PLATE GIRDER & COMPONENTS

 PLATE GIRDER

When span and load increases, the available rolled section may not be sufficient, even after strengthening with cover plates. Such situations are common in the following:

1. Large column free halls in the lower floor of a multistory building..

2. In a workshop, where girders are required to carry crane beams.

3. In road or railway bridges.

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Fig.1: Typical Plate Girder

Before welding technology advanced it was common practice to use riveted/bolted plate girders. Flange and web plates are connected to each other using angles and rivets/bolts. Many railway bridges of 24 m to 46 m were built like this. This practice of using riveted/bolted plate girder is given up in 1960's. Now-a-days only welded plate girders are built which are aesthetically good and at the same time light compared to riveted/bolted plate girders. Hence, in this chapter, design of only welded plate girder is discussed.

ELEMENTS OF PLATE GIRDERS

The following are the elements of a typical plate girder :

1. Web

2. Flanges

3. Stiffners

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1.WEB

Web of required depth and thickness are provided to :

(a) Keep flange plates at required distances

(b) Resist the shear in the plate girder

2.FLANGE

Flanges of required width and thickness are provided to resist bending moment acting on the beam by developing compressive force in one flange and tensile force in another flange.

3.STIFFENERS

Stiffeners are provided to safeguard the web against local buckling failure. The stiffeners provided may be-classified as

(a) Transverse (vertical) stiffeners and

(b) Longitudinal (horizontal) stiffeners

Transverse stiffeners are of two types :

(i) Bearinglend stiffeners

(ii) Intermediate stiffeners

End bearing stiffeners are provided to transfer the load from beam to the support. At the end certain portion of web of beam acts as a compression member and hence there is possibility of crushing of web. Hence web needs stiffeners to transfer the load to the support. If concentrated loads are acting on the plate girder (may be due to cross beam) intermediate stiffeners are retired.

To resist average shear stress, the thickness of web required is quite less. But use of thin webs may result in to buckling due to shear. Hence when thin webs are used, intermediate transverse stiffeners are provided to improve buckling strength of web..

Many times longitudinal (horizontal) stiffeners are provided to increase the buckling strength of the web. If only one longitudinal stiffners is provided, it will be at a depth of 0.2 d from the compression flange where d' is the depth of the web. If another longitudinal stiffener is to be provided it will be at mid depth of the web.

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EXTRA KNOWLEDGE 

BRICK COMPOSITION & MANUFACTURE

BRICKS 

Bricks are obtained by moulding clay in rectangular blocks of uniform size and then by drying and burning these blocks. As bricks are of uniform size, they can be properly arranged, light in weight and hence bricks replace stones. 

Composition 

Composition – Following are the constituents of good brick earth. 

Alumina: - 

It is the chief constituent of every kind of clay. A good brick earth should contain 20 to 30 percent of alumina. This constituent imparts plasticity to earth so that it can be moulded. If alumina is present in excess, raw bricks shrink and warp during drying and burning. 

Silica

-A good brick earth should contain about 50 to 60 percent of silica. Silica exists in clay either as free or combined form. As free sand, it is mechanically mixed with clay and in combined form; it exists in chemical composition with alumina. Presence of silica prevents crackers shrinking and warping of raw bricks. It thus imparts uniform shape to the bricks. Durability of bricks depends on the proper proportion of silica in brick earth. Excess of silica destroys the cohesion between particles and bricks become brittle. 

Lime 

– A small quantity of lime is desirable in finely powdered state to prevents shrinkage of raw bricks. Excess of lime causes the brick to melt and hence, its shape is last due to the splitting of bricks. 

Oxide of iron

- A small quantity of oxide of Iron to the extent of 5 to 6 percent is desirable in good brick to imparts red colour to bricks. Excess of oxide of iron makes the bricks dark blue or blackish. 

Magnesia- 

A small quantity of magnesia in brick earth imparts yellow tint to bricks, and decreases shrinkage. But excess of magnesia decreases shrink leads to the decay of bricks. 

 The ingredients like, lime, iron pyrites, alkalies, pebbles, organic matter should not present in good brick earth

Type of brick closer (cut) with help of 3D video Presentation.

Manufacture of bricks: 

The manufacturing of brick, the following operations are involved 

1. Preparation of clay 

2. Moulding 

3. Drying 

4. Burning 

(i) Preparation of clay :- The preparation of clay involves following operations 

a) Unsoiling :- Top layer of 20cm depth is removed as it contain impurities.

 b) Digging: - Clay dug out from ground is spread on level ground about 60cm to 120cm heaps. 

c) Cleaning:-Stones, pebbles, vegetable matter etc removed and converted into powder form. 

d) Weathering:- Clay is exposed to atmosphere from few weeks to full season. 

e) Blending:- Clay is made loose and any ingradient to be added to it is spread out at top and turning it up and down in vertical direction. 

f) Tempering:- Clay is brought to a proper degree of hardness, then water is added to clay and whole mass is kneaded or pressed under the feet of men or cattle for large scale, tempering is usually done in pug mill as shown in the 

Process:- Clay with water is placed in pug mill from the top. When the vertical staff is rotated by using electric pair, steam or diesel or turned by pair of bullocks. Clay is thoroughly mixed up by the actions of horizontal arms and knives when clay has been sufficiently pugged, hole at the bottom of tub, is opened cut and the pugged earth is taken out from ramp for the next operation of moulding. 

(2)  Moulding: 

Clay, which is prepared form pug mill, is sent for the next operation of moulding. Following are the two ways of moulding. Hand Moulding & Machine moulding

(3) Drying:

The damp bricks, if burnt, are likely to be cracked and distored. Hence moulded bricks are dried before thay are taken for the next operation of burning. Bricks are laid along and across the stock in alternate layers. The drying of brick is by the following means 

(i) Artificial drying – drying by tunnels usually 1200C about 1 to 3 days 

(ii) Circulation of air- Stacks are arranged in such a way that sufficient air space is left between them free circulation of air. 

(iii)Drying yard- special yards should be prepared slightly higher level prevent the accumulation of rain water 

(iv) Period for frying – usually about 3 to 10 days to bricks to become dry 

(v) Screens – screens are necessary, may be provided to avoid direct exposure to wind or sun. 

(4) Burning: 

This is very important operation in the manufacturing of bricks to impart hardness, strength and makes them dense and durable. Burning of bricks is done either in clamps or in kilns. Clamps are temporary structures and they are adopted to manufacture bricks on small scale. Kilns are permanent structures and they are adopted to manufacture bricks on a large scale. 

Types of Brick Bond with 3D video Persentation

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CEMENT & ITS COMPOSITION

 CEMENT 

 Cement in its broadest term means any substance which acts as a binding agent for materials natural cement (Roman Cement) is obtained by burning and crushing the stones containing clay, carbonates of lime and some amount of carbonate of magnesia. The clay content in such stones is about 20 to 40 percent. Natural cement resembles very closely eminent hydraulic lime. It is not strong as artificial cement, so it has limited use in practice.   Artificial cement is obtained by burning at very high temperature a mixture of calcareous and argillaceous materials in correct proportion. Calcined product is known as clinker. A small quantity of gypsum is added to clinker and it is then pulverized into very fine powder is known as cement. Cement was invented by a mason Joseph Aspdin of leeds in England in 1824. The common variety of artificial cement is known as normal setting cement or ordinary cement or Portland cement. 

 

 Ingradeints 

Ordinary Portland cement contains two basic ingredients, namely argillaceous and calcareous. In argillaceous materials, clay predominates and in calcareous materials, calcium carbonate predominates. Good ordinary cement contains following ingradients.  

1. Lime (cao)    ………. 62% 

2. silica (Sio2)   ………. 22%  

3. Aluminca(Al2 u3)   ………. 5%

 4. Calcium sulphate (CaSo4)  ……….  4%

 5. Iron Oxide (Fe2 O3)   ………. 3% 

 6. Magnescia (Mgo)   ………. 2% 

7. Sulphur   ………. 1%

 8. Alkalies   ………. 1% 

 

Functions of Ingradients: 

1. Lime:

 Lime is the important ingredient of cement and its proportion is to be maintained carefully. Lime in excess makes the cement unsound and causes the cement to expand and disintegrate. On the other hand, if lime is in deficiency the strength of the cement is decreased and it causes cement to set quickly 

 

2. Silica: 

This also an important ingredient of cement and it gives or imparts quick setting property to imparts strength to cement. 

 

3.Alumina: 

This ingredient imparts quick setting properly to cement. Express alumina weakens the cement.   

 

4. Calcium Sulphate: 

This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement. 

 5. Magnesia: 

The small amount of this ingredient imparts hardness and colour to cement. 

 6. Sulphur: 

A very small amount of sulphur is useful in making sound cement. If it is in excess, it causes the cement to become unsound. 

 

7. Alkalies:

 Most of the alkalies present in raw material are carried away by the flue gases during heating and only small quantity will be left. If they are in excess in cement, efflorescence is caused.  

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 The main types cements 

 i) Acid resistance cement

 ii) Blast furnace cement 

iii) Coloured cement 

iv) Expanding cement 

v) High alumina cement 

vi) Hydrophobic cement 

vii) Low heat cement 

viii) Pozzolona cement

 ix) Quick setting cement 

x) Rapid hardening cement

 xi) Sulphate resistance cement 

xii) White resistance cement 

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