Bridges as a Very Integral Part of the Transportation System - Essay Example

?As junior site manager it is very essential that I have knowledge about all the components and structures that are made in a highway and in its adjacent areas. The most important among which are the roads and connecting bridges. In addition frame structures are also a common when we move on a highway through industrial areas. In this report the first component I would discuss would be Bridges. Bridges Bridges are very integral part of the transportation system and if the bridge fails the system might also fail resulting into heavy destabilization of logistics services of the area. Another aspect that makes bridges a key element is that it has the highest cost per kilometer in the system, which makes its selection and length a detrimental part of the project and its cost. The next stage is the selection of the bridges, there are several types of bridges and they differ on many basis which can be material, span and inter-span relation, geometry of members and bridge, maximum loading and its patterns and lastly and most importantly the form of superstructures. Generally the first and the primary basis of differentiation is the superstructure, there are many types if bridges on that basis which include Truss, Beam/girder, Arch, Suspension, Swing, bascules and many others, some of which are shown in the figure below. For the time being we will consider only three most important as a part of our discussion, which are Beam bridges, Arch Bridges and Suspension Bridges. Beam Bridges The beam bridges are the most basic and ancient type of bridges they consist of just plank or beam that runs along the abutment/banks. The figure shows some of the components of the beam bridge, These beam bridges are normally used for smaller spans which are from 10-50m which is normally a short range for bridges. The primary reason for this is the force distribution pattern of the beam to the abutment. Then these beams are made of materials that must withstand high tension and compression forces and the rough diagram below shows the distribution of these forces in the beam. Because the materials used in the construction are normally heavy sections of RCC, Prestressed concrete and heavy weight steel beams which makes the span less. Then the material used for the construction of these types of bridges also determines the type of on site activities and requirements. In highways beam bridges are used for underpasses and other small intersectional roads. Whether its concrete bridge or it is a steel bridge in either case the beams are precasted or pre fabricated and they are transported to the site where they are connected through simple construction processes. As the construction of components are done in some fabrication facility far from the actual site that gives these type of bridges a green signal in environmental issues when compared to other types and do not need any plants onsite. Then there is rare use of heavy machinery in its construction so no major problems regarding technical aspects are encountered. As shown in the figure above as well these bridges are made with the several spans so no problem is faced with the expansion joints which are normally given in between the spans. The following picture shows during the construction of Beam Bridge and the next picture shows the completed state. Arch Bridges The second oldest type of bridges are the Arch Bridges, this type of assembly is seen in several roman bridges. As shown in the figure Arch bridges are curved structures which have very high resistance to bending moments, this is achieved when the material is compressed under load, as a result the columns become thin compression members and transfer these compression forces into the abutments and foundations. The force transfer and parts of Arch bridge is shown in the figure below. Arch types of bridges are good choice for crossing valleys and rivers since the arch doesn’t require piers in the center. On the other side there is a restriction as well which is that Arch bridges can only be used where the foundation or ground is solid, stable and has high bearing capacity. If this is not possible then construction of heavy abutment is mandatory which will make this type of bridge lesser economically feasible. The first arch bridges were made of stones and masonry which are weak material and thus the span of these bridges were only limited to 15 m. But with vast advancements materials such as concrete & steel and better understanding of structural responses and engineering we are now able to utilize Arch bridges for spans upto 300 m in case of concrete and as high as 500m while using Steel as the material of constructions. Then comes the construction process and principle of works in Arch Bridges, the first step is the diversion of water stream where arches are to be found and there gravels are excavated to a good depth, from which foundation columns/peirs are raised to the base of the arch or a point also known as Springing. Then falsework centering is fabricated. It is to be noted that in case of multiarch system all arches must be raised at same time. The thrust is taken into the earth through the footing of the end each arches or it may be done by large inclined planes ramps to the bridge. Later in the construction several arches are made over the centering. After the completion of arch barrel, the arches are stabilized with infill within he arches laid in horizontal running bond courses. These two walls are also known as Spandrel which are infilled with less compacted materials. Lastly it’s the road which is made and railings/parapet wall for protection and guidance of traffic. There are several other techniques of constructions which vary with span length, materials and location of the bridges. The longest arch bridge in the world (until last year) was the New River Gorge Bridge in West Virginia, built in 1977. It has a central span of 1700 feet and a total length of 4224 feet. The Lupu Bridge in Shanghai now exceeds it by 105 feet. The New River Gorge Bridge is still the highest bridge; it rises 360 feet above the river and weighs 88,000,000 lbs. The following pictures show some of Arch bridges of different materials and in diverse location. Suspension Bridges A typical suspension bridge is a continuous deck with one or more towers erected above piers in the middle of span. The deck maybe of truss or box girder. Cables pass over the saddle which allows free sliding. At both ends large anchors are placed to hold the ends of the cables. Suspension bridge needs to have very strong main cable; Cables are anchored at the abutment so abutment has to be massive to withstand the massive forces. A typical suspension bridge is a continuous girder/deck/truss with one or several towers erected above piers in mid span. To allow free sliding cables run over the saddle as shown in the figure. Large anchorage is provided at the abutment on both ends to grip the cables in high tension. The main cables need to be extremely strong in tension as compared to suspender cables. Then comes the force transfer from the deck to vertical cable to the main cables and eventually into the foundation and abutments as shown in the figures and diagrams below. Now the major question of suitability and advantages of having suspension bridges over the other ones. Suspension bridges are used where a large span are required in minimum no of piers and is normally used for large water bodies such a rivers or even connecting two countries. The span of suspension bridges can go as high as 2000m. The Akashi Kaikyo Bridge is the largest suspension bridge in the world, with a span of 1991 m, and the Great Belt Bridge is the second largest, with a span of 1624 m. The construction process is very length and extended where as I would try to summarize some of the activities that a junior site manager must know about. The first process that involves the construction of underwater piers, the procedure of pile driving is dependant on the bed conditions. Whereas for towers on dry land or onshore deep excavation and foundations are made. Due to the high cost of steel the concrete is used for the construction of tower these days. Once the towers are constructed then the saddles are placed on the top of these towers so that later the main cables are to be put onto them. The anchorage is then constructed or it can also be build simultaneously with the towers, these are usually constructed to withstand the high tensile forces of the main cables. For which the old concept of huge concrete mass is used to hold bridge’s cables together. After this stage next come the construction of Catwalks which are temporary suspended walkways. These are erected by the help of guide wires positioned into the place by winches at top of the towers. The shape of the Catwalks is similar to that of the curve of the main cables and which is also described as catenary., usually the catwalks are 8-10 feet in dimension. Then comes gantries which are positioned on these catwalks and this will support the main cable spinning reel as well. The cables are connected by the help of winch and main cable spinning device is fixed. Later the high strength usually made of high strength galvanized steel wire stranded ropes are pulled by traveler from the opposite side and the cable strand is complete, after which the workers connect it to the remove the individual wrap of the cables. After the work on the main cables is complete then comes the suspender cables, which are cut to exact dimensions and installed in precise positions and level. These cables upon completion will give the final iconic parabolic shape to the suspension bridge and an arc is made. These types of bridges were usually made with steel in the start of their inception but as time proceed steel became expensive, on the other side new, better high strength concrete was made some of the components started to shift there basics, this include the towers, anchors and the deck where as there is no parallel to the tensile capacity of steel so all parts in tension are still made by steel. The suspension bridges although have a huge price tag but no other options are available for long spans and its complete understanding of mechanics and engineering have made it a reliable choice as well. Industrial Buildings A junior site manager must have basic knowledge of frame structures, normally it is seen that industrial areas are established on the highways, the reason for which are firstly it provides best place for easy and effective logistics and secondly they are away from urban areas. On the other hand many of these framed structures are utilized in highways as intersections, toll plaza and short span portal frame bridges and many others. The framing of structures is primarily differentiated on the basis of materials of construction materials. According to which there are three types of frames timber frames, steel frames and concrete frames. In each of these types there are further classifications based on geometry, spaces, force systems and many others. At present only two out of the three materials are used for framing, which include steel and concrete. Both of these have their respective advantages and disadvantages. On one side steel is a material that is recyclable, quick to construct, has lesser environmental impact on site area and is more reliable for designer as it shows homogenous behaviors. Whereas on the other hand concrete structures have lesser initial cost, used for more massive structures, has less maintenance requirements and can be made from variety of material and aggregate combinations. For this purpose we shall discuss both types of structure at length. Steel Frame Structures Steel frame structures are built in mainly two stages first is the fabrication phase and the second is the erection or assembling phase. As the names are self explanatory the fabrication is the phase which involves processing and manufacturing of Cold-formed steel products. This is done in facility that is far away from the site area. This process start with the hot rolled coils and then later these are hot coil has been rolled to the desired thickness and after it has cooled, the ribbon of steel passes through a series of rollers to form the desired products. These products are made of many shapes and sizes some of which are shown in figure below. After these sections and members are fabricated they are designed by the designers to form several types of structures. The next phase is the erection phase where these components are connected to each other to form a frame structure. Erection includes all rigging, hoisting, or lifting of members to their proper places in the structure and making the finished connections between members. Steel construction is classified into mainly three types which include wall-bearing construction, skeleton construction, and long-span construction. In the wall-bearing construction the exterior and interior masonry walls are bearing loads of all the steel beams and components. This type is used where there is only one storey and the loads are not excessive, normally used for houses. Then comes the skeleton construction in which all the load bearing components are made of different steel sections and a skeleton is formed by the connecting steel columns and beams as shown in the picture below. The next type of construction is long span construction, this is almost same as that of skeleton construction the only difference is that this type has a longer span and consequently lesser no of columns are required. This is achieved by introducing truss or bar joist into the roofing system, as shown in the figure above. The structures made from steel frames include pre-engineered metal building, Towers and truss bridges, agricultural buildings, green house, warehouses, storage facility, air hangers and Industrial buildings. Among these, the building of our interest is the industrial building as these come in interaction several times. The post common type of steel frame structure that is used for industrial buildings is Portal Frame structure. Portal Frame Structure Before the portal frame, the truss and column arrangement was used which required excessive connecting members and labour. Whereas the rigid portal frame enables us to have more span upto 45m and highly economical structures. This type saves cost due to lesser fabrication and simpler erection. The main components of the portal frames are shown in the figure below. As shown in the figure above this type of portal frames are used in facilities like factory buildings, large span storage buildings and Heavy industrial process plant structures. The portal frames are highly suitable for covering large areas of land in lesser cost and for more space requirement. The rapid construction and design of such structures make it most suitable for facility which have money to time relation and the early commissioning of the building can save some time and cost to the owner. Then it is also suitable for temporary works as well the reason for that is the structure can be disassemble and moved to some other place. This is not possible for concrete structures. The portal frame in specific and steel frames in general are more environment friendly as to concrete frames, there are several reasons for that firstly the components used in steel frames are fabricated some place far from the site hence no wastage is seen on site and no environmental issues are faced due to neat process of errection. Secondly the production of concrete take a lot of aggregate which is brought from excavation of land, on another level very high amount of water is used for concrete production which makes it steel frame structures as more eco-friendly. On the other side there are some negative aspects to steel frames as well, which include the possibility of corrosion to the members and high price tag for the maintenance works. Another setback for using steel frame is that extra insulation is to be provided to maintain certain temperatures in the interior space and the insulation material cost some times become excessive. Roads and Highway Pavements The road and highways are normally also known as pavements when discussing in engineering terms. Pavements and all roads are generally classified into two major types first is the flexible pavement which normally consists of bituminous materials and the second is Rigid pavement where the basic material of construction is cement and concrete. Both of the types have very different construction and load distribution patterns. Thus the materials used along with the construction differ immensely. We would carry on the discussion on both Flexible and Rigid types simultaneously on each aspect. As the figure shows the main components of Flexible pavement are Subbase, base and wearing surfaces of various types. Whereas on the other side the Rigid Pavement has only Subbase, Portland cement concrete slab as its main component. The next figure is a section in detail of roads both rigid and flexible and this will help us understand the different components and parts to be constructed, it is a further extension to the figure shown before. Now there is a question of the suitability of these types of roads. The rigid road are normally used where the ground condition beneath the road are not supportive and have less bearing strength in addition if the water table in the region is high. Another reason for the construction of rigid pavement is its strength against the heavy loading. Rigid pavement is used in those areas of the transportation system where heavy loading is expected and weak soil conditions are encountered. Where as on the basis of economics Rigid roads have high construction cost as compared to flexible/ bituminous materials. On the other side, flexible pavements can be used anywhere and it does not need that much technical expertise as compared to Rigid. Flexible pavements are normally less cost than rigid and as there is almost no usage of cement or steel. Flexible pavement is used for lesser important roads as well. It is also used in areas where higher road grip and resistance is required, for instance mountainous areas and high grade roads sections. Flexible pavement needs more drainage and preventive measure against high water table is mandatory for longer service life of flexible pavements. ROADS COMPONENTS, MATERIALS AND THEIR FUNCTION Flexible pavements Surfacing: Normally consists of bituminous surface dressing or a layer of premixed bituminous material. Where premixed materials are laid in two layers, these are known as the wearing course and the base course (or binder course). The functions of the surfacing are to (i) provide a smooth wearing surface (ii) reduces the dust, the dust affects the visibility and increases chances of accident (iii) provide a sealing surface so that water should not go into the subgrade. Base: Normally consists of highest quality crushed stone or gravel, or of gravelly soils, decomposed rock, sands and sand-clays stabilized with cement, lime or bitumen. (i) This is the main load-spreading layer of the pavement. It reduces the vertical compressive stress induced by traffic in the subbase and the subgrade to a level at which no unacceptable deformation will occur in these layers. (ii) Unbound bases perform their function solely by virtue of their thickness and state of compaction. Bound bases perform the same function by a combination of thickness and stiffness. The base courses are generally extended about 1 ft beyond the edge of the pavement in order to support the loads applied at the edge of the pavement. Subbase: Normally consists of materials of lower quality than that used in the road base such as unprocessed natural gravel, gravel sand, or gravel-sand-clay. It has the following functions: (i) It is a secondary load-spreading layer underlying the road base. It enables the traffic stresses to be reduced to a level acceptable in the subgrade. It can accept greater compressive stresses than the subgrade, and thus reduces the deformation of the pavement under traffic loading. (ii) It provides a working platform over which the construction plant can operate without the formation of flitting when the layer above is being placed. It is noted that flitting to depth of 40 mm can be reshaped and compacted efficiently. (iii) If thick enough, it prevents frost from penetrating into frost susceptible subgrades. (iv) Under special circumstances, it may also act as a filter or a drainage layer. (v) Prevents the clay working up into the base. (vi) Prevents swelling of subgrade soil. (vii) Its material being cheaper than that for base, total cost for the road construction may reduce. Capping Layer: (selected or improved subgrade): A capping layer is sometimes provided above the natural subgrade where very weak subgrade (CBR < 4%) is encountered. It consists of better quality subgrade material imported from elsewhere or existing subgrade material improved by mechanical or chemical stabilization. Subgrade: This is the upper layer of the natural soil which may be undisturbed local material or may be soil excavated elsewhere and placed as fill. In either case it is compacted during construction to give added strength. The surface of the subgrade is termed the formation level. Rigid Pavements Concrete Slab: It acts as a wearing surface and provides the major portion of the structural capacity. Therefore structural strength of concrete is the main consideration. For this reason, minor variations in subgrade strength have little influence upon the structural capacity of the pavement. Concrete pavements may be reinforced with steel mesh or they may be unreinforced. Under heavy traffic loading, wearing surface is provided by a bituminous carpet, the concrete slab then becomes the base and only has to distribute the load. Subbase: Subbase lends some structural capacity to the pavement; however, its contribution to the total load carrying capacity is relatively minor. It may be used under the concrete slab for various reasons, including (i) control of pumping (ii) control of frost action (iii) drainage (iv) control of shrink and swell of subgrade (v) expedition of construction Load Distribution Characteristics A rigid pavement, because of its rigidity and high modulus of elasticity, tends to distribute the load over a relatively wider area of soil. If there is some permanent set in the subgrade soil, the slab will deflect under the wheel load. If the mobilized elasticity is less than that of the concrete slab, the slab will move to its original position. However, if the mobilized elasticity is more i.e. settlement under the wheel load is more than the elastic limit, the slab will fail, cracks will be pronounced in the slab. The load carrying capacity of flexible pavements is brought about by the load distribution characteristics of the layered system. Since there is no rigid connection between the various constituent particles, any permanent set in the subgrade will move upto the top layer. Factors Affecting Road Performance There are several issues and factors that affect the pavement some of which are common for both where as others are specific. Some of which are mentioned in the followings. A. Rigid and Flexible (applies to both) 1. Traffic a. Gross load and Lyre pressure b. Properties of subgrade and paving materials c. Repetition of load d. Radius of Load influence e. Speed f. Axle and wheel configuration 2. Climate a. Rainfall b. Shrinkage and swell c. Freeze-thaw and wet-dry d. Frost heave 3. Geometry a. Traffic distribution across pavement 4. Position a. Cut-and-fill sections b. Depth to water table c. Landslides and related problems d. Deep soft deposits B. Rigid Pavements 1. Climate a. 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