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PREFABRICATED STRUCTURES
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UNIT 1- INTRODUCTION NEED FOR PREFABRICATION ADVANTAGES MATERIALS
MODULAR COORDINATION STANDARIZATION TRANSPORTATION ERECTION
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DEFINITION Prefabricated building is the completely assembled and erected building of which the structural parts consist of prefabricated materials
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AIM To effect economy in cost To improve quality
To speed up construction since no curing is necessary To use locally available materials
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ADVANTAGES Work time is reduced Less workers needed
Work done with better technology Interruption in connection can be omitted
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Materials of Construction
Materials like concrete,steel,treated wood,aluminum etc… Special characteristics while choosing materials Easy available Economy Easy workablity Light weight Durablity
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MODULAR COORDINATION Modular coordination is a concept of coordination of dimension and space in which buildng components are positined in terms of MODULE(M) 1M= 100mm
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Dimensions of pre fab components
Flooring and Roofing Length- multiple of 3M Width – multiple of 1 M Thickness – multiple of M/4 BEAMS Length – multiple of 3M Width – multiple of M/4 Depth – multiple of M/4
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Thickness – multiple of M/4 Staircase
Column Height – multiple of 1M for ht up to 2.8M and ht above 28M is multiple of 2M Lateral dimension- multiple of M/4 Wall Thickness – multiple of M/4 Staircase Width – multiple of 1M
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lintels Sunshade Length – multiple of 1M Width – multiple of M/4
Depth – multiple of M/4 Sunshade Projection- multiple of 1M
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STANDARDIZATION Factors influencing :
Reducing the number of joints by using largest type of elements To select the suitable type of member for each element from the point of production,assembly ,economy To limit the size there by reducing the transportation and by reducing the over all weight
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ADVANTAGES Easier in design Easier in manufacture
Makes repeated use of specified equipments in erection and completion easier and quicker
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Types of pre cast RC elements
i) precast concrete foundation structures Use of pads should permit larger loads and poorer soil condition Load bearing walls consist of wall blocks,wall panels,shell units…. Entire scheme should be simple in design and easy construction
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Prefabrication pad locks
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Isolated column footing
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Combined footing under two columns
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Cross section of completed prefab footing
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Precast RC slab units 1.slabs with precast funicular shell units
Funicular shells are ideal for shell roofs These precast shell are also called waffle shells, can be either square,rectangular or triangular They can be cast in a factory or at site with simple moulds with concrete or surki mortar No reinforcement is req in body of shell since no tensile stress is developed However 6mm mild steel bar is used in edges of ribs to take care during transportation
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Section of shell units
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Slabs with RC joists and hollow clay blocks
In this joist with bottom flange of width slightly greater than web width are precast in site and laid over support Space bwn them is adjusted by providing clay blocks The precast joist are provided with reinforcemnt to take shear and flexure tension.
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Celcrete slab units Natural light wt agg is not available in india so autoclaved aerated concrete is used to produce slab units Also called foam or cellular concrete Density is kg/m3 C.s is 3MPa-5MPa This foam concrte is used in prefab bldg
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Pre cast concrete column
Bldg constructed by using precast columns and beams are generaly designed as framed structures Connection bwn column and beam is designed in such a way shear force and moments are transferred through structural embers Connection can be done by placing insitu concret or welding of projectng steel renforcements In some case bolted connection also used to connect precast columns later coverd wit insitu concrete
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Precast water tank elements
Water tank can be cast either as part or whole Large water tank cannot be prefab at factory bcoz of over wt . Small segemnts are precasted and bolted together at site In order to avoid corrosion stainless steel bolts are used Tanks up to 2.5m ht and 40m in dia have been built using such segments
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Precast pavement slabs
Pressed pavement slabs can be made from palin concrete for covering large open lands These elelments can be pressed to obtain smooth finish and also enhance durablity Excess strength is achieved by pressing by squeezed out in wet state
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Precast retaining wall
Cantilever type retaining wall itself can be pre casted if the ht is with in 1.5m Normally in railway platform, verandah in residential bldg are provided with retaining walls of about this ht These retaining wall can be bwn 1.5m-2m length depending up on facility available Weep hole are provided on vertical face to drain water from fill.
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Transportation Transportation of precast elements inside the factory and to the site is based on not only economy but also design aspect to avoid jerk as it s placed in final position Inside the factory its based on method of production selected for manufacture
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Transport from factory stacking yard to site of erection
It s planed based on traffic rules Size is restricted by the availability of suitable transport equipments such as tractor cum trailer While transporting through wagons, trucks , bullock cart etc care should be taken such that excessive cantilever is avoided Also care should be seen at sharp bends and uneven roads
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Erection Factors to be considered Precast element position
Crane capacity Crane rotation radius Cleaning of elements and site for erection
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Types of crane 1.stationary erection 2. cranes on rails
Guyed derrick Climbing carne Tower crane with fixed base 2. cranes on rails Portal crane Tower crane 3.Mobile crane moving on ground Truck mounted Crawler mounted
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Stationary cranes A.guyed derrick
Used on framed buildings for errection of floor panel ,columns . Lighter in weight Can be shifted from floor to floor operating from an erected floor.
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b.Climbing crane Tall building over 20storeys are constructed this type crane is used Horizontal jib and balancing counter weight is placed on top of shaft It operates around 360 degree and can be lifted up to new position as buldg goes on completion and dismantled This is used in longest areas
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C . Tower cranes with fixed base
Places were rail mounted and climbing cranes are not possible tower cranes can be installed These cranes are fixed at base and braced horizontally to the building portion already completed as building go up ,the vertical mast can be hoisted up by special hydraulic jack.
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2. Cranes on rails A. portal cranes
In storage yards and in buildings where the travel distance is short portal crane is used Consist of two vertical legs and horizontal beam Travelling trolley is attached to bottom flange of beam tht carries load horizontally It is very stable
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Mobile cranes moving on ground
A . truck mounted crane For quick movements from place to place cranes are mounted on track with operating cabin and separate chassis Chassis will have generator and motor Before operating the crane with loads,crane is taken off from pneumatic tyres and load transferred to the ground by simple hydraulic legs
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Crawler mounted cranes
It s used in excavation ,dredging and material handling. Should be used on a hard and compacted soil On loose soil it is difficult to move the crane with load It can operate 360 degree
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Methods for manufacture of pc elements
factory prefabrication Site prefabrication
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Factory pre fabrication
Done in centrally located plant High level of mechanization Preparation of Moulds Concreting Vibration curing
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Major disadvantage Extra cost –transportation from plant to site
Shape and size is limited due to lack of transportation
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Site prefabrication Manufactured at site or near the site
Normally done in open space with local labours Reduction of transport cost draw backs: Not suitable for high degree mechaniztion Continuity of work is not available
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Process involved Main process Auxiliary process Subsidary process
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Main process Assembling moulds Placing reinforcements in position
Fixing of tubes if necessary Pouring of concrete Vibrating Demoulding curing
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Auxiliary process It s necessary for successful completion of process covered by main process Testing products
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Subsidary process Storage of materials
Transport of cement and aggregate Transporting and stacking the precast elements Repairs and maintenance of tools,machines etc…
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Various methods of manufacture of precast unit can be broadly classfied in two methods
Stand method Flow method
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Stand method Moulds remain stationary at factory
Various elements required for work is casted in factory itself and then transported to site
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Flow method Precast unit under consideration is in movement according to the various process involved in it. Moulds are transported to site for required elements
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Cycle of precasting Preparation of concrete
Preparation of reinforcement Preparation of moulds Assembly of reinforcement in mould Casting and compaction Curing
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