
1. Monopile Structures
As shown Fig.1, monopole has the simple fabrication and installation.The tower of the turbine directly sits on one pile. Monopile foundations are one of the most frequently used support structure to date. Most of the offshore wind farms in shallow waters are monopole structures, which have the advantage of simple design for manufacturing.
However, failure of the grouted connections between the monopile andthe transition piece is one of its disadvantages. This transition piece is responsible for connecting the monopile to the turbine tower. In addition, there is no proven solution using monopiles for larger turbines with 5 MW or more powerful turbines.
Fig. 1. Wind turbine on the monopile foundation
2. Tripod and Lattice
As shown in Fig.2, the turbines directly sit on a tripod or a lattice, which are supported on the pile foundations. The tower can be further stabilized by the tripod.
Fig.2. Wind turbine on tripod
3. Gravity Foundations
As shown in Fig.3, this type of foundation achieves its stability solely by providing sufficient dead loads by means of its own gravity. Ballast can be pumped-in sand, concrete, rock, or iron ore to add extra weight. Gravity structures are suitable for modest environmental loads such as wave load that are relatively small and dead load is significant or when additional ballast can easily be provided at a modest cost.
The gravity base structure is especially suited where the installation of the support structure cannot be performed by a heavy lift vessel or other special offshore installation vessels, either because of non availability or prohibitive costs of mobilizing the vessels to the site.
Gravity-base foundations are the second most popular sort of support structure to date. They have been used mostly to support smaller turbines in shallow waters near shore locations with a rocky seabed where the operation of piling is extremely complicated and expensive. However, for waterdepth beyond 35 m, the new generation of wind farms are needed.
Fig.3. Wind turbine on the gravity-base foundation
4. Floating Structures
Floating structures are especially competitive at large water depths where the depth makes the conventional bottom-supported structures non-competitive. Detailed design guideline can refer to DNV-OS-J103,” Design of Floating Wind Turbine Structures”.Fig.4 shows a floating form using space frame style floater.Fig.5 gives the other two examples.
Fig.4. Floating wind turbines
Fig.5 Floating wind turbines examples
5. Jacket Foundations for Offshore Wind Structure
As shown in Fig.6, jacket foundation uses four-legged jackets to supportthe OWTs, which can support larger OWTs such as 6 MW turbines. Jacket foundations provide a solution for foundations in offshore wind farms in water depths of 35 m and beyond which is less risky, less expensive, and more reliable than monopiles and gravity-base foundations.
While choosing the correct type of platform, several factors need to be considered: such as its function and the water depth. Based on these two factors the oil platforms are designed into two major categories: one is movable oil platform, the other kind is fixed platform.
1. Movable Oil Platform
1.1 Drilling Barges
Drilling barges are shown in Fig.1. They are mostly used for shallow,in land waters; this would typically be lakes, rivers, and canals. When the drilling barge tends to be large floating platforms must be moved using a tugboat to the location of drilling. They are not suitable for large open waters as they are not able to with stand water movement.
Fig.1. Drilling Barge
1.2 Drillship
As shown in Fig.2, drill ships are ships with the drilling platform installed inthe middle of the ship deck to allow the drilling string to reach the sea bed through ship’s. Drill ships can drill even in deep waters. Drill ships maintains its position using dynamic positioning systems and other sensors. The ships also use satellite-positioning technology with motors integrated below the hull to position the ship directly above the drill site.
Fig.2. DrillShip
1.3 Jack-Up Platforms
Jack-up platforms (Fig.3) are mobile drilling platforms made of floatable deck with three or four legs lowered on to the seabed upon being towed to the drill site. The platform is then towed to the location with raised legs. It can raise its hull over the surface of the sea. Jack-up rigs are suitable for shallow waters with a maximum depth of water of up to 130 m in gentle water and up to 70 m in harsh waters. Jack-up platforms tend to be a safe alternative than drilling barge since during operation they operate similarly to a fixed platform.
Fig.3. Jack-up rig
1.4 Submersible Platforms
As shown in Fig.4, submersible platforms are mobile structures which are designed that can be floated to location and lowered onto the sea floor for offshore drilling activities. However, due to its own feature, it is only limited to shallow waters.
It normally consists of two platforms one is on top of another. The upper platform is the actual drilling platform, the lower platform provides buoyancy during towing of the platform between drilling sites.Once the platform has been positioned, air is let out of the hull and then it submerges to the sea floor. Due to the design of the platform, it is limited to shallow waters.
Fig.4. Submersible Platform
1.5 Semi-Submersible Platforms
As shown in Fig.5, semi-submersible platforms are similar in design to submersible platforms with same principles. As the lower hull can be inflated and deflated this type of platform is only partially submerged and uses water to fill the bottom hull for buoyancy. A semi-submersible platform can be used to drill in deep water depths of 1800 m.
The depth of the lower hullthat submerges into the water is predetermined, and the platform is held in position by anchors weighing upwards of 10 tones. The dynamic positioning can also be used for the same purpose, and this can ensure safe and stable conditions of the platforms.
Fig.5 Semi-submersible platform
2. Fixed Type Oil Platform
The fixed platforms are normally used in shallow waters. They have the legs which are made of concrete or steel tubular structures anchored to the seabed to ensure stability of the platforms. However, the cost of these types of permanent platform would be too much to use in deep water conditions, therefore, if most of them are used in the shallow sea water, except for compliant tower (CT), which can be used for>600 m. There are several types of fixed platforms such as, Jacket platforms, tension leg platform(TLP), gravity platform, or even an artificial island.
2.1 Jacket Offshore Platforms
As shown in Fig.6, jacket platforms are one of the most commonly used fixed type of platform, about 95% of the offshore platforms in the world are using jacket platform. The platform is supported by steel space frame which consists of plate girder or deck truss structure supported by welded tubular that is piled to the sea floor. The steel frame is called jacket. It is used in water when the depth does not exceed 500 m.
Fig.6. Jacket offshore platforms
2.2 Tension Leg Platforms
Fig.7 shows the TLPs. TLPs is evolved from semi-submersible platforms, which consist of a buoyant floating platform kept in position by pretensioned anchors fixed to the seabed using vertical mooring lines called tension leg. Although vertical movement is restricted, they do allow for significant sideway movements. The pretensioning of the tethers is huge in size to avoid compression due to waves.
Typically, thetethers consist of a 12- to 15-m-long section connected by welds or threaded joints, with a pile or gravity skirt foundation into the seabed.To further increase the stability of the platform the lower hull is filled with water during drilling to resist movement of ocean and wind forces.The TLPs can operate in waters up to 2.13-km-deep waters. Sea star platforms are a miniature version of a TLP made for waters of depths up to 1 km.
Fig.7. Tension leg platform
2.3 Gravity Platforms
Gravity platforms consist of a steel deck and concrete framework as well as as teel skirt foundation where petroleum is normally stored in the skirts. Gravity platforms have been made for water depths of up to 300 m in harsh waters.
They are built in an upright position and towed out to the off shore drilling site and installed by ballasting, where a heavy substance is used to provide floating stability.
Gravity platforms are cost-efficient option dueto reusability of a platform as the structure can be towed to another location.A gravity platform is shown in Fig.8
Fig.8 A gravity platform
2.4 Spar Platform
This is one of the largest fixed type offshore platforms. The spar is a lowmotion floater that can support full drilling assistance of flexible or steel cat-enary risers.
A large platform consists of a large cylinder that does not extendall the way to the seafloor but is tethered to the seabed by cables and lines.
A cylinder is used to stabilize the platform in the water and allows the move-ment to absorb the force of the hurricanes.
There are different types of spars: original cylindrical classic spar, truss spar, the cell spar, the Arctic Spar, and spar with storage.
Three main types of configurations of Spar platform cylinders are mentioned below:
Conventional spar: One-piece cylindrical hull, as shown in Fig.9.
Truss spar: Midsection is made of truss elements as shown in Fig.10.
Cell spar: Midsection made of multiple vertical cylinders, as shown in Fig.11.
Fig.9 Conventional spar
Fig.10 Truss spar
Fig.11 Cell spar (Skaug, 1998)
2.5 Compliant Towers
A CT is a fixed rig which is similar to the fixed platform, but consists of anarrow tower attached to piled foundations on the seafloor and extend up to the platform shown in Fig.12.
In deep water, in order to prevent excessive amplification of wind, waves, and current, the natural period of the bottom founded structure should be substantially different to the dominant period of hurricane. Therefore, one of the methods to achieve this is to be able to control the mass and stiffness of the rigs.
The advantage of CTs use the flex elements such as flex legs and axial tube, which can control its natural periods therefore resonance is reduced and wave forces are de-amplified. Therefore, they are designed to sustain significant lateral deflections and forces, and are typically used in water depths ranging from 450 to 900 m.
The concept of outrigger dates back to 50 years, it origined in deepbeams. It has been derived from deep beam into concrete walls, and now in the form of one or several story outrigger trusses.
Outriggers are one of the most widely used systems for relative regular floor plan. It is constructed using steel trusses, girders, concrete walls, or deep beams to connect the core and the columns at the perimeter.
The outrigger trusses are normally one-story high, some even occupy several storys. The cores are normally located at the center of the building, whereas the outriggers extend out to the outer columns (as it is shown in Fig.1).
Therefore, the outriggers and the outer columns work together as a further restrain to the core wall. Under lateral load, the belt trusses act as lever arms that directly transfer axial stresses to the perimeter columns. The bending, axial tension, and compression of the outer columns connected to the outriggers help resist the external moments of the structure. This resistance enhances the overall stiffness of the core, helps in reducing the lateral deflections, and overturning moments.
The outrigger columns work together particularly helping to restrain the rotation of the core. Overall, major advantage of using the outrigger is to resist the rotation of the core and significantly reduce the lateral deflection and overturning moment.
One of the famous examples of this system is Shard, London BridgeTower (Fig.2). It has core wall at the center and outrigger truss at highlevels, inside the plant room, to connect the central core and outer raking columns.
Fig.1. Outrigger structures
Fig.2. The Shard.(Adapted and reuse with the permission of Asset bank, City, Universityof London)
If the outrigger is used together with external tube systems, it can more evenly distribute the large vertical forces applied by outriggers across the multiple columns. Analysis and design of a core-and-outrigger system requires the use of computer program. This is because the distribution of forces between the core and the outrigger system are determined by the relative stiffness of eachelement: the core, the outrigger, and the columns. Therefore, it is difficult to calculate manually.
2. Types of Outriggers
There are several different types of outrigger system, such as steel outriggers, concrete outriggers, and hybrid outrigger (using both concrete and steel material). Among them, steel outriggers are most conventional type outriggers.
The famous examples are: Twin Tower (collapsed in the 9/11 attack) and the Shard in London. Concrete outriggers are used in some tall buildings. One of the famous examples is 432 Park Avenue building in NewYork.
With the development of the construction technology, new types of outriggers such as hybrid outriggers and damped outriggers have emerged in the construction projects.
2.1 Steel Outriggers
Steel outrigger systems are extensively used in a lot of tall buildings as most of tall buildings are either steel or composite structural system. In the conventional design, the outrigger is designed to be a story height truss.
2.2 Concrete Outriggers
The benefit of concrete outrigger system verses steel is high stiffness and lowcost. Under wind load cases, the outrigger system needs to be of stiff concrete deep beam or of concrete wall which can be easily achieved by this.
Fig. 3 shows a typical outrigger using concrete wall. This type of system is more common in a concrete structure rather than in a steel frame structure.
Fig.3. A typical outrigger using concrete wall modeled using ETABS
2.3 Hybrid Outriggers
The steel outrigger is not as stiff as concrete outrigger. However, a pure concrete outrigger system is very brittle.
An innovative type of steel-concrete hybrid outrigger truss was developed in two 370-m tall mega-high-rise towers in Raffles City Chongqing, in which the steel truss is embedded into the reinforced concrete outrigger wall as shown in Fig.4. Both the steel truss and the concrete outrigger wall work compositely t oenhance the overall structural performance of the tower structures under extreme loads.
Fig.4. Fused outriggers (a concept originally developed by Arup)
2.4 Damped Outrigger
In the event of severe earthquake, the overall structural system should be able to dissipate energy and maintain its robustness against the collapse. Additional viscous dampers can be installed in the outrigger for a nonlinear response and tuned to meet multilevel performance objectives.
In case the dampers fail, the outriggers which is designed to yield in a ductile manner will remain intact. Thus, it can reduce wind-induced vibration and can also be used as fuse to protect the building under a severe earthquake condition.
Fig.5. Arup Damped Outrigger system adopted in St Francis Towers
