SUPER STRUCTURE CONSTRUCTION

UNIT IVSUPER STRUCTURECONSTRUCTIONSYNOPSIS:

Launching girdersBridge decksOffshore platformsSpecial forms for shellsBraced domes andSpace decksHigh Rise StructuresMaterial handling

Transmission TowerSky ScrapperCooling TowerErecting light weight components on tall structuresSupport structure for heavy equipments and conveyorsErection of articulated structures 2SITE ERECTION METHODSSlide Slowing MethodA method of construction of the super structure is to erect girders, whether steel or precast concrete girders, over temporary supports by the side of the pier, opposite to the span and when ready, slewing same into position.Move the unit, full or part of the deck, if any, is added after the basic girder structure with adequate bracings is slewed inThis method is adopted when the erection or casting of the girders is being done simultaneously with the construction of the piers as in new construction, in order to avoid delay laterWhere pier height is too much it is adopted and in existing bridges case, this method is adopted to replace the existing structure with stronger unit

Common for both railway and road bridges but one difference is deck, except for linking of rails in case of railway bridges and weathering course & parapet in case of road bridgesFor new bridges the staging for casting will have to be put up by the side of the piers on one side In case of replacement, similar staging will have to be erected on the other side also for receiving the old girders and dismantling them into parts before being taken away to storesIn both cases, some temporary arrangements over the piers or adjacent to them in the alignment for the purpose of slewing in will also be necessary

LAUNCHING METHODSEnd LaunchingThis method is adopted mainly for long span prestressed concrete and steel girders and mostly on new constructionThe girder is cast or built up on the approach bank, and it is longitudinally traversed over the opening it has to span and lowered in positionFor this purpose, a small temporary intermediate staging has to be provided in the gap between piers for taking the girder across the gapFor steel girder bridges , staging are arrange them one behind the other, linked with temporary links and launch them together

Launching GirdersFully cast prestressed concrete girders are not launched independently as the cantilevering stress developed is considerable and the design difficultThe method adopted is to first launch a steel or aluminium supporting frame or girder so that it spans over the gapOnce the launching of this girder over the gap on one side is over, the first main girder is moved over this temporary girder or frame, supported at intervals or pulled acrossWhen the full length of the main girder has come over the span, it is jacked up and temporarily held in position and the launching girder can be side slewed to take the position of the next girder over the span.The main girder launched earlier is then lowered into position with the helps of jacksThe next girder cast and ready on the approach is then launched longitudinally over the temporary girder again and the process repeated till all the permanent girders are put in positionThe launching girder can then be moved over to the next span and it can take the position required for taking on the first girder of the next spanThe girders of the next span can then be longitudinally taken over the girders erected in the previous span and then over the temporary girder on the next span and the process repeatedErection of Concrete Girders with Cranes/ DerrickIf the bed is dry, the girders can be cast on the bed by the side of the span and they can be erected by using two mobile cranes one on either and with the help of a suitable derrick in the centre or one derrick each on either endIf the height of the pier is not much and girders are too heavy to be handled by the available crane or derrick, the girder can be jacked up from either end on temporary trestles to pier top level and then side slewed in position launched and slewed has the minimum weightThe full deck slab can be cast subsequentlyIn the case of prestressed concrete girders there will be transverse prestressing also involvedThe diaphragm with necessary ducting should be cast after all the girders are launched correctly and adjusted in positionPart prestressing is done before individual girders are lifted or launched and remaining cables are tensioned, some before and balance after or all after the deck is cast according to the designThis stage prestressing is done to take the maximum advantage of prestressingIn doing such post tensioning, extreme care has to be exercised in following the sequence that has been given by the designers and any small change can cause a crack in the system or unwanted lateral deflection in the individual girderCantilevering MethodFor very large spans, the method which is now used is the cantilevering methodIn this method, the erection starts from the abutment end and the erection of the members ahead is done by using a crane which travels on the top boom of the previously erected part of the structure in case of steel and by using the support on the previously erected part structure in case of prestressed concrete constructionTypes : Howrah Bridge Steel cantileversSydney Harbour Bridge Steel archSecond Godavari Bridge Steel simply supported

BRIDGE DECKSThe grade line of highway or railway track and the clearance required under the bridge decide the use of deck type or through type bridgeIncase the sufficient clearance is unavailable under the bridge, the deck type bridges are the advantages over the through type bridgesThe deck type bridges are relative economicalThe height of piers and abutments are reduced are reduced in the deck type bridges

SPACE DECKSOf recent years a range on standard section mild steel tubes has been manufactured in increasing quantity .The advantages of a tubular section are that it has a uniform cross section and its position does not have to be adjusted for maximum strength as does an angle section and the surface area of a tubular section is considerably less than standard tubular sections welded together. Erection of Space decks The standard units of this type of roof consists of pyramid frames, the tray and base of each unit consisting of light steel angles welded together to which tubular steel diagonals are welded.

The diagonals are welded to the socketed couples at the apex of each pyramidThe units are joined by bolting the angles of adjacent trays together and threading tie bars between couplers of adjacent units. By adjusting the tie bars the top of the deck may be flatThe finished deck is extremely rigid and can be provided for comparatively long spans with few internal columnsThis type of roof is more expensive than a pitched lattice steel roof structureThe deck is connected to internal and external I section or hollow rectangular section columns. The cheapest and most commonly used form of coverings consists of wood wool laid on and dipped to the deck and covered with felt roofing.

SPECIAL FORMS FOR SHELLA structural shell is covered surface structure. It is generally capable of transmitting loads in more than 2 directions to supportThese structures are highly efficient structurally when they are so shaped, proportioned and supported that they transmit the loads without bending or twistingA shell is defined by its middle surface halfway between its outer surface and inner surfaceDepending upon the geometry of the middle surface, shell may be classified as Dome, Barrel arch, Cone, Hyperbolic paraboloid

A thin shell has relatively small thickness compared to other dimensionsIt should not be so thin that the deformation would be large compared with the thicknessThe shell shearing stresses normal to the middle surface should be negligibleThin shells usually are designed so that normal shears, bonding moments and torsions are very small except for relatively small portions.DOMESRibbed domes are the earliest type of braced domes .A ribbed dome consists of a number identical meridional solid girders or trusses, interconnected at the crown by a compression ring.The rings are also connected by concentric rings to form grids in a trapezium shapeIt is stiffened by a steel or reinforced concrete tension ring at its base.SCHWEDLER DOMES A schwedler dome also consists of meridional ribs connected together to a number of horizontal polygonal rings to stiffen the resulting structure. So that it will be able to take unsymmetrical loadsEach trapezium formed by intersecting meriodional ribs with horizontal rings is subdivided into two triangles by a diagonal memberSometimes the trapezium may also be subdivided by two cross diagonal membersThis type of dome was introduced by a German engineer J.W.Schwelder in 1863The great popularity of schwedler domes is due to the fact that, on the assumption of pin connected joints, the structure can be analyzed as statically determinateIn practice, in addition to axial forces, all the members are also under the action of bending and torsional momentsMany attempts have been made in the past to simplify their analysis, but precise applied to find the actual stress distribution

HIGH RISE STRUCTURESTransmission TowerTransmission towers are tall structures with relatively small cross section and with a large ratio between the height and the maximum. Towers are also called as masts or pylon.Tower is a single cantilever freely standing , self-supporting structures fixed at its base.Transmission towers structures that are pin connected to its foundation and braced with guys or other elements. Similar examples are water towers, radio, television towers, radio relay system etc.,These towers of power transmission lines are used to support transmission cables carry transmitting voltages exceeding 1,32,000 voltsOver long distance, since these cables carry heavy voltage, they provide necessary clearance, where the transmission cables have maximum sagOn general the towers may be built up with three or more legs. But generally all towers are built up with four legs and spared suitably. This is because four leg gives sufficient stability to the structures.There are different types of them:Self supporting typeFlexible type Semi-flexibleLine towerAngle towerSelf supporting wide baseGuyed type

Self supporting type towers are generally rigid in both the transverse and longitudinal directionsFlexible type towers are not rigid in the longitudinal direction ie., in the direction along the transmission cablesLine towers, when there is a straight portion of the power line, line towers are preferredAngle towers, when the direction of the power line is changed angle towers are providedThe height of these towers should be within 20 40mThe towers of such height provide 6 10m clearance from the ground surface to the point of maximum sag of the cablesThese towers are also known as single, double or multiple circuit towers depending upon the no of circuits supported by the towers

Single diagonal bracing tower have long free length and double diagonal bracing or cross diagonal towerRadio and television towers are self supporting towers. For there towers rigid diaphragms are provided at the top and at several intermediate sections to the increase the stiffness of cross sections. In masts they are guyed with wire ropes at one or more levelsThe vertical loads acting on towers are the dead load and live loadThe vertical loads on the power transmission line towers include self weight of towers insulators, fittings, ice coating, line man with toolsThe self weight of towers is found by a general existing Ryles empirical formula (For suspension , dead load towers)

SKYSCRAPERSSkyscraper (high rise) is a tall habitable building.Emporis data committee defines skyscraper as a building which is 35 meters or greater in height , and is divided at regular intervals into occupiable levels.Some structural engineers define a skyscraper as any vertical construction for which wind is a more significant load factor than weight.The first skyscrapers is considered the Home Insurance Building in ChicagoTen storey structure was constructed in 1884 1885. Another contender for the title is the 1892 ten storey Wainwright Building by Louis Sullivan, which still stands in St. Louis, Missouri.The crucial developments for skyscrapers were steel, reinforced concrete, water pumps, elevators.Until the 19th century, buildings of over six stories were rare. It was impractical to have people walk up so many flights of stairs, and water pressure could only provide running water to about 50 feet (15m)The weight-bearing components of skyscrapers differ substantially from those of other buildings.Their walls can support buildings up to about four stories, while skyscrapers are larger buildings that must be supported by a skeletal frameThe walls than hang off this frame like curtains hence the architectural term curtain wall for tall systems of glass that are laterally supported by these skeletal frames. Skyscrapers was a nautical term for a tall mast or sail on a sailing shipRankBuilding, CityYearStoryesHeight inMeters Feet1Freedom Towers, New York200512154117762Taipei 101, Taipei, Taiwan200410150916713Petronas twin tower, Kuala lumpur, Malaysia19988845214834Sears tower, Chicago197411044214515Jin Mao Buildings, Shanghai19998842113806Two International Finance Centre, Hong Kong20038841513627CITIC Plaza, Guangzhou, China19968039112838Shun Hing Square, Shenzhen, China19966938412609Empire State Building, New York1931102381125010Central Plaza, Hong Kong1992783741227Worlds 10 Tallest Buildings

COOLING TOWERCooling towers are widely used for cooling large quantities of water in thermal power station , refineries , atomic power plants , steel plants , air conditioning and other industrial plant.A cooling towers incorporates a draft tower distributing and spraying devices and a cold water basin.A typical section of hyperbolic cooling tower of height 100mThe draft is induced naturally or mechanically. Hot water is pumped to a certain height and then distributed through a piping system of nozzles, where it splashes over a system called Filling or StackingThe filling may be of wood or transite material placed in several layers

The water splashes drips and flows through the layer of fillingThe air which is forced upward either mechanically or by atmospheric pressure difference rises through the driplets or films of the hot water and consequently cools the water through evaporation and convectionThe cooled water is collected in a pond at the bottom of the towers and then recirculated for industrial useHyperboloid of revolutionThe hyperboloid of revolution of one sheet is generally used for cooling towers of thermal stationThe great advantage of this type of shell is that it is generated by two families of intersecting straight lines and the form work can be achieved by straight boards warped only slightly over the lengthsThe intersecting grid of straight lines forms rhombuses of intersection

The shell surface can also be built of pre-cast rhombic elements which are repeated along the complete circumferences at fixed heightsThe generation of the hyperboloid of revolution is by intersecting straight lines

MATERIAL HANDLINGMaterial handling is the movement, storage, control & protection of materials, goods & products throughout the process of manufacturing, distribution consumption and disposal.The focus is on the methods, mechanical equipment systems and related controls used to achieve these functions Material handling management consists use of a proper technique for moving, transporting, storing or distributing materials with or without the help of mechanical appliances. Because of various complexities in the construction process, management must be fully acquainted with various material handling devices so that unit cost of construction can be down

All material handling activities must be simplified and must be fully safe preventing any accidentsAlso material movement planning must be economical is terms of time and labour Material handling objective:The right amount The right materialAt the right time In the right position In the right sequenceFor the right costThe handling of material depends on three items. They identified as follows Identification of source of building materialIdentification of quantity and quality of materialIdentification of cost of building materialPrinciplesOrientation principlePlanning principleSystem principleUnit load principleSpace utilization principle

Standardization principleErgonomic principleEnergy principleEcology principleMechanization principleFlexibility principleSimplification principleGravity principle

Safety principleComputerization principleSystem flow principleLayout principleCost principleMaintenance principleObsolescence principle

MATERIAL MANAGEMENTMaterial management is concerned with ensuring that the quantity and quality required are on the job as per requirementThe construction manager is concerned about the nature of material handling problems like quantity , quality , price , delivery date , mode of transportation , inspection , counting , cost , storage and protection.Considerable time and effort is spent on a practical network that will satisfy and resource limitationsHowever, such a schedule is not used unless supported by the timely delivery of materialsThe material for delivery to the site can be broadly classified as follows,Bulk itemsStandard itemsFabricated itemsBulk items - like earth , bricks , cement , sand , stone chips , steel sheet and asphalt that are obtained in a semi-processed stateStandard items - all shelf items that can be procured from the market like pipes and pipe fittings.Fabricated items are the items that are specially built as per specification and sizes for the project. Built as per specification like roof truss, doors, windows, electrical panel etc.ERECTING LIGHT WEIGHT COMPONENTS ON TALL STRUCTURETall buildings are generally multi storey structures where greater part of construction of beams and stanchreonsErection Procedure for Multi Storey Building The order of erection follows pattern of columns, girder and beams or columns, trusses and purlins. Buildings are generally erected by cranes and beyond the reach are erected guy derricksThe step wise operation is as followsGuy derrick mast is assembled on ground with its base in approximate required location. The mast is tipped up vertically and guys are anchored to column bases. The boom is inserted and topping lift and load lines are served. The derrick is ready to operate. The first tier steel is erectedThe bottom is removed from boom seat by picking with topping lift falls, revolved 18degree and placed in a temporary jumping shoe. The top of boom is guyed off with temporary guysThe load falls are attached to mast above its center of gravity, the mast guys are moved to top of next tier and mast is raised to its new position. The mast guys are adjusted and load falls unhookedThe temporary guys of boom are removed and the topping lift falls are used to raise the boom and place it in the boom seat. The derrick is now ready to operate and the next tier of steel is erectedThis operation is repeated until all tiers are erectedSafety RulesSee that the equipment is not over loaded. Be certain of load to be liftedAll bolts and splice material on lattice derricks and crane sections have been insertedDo not open the legs of brother chains to too large an angleThe rings of chain slings is trig enough for crane hookAvoid sudden shocks when liftingSUPPORT STRUCTURE FOR HEAVY EQUIPMENT Column BaseFoundation is necessary for a column to distribute the column load on sufficient area of the soil so that the bearing capacity of the soil is not exceeded, it is also equally important that the column load be applied on sufficient area of the concrete foundation so that bearing strength of the concrete is not exceededA steel base plate is therefore used to spread the column loaf on sufficient area of the concrete foundationBase plate used may be of following typesSlab basesGusseted basesSlab bases : In this case the column stands directly on the base plate the bearing end of the column is machined so that the column load is transferred to the slab base by bearingGusseted base: Gusseted base plates are used in columns carrying heavy loads. In this case fastenings are used to connect the base plate and the column in the form of gusset plate, angles etc., In case the end of the column is sufficiently machined so as to provide full bearing on the base plate, it is usual to assume that half the column load is liable to be transferred to the base plate through the fastenings and the balance load is transferred to the base plate by direct bearing. Suppose the ends of the column and gusset plates are not exactly faced for full or complete bearing. Then it is usual to design the fastenings to transmit all the forces to which the base is subjected.

Grillage foundation: It is provided for a column carrying heavy load when it has to transfer its load to a soil of low bearing strengthThis foundation consists of two or more layers or tiers of steel beams, the layers being provide one above the other at right angles at each other . The beams are completely encased in well compared concrete. Generally only two tiers of beams are usedThe column rests on a base plate through which the load is transmitted to the upper tiers beamsThese beams in turn transfer the loads to the lower tier beams. From the lower tier beams the load will be transmitted to the soil.

ERECTION OF ARTICULATE STRUCTUREBow String Girder Type BridgesThe bow string girder type bridges derives its name from its shape. The arch rib and the tie respectively resemble bow and string. The flooring of bridge rests on ties and the load is transmitted to the arch rib through suspendersSuitable bracing may also be provided in case of steel bow string girdersThe bow string girder type bridge removes the following two disadvantages of the arch bridgesThe horizontal thrust is resisted by ties, hence the reactions at supports are vertical and not inclined as in case of the arch bridges. As a result of vertical reactions the supports require lighter sections

The bow string girders project above the formation level of road or railway line. Hence, the question of accommodating the rise between the level of approaches and the springing level of arch of arch close not ariseThe bow string girder type bridges are therefore very much suitable for multiple spans and at places where the available clearance is restrictedDeck Type and Through Types Truss BridgesIn the deck type truss bridges, the floor system rests on the top chord. In the through type truss bridges, the floor system rests on the bottom chord. Through type warren truss bridge with verticalsAlthough the parallel chord trusses are used for the through type truss bridges, but the lines of various numbers of parallel chord truss bridges are more in harmony with deck type truss bridges

Component Parts of Truss BridgeThe various component parts consists ofThe main vertical trussesThe floor systemThe bottom lateral bracingThe top lateral bracingThe portal bracingThe sway bracing

Cable Stayed Bridge Cable stayed bridges may look similar to suspensions bridges both have roadways that hang from cables and both have towersBut the two bridges support the load of the roadway in very different ways. The difference lies in how the cables are connected to the towersIn suspension bridges, the cables ride freely across the towers, transmitting the load to the anchorages at either endIn cable-stayed bridges, the cables are attached to the towers, which alone bear the loadThe cables can be attached to the roadway in a variety of ways. In radial pattern, cables extend from several points on the road to a single point at the top of the towerIn a parallel pattern, cables are attached at different heights along the tower, running parallel to one otherConstruction Sequence of Cable Stayed BridgeThe cable stayed bridges are similar to the suspension bridge except that there are no suspenders in the cable stayed bridges and the cables are directly stretched from towers to connect with the deckingThus no special internal anchorage is required for the cables as in case of suspension bridges because the anchorage at one end is done in the girder and at the other on top of towerEach because in the girder introduces horizontal and vertical forcesThe cables can be arranged with two plane system or one plane system. The two plane system requires additional width to accommodate the towers and deck anchoragesBut in case of one plane system the anchorage at deck level can be accommodated in the traffic median and it results in the least value of total width of the deckIn principle the cable stayed bridge essentially of the following three elements Bridge deckPylons or towerStay cablesThe stay cables are the principal structural elements because they play an important role in the design, stability and performance of the structure as a wholeThe different types of stay cables are locked coil wire ropes or stranded ropes long lay spiral strand cablesparallel or semi parallel wire cables

The deck in case of the cable stayed bridges is supported by a number of cables meeting in a bunch at the tower, known as the harp formThe use of multiple cables facilities smaller distances between points of supports for the deck girders and it results in the reduced structural depthThe cable stayed decks are less prone to the wind induced oscillations than the suspension bridges because of the damping effect of the inclined cablesThe brick deck may be in the form of steel prone or RCC or prestressed concrete girders. The concrete girders possess the following advantagesAs their damping effect is very high, the vibration effects are also smallThey possess much higher stiffness and hence, they exhibit comparatively less deflection