Civil Engineering Construction and Types of Bridges - Assignment Example

Civil Engineering Construction-Superstructure 524468) Introduction This report discusses in detail the different types of bridges and roads that are laid out across the length of two connecting points. It also covers the different types of frames that are used in the construction of bridges and other strength bearing parts. Types of bridges The early form of bridges were the beam bridges, arch bridges and the suspension bridges all constructed from natural resources like wood, vines and fallen tree trunks. (Hill James et al, n.d ) The modern day bridges essentially consist of a combination of all the three bridges but to a more refined scale using materials like steel and concrete. 1. Arch Bridges These are one of the oldest forms of bridge that were introduced by the Romans. In this method the total downward weight of the arch bridge is distributed along the arch and the load taken up by abutments kept at the outer end of the bridge. Source: Sydney Harbour Bridge, Sydney, Australia This abutment or a kind of stopper prevents the arch from straightening out and keeps the structure of the bridge intact. The olden day arch bridges were constructed of wedge shaped stones that were arranged neatly to take the shape of an arch. The Pont du Gard aqueduct built near Nimes in France is an excellent example of a arched construction standing the test of time. (Hill James et al, n.d) The modern day construction involves the use of concrete in construction. These are also reinforced using steel. These new methods of construction make it possible to construct bridges up to 800 ft spans. (Beniwal Tarun et al, 2007) Arch bridges can be classified according to their material of construction. (i) Stone Arch Bridge- These were usually built in the olden days and involved constructing the bridge using wedge shaped stones. (ii) Steel Arch bridge- This consists of truss sections made of steel. These steel sections may be prefabricated and assembled at site. (iii) Concrete Arch Bridge- In this method precast concrete segments in the form of wedges are arranged in arches that are supported on temporary sections. Sometimes two arches may be used and these are supported in the middle using a vertical support called a spandrel. (Hill James et al, n.d) 2. Beam bridges Beam bridges involve a simple design of a beam that is supported at its ends. The loads that act on the bridges act as reactions at the ends. A single span can usually support a span of 250 ft. Pre-stressed concrete that are adequately reinforced using steel rods are used in construction. The concrete can sustain the compressive loads while the steel can take the tensile loads. (Hill James et al, n.d ) Concrete bridges have the advantage that the raw materials are easily available and skilled labour is not required for laying. Three methods are commonly used for construction of bridges. (i) Balanced cantilever Method- This method involves construction of a false work that supports a counterbalance weight and thus prevents the cantilever from tilting over during progressive construction. The framework for constructing the next cantilever is carried by a gantry. (Beniwal Tarun et al, 2007) Thereafter reinforcements and stress bearing tendons are placed and the first segment cast using concrete. After one segment is cast the gantry moves along on rails supported on the deck to start a new cycle which would be cast and joined to the previous concrete segment. Source: Lake Pontchartrain Causeway, Louisiana, United States (ii) Span-by-Span Method- Compared to the balanced cantilever method this method is used for shorter spans. No counterbalance weights are required as it proceeds in one direction completing one span at a time. This uses the gantry system for effective site adjustments which is supported from piers. This method is faster and involves far greater flexibility. (Beniwal Tarun et al, 2007) Source: Beniwal Tarun et al, 2007, Span by Span Method (iii) Incremental Launching Method- This involves casting small lengths of 10-30m on a stationery framework that is located just behind the unit that had been cast prior to it. The assembly is then post tensioned to ensure continuity. This assembled piece is pushed ahead to make room for the next casting. To facilitate movement lateral low friction guides along with bearings are placed at supports. For building bridges of large spans temporary piers may be constructed to limit the sagging bending moments arising due to weight of the structure.( Beniwal Tarun et al, 2007) Source: Beniwal Tarun et al, 2007, Travelling Gantry 3. Suspension Bridges Suspension bridges can be used in construction of bridges exceeding 500m span length. Two towers that support cables which support the bridge deck are used in construction. These cables are eventually supported at either end by anchoring them inside huge concrete blocks and preventing it from pulling off. (Hill James et al, n.d ) Considered to be costly in construction, the bridge consists of the following parts. Source: Golden Gate Bridge, California, USA (i) The tower- These are fabricated in shop from box girder steel sections. (ii) Cable- The cable consists of thousands of galvanised steel wires that are bound together and coated with paint to prevent corrosion since these wires are exposed to the elements. Source: Beniwal Tarun et al, 2007, Cable (iii) Hangers- These are the vertical bands that are attached to the suspension cable and fixed to the deck. (iv) Deck Unit- The two types of deck used are (i) Trussed girders- These involve sections placed progressively using cranes. Disadvantages include high cost and limited flexibility in fabrication. (ii) Steel box sections- These sections are fabricated at shop and transported to site. Transported sections are complete in all aspects as these include handrails and toe plates. The parts only need to be joined together by bolts or by limited welding processes. After the sections are laid, a concrete surface is used to finish up the surface. (Beniwal Tarun et al, 2007) (v) Anchorages- This is the most important part of the suspension bridge as the entire load is borne by this anchor. Rock anchors involve piercing huge rocks that may be found at the span ends. Source: Beniwal Tarun et al, 2007, Anchorage using anchor bolts. The cable may also be inserted into the hole of suitable depth and the ends grouted. Other designs involve drilling a U shaped tunnel into the rock and directing the cables into it to form a loop. This is called Tunnel anchorage. (Beniwal Tarun et al, 2007) Further combination includes the looped cables being affixed to concrete using anchor bolts. Structures in Industrial Buildings 1. Steel frame structure Steel frames used in the construction of industrial buildings are light and offer cheaper, better quality options compared to traditional materials. Cold formed steel frames posses high quality and are non combustible thereby adding to the safety feature. (Steel frame construction, 2008) These are also highly resistant to corrosion. Design of structures against seismic and wind failure are better attained using steel frames. Another advantage is that close to 25% of steel building structures can be recycled. These are manufactured from coils of steel which are passed through rollers to attain the desired shape profiles. The C shape is mostly used in industrial building construction. These steel frames are extensively used in the construction of floors, walls and roofs. Steel floor joists of thickness 0.034 to 0.101 inches are commonly used to create continuous spans. Walls employ ‘C’ sections of depth 2 ½ inches to 8 inches and depth 0.034 to 0.071 inches. (Steel frame construction, 2008) These structures also facilitate installing insulation boards on the outer walls. These frames can also be used in the fabrication of steel trusses on which the roof can rest. Dome roof and cone roof can be stiffened either externally or internally using ‘C’ shaped structures. The framing process involves three methods (i) Stick built- This involves using these ‘C’ sections to act as structural supports. This is similar to a wooden framed panel except the wooden structures are replaced by steel members. (Steel frame construction, 2008) (ii) Panelized- These are assembled at shop and delivered to site. In situ panels are used extensively in the construction of industrial buildings and hotels. The construction is quick and involves lot of flexibility. (Steel frame construction, 2008) (iii) Engineered- These involve using the ‘C’ sections to take loads. Sizes are chosen as per its section modulus and used in design of stiffeners. Source: Yakut Ahmet, 2003, RC frame construction with hollow clay tile masonary. 2. Concrete frame structure These consist of horizontal members called beams and vertical sections called columns arranged and cast insitu. This reinforced concrete construction is used in the construction of non ductile RC frames and ductile frames. (Yakut Ahmet, 2003) Disadvantages include poor concrete mixing leading to non homogenous laying. Reducing steel reinforcements can affect the strength of these concrete frames leading to potential failures. 3. Portal frames Portal frames are now extensively used in the construction of industrial structures. Rolled sections are used for supporting roofs and side cladding strengthened by purlins. Rafters at slopes of 1 in 3 to 1 in 10 are usually used. (IS800: 1984) These are supported at the ends on stanchions. The stanchions are fixed to the foundation by a pinned support or a fixed support. The eaves are capable of taking the lateral loads and the bending moments arising due to the rafter connections. Plastic analysis is used in design these portal frames. The other variant is the haunched portal frame. This is generally used in the construction industry with rafters and columns designed to take bending moments and buckling loads. Haunches at the eves and the top position give this kind of framework its name. (IS800: 1984) Field joints are kept at suitable locations for flexibility in construction. The haunched frame reduces the moment on the rafter but correspondingly increases it on the column. This idea behind this is to reduce the total weight since rafter lengths are more than column lengths. Haunches are usually constructed with the same rafter section with twice its depth. (IS800: 1984) This facilitates using the same rafter section to construct the haunch. Source: IS800: 1984, Haunched Frames Types of Roads Roads have undergone changes through periods in history. Well established roadways have proved very beneficial for the development of trade and commerce. Roads can be broadly classified as highways, expressways, urban roads, dirt roads, cobbled roads and arterial roads. The method of construction also varies with the vehicle density on these particular roads since a road with more number of plying vehicles needs a more robust construction to take the bearing loads. Constructing Highways Before the actual construction begins, stakes are set up on the ground to locate the reference points where the excavation needs to begin. Reference points are usually located at 70 to 100m intervals. (Highway Construction, 2005) Improper location of reference points leads to undercutting which causes steep slopes. If cuts are made wider then it would result in large volume of earth being excavated. Source: Highway Construction, 2005, Clearing and Grubbing for laying roads. The next step involves clearing and Grubbing of the road laying area. The clearing phase involves removing obstructions that may be in the form of trees. Grubbing involves removing the remaining tree stumps and any other kind of vegetation cover hindering the laying of roads. Tree stumps are cut at heights of 1.2m from the ground. This facilitates easy removal of the stumps along with the root below the ground using bulldozers since an adequate leverage is obtained by keeping this height. (Highway Construction, 2005) The organic materials removed can be laid along the road sides as these function as embankments preventing moisture ingress. The next step involves the compaction techniques which prevent soil from being eroded away. Constructing embankments on either side of the roadway also prevent erosion. These are constructed as side cast embankments which require no compaction. Layers of 30cm thickness can be placed, compacted and the subsequent layer again lay out. Controlled compaction of embankments involves 20cm thick layers that are compacted using water and rolled using vibrators. (Road Construction, 2007) The embankment obtained is thus more compact and denser. The final process involves laying of the asphalt concrete on the surface of the road. The thickness of the concrete and the compaction provided define the type of road that is being constructed. Highways less than 7.8m entering urban cities have a 30mm mix thick asphaltic concrete. Roads wider than 7.8m are laid with asphaltic concrete of 35mm thickness. Intersections shall have pavements extending 10m on all four sides. These have a minimum thickness of 125mm with a 35mm surface of asphaltic concrete. (Road Construction, 2007) Suitable areas on these highways need to be chalked out for bus bays. Zebra crossing need to be painted for traffic movement and proper drainage pits of dimensions 450cm x 450cm should be constructed to ensure collection of rainwater. Rural Road construction The initial activities that go into road construction prior to actual paving of the top surface remain the same. However a number of alternate materials can be used in rural roads which tend to be constructed in and around marshy lands containing loose soils and having problems in drainage. Jute Geo Textile (JGT) is mixed and laid in soil. (Nath Pawan et al, 2004) This possesses high moisture absorption, tensile strength and is not harmful to the environment. Another technology used in constructing rural roads includes the use of flexible concrete pavement technology. This consists of placing plastic cells of dimensions 150 x 150 x100 over the prepared road area and then laying concrete into these cells and further compacting with an earth hammer. Waste plastic Blended Bitumen is also being used for road construction in rural areas. Tests have proved that adding 8.8% by weight of polyethylene plastic into bitumen improves its strength and prevents fatigue failure due to cyclic loads. (Nath Pawan et al, 2004) Construction of Cobbled roads These consist of rectangular blocks of stone that are neatly trimmed at the edges and laid out in specific patterns. These stones usually range between 64mm and 256mm. The stones up to 128mm are called small cobbles while those between 128mm and 256 mm are called large cobbles. (Cobbles and Duckstones, 1997) Cobbles have the advantage of being easy to pack between sand bases. These cobbles are at then smoothened on the top surfaces using hand tools before arranging for road laying. The surfaces thus provided are hard and are not susceptible to erosion. These are aesthetically very beautiful and provide very comfortable walking surfaces. The advantages of cobbles include moderate price and the laying work also does not require too much skill. Maintenance is also minimal. However, the laying of cobbles involves lot of time and hence the laying costs are high. Source: Cobbles and Duckstones, 1997, Cobbled Road Conclusion The road project involves 30 miles of motorway and therefore covers a whole gamut of civil engineering structures. These include different types of bridges like the beam, arched and suspension bridges. Industrial buildings planned along the roads are constructed by using steel, concrete or portal frames. The report also touches upon the types of roads that into be constructed for highways and the additional requirement of the road when the highway enters the city urban area. Dirt roads and rural roads do not require such strict guidelines due to lower vehicle density. Apart from these three topics the design of the motorway along the 30 mile length also includes the construction of retaining walls and ground water control mechanisms to increase the longevity of the road built. Bibliography 1. Hill James et al, n.d, Construction of bridge and structures, A2F04: Committe on Construction of Bridges and Structures. 2. Road Construction, 2007, Engineering Quality Standards, Manukau City Council. 3. Beniwal Tarun et al, 2007, Modern Practice on Bridge Construction on World railways. 4. Highway Construction, 2005, Transportation Engineering Technology, National Institute for Certification in Engineering Technologies. 5. Yakut Ahmet, 2003, Reinforced Concrete Frame Construction, Middle Esat Technical University, Turkey. 6. Nath Pawan et al, 2004, A Critical review of Innovative Rural road construction Techniques and their impacts. 7. Steel frame construction, 2008, Available at http://www.steelframingallinace.com, [accessed on 19th April 2011] 8. IS800: 1984, Code of practice for use of structural steel in general building construction. 9. Cobbles and Duckstones, 1997, Available at http://www.pavingexpert.com, [accessed on 19th April 2011] Read More