Slab 3c1d11

Slabs are constructed to provide flat surfaces, usually horizontal, in building floors, roofs, bridges, and other types of structures. The slab may be ed by walls, by reinforced concrete beams usually cast monolithically with the slab, by structural steel beams, by columns, or by the ground Slabs.@@@@ Economics and development of methods of construction, the suitability of particular sets of requirements, and advance in the method of analysis of slabs have all ed to shape the current practice, and these factors will just as surely continue to change the types of slabs that are built. Slab may be divided into two general categories: beam less slab and slabs ed on beams located on all sides of each . (Reinforced concrete slabs)(Robert bark). Actual floor systems in buildings come in all sizes, shapes, and forms. There are so many variables to any floor system such as different spans, offset spans, cantilevers, and the extent of continuity, the effects of beams, columns and walls on the slab system, etc. that it is difficult to cover all situations in a limited series of charts. With the range of sophisticated computer packages now available for concrete floor design, it is strongly recommended that once a floor system and initial sizes are chosen then a series of trial runs be carried out to determine the appropriate thicknesses and details as part of the preliminary design process (guide to long span concrete floors).

1. Flat slab:A flat slab is a one-way or two-way system with thickenings in the slab at the columns and loadbearing walls called 'drop s' figure1. Drop s act as T-beams over the s. They increase the shear capacity and the stiffness of the floor system under vertical loads, thus increasing the economical span range. This form of construction has become less popular in recent years because of the limit on economical spans of about 9.5m for reinforced slabs and about 12 m for pre-stressed slabs.

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The plan dimensions of the drop s are a minimum of 1/3 of the span in the direction under consideration, usually rounded to the nearest 100 mm. The overall depth of the drop is typically taken as 1.75 to 2 times the depth of the slab, again rounded to suit timber sizes or the nearest 25 mm. The principal features of a flat slab floor are a flat soffit, simple formwork and easy construction. The economical span 'L' of a reinforced concrete flat slab is approximately D x 28 for simply ed, D x 32 for an end span and D x 36 for an interior span, Prestressing the slab increases the economical span to D x 35, D x 40 and D x 45 respectively, where D is the depth of the slab excluding the drop . (Guide to long span concrete floors). Where added strength over a is required, thickened slabs around columns are used to increase the local shear capacity. A drop cap, also referred to as a column capital, is defined by the ACI 318–05/08 as a thickening, which does not extend into the span beyond one-sixth of the span length. A drop , on the other hand, extends into the span one-sixth of the span length or beyond. It is permitted by the ACI to increase the negative moment capacity of the slab by using the increased thickness of the slab in drop regions. Traditionally, drop caps and s around columns are used for two principal reasons: 1. Column capitals, or drop caps, are used to improve the punching shear capacity of the column/slab t only. 2. Drop s are used to increase the bending moment capacity of the t, reduce deflection, and increase the punching shear capacity. (Reinforced concrete design of tall buildings).

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Figure1: Flat slab floor system.

It can be used where : 1. To provide plain ceiling surface giving better diffusion of light 2. Easy constructability with economy in the formwork 3. Larger head room or shorter storey height & pleasing appearance. 4. This kind of slabs are provided in parking 5. Flat slabs are generally used in parking decks, commercial buildings, hotels or places where beam projections are not desired.

Advantages: 1. It minimizes floor-to-floor heights when there is no requirement for a deep false ceiling. Building height can be reduced 2. Auto sprinkler is easier. 3. Less construction time. 4. It increase shear strength of slab 5. Reduce the moment in the slab by reducing the clear or effective span.@@@@ 6- Simple formwork 7- No beams—simplifying under-floor services outside the drops. 3

8- Minimum structural depth. 9- Usually does not require shear reinforcement at the columns.

Disadvantages: 1-Medium spans. 2- Generally not suitable for ing brittle (masonry) partitions. 3-Drop s may interfere with larger mechanical ducting. 4-Vertical penetrations need to avoid area around columns. 5-For reinforced flat slabs, deflection at the middle strip may be critical. (guide to long span floors).

There are four different types of flat slabs:1. Slab with drop and column without column head. 2. Slab without drop and column with column head. 3. Slab with drop and column with column head.@@@@@

2-Flat Plate: A flat plate is a one- or two-way system usually ed directly on columns or loadbearing walls. It is one of the most common forms of construction of floors in buildings. The principal feature of the flat plate floor is a uniform or near-uniform thickness with a flat soffit which requires only simple formwork and is easy to construct. The floor allows great flexibility for locating horizontal services above a suspended ceiling or in a bulkhead. The economical span of a flat plate for low to medium loads is usually limited by the need to control long-term deflection and may need to be sensibly pre-cambered (not overdone) or pre-stressed. An economical span for a reinforced flat plate is of the order of 6 to 8 m and for pre-stressed flat plates is in the range of 8 to 12m. The span 'L' of a reinforced concrete flat-plate is approximately 4

D x 28 for simply ed, D x 30 for an end span of a continuous system, to D x 32 for internal continuous spans. The economical span of a flat plate can be extended by pre-stressing to approximately D x 30, D x 37 and D x 40 respectively, where D is the depth of slab.

Figure2: Flat plate floor system. Advantages: 1- Simple formwork and suitable for direct fix or sprayed ceiling. 2- No beams—simplifying under-floor services. 3- Minimum structural depth and reduced floor-to floor height. 5

Disadvantages: 1- Medium spans. 2- Limited lateral load capacity as part of a moment frame. 3- May need shear heads or shear reinforcement at the columns or larger columns for shear. 4- Long-term deflection may be controlling factor. 5- May not be suitable for ing brittle ( masonry) partitions. 6- May not be suitable for heavy loads. (guide to long span floors)

3. Conventional slab (beam and slab):The slab which is ed with Beams and columns is called conventional slab. In this kind of slab the thickness of slab is small whereas depth of beam is large and load is transferred to beams and from beams to columns. It requires more formwork when compared with the flat slab. and there is no need of providing column caps in conventional slab. The thickness of conventional slab is 4″ or 10cm. 5″ to 6″ inches is recommended if the concrete will receive occasional heavy loads, such as motor homes or garbage trucks.@@@@@ It is a very traditional system .The relatively deep beams provide a stiff floor capable of long spans, and able to resist lateral loads. However, the complications of beam formwork, co-ordination of services, and overall depth of floor have led to a decrease in the popularity of this type of floor. The traditional reinforced concrete beam-and-slab floor has an economical span 'L' of D x 15 for a single span and D x 20 for a multispan, where D is the depth of the slab plus beam. The depth of slab between the beams can be initially sized using the span-to-depth ratios for a flat plate. Pre-stressing is not normally used with this system. ( guide to long span floors) Reinforcement is provided in conventional slab and the bars which are set in horizontal are called Main Reinforcement Bars and bars which are set in vertical are called Distribution bars. These types of slabs are used in constructing floors of multi-stored building.@@@@@@

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Advantages: 1. Traditional effective solution. 2. Long spans.

Disadvantages: 1. Penetrations through beams for large ducts difficult to handle. 2. Depth of floor. 3. Greater floor-to-floor height. (guide to long span floors).

4-Band Beam and Slab: This system consists of a series of parallel, wide, shallow beams (known as band beams or thickened slab bands) with the floor slab spanning transversely between the bands. (guide to long span floors). When the arrangement of a uniform floor slab is such that the spans in one direction are substantially longer than in the perpendicular direction, the length of the longer spans typically governs the slab thickness. The unfavorable effects of longer spans can be reduced if a band beam, also called slab band, is used in the long span direction.(reinforced concrete design of tall buildings). The floor slab is designed as a continuous slab, with the shallow band beams carrying all loads from the slab. Band beams or thickened slab bands are a two-way slab system designed in accordance with Clause 7.5 of AS 3600 and are not treated as beams, except for shear under Clause 8.2 of AS 3600. Sometimes, composite concrete/metal decking is used for the slabs, provided the slab spans are not too large. In a single-span floor, the spacing of the band beams may coincide with the columns, or the bands may be more closely spaced to reduce the thickness of the slab spanning between walls or beams. For single span reinforced concrete floors the economical span 'L' of the band beam is D x 20 to D x 22 depending on the width and spacing of the band beam, where D is the depth of the slab plus band beam. Pre-stressing the band beam gives economical band-beam spans in the range of D x 24 to D x 28. For internal spans the slab thickness is based on the clear span between band beams, and for an external bay is from the edge of band to the column line of the external band. The 7

depth of the band is typically 1.5 to 2 times the depth of the slab and the minimum economical span for a band beam is about 7–8 m. In multiple spans using reinforced concrete, the economical slab of the band beam 'L' is approximately D x 22 for 1200-mm-wide band beams and D x 26 for a 2400-mm-wide beams at 8400-mm centres. Pre-stressing increases the economical span 'L' to D x 24 to D x 28 for similar beam widths. D is the depth of slab plus band beam in each case. The maximum span for reinforced concrete bands should not normally exceed 12 m. Above this span, bands should be pre-stressed. The slab band width should be between band-spacing/3 to band-spacing/4 and, where possible, should be based on a module of a standard sheet of ply of 2.4 m x 1.2 m.

Figure3: Band beam and composite/metal decking slab. Advantages: 1-Relatively simple formwork 2- Shallow beams to allow services to run under the floor. 3- Minimum structural depth and reduced floor-to floor height. 4- Long spans. 5- Good cost/time solution. 6- Allows the use of flying forms. Disadvantages: 1- Long-term deflection may be controlling factor and post-tensioning may be required. 8

2- May need service penetrations through beams which are difficult to handle. (guide to long span floors).

5-Hollow core ribbed slab:Hollow core ribbed slabs derive their name from the voids or cores which run through the units. The cores can function as service ducts and significantly reduce the self-weight of the slabs, maximizing structural efficiency. The cores also have a benefit in sustainability in reducing the volume of material used. Units are generally available in standard 1200 mm widths and in depths from 110mm to 400 mm. There is total freedom in length of units. This kind of slab is used where the construction has to be done fast. These kind of slabs are Pre cast slabs which are readymade. The hollow core ribbed slabs have between four and six longitudinal cores running through them, the primary purpose of the cores being to decrease the weight, and material within the floor, yet maintain maximal strength. To further increase the strength, the slabs are reinforced with 12mm diameter steel strand, running longitudinally. By using tower cranes Hollow slabs are inserted between the beams. The gaps between the slab is filled with screed .Screed is a concrete material generally we use 20mm aggregate in concrete whereas in screed we use baby chips (small broken stones) as aggregates. Hollow core ribbed slabs have excellent span capabilities, achieving a capacity of 2.5 kN/m over a 16m span. The long-span capability is ideal for offices, retail or car park developments. Units are installed with or without a structural screed, depending on requirements. Slabs arrive on-site with a smooth pre-finished soffit. In car parks and other open structures, pre-finished soffits offer a maintenance free solution. 9

Advantages: 1. Using hollow core ribbed slab not only reduces building costs it also reduce the overall weight of the structure. 2. Excellent fire resistance and sound insulation is the another attributes of hollow core slab due to its thickness. 3. It eliminates the need to drill in slabs for electrical and plumbing units. 4. Easy to install and requires less labor. 5. Fast in construction. 6. No additional formwork or any special construction machinery is required for reinforcing the hollow block masonry. Disadvantages: 1. If not properly handled, the hollow core ribbed slab units may be damaged during transport. 2. It becomes difficult to produce satisfactory connections between the precast . 3. It is necessary to arrange for special equipment for lifting. and moving of the precast units. 4. Not economic for small spans. 5. Difficult to repair and strength.

6- Hardy slab:These kind of slabs are generally seen in Dubai and China. This slab is constructed by hardy Bricks. Hardy bricks are hollow bricks and made up of concrete Hollow blocks are used to fill portions of the slab thickness it saves the amount of concrete and hence the own weight of the slab is reduced. This kind of slab has a more thickness when compared with the conventional one. The thickness of hardy slab is 0.27m. The method of installing Hardy slab is different from 10

normal and it is clearly explained below: This is a one of the types of concrete slabs. The dimensions of Hardy brick is 40cm x 20cm x 20cm

The process of execution is as follows: Step 1: Formwork is arranged and then shutters are fixed on the formwork. Step 2: Hardy blocks are placed on the shutter with one brick gap on the entire Shutter. Step 3: The gaps between the bricks are called as rib. Reinforcement is provided in a form of beam within the gap. Step 4: After placing the rib, the plane steel mesh is placed on entire slab area resting on ribs. Step 5: Now pouring of concrete is done on slab.

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Where to use? Hardy slab is used where temperatures are very high. To resist the temperature from top the slab thickness is increased. Heat coming from walls are resisted by using special bricks which has thermacol in it. Thermacol is a best insualtor of sunlight.

Advantages: 1. Reducing slab weight by reducing amount of concrete below neutral axis. 2. Ease of construction, especially when all beams are hidden beams. 3. Economic for spans > 5m with moderate live load: hospitals, office and residential buildings. 4. Improved insulation for sound and heat. 12

Disadvantages: 1. If not properly handled, the hollow core ribbed brick units may be damaged during transport. 2. Not economic for small spans. 3. Difficult to repair and strengthen

7- Ribbed (Waffle) slab:Ribbed floors consisting of equally spaced ribs are usually ed directly by columns. They are either one-way spanning systems known as ribbed slab or a two-way ribbed system known as a waffle slab. This form of construction is not very common because of the formwork costs and the low fire rating. A 120-mm-thick slab with a minimum rib thickness of 125 mm for continuous ribs is required to achieve a 2-hour fire rating. Slab depths typically vary from 75 to 125 mm and rib widths from 125 to 200 mm. Rib spacing of 600 to 1500 mm can be used. The overall depth of the floor typically varies from 300 to 600 mm with overall spans of up to 15 m if reinforced, longer if post-tensioned. The use of ribs to the soffit of the slab reduces the quantity of concrete and reinforcement and also the weight of the floor. The saving of materials will be offset by the complication in formwork and placing of reinforcement. However, formwork complication is minimised by use of standard, modular, reusable formwork, usually made from polypropylene or fiberglass and with tapered sides to allow stripping. For ribs at 1200-mm centres (to suit standard forms) the economical reinforced concrete floor span 'L' is approximately D x 15 for a single span and D x 22 for a multi-span, where D is the overall floor depth. The one-way ribs are typically designed as T-beams, often spanning in the long direction. A solid drop is required at the columns and loadbearing walls for shear and moment resistance. (guide to long span floors). This kind of slab is majorly used at entrance of hotels, Malls, Restaurants for good pictorial view and to install artificial lighting. This atype of slab where we find hollow hole in the slab when the formwork is removed. Firstly PVC trays (pods) are placed on 13

shuttering then reinforcement is provided between the pods and steel mesh is provided at top of the pods and then concrete is filled. After concrete sets the formwork is removed and PVC pods are not removed. This forms hollow hole in it in which hole is closed at one end. The concrete waffle slab is often used for industrial and commercial buildings while wood and metal waffle slabs are used in many other construction sites. This is a one of the types of concrete slabs.

Where to use? A waffle slab is a type of slab with holes underneath, giving an appearance of waffles. It is usually used where large spans are required (e.g auditorium, cinema halls) to avoid many columns interfering the space. Hence thick slabs spanning between wide beams (to avoid the beams protruding below for aesthetic reasons) are required .The main purpose of employing this technology is for its strong foundation characteristics of crack and sagging resistance. Waffle slab also holds a greater amount of load compared with conventional concrete slabs. Types of Waffle slabs: Based on the shape of Pods (PVC Trays) They are classified into different types some of them are: 14

1. Triangular pod system 2. Square pod system

Advantages: 1. Waffle slabs are able to carry heavier loads and span longer distances than flat slabs as these systems are light in weight 2. Waffle slab can be used as both ceiling and floor slab. 3. Suitable for spans of 7m – 16m ; longer spans may be possible with posttensioning. 4. These systems are light in weight and hence considerable saving is ensured in the framework as light framework is required.@@@@

Disadvantages: 1- Depth of slab between the ribs may control the fire rating 2-Requires special or proprietary formwork 3- Greater floor-to-floor height 4- Large vertical penetrations are more difficult to handle. (guide to long span floors).

8- Dome Slab:These kind of slab is generally constructed in temples, Mosques, palaces etc. And Dome slab is built on conventional slab. Thickness of Dome slab is 0.15m. Domes are in semi-circle and shuttering is done on conventional slab in dome shape and concrete is filled in shuttering forming dome shapes. This is a one of the types of concrete slabs.

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9- Pitch roof slab: Pitch roof is an inclined slab, generally constructed on resorts for a natural look. Compared to traditional roofing materials Tile-sheets used in pitch roof slab are extremely lightweight. This weight saving reduces the timber or steel structural requirements resulting in significant cost savings .Tile-sheets are tailor made for each project offering labor cost savings and reduced site wastage. And the thickness of slab is depends on the tiles we using it may be 2″-8″. This is a one of the types of concrete slabs.

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Advantages: 1.It sheds off rain water better. 2. It gives you internal storage or room space. 3. It is less likely to leak. 4. Roof coverings are cheaper. 5. If it is a standard pitch, building materials are more cost-effective

Disadvantages: 1. This type of slabs are not suggested for long spans. 2. Repairs in slabs ike plumbing repair or electric wiring on slabs is difficult.

10- Slab with arches: This is a type of slabs which is generally adopted in construction of bridges. Bridges are subjected to two loads moving load from the vehicles and Wind load. These slabs are adopted at a place where there is a need of redirecting wind load and if there is a long curve in direction of slab these slabs are adopted. It resists the fall of bridge due to heavy wind load. They were originally built of stone or brick but these days are built of reinforced concrete or steel. The introduction of these new materials 17

allow arch bridges to be longer with lower spans. This is a one of the types of concrete slabs.

11.Post tension slab: The slab which is tensioned after constructing slab is called Post tension slab. Reinforcement is provided to resist the compression. In Post tension slab the reinforcement is replaced with cables/ steel tendons. Post-tensioning provides a means to overcome the natural weakness of concrete in tension and to make better use of its strength in compression. The principle is easily observed when holding together several books by pressing them laterally. Linder such pressure the whole row gains enough stiffness and strength to ensure its integrity. In concrete structures, this is achieved by placing high-tensile steel tendons/cables in the element before casting When the concrete reaches the desired strength the tendons are pulled by special hydraulic jacks and held in tension using specially designed anchorages fixed at each end of the tendon This provides compression at the edge of the structural member that increases the strength of the concrete for resisting tension stresses. If tendons are appropriately curved to a certain profile, they will exert, in addition to compression at the perimeter, a beneficial upward set of forces (load balancing forces) that will counteract applied loads, relieving the structure from a portion of gravity effects. This is a one of the types of concrete slabs. 18

Current openings for freshers civil engineers package 4.5 lac to 8 lac per year apply here and submit your resume here In this type of slab cables are tied instead of reinforcement. In Steel reinforcement the spacing between bars is 4inch to 6inch where as in Post tension slab the spacing is more than 2m. This is a one of the types of concrete slabs.

Advantages: 1. It allows slabs and other structural to be thinner 2. It allows us to build slabs on expansive or soft soils 3. Cracks that do form are held tightly together 4. Post tension slabs are excellent ways to construct stronger structures at an affordable price. 5. It reduces or eliminates shrinkage cracking-therefore no ts, or fewer ts, are needed 6. It lets us design longer spans in elevated , like floors or beams Disadvantages: 1. The post tension slab can be made only by skillful professionals.

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2. The main problem with using post tension slab is that if care is not taken while making it, it can lead to future mishaps. Many a times, ignorant workers do not fill the gaps of the tendons and wiring completely. These gaps cause corrosion of the wires which may break untimely, leading to some failures unexpectedly.

12. Pre tension slab : The slab which is tensioned before placing the slab is called Pre tension slab. The slab has same features of Post tensioning.

13. Cable suspension slab: If the span of the slab is very long, then we go for cable suspension slab which is ed on cable such as London bridge, Howrah bridge etc. Generally in construction of houses for every 4m we provide a column wheras in cable suspension slab for every 500m we provide a column. This kind of slab is provided where length of span is more and difficulty in building columns. The slabs are tied with cables and these cables are ed to columns.

14. Low roof slab: The slab which provided above the door for storage purpose is called. Low roof slab. Slab is closed at all ends and open at one end. This slab 20

lies below the actual slab and above the door sill level. These types of concrete slabs are used in houses.

15. Projected slab: The slab which has one side fixed and the other side is free is called as Projected Slab. These type of slabs are generally constructed in hotels, Universities, function halls, etc. to use that area for dropping or picking up zone and for loading and unloading area. This is a one of the types of concrete slabs.

16. Grads Slab/ Slab on grade: The slab which is casted on surface of the wearth is called a Ground slab. This type opf slab is used in Basement floor. There are two types of Grade slabs: 1. Usually after casting Plinth beams. Sand is filled at an height of 0.15m and then Sand level is rammed. Then PCC is poured on sand 21

upto a height of Plinth beams. Its an economic way of constructing ground slab which use majorly in India.

2. In high rise buildings after constructing Plinth beam the termite control is done in between the beams and then polythene sheet is laid to avoid termites inside the slab and then steel mesh is provided and concrete is filled. This costs more when compared with the previous one and requires more concrete than first one.

17. Sunken slab : Slab which is provided below the washrooms to hide the sewage pipes or sewerage pipes is called Sunken slab. Since the pipes that carry water are concealed below the floor, care has to be taken to avoid leakage problems. After casting sewage pipes in the slab the slab is filled with coal or broken pieces of bricks. There are two types of sunken slab. 22

1. The slab which is provided below the normal floor level at a depth of 200mm to 300 mm and filled with broken pieces of bricks is called Sunken slab

2. The Slab which is provided above the normal floor level at a height of 200mm to 300mm and filled with coal or broken pieces of bricks called sunken slab.

18. Miscellaneous slab: 1. chajja family: a) chajja :Small projection outside the door & windows. Purpose is protection from sun light & rains. Normal width of chajjah is between 2′ to 2’6″. Sometimes a box window is built which creates a box just outside the window. This also serves as a protection against sun & rains. In such case the upper slab acts as a sun shed. Box window allows occupants to maintain greenery by keeping plants in earthen pots. b) Porch: This is a projection outside the main door and the width could 23

be from 5′ to 6′. This can be done with either a cantilever slab or with cantilever inverted beams with sunk slab. Main Purpose is protection from sun light & rains. Porch also acts as a central element in the front elevation. Porch can also be used as a small sit out and for this access is given from the first floor.

c) Portico: Portico is a longer porch. Main difference between porch & portico is that the portico usually has a simply ed slab testing on simply ed beams (porch has cantilever beams). Portico are more ornamental in nature and have a provision that car can be parked below till engers board or alight. Porticos are generally provided for monumental buildings college’s institutions public buildings or a posh spacious bungalow.

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d) Loft: Loft is a cantilever slab of 2′ to 2’6″ projecting out from kitchen or bedroom wall at a height of 7′.Used for storage. e) Mezzanine: It is the intermediate floor between ground floor and the first floor. Details: In the shops the roof height is kept at about 16′.In half unit . A slab is given usually at 7′ height in half of this unit. This is known as the mezzanine floor. It can be used as the office or studio or private space etc. A stair case takes the people from the ground floor.

f) Stilts and stilts floor: Although these terns appear similar, there is a slight difference between these two

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On ground floor if there are only columns but no peripheral walls, it is called “stilts “.This floor is usually kept slightly above the formation level of the plot .(Formation level is the level of plot after it has been properly leveled and developed. This shall be slightly above the natural ground level). There are many uses but most common use of stilts is parking of vehicles. That’s why it is normally called as Parking. Stilts floor is that floor which is just above the stilts. There seems to be some variation in the usage of this term. Some people call this floor as first floor whereas others designate the floor above stilts floor as first floor

(https://constructionqueries.wordpress.com/2014/11/21/family-of-chajjah-porch-portico/)

2. Lintels:  Lintels are provided Inside building above the doors and windows to re direct the top load. There are two types of lintels  Pre cast Lintels: Lintels which are manufactured in factories is called Pre cast Lintels 26



Cast in situ: Lintels are casted at site itself is called Cast in situ lintels. The length of lintel is more than door length and has a width of wall, thickness of lintel is 0.1m

3. Sun shade slab: 

Sun Shade is provided outside building above the Doors and windows are called Sun shade slab. The slab stops rain to come inside the building and direct sunlight. This is a one of the types of concrete slabs.

Reference @@@@@ https://www.quora.com/What-are-the-16different-types-of-slabs-in-construction.

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19. Slab and Joist This system consists of a slab spanning between beams (which span between columns) and usually an intermediate joist in one direction. Frequently, the slab thickness between the joists is controlled by requirements for fire-resistance. For example, a 2-hour fire resistance rating requires a 120-mm slab thickness, which is capable of spanning approximately 4 m. For this widely spaced rib or joist floor the economical span 'L' is D x 18 for a single span and D x 22 for a multi-span. Prestressing the joists permits the span to be increased to D x 24 and D x 28 respectively, where D is the depth of floor including the slab and joist.

Advantages: 1. Thin slab s possible 2. Suits industrial structures 3. Long spans 4. Vertical penetrations between beams easily accommodated.

Disadvantages: 1. More formwork 2. Joists and beams intrude on services 3. Depth of floor 4. Greater floor-to-floor height. (guide to long span floors).

Based on length and breadth of Conventional Slab is classified into two types: 1. One way slab: 28

One way slab is a slab which is ed by beams on the two opposite sides to carry the load along one direction. The ratio of longer span (l) to shorter span (b) is equal or greater than 2, considered as One way slab because this slab will bend in one direction i.e in the direction along its shorter span. However minimum reinforcement known as distribution steel is provided along the longer span above the main reinforcement to distribute the load uniformly and to resist temperature and dhrinkage stresses.

In general length of slab is 4m. But in one way slab one side length is 4m and other side length is more than 4m. So it satisfies the above equation. In one way slab main reinforcement is provided in shorter span and distribution reinforcement is provided in longer span. Distribution bars are cranked to resist the formation of stresses. Example: Generally all the Cantilever slabs are one Way slab. Chajjas and verandahs are an practical example of one way slab.

2. TWO WAY SLAB: Two way slab is a slab ed by beams on all the four sides and the loads are carried by the s along both directions, it is known as two way slab.

In two way slab, the ratio of longer span (l) to shorter span (b) is less than 2.The slabs are likely to bend along the two spans in this load is transferred in both the directions to the four ing edges and hence distribution reinforcement is provided in both the directions.

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In this kind of slab the length and breadth of slab is more than 4m. So distribution bars are provided at both the ends in two way slab. To resist the formation of stresses. These types of slabs are used in constructing floors of multi storeyed building. Learn More about Two Way Slab

CHOICE OF TYPE OF SLAB FLOOR The choice of type of slab for a particular floor depends on many factors. Economy of construction is obviously an important consideration, but this is a qualitative argument until specific cases are discussed, and is a geographical variable. The design loads, required spans, serviceability requirements, and strength requirements are all important. For beamless slabs, the choice between a flat slab and a flat plate is usually a matter of loading and span. Flat plate strength is often governed by shear strength at the columns, and for service live loads greater than perhaps 100 lb/ft2 (4.8 kN/m2 ) and spans greater than about 20 to 24 ft (7 to 8 m) the flat slab is often the better choice. If architectural or other requirements rule out capitals or drop s, the shear strength can be improved by using metal shear heads or some other form of shear reinforcement, but the costs may be high. Serviceability requirements must be considered, and deflections are sometimes difficult to control in reinforced concrete beamless slabs. Large live loads and small limits on permissible deflections may force the use of large column capitals. Negative-moment cracking around columns is sometimes a problem with flat plates, and again a column capital may be useful in its control. Deflections and shear stresses may 30

also be controlled by adding beams instead of column capitals. If severe deflection limits are imposed, the two-way slab will be most suitable, as the introduction of even moderately stiff beams will reduce deflections more than the largest reasonable column capital is able to. Beams are also easily reinforced for shear forces. The choice between two-way and beamless slabs for more normal situations is complex. In of economy of material, especially of steel, the two-way slab is often best because of the large effective depths of the beams. However, in of labor in building the floor, the flat plate is much cheaper because of the very simple formwork and less complex arrangement of steel. The flat slab is somewhat more expensive in labor than is the flat plate, but the forms for the column capitals are often available as prefabricated units, which can help limit costs. The real cost parameter is the ratio of costs of labor relative to material. Few two-way slabs are built in areas of high labor costs unless there are definite structural reasons, and many are built where steel is the most costly item. Hollow-tile slabs are still built in some places, but only where the cost of both steel and cement is very high relative to labor. Local customs among builders, designers, and s should not be overlooked when selecting the slab type. There is a natural human tendency to want to repeat what one has previously done successfully, and resistance to change can affect costs. However, old habits should not be allowed to dominate sound engineering decisions. If a flat plate or flat slab is otherwise suitable for a particular structure, it will be found that there is the additional benefit of minimizing the story height. In areas of absolute height restrictions, this may enable one to have an additional floor for approximately each 10 floors, as compared with a two-way slab with the same clearstory heights. The savings in height lead to other economies for a given number of floors, since mechanical features such as elevator shafts and 3 piping are shorter. There is less outside wall area, so wind loadings may be less severe and 31

the building weighs less, which may bring cost reductions in foundations and other structural components. There are other cost savings when the ceiling finishes can be applied directly to the lower surfaces of the slabs. Beamless slabs will be at a disadvantage if they are used in structures that must resist large horizontal loads by frame action rather than by shear walls or other lateral bracing. The transfer of moments between columns and a slab sets up high local moments, shears, and twisting moments that may be hard to reinforce for. In this situation, the two-way slab is the more capable structure because of the relative ease with which its beams may be reinforced for these forces. In addition, it will provide greater lateral stiffness because of both the presence of the beams and the greater efficiency of the beam-column connections. The possible choice of a precast one-way floor system, consisting of prestressed concrete placed side-by-side and spanning between the beams, girders, or walls and generally covered by a cast-in-place concrete topping slab, should not be overlooked.(2. file:///C:/s/RUM.RUM-PC/s/Documents/F_slabs.pdf)

Airdeck reinforced concrete decking can be used to make two-way lightened slabs with spans up to 14 x 14 m, or up to 17 x 17 m with posttensioning. The decking planks incorporate the lower reinforcing, a 32

reinforcing grid and a 300 x 300 mm grid of recycled polypropylene coffers that lighten the slab. The coffers are available in four sizes for slab thicknesses of 280 to 450 mm. The planks are lifted into place with a crane, using the sheer reinforcing grid for anchoring, and they are ed by a network of props that are generally spaced every 1.8 m. The rest of the reinforcing is set into place and the concrete is poured. The system can be adapted for irregular floor layouts. Its use significantly reduces construction costs and increases the speed of construction in buildings with repeated floors. Airdeck can incorporate a radiant slab climate-control system, in which the mass of the concrete slab is cooled or heated by water. (3. http://www.tectonicaonline.com/products/1715/span_slabs_large_decking_concrete_airdeck/ #) Beeplate® is a system for building two-way large-span slabs using hollow coffers in a staggered layout. Spans of 10 to 16 m can be made with slab thicknesses of 34 to 60 cm. Beeplate fixed coffers are laid out over two layers of Bamtec rolled reinforcing using clip-on spacers to form a honeycomb pattern. Edge reinforcing and additional reinforcing as required are then laid out. Once the two upper layers of rolled reinforcing and the sheer reinforcing are laid out over the coffers, the slab is ready for the pour. Concrete is poured in two stages, the second a wet over wet pour. The honeycomb structure creates a very compact hollow slab with a reduced thickness, lowering the amount of concrete required as well as the weight of the slab, as well as the weight of reinforcing and foundations. Two types of coffers are available: 20 cm deep with a diameter of 70 cm, or 31 cm deep and ø 66 cm. The two can be laid over one another in thick slabs.(4. http://www.tectonica-

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online.com/products/1712/slab_honeycomb_lightened_bamtec_beeplate/ )

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