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Chapter: Civil - Prefabricated Structures - Prefabricated Components

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Prefabricated Structures: Joints of different structural connections

a. jointing of column to footing b. jointing of column to beams on top of column. c. jointing of column to beam at an intermediate. d. jointing of beams f. Forming of joints of arched structures g. joining of joints of post tensioned structures. h. joining of precast to monolithic reinforced concrete structures.

Joints of different structural connections:-

 

a.     jointing of column to footing

b.     jointing of column to beams on top of column.

c.      jointing of column to beam at an intermediate.

 

d.  jointing of beams

f.       Forming of joints of arched structures

g.     joining of joints of post tensioned structures.

h.     joining of precast to monolithic reinforced concrete structures.

 

a. Joining column to footing:-

 

This joint is usually rigid but also can be hinge. A rigid joint can be made by placing the column into a calyx of the footing or by using a welded joint the figure shows the three variations of the this method.

a)  Can be used for smaller b) for average and for large footings

 

The opening of the calyx is 6-10 cm greater in all directions than the c/s of the column. This is enable the vibrator to be operated while concreting at the bottom of the calyx & checked by leveling before concreting. A similar steel plate is also put on the lower end of the column when positioning the column. These two steel plates must be on each other. The dimensions of these steel plates are from 100 x 100 x 10 to 150 x 150 x 10mm an chored in to the concrete after the column is put in placed properly plumbed tow advantages of the calyx joint should be mentioned

 

1       The placing plumbing and fixing of the column as well as the subsequent filling of the calyx with concrete is far simpler and requires less time then in the case of a welded joint.

2       The method is least sensitive to inaccuracies occurring during the construction.

 

The disadvantages of the calyx Joint is more suitable for small columns. In the case of large columns requiring a calyx depth of which is greater than 1.0m.

 

One members of joining a merge like joint consists either of placing to beams on to as small cantilever protracting trans the column or of putting it on the bottom of an adequately shaped opening deft out of the column shaft. The beam rests temporarily on a tongue like extensist on a steel plate placed in this opening on the supporting surface the tongue is also furnished with a steel plate anchored in to the concrete. The other parts of the tongue are supported after the placing has been finished with concrete cast through an opening left for this purpose.

 

d. Lengthing of columns:-

 

Columns are usually lengthened at floor levels. An intermediate lengthening should be avoided it possible.

 

The lengthening of columns can be executed similarly to the joining with fooling, accordingly reinforcement are joined by overlapping looped steel bars a welding. Thereafter the stirrups have to be placed & finally the joint must be concreted.

 

e) Joining of Beams:-

 

The functions of beams can be affected either by overlapping the protracting steel bars or by welding them together.

 

Figure shows the tinge like joint of purlins. In this method the whole shear must be borm by both cantilever (i.e) by two separate structures therefore it is expedient to form this joint atleast for large girders.

 

The method illustrated in the fig presents a dry joint of beams which is called a bolted form. The advantages of this joint is immediate bearing capacity.

 

f) Forming of functions of arched structures:-

 

Precast arches are usually produced and assembled in the form of three hinged structures. When the constant load has already been applied the centre joint is frequently eliminated. The omission of the centre joint increases the rigidity of the structure. Naturally arched structures can also be precast in a piece i.e in the form of two hinged ones.

 

Hinges of arched structures can be made by using either steel shors are more expensive, but the centre transmission of forces is enhanced by their use.

Figure shows the centre hinge of a three hinged arch with a span of 12.5 m for this kind of each the joint can also be formed from the concrete itself. The latter method require exactly executed and adequately reinforced concave & convex surfaces with hear against one another.

The joining of a smaller hinged structure is show in below mentioned.

 

The arrangements the centre junction of the end hige of an arched structure. This method was used in the construction of the ball for the middle rolling train in D.O.Sayar. The structure was precast & assembled in the form of a three-hinged arched transformed later in to a two -hinged one.

 

g. Design of joints for post tensioned structures:-

 

Post tensioned structures can generally be joined for more simply then the usual reinforced concrete structures, By using post tensioning it can be ensured that in the entire structure. The joints included only compressive can develop consequently the problem of joining can be solved in a very easy manner namely by placing plane surfaces side by side and then filling the gaps with cement mortar.

 

precast member. Thus in post tensioned structures the forming of joints does not cause difficults.

 

Sketches solutions of principles relating to the joining of post tensioned structure are to be illustrated in the fig. All these joints are of course rigid and moment bearing. It is not permissible for the mortar which is to be poured into the gaps to follow in to the ducts of the stressing lables to arid this cable ducts are jointed by placing a shore piece of tube or rubber ring into the duct itself.

 

A regid point of their kind established between a columned two girders supported by the former after the casting of the gaps & hardening of the mortar, the short inserted cables are stressed and so rigid joint is established.

 

h) Joining of precast to monolithic reinforced concrete structures:-

 

If frequently occurs that a monolithic beam has to be joined to a precast column in this by placing the protruding from the column or into an opening formed in the column's shaft.

 

When making the joint, first of all a 2.5 cm deep cavity is chielled out of the side of the precast column. The bottom of this cavity should be roughened so as to attain a better bank between the concrete of the monolithic beam and the precast column.

 

1.Explain about Roofing members in etail.

 

Roofing members are classified as,

 

i.Reinforced planks.

 

ii.Light weight  concrete members.

 

iii.Small reinforced roofing

 

members. iv.purlins

 

v.Large reinforced roofing

 

members. Reinforced planks:

 

Reinforced planks made of hollow tiles.The reinforced planks with longitudinal circular holes.Thickness of these tiles is 60mm,80mm & 100mm & the width is 200mm & length is vary from 360mm to 400mm. On the upper side one longitudinal groove is provided.

 

Reinforcement is placed into these grooves which are subsequently filled with cement mortar.In this way ,roofs of length 2 to 3m & thickness of 60 to 100mm & width  200mm can be constructed.

 

The end tiles resting on the support are provided with 3.11mm dia stirrups protruding from the tile.There are kept together over mortar of 40mm thickness & in further concreting of joint.the joint is completed.

 

Light weight concrete roofing members:

 

Light weight concrete roofing members play a role in addition to space bordering & load bearing in heat insulation.The thickness varies from 7.5 to 25cm for reinforcement of Light weight concrete roofing members.weiding nets is used.steel reinforcement is given additional coating to prevent any corrosion care is taken to give good bonding of reinforcement with concrete.

 

The unit weight of these members is 750kg/m3 & width of 50cm.Its varies from 1.75mm to 6m.precast members can be made either in usual way using light weight materials.sand as aggregate

 

& combination of high strength concrete.The top & bottom layer of about 2 to 3cm thickness iss provided with high strength concrete.Its consists of prestressed 2.5mm ddia embedded in these layers.The middle portion is made with light weight concrete.

 

Small  reinforced concrete roofing

 

members:

 

The    Small   reinforced  concrete  roofing  members  is  essentially  precast   simply

 

supported ribbed concrete slab width varying from 450 to 120cm & length varying from 2 to 4m.

 

Purlin s:

 

Purlins are precast concrete beams supported by the main girders serving the purpose of bearing for the roof covering.The cross section of purlins iss generally rectangular but it can also have trapezoidal,T,L and I shape.

 

Precast purlins can be simply supported or cantilever beams & for the bearing of loads beyond these weight simply supported purlins can be transformed into continuous beams.It is very simple & easy to place.For cantilever purlins placing of hinges should be determined in a manner to develop positive & negative moments equal to each other.This can be arrived by placing the hinges @

 

0.1451 from the support where I is the spacing between the

 

supports. Large reinforced concrete roofing members:

 

Large reinforced concrete rest on the main girders.These are generally used for large hall structures & these are most advanced type of precast structures.Members are manufactured corresponding to spacing of the frame length of about 6 to 10m & width of 1.3 to 1.8m. As they are most supported on main girder purlins are not required.

 

Four kinds of members exist:

 

1.Normal members.

 

 

 

2.Intermediate

members.

 

3.Members with cornice.

 

4.Members with gutter & eves border.

 

2.Write a detailed note on shear walls.

 

The types are classified as

 

1.Rectangular type or Bar bell type.

 

2.Coupled shear wall.

 

3.Framed shear type.

 

4.Core type shear wall.

 

When walls are to carry only compressive force,they can be designed as plain concrete,when walls are subjected to tensile forces(due to wind force)due to eccentric loading& earthquake load the walls are RC walls,shear walls are previously provided to resist the wind forces only.Hence became popylar & to prvide shear walls to resist EQ forces.

 

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