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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.

Bridge Pavement functions and classifications

Bridge Pavement Functions and Classifications

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 for Tall Buildings

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.

Offshore Wind Turbines Support Structures Types

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.

Fig.6. Steel jackets on a barge

Movable and Fixed Oil Platforms Types

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.

Fig.12. Compliant tower

Outrigger Structures Types

1. Introduction

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

Shear Wall and Core System in Tall Buildings

Shear walls are widely used for both tall buildings and low-rise buildings. They are important structural members used in the lateral resisting system. They work as a deep vertical cantilevered beam supported at the ground. They also carry vertical load together with columns.

Some structures may require coupled shear walls, where girders and the floor system join the two or more walls together as a coupled system to provide morestiffness.

In tall buildings, shear walls are generally located at the center of the building, normally in the form of core wall system to accommodate the vertical translation system such as lifts for the tall building. It is a very common form of lateral load support system in tall buildings.

1. Type of Cores

There are two major types of cores: concrete core and steel framed cores. Concrete walls are used widely in the tall building design; on certain occasions, steel core can be found in buildings built before 9/11, they being much lighter, can save the cost of the foundation. However, they are gradually abandoned after 9/11 attack.

1.2 Concrete Core

Fig.1 shows a concrete core for a tall building under construction. Reinforced concrete cores are a more standard option for tall buildings in general, as seen from the history, concrete structure is dominant in the market because they provide more stiffness than steel cores, and it is relatively cheaper to use a concrete core in certain countries such as China.

In certain countries such as China, the steel production was not sufficient in the past; therefore, most of the tall buildings were built in concrete. In addition, some codes require that the core of the building be constructed using reinforced concrete in case of fire and for emergency safety.

When designing a concrete core, there are several issues that must be considered:

Constructability and construction sequencing: The erection sequence for building with a concrete core will see the casting of the core proceed ahead of the steel framing. The core and elevator shafts can therefore allow for the use of a climbing crane rather than having to use separate tower cranes.
is also quite common to install the steel embedment such as steel plate in the core to further strengthen the core. One of the project example is the China Zun Tower in Beijing.
Due to the shortening effect, differential movement may incur especially in the structural system of the building, which is a combination of steel and concrete, the tendency of concrete to creep or shrink over time and thermal expansion of steel need to be considered.

Fig.1. Concrete core of a tall building under construction

1.2 Steel-Framed Cores

Steel core was quite common for tall building design before 9/11 attack. Most tall buildings in the United States at that time were predominantly using the structural steel for the cores. The twin towers in World Trade Center is one of the examples. It used the steel core at the center of the building. Another example is Swiss Re Tower in London, also called as 30 St Mary Axe or Gherkin.

The main reason for using steel core is that it provides a light weight structure solution. The total weight of the structure is quite important for tall building design, as it will directly affect the foundation design. Therefore, a light weight solution will make a cost-effective foundation design possible.

However, the investigation of NIST NCSTAR “Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers,” shows that fire was the major cause of the collapse of the World Trade Center as majority of its structural members were steel. So for supertall buildings designed after 9/11, steel core is rarely used.

When designing the steel-framed cores, one need to check the lateral loads, fire protection, constructability, and erection sequencing. Among them, the most important issue is the fire protections, due to the lessons learnt from the twin tower. Different fire protection strategies can be considered, such as intumescent paint, board, spray, etc.

2. The Importance of Core Design

The structural system of Twin Towers is theso-called framed tube system. An internal steel core was used for TwinTowers, the floors were made of the steel composite truss floor system, under the fire, the truss starts sagging, which pulled inward on the perimeter columns: “This led to the inward bowing of the perimeter columns and failure of the south face of WTC 1 and the east face of WTC 2, initiating the collapse of each of the towers”.

As a result of 9/11 attack, more and more design engineers began to focus on how to design a tall building to be able to resist a similar attack. Therefore, a concrete core became one of the major choices for consideration. As discussed, core is also part of evacuation route when hazards happen,therefore, concrete core would also be a good option.

Foundation types and selection criteria

There are a number of foundation types available for geotechnical engineers.

1. Shallow foundations

Shallow foundations are the cheapest and most common type of foundations (Fig.1).

Shallow foundations are ideal for situations, when the soil immediately below the footing is strong enough to carry the building loads. In some situations soil imme-diately below the footing could be weak or compressible. In such situations, other foundation types need to be considered.

Fig.1. Shallow foundations

2. Mat foundations

Mat foundations are also known as raft foundations. Mat foundations, as the name implies, spread like a mat. The building load is distributed in a large area (Fig.2).

Fig.2. Mat foundations

3. Pile foundations

Piles are used when bearing soil is at a greater depth. In such situations, the load has to be transferred to the bearing soil stratum (Fig.3).

Fig.3. Pile foundations

4. Caissons

Caissons are nothing but larger piles. Instead of a pile, a group few large caissons can be utilized. In some situations, caissons could be the best alternative (Fig.4).

Fig.4. Caissons

5. Foundation selection criteria

Normally, all attempts are made to construct shallow foundations. This is the cheapest and fastest foundation type. The designer should look into bearing capacity and settle-ment when considering shallow foundations.

The geotechnical engineer needs to compute the bearing capacity of the soil immediately below the footing. If the bearing capacity is adequate, settlement needs to be computed. Settlement can be immediate or long-term. Immediate and long-term settlements should be computed (Fig.5).

Fig.5. Different foundation types

The Fig.5 shows a shallow foundation, mat foundation, pile group, and a caisson. A geotechnical engineer needs to investigate the feasibility of designing a shallow foundation due to its cheapness and ease of construction.

In the previous situation, it is clear that a weak soil layer just below the new fill may not be enough to support the shallow foundation. Settlement in soil due to loading of the footing also needs to be computed.

If shallow foundations are not feasible, then other options need to be investigated. Mat foundations can be designed to carry large loads in the presence of weak soils. Unfortunately, cost is a major issue with mat foundations. Piles can be installed as shown in the figure ending in the bearing stratum. In this situation, one needs to be careful of the second weak layer of soil below the bearing stratum.

Piles could fail due to punching into the weak stratum (Fig.6).The engineer needs to consider negative skin friction due to the new fill layer. Negative skin friction would reduce the capacity of piles (Fig.7).

Due to the new load of the added fill material, weak soil layer 1 would consolidate and settle. Settling soil would drag the piles down with it. This is known as negative skin friction or down drag.

Fig.6. Punching failure (soil punching into the weak soil beneath due to pile load)

Fig.7. Negative skin friction

Different Types of movable Bridges

Introduction

One of the great beneficiaries of globalization is the transport sector, especiallymaritime transport. With cost between the Far East and Europe of about$2 for aDVD player and$30 for a television set, even the longest way pays off! This hasled to an explosionlike increase of container traffic (e.g., between 2004 and 2005 inShanghai by 24%, in Dubai by 17%, and in Hamburg by 17%) .

Consequently, the number and size of container ships has increased permanently (Fig.1).

Fig.1. Development of container ships.

In places with sufficient space for long-ramp bridges, normally high-level bridgesare built (Fig.2). In places with restricted space, road bridges may still be built ashigh-level bridges, but railway bridges as low-level movable bridges (Fig.3).

Because in many ports high-level bridges are unfeasible due to the very restricted space, movable bridges have experienced a veritable renaissance during the lastdecades.

Fig.2. High-level bridge for road and railway traffic: The Za ́rate-Brazo Largo Bridgesacross the Parana ́River, Argentina

Fig.3. A high-level bridge for long-distance road traffic and a low-level bridge for local roadand railway traffic: the Strelasund Crossing at Stralsund, Germany

Lift bridges

Lift bridges are suitable for great spans, but their clearance is limited by the lift towers, which have a great impact on the environment, even when the bridge is closed (Fig.4). The cables linking the bridge and the counterweights may suffer fromsignificant wear.

Fig.4. Kattwyk lift bridge at Hamburg, Germany

The lift bridge has a free span of 50 m and a clearance above the low-water level of13.5 m when in service, and 40 m when opened. The lifting height, therefore, is 26.5 m.It consists of the bridge deck, a steel bridge with orthotropic plate, and four roundedtowers made of reinforced concrete (r.c.), which hoist (and hide) the concrete coun-terweights and machinery. Due to the graceful design of these towers, the often uglyappearance of lift bridges is avoide.

Swing bridges

Swing bridges are also suitable for great spans and do not limit the clearance. The biggest bridge of this type crosses the Suez Canal at El Ferdan, Egypt, with a free spanof about 300 m (Fig.5).

Fig.5.Swing bridge across the Suez Canal at El Ferdan, Egypt

The disadvantages of swing bridges include the following:

When opened, they occupy the embankment over a length of about their main span.
Due to geometrical reasons, it is impossible to have separate bridges for railways and high-ways in close vicinity.

Bascule bridges

Bascule bridges may have a single flap or two flaps and are also adequate for longspans without limiting the clearance. The connection between the two flaps may trans-mit shear forces only, or shear forces and bending moments. For great heights above the water, the counterweight may be attached to the reararm as a pendulum (Fig.6), for reduced heights it has to be integrated with it.

Fig.6. Sample of a bascule bridge with hang-on counterweight: Bridge across the Bay of Cadiz, Spain

Balance beam bridges (draw bridges)

Drawbridges, the precursors of bascule bridges, are most probably the oldest type ofmovable bridge (Fig.7). Compared to bascule bridges, they have the advantageof rather simple piers and a high architectural potential (Fig.8), but the disadvantage that they permit only rather reduced spans.

Fig.7. Vincent vanGogh – Langlois Bridge at Arles, France.Courtesy of Rheinisches Bildarchiv Köln

Fig.8. Diffené ́Bridge at Mannheim, Germany

Suspension Bridge Classification

In general, the suspension bridges can be classified according to theirspan numbers, the connection between stiffener girders, the layout of sus-penders, and anchoring conditions, etc.

1. According to Span Numbers

Based on the number of spans and towers, there are single-span, two-span,or three-span suspension bridges, as shown in Fig. 2. Among them,three-span suspension bridges with two main towers are the most commonlyused in engineering practice, like the Rainbow Bridge in Tokyo as shown in Fig. 3.

Fig.1 A suspension bridge in Izu, Japan

Fig.2. Suspension bridge classification according to span numbers. A) Single span.(B) Three-span. (C) Four (or multi) -span.

The Tsing Ma Bridge in Hong Kong and the Pingsheng Bridge in Guangdong are typical single-span suspension bridges, as shown in Figs. 4 and 5.

For multispan suspension bridges with more than two towers, the horizontal displacement of the tower tops due to live loads can be a concern and measures for controlling such displacement becomes necessary.

The Tamate Bridge built in 1928 in Japan is a typical multispan suspensionbridge, which is still in use now. Since then, several bridges were built inFrance (Pont de Château neuf-sur-Loire, 1932; Chatillon Bridge, 1951; and Bonny-sur-Loire Bridge, etc.), Switzerland (Giumaglio Footbridge).

Fig.3. The Rainbow Bridge, Tokyo

Fig.4. The Tsing Ma Bridge in Hong Kong

Fig.5. The Pingsheng Bridge Guangdong, China

Mozambique (Samora Machel Bridge, 1973), and Nepal (Dhodhara-Chandani Suspension Bridges, 2005). These bridges are generally built ina relatively short span except the Taizhou Yangtze River Bridge in China, which has three main towers and two main spans with a span length of1080 m, currently are the largest such suspension bridges.

2. According to Stiffening Girders

Based on the continuity, there are two types of stiffening girders, namelytwo-hinge or continuous types, as shown in Fig.6. Two hinge stiffening girders are commonly used for highway bridges, while the continuous stiffening girder is often used for combined highway-railway bridges to ensure the continuity between adjacent spans and to secure the smooth operation of the trains (Alampalli and Moreau, 2015).

The Akashi Kaikyo Bridge, the longest suspension bridge in the world, was designed with atwo hinged stiffening girder system.

Fig.6. Suspension bridge classification according to stiffener girders. (A) Two hinged stiffening girder. (B) Continuous stiffening girder.

3. According to Suspenders

In suspension bridges, suspenders (or hangers) can be designed as either ver-tical or diagonal, as shown in Fig.7. Vertical suspenders are more oftenused in suspension bridges, but diagonal hangers are sometimes used for the sake of increasing the damping and improving the seismic performance ofsuch bridges. For higher stiffness of a cable supported bridge, a combinedsuspension and cable-stayed cable system can also be used.

Fig.7. Suspension bridge classification according to suspenders. (A) Vertical sus-penders. (B) Inclined suspenders.

4. According to Anchoring Conditions

Based on anchoring conditions, the suspension bridges can be classified intoexternally anchored or self-anchored types, as shown in Fig.8. For externally anchored suspension bridges, the anchorages need to be built on both ends ofthe bridges to sustain the tensile forces from the main cable, which is the mostcommon type of suspension bridges.

As for self-anchored suspension bridges,the anchorages are not necessary and main cables are connected directly to thestiffening girders. In this case, however, relatively large axial compressiveforces need to be carried by the main girder and this should be consideredin the design. The San Francisco Oakland Bay Bridge and Konohana Bridgein Osaka (Fig.9) are typical self-anchored suspension bridges.

Fig.8. Suspension bridge classification according to anchors. (A) Externallyanchored suspension bridges. (B) Self anchored suspension bridges.

Fig.9. The Konohana Bridge (self-anchored suspension bridge) in Osaka, Japan.