PREFABRICATED STRUCTURES

UNIT 1- INTRODUCTION NEED FOR PREFABRICATION ADVANTAGES MATERIALS MODULAR COORDINATION STANDARIZATION TRANSPORTATION ERECTION

DEFINITION Prefabricated building is the completely assembled and erected building of which the structural parts consist of prefabricated materials

AIM To effect economy in cost To improve quality To speed up construction since no curing is necessary To use locally available materials

ADVANTAGES Work time is reduced Less workers needed Work done with better technology Interruption in connection can be omitted

Materials of Construction Materials like concrete,steel,treated wood,aluminum etc… Special characteristics while choosing materials Easy available Economy Easy workablity Light weight Durablity

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

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

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

lintels Sunshade Length – multiple of 1M Width – multiple of M/4 Depth – multiple of M/4 Sunshade Projection- multiple of 1M

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

ADVANTAGES Easier in design Easier in manufacture Makes repeated use of specified equipments in erection and completion easier and quicker

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

Prefabrication pad locks

Isolated column footing

Combined footing under two columns

Cross section of completed prefab footing

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

Section of shell units

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.

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 500-900kg/m3 C.s is 3MPa-5MPa This foam concrte is used in prefab bldg

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

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

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

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.

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

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

Erection Factors to be considered Precast element position Crane capacity Crane rotation radius Cleaning of elements and site for erection

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

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.

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

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.

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

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

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

Methods for manufacture of pc elements factory prefabrication Site prefabrication

Factory pre fabrication Done in centrally located plant High level of mechanization Preparation of Moulds Concreting Vibration curing

Major disadvantage Extra cost –transportation from plant to site Shape and size is limited due to lack of transportation

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

Process involved Main process Auxiliary process Subsidary process

Main process Assembling moulds Placing reinforcements in position Fixing of tubes if necessary Pouring of concrete Vibrating Demoulding curing

Auxiliary process It s necessary for successful completion of process covered by main process Testing products

Subsidary process Storage of materials Transport of cement and aggregate Transporting and stacking the precast elements Repairs and maintenance of tools,machines etc…

Various methods of manufacture of precast unit can be broadly classfied in two methods Stand method Flow method

Stand method Moulds remain stationary at factory Various elements required for work is casted in factory itself and then transported to site

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

Cycle of precasting Preparation of concrete Preparation of reinforcement Preparation of moulds Assembly of reinforcement in mould Casting and compaction Curing