Various types of RCC Slabs | Design of RCC Structures

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Reinforced Cement Concrete Slab

  • A Reinforced Concrete Slab is the one of the most important component in a building. It is a structural element of modern buildings. Slabs are supported on Columns and Beams.
  • RCC Slabs whose thickness ranges from 10 to 50 centimetres are most often used for the construction of floors and ceilings.
  • Thin concrete slabs are also used for exterior paving purpose.
RCC Slab Construction
RCC Slab Construction
  • In many domestic and industrial buildings a thick concrete slab, supported on foundations or directly on the sub soil, is used to construct the ground floor of a building.
  • In high rises buildings and skyscrapers, thinner, pre-cast concrete slabs are slung between the steel frames to form the floors and ceilings on each level.

  • While making structural drawings of the reinforced concrete slab, the slabs are abbreviated to “r.c.slab” or simply “r.c.”.

Design of various types of slabs and their reinforcement

For a suspended slab, there are a number of designs to improve the strength-to-weight ratio. In all cases the top surface remains flat, and the underside is modulated:

  • Corrugated, usually where the concrete is poured into a corrugated steel tray. This improves strength and prevents the slab bending under its own weight. The corrugations run across the short dimension, from side to side.
  • A ribbed slab, giving considerable extra strength on one direction.
  • A waffle slab, giving added strength in both directions.

Reinforcement design

  • A one way slab has structural strength in shortest direction.
  • A two way slab has structural strength in two directions.

These slabs could be cantilevered or Simply Supported Slabs.

Construction

  • A concrete slab can be cast in two ways: It could either be prefabricated or cast in situ.
  • Prefabricated concrete slabs are cast in a factory and then transported to the site ready to be lowered into place between steel or concrete beams.
  • They may be pre-stressed (in the factory), post-stressed (on site), or unstressed. Care should be taken to see that the supporting structure is built to the correct dimensions to avoid trouble with the fitting of slabs over the supporting structure.
  • In situ concrete slabs are built on the building site using formwork. Formwork is a box-like setup in which concrete is poured for the construction of slabs.
  • For reinforced concrete slabs, reinforcing steel bars are placed within the formwork and then the concrete is poured.
  • Plastic tipped metal, or plastic bar chairs are used to hold the reinforcing steel bars away from the bottom and sides of the form-work, so that when the concrete sets it completely envelops the reinforcement.
  • Formwork differs with the kind of slab. For a ground slab, the form-work may consist only of sidewalls pushed into the ground whereas for a suspended slab, the form-work is shaped like a tray, often supported by a temporary scaffold until the concrete sets.

Materials used for the formwork

  • The formwork is commonly built from wooden planks and boards, plastic, or steel. On commercial building sites today, plastic and steel are more common as they save labour.
  • On low-budget sites, for instance when laying a concrete garden path, wooden planks are very common. After the concrete has set the wood may be removed, or left there permanently.
  • In some cases formwork is not necessary – for instance, a ground slab surrounded by brick or block foundation walls, where the walls act as the sides of the tray and hardcore acts as the base.

Span – Effective Depth ratios

  • Excessive deflections of slabs will cause damage to the ceiling, floor finishes and other architectural details. To avoid this, limits are set on the span-depth ratios.
  • These limits are exactly the same as those for beams. As a slab is usually a slender member the restriction on the span-depth ratio becomes more important and this can often control the depth of slab required in terms of the span – effective depth ratio is given by,

Minimum effective depth = span/(basic ratio x modification factor)

The modification factor is based on the area of tension steel in the shorter span when a slab is singly reinforced at midspan, the modification factors for the areas of tensions and compression steel are as given in the figure 2 and 4 of the code.

Solid Slab spanning in two directions

  • When a slab is supported on all four of its sides, it effectively spans in both directions, and it is sometimes more economical to design the slab on this basis. The moment of bending in each direction will depend on the ratio of the two spans and the conditions of restraint at each support.
  • If the slab is square and the restraint is similar along the four sides, then the load will span equally in both directions. If the slab is rectangular, then more than one-half of the load will be carried in the shorter direction and lesser load will be imposed on the longer direction.
  • If one span is much longer than the other, a large portion of the load will be carried in the shorter direction and the slab may as well be designed as spanning in only one direction.
  • Moments in each direction of span are generally calculated using co-efficients which are tabulated in the code.
  • The slab is reinforced with the bars in both directions parallel to the spans with the steel for the shorter span placed farthest from the natural acis to five the greater effective depth.
  • The span-efective depths are based on the shorter span and the percentage of the reinforcement in that direction.

92 thoughts on “Various types of RCC Slabs | Design of RCC Structures”

      • A concrete block or specimen is sent to the lab for testing the quality of the concrete. Load is applied on the concrete block after 28 days of curing to determine the crushing strength of the concrete.

        The strength of the concrete is calculated by dividing the maximum load at failure by the average cross sectional area of the concrete block or member.

  9. For simply supported slab
    where as Room size of 3m X 4.5m
    As per direction of reinforcement, What type slab will come ? 1> one way slab 2> two way slab

    • Hello,
      Since the room dimensions are 3mx4.5m, two way slab would be perfect and safe. If the size of the room was 2mx4m, then you could have go for a one way slab.

      I hope I have made it clear.

  13. i am civil engineering student rivers state polytechnic bori, while i was dabbling into the internet i found site more interesting and educative, please could assist me more in engineering mathmatics, like structral mechnics, engineering hydrology and structural calculation.

    Regards emma

  16. Hi,
    Dear sister it is very benifacial for the student.i admit and i proude of my nighbours(idian’s brothers and sisters) wich are taking intrest in to distribute the knowledge.after that sister what is cintiliwer beam and also its desiging?thanks and best wishes 4 u.

  19. Hello, I’m constructing a warehouse with basement and ground floor. we initially planned to do the RCC, but after completion of Basement floor, now we decided to use the pre-fabricated slabs for ground floor, because it is very economical (save 50%). size of hall is 110 x 80 ft. plz tell me some benefits of using prefab technology.
    Thanks

    • One way slab is generally rectangular in shape and two way slab is square in shape.

      The reinforcement requirements for a rectangular shaped slab and a square shaped slab is different. Depending on the span and shape of the slab, it is classified under one way or two way.

      Hope it is clear now.

      • two way slab need not be square itself in shape can be of rectangle also.
        In simple words lenght to width ratio of slab if it exceeds 2.0 its one way slab, if not two way slab.

    • r=(b*m1)/(a*m2)
      b = long direction
      a = short direction
      m1 = factor depend slab system in long direction
      m2 = factor depend slab system in Short direction
      if r > 2.0 , so the slab was One Way Slab
      if r =< 2.0 , so the slab was Two Way Slab
      if r = 1.0 , reverse the directions ( the short direction become the older one )
      ——————————————
      m1 & m2 determine from the following :
      1. if the slab simply supported , m = 1.0
      2. if the slab continuous from two sides ( in the same direction of study ) , m = 0.87
      3. if the slab is simply from side and continuous from other side ( in the same direction of study ) , m = 0.76
      ——————————————–

  21. How do we consider a water tank of 2000 litres (sintex tank) in the design of a two way slab or One way slab? As a concentrated load or udl?

    I am planning to do an inverted T-beam designof a slab and place the water reservoir on those beams

  25. hello,
    I am pg student. i am having my project work in scaffolding design and i dont have any material regarding designing standards of scaffolding and formworks so please guide me.

  26. i am trying to design a slab spanning 25 feet X 19 feet. as a oneway with steel 10mm in short at 6″ c/c and 8 mm distribution at 8″ c/c plz guide me is it ok or not???????

  30. awesome awesome awesome..u hav really summed it very well..it helped me a lot to do a quick recap of the rcc slab construction for my seminar.. 🙂 thankz..

  34. Dear mam, today fortunately I visited ur blog and found it quite informative. I request u to plz notify me new posts by email.thanx and regards. .Ajay sharma

    • Hello Ajay,
      I have added your email address in the RSS feed list. After you confirm the subscription, you will automatically receive all the new posts posted on the Civil projects blog.

      Cheers 🙂

  36. hello BENZUJK madam, pl remember a 2 way or 1way slab is classified by the ratio of large span and short span .if the ratio increases 1.5 the slab is taken as two way and reinforcement must be designed by code method which gives coefficients for long and short span and the bending moments can be calculated depending on if they are fixed on 2 supports or continuous..the short span takes more load and for cornors for torsion spl square rods should be provided..thank you if you read with patience

    d.ramachandran.B.E(CIVIL)

    • Hello sir,
      I think u have lot of field knowledge. so please give me answers of below questions so that i will be thankful to u.
      1) what are the disadvantages of dropping concrete from certain height.
      2) what is the Initial and final setting time of cement.

      Regards,
      Abdul kaleem
      kaleem1004@gmail.com

  39. In a villa construction with load bearing wall having a room size 4.15m x 5.18. could you please advise the reinforcement which I am planning is correct or not. The slab thickness 110mm with reinforcement 8mm@100mmc/c shorter span and 8mm @120c/c at longer span. I hope your valuable advise.

    Regards
    Baiju

  44. hello ma’am,i hope u r doin gud, few queries.
    i saw that roofing can be done by pouring concrete over the steel corrugated sheet, is der need for reinforcement,should we give neccesary support like i section r any at bottom, wat is da max load dat it can take n mix proportion dat can be used for concreting..plz help me out

  45. Hello mam,

    please give me the answer of request no : 46. I am eagerly waiting for ur answer so please check it and try to reply as early as possible.

    Regards,
    Abdul kaleem

    • Hello Adam,
      Ribbed and Waffle slab are almost the same with a few differences.

      Ribbed slab has a thick top slabs with crisscross beam network below. The depth of the ribs (beams) is less than the depth of the beams in the waffle slab. The beams are deeper in waffle slab. Also, the top slab is comparatively thinner than the top slab in a ribbed one.

  53. Any suggestions – steps to green roof?
    In Lahore got constructed 1500 sq ft
    1-RCC slab roof – lenter.
    2-Over the lenter slab completed a half inch layer of thin gravel + cement + Wapro chemical (water, heat resistant).
    3-Now going to spread a sheet of plastic.
    4-Then a 6 inch soil layer
    5-Then a layer of tiles an inch thick.
    What should be the next step?

  55. sambhaji :
    normally how many days should we water to slab? also how many days we should keep slab centering supports?

    Amjad :
    Any suggestions – steps to green roof?
    In Lahore got constructed 1500 sq ft
    1-RCC slab roof – lenter.
    2-Over the lenter slab completed a half inch layer of thin gravel + cement + Wapro chemical (water, heat resistant).
    3-Now going to spread a sheet of plastic.
    4-Then a 6 inch soil layer
    5-Then a layer of tiles an inch thick.
    What should be the next step?

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