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Chapter: Civil - Prefabricated Structures - Prefabricated Components

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Prefabricated Components: Characteristics of Materials

Light weight for easy handling and transport and to economics on sections and sizes of foundations.

 

Characteristics of Materials

 

Easy availability

 

Light weight for easy handling and transport and to economics on sections and sizes of foundations.

 

v Thermal insulation property

v Easy workability

v Durability in all weather conditions

v Non Combustibility

 

v Economy in cost

v Sound insulations

Modular Coordination Definitions

Basic Module

 

The fundamental module used in modular coordination the size of which is selected for general application to buildings and components.

 

Modular coordination

 

Dimensional coordination employing the basic module or a multimodule. The purposes of modular coordination are

 

(i)    To reduce the variety of component size produced.

(ii) To allow the building designer greater.

(iii) Prefabricated components

 

Behaviour of Structured Components:-

 

Precast reinforced structures are many types, because almost all reinforced concrete structures can be prefabricated.

 

The members of structural components can be classified in to

v Lead-carrying structural members

v Space-bordering members

v Surface forming structural members

 

Load carrying structures generally act in their own plans.

 

Space bordering members have as a rule of plane surface. These also play a role in the carrying of loads, but this role is of a lower order. Thus.

 

Space-bordering members are

 

Load-carrying structures of a lower order, the load is transferred from the structures of lower order through the structures of higher order to the footings.

 

Load carrying structures can be classified in various ways from the view point of fabrication, load carrying structures can be divided in to main groups according to their disuniting in to members.

v of straight members

v of entire rigid frames

v of members joined at points of minimum moments

v of orched members limited in to two-hinged of three hinged arches.

 

Roofing slabs or corrugated asbestos cement sheets laid on purlins

 

These are the most popular forms of roof covering used in central Europe. This is not surprising considering the simplicity of manufacture of purlins and the availability from stock of factory made lightweight roofing slab and panels.

The structural system of the purlins may be

a) Freely supported beam        b) The cantilever girder

c) The continuous girder.

 

The connection of the purlins over the support are designed only to absorb a limited BM. Normal purlins spans between 5 and 10m. The purlins are spaced at intervals of 2 to 3m.

 

Purlins:-

 

Purlins are usually solid web members. For long spans they may lattice girders or trussed beam.

 

Freely supported purlins are designed as let flanged or fish-belly members. Purlins designed as cantilever grilles (articulated griders) are usually llel flanged members.

 

The c/s feature depend on the spans of purlins & on the slope of the roof. The purlins for flat roofs are usually rectangular T-section or (prestressed concrete) T-Section members for steeply sloped roofs it the loaded also in biaxial bending L-Sections or channel sections are used.

 

Purlins section & the approximate spans associated with them for a purlins spacing of 3m are indicated for flat roofs. The dimensions relate to freely supported purlins.

 

Purlins sections with associated spans for a purlins spacing of about 1.25 M in the case of steeply sloped roofs with corrugated

 

professor von Halazz. They are convenient to manufacture with the legs of the channel upwards. whereby ray thin webs can be produced.

 

This type of purlins may be conventionally reinforced or by prestressed. Also they may be freely supported or be continuous over several spans. In case of L-shaped purlins usually only the flange of the section is supported.

 

The fishbelly girder is very favourable with regard to material requirements & the patterns of forces in the girder, but it has the disadvantages of being rather unsatisfactory from the point of view of architectural aesthetics. When it is used generally designed as a reinforced concrete purlins.

 

Structural connection to the main beams may be constructed as follows:-

 

a)     By supporting the entire c/s of the purlins, the latter being secured to the beam by means of dowels, projections concreted on to the beam so-called shoes thickening the top flange of the beam with insist concrete or performed recesses in the beam.

 

b)      The two last-mentioned methods are more particularly suitable for purlins of rectangular section. If the main beam is not provided with recesses to receive the ends of the purlins, it is desirable to apply an insite concrete topping to it, else there will be gap between its top flange & the roofing slabs, which not only looks rather unrightly but also aolds unutilisable, extra space to the enclosed volume of the buildings on the other hand, when recesses have to be formed in the top of the beams, concreting

 

presents-difficulties Besides the recesses weaken the top flange unless it is possible the supports. Because of this local strengthening it is possible to reduce the depth

 

of the purlin over the rod beam. There are too possibilities 1) The end of each purlins is reduced the depth to form a nib which rests on the beam. 2) The diaphragm on the top flanges of the roof beam is provided with nibs or corbels for supporting the purlins.

 

Purlins based on the principles of the cantilever girder. Provided with hinges which are so located as to ensures statical determining have not proved satisfactory. The reason for this are a) Halved joints:- The requirements of careful workmanship are more stringent than are normally considered acceptable in practice.

 

By large force will develop, more particularly in the purlins of long buildings, the failure of one member is liable to result in the collapse of the whole structure. This has indeed happened in a number of buildings.

 

The roofing slabs laid on the purlins of flat roofs are usually 0.5 TO 1.0m wide & have sparts of about 2.0 to 5.0m these slabs also comprise the thermal insulation and where necessary, also the cement mortar screed for the gravel roof.

 

The wt of slabs varies bet 10 & 150kg/m depending on the bulk density of the insulating materials and on the span.

a)     Ordinary concrete : 'Waffle' slabs with a shell thickness of only 1cm have been produced by the firm of press AG. The concrete bad a specified 28 days strength of 600kg /m2.

b)    layer, are manufactured on vibrating tables in Hungary the output per vibrating table is about 100 slabs per day. The slabs have standardized dimensions of 0.5 x 3.0m.

 

 

c)     Roofing slabs can be produced in a simple manner from perforated bricks or fixed insulating clay. The reinforcement is passed through cavities. These slabs are 20cm wide & 8 to 10cm thik for spans upto 2.8 m.

 

d)    I, perforated bricks or light wt bricks are used in conjunction with reinforced concrete ribs to form slabs 0.87 m wide & upt 3m span.

e)     Pumice concrete slabs ('Planks') have been produced in germany for a good many gears, they are available in 3 forms slabs with circular cavities, 'waffle' slabs comprising longitudinal and transverse ribs, & sold slabs-1.

 

f)      Duris of roof slabs are used in many countries. The standard slabs are 50cm wide & 2 to 4 long. This concrete is not hygso scopic to that these slabs can also be used in damp surroundings without suffering any appreciable loss of Thermal insulating capacity.

 

g)     Hebel aerated concrete roofing slabs used in Germany and Switzerland are likewise 50cm wide 2 to 6m long. They are reinforced & light wt concrete of with they are made has a compressive strength of 35 to 50kg/cm2.

 

Large individual precast units are better able to fulfill the requirements of industrialized old than purlins & relatively small roofing slabs can they will therefore gradually suppressed the more conventional form of construction with roofing slabs & purlins. The advantages are

a.     The number of units is substantially reduced

 

b.     The slab & web of the unit are concrete as single monolithic whole with structural co-operation of the slab.

 

c.      Window bracing for roof are dispensed with as it is readily possible to inter connect a number of large roof units.

 

d.     Large roof units generally produce an aesthetically more pleasing result.

The requirements applicable to roof units may be summarized as follows.

 

1.     The units should readily lend themselves to type standardization & to quantity production with the minimum of labour.

 

2.     They should require the least possible quantities of material both in terms of concrete & of reinforcing steel.

 

3.     They should be suitably transportable & capable of being conveniently stacked.

 

4.     When joined together they should act as rigid diaphragms, so that no other form of wind bracings needed.

 

Countries they are reinforced & the light wt which is aerated concrete.

 

(k) Hollw prestressed concrete concrete slabs of the schater system are produced with light wt concrete core d are suitable for span up to 6.5 m.

 

Many other types of roofing slabs are manufactured from a variety of light wt materials.

 

How roofing slabs are supported on the purlins.

 

The standard specifications of some countries require that roofing slabs be designed as freely supported members without taking structural continuity into account. However it is normal practices always to install reinforcing bars in the longitudinal joints not as a means of establishing true continuity but merely to inter connect and locate the slab.

 

Structural continuity can be achieved by appropriate forms of construction with prince or other light wt concrete slabs. Tests have confirmed that the failure load is increased as a results of such arrangements.slabs.

 

2) Roof units with 3D structural action such as singly & doubly curved shells & fololed plate structures.

 

Hollow roofing slabs with rectangular cavities.

 

Slab type roof units are subdivided according whether they have structural longitudinal & transverse ribs or only structural longitudinal ribs.

 

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