Some useful guidelines for RCC slab design

In this exclusive civil engineering tutorial, you will get some useful guidelines for designing any RCC slab.

A) Effective span of slab – It should be least of the two

1) L = clear span + d (effective depth)
2) L = Center to center spacing among the support

B) Depth of slab: The depth of slab is influenced by bending moment and deflection criterion. The trail depth is achieved with the following :-

Effective depth d = Span/((L/d) Basic x modification factor)

To get modification factor, the percentage of steel for slab is taken from 0.2 to 0.5.

The effective depth d of two slabs is also taken as cl.24.1,IS 456 on the condition that short span is 3.5m and loading class is <3.5KN/m2.

Categories of supports: Fe-250 – L/35, Fe-415 – L/28

Continuous support: Fe-250 – L/40, Fe-415 – L/32

The following thumb rules are commonly applied :-

One way slab d = (L/22) to (L/28). Two way simply supported slab d = (L/20) to (L/30). Two way restrained slab d = (L/30) to (L/32)

Load On Slab: The load on slab contains dead load, floor finish and live load. The loads are measured according to unit area (load/m2).

Dead Load = D x 25 kN/m2 (Here D denotes thickness of slab in m). Floor finish (taken as) = 1 to 2 kN/m2. Live load (taken as) = 3 to 5 kN/m2 (based on the occupancy of the building)

Nominal Cover
For mild exposure – 20 mm
For moderate exposure – 30 mm

When the diameter of bar does not go beyond 12 mm or cover is decreased by 5 mm. For main reinforcement up to 12 mm diameter bar and for mild exposure, the nominal cover is 15 mm.

Least reinforcement: The reinforcement in either direction in slab should not remain under :-

0.15% of the total cross sectional area for Fe-250 steel. 0.12% of the total cross sectional area for Fe-415 & Fe-500 steel

Distance of bar: The maximum distance of bars should not surpass. Main steel – 3d or 300 mm which is lower. Distribution steel – 5d or 450 mm whichever is lesser

Here, d denotes the effective depth of slab. The least clear spacing of bars should not be under 75 mm (desirably 100 mm). Highest diameter of bar. The highest diameter of bar should not go over D/8, here D denotes the total thickness of slab.

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Published By

Rajib Dey

www.constructioncost.co

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Some important checklists for RCC slab and beams

The following checklists are extensively utilized for RCC slab and Beams.

1. Examine the bottom line, level & width of beam properly.
2. Examine the side line, level & plumb of beam properly.
3. Beam to beam measurements should be examined according to architectural drawings.
4. Individual level & diagonal of each slab bay should be examined properly.

5. The thickness level of slab should be marked with nails on the peripheral beam’s exterior sides.
6. Examine the thickness of slab and depths of beam properly.
7. Support props for slab & beam bottoms should arrange in line & plumb. Bamboo bracing should be accomplished at around 4′ ht from floor level. Support props for double staging terraces should be examined cautiously.
8. Line of external /peripheral beam’s sides should be examined & cross supported to get rid of buldging out of sides.
9. Junctions of columns & beam should be verified to make them water resistant.
10. De-shuttering oil should be provided to beam/slab shuttering.

11. Formwork of stair case should be examined for dimensions of tread and riser, level of treads, plumb of risers.
12. Reinforcement should be examined for beams and slab as per R.C.C drawing
13. Ensure to provide exact cover for bottom /sides of beams.
14. Proper cover should be provided for slab.
15. Electrical points, electrical piping (conduits)/fan hooks should be examined as per drawing.
16. Reduction of column according to drawing should be performed if any.
17. There should be exact numbers of chairs should be arranged for slab.
18. Dowels (if any) should be arranged for elevational feaures / future expansion.
19. Parapet (Pardi) bars should be provided for balconies/staircase etc.

20. Ring (stirrups) should be arranged at the free end of each column reinforcement.
21. Distances among plates /planks should be filled. Taping should be performed at ply joints for the protection of ply shuttering.
22. Before starting pouring work, consent of the architect and R.C.C Consultant should be obtained.
23. Prior to start concreting work, verify the accessibility of necessary labour strength, mixer, lift, vibrators, masons, weigh batcher, diesel/petrol and plastic sheets etc before start of concreting.
24. Hidden beams /inverted beams/cantilever beams should be examined properly.
25. Based on the approved drawings, verify the sunk.
26. Packing underneath support props should not be provided. As an alternative, single wooden plank should be inserted as packing.
27. For large slabs, exact location of concrete joints should be determined beforehand as per approval of RCC consultant.
28. Accessibility of raw material for concrete /RMC per grade of concrete, water , electricity should be examined beforehand prior to start concreting work.

29. Initially, concreting should be performed for the beams & then slabs.
30. Compaction of concrete should be accomplished with vibrators & tamping rods.
31. The top level of the slab should be completed with mason & there should be restrictions to walk on the fresh finished concrete.
32. Carpenter should be provided under the slab shuttering throughout concreting to keep the form work tight.
33. For protection against rains, large plastic sheets should be used to wrap finished concrete.
34. Cast 6 cube moulds for testing.
35. Curing of slab should be accomplished with ponding method by making ponds in cement & sand mortar (1:10) of size around 5′ X 5′. Curing of beams and slabs should be performed for 7 to 10 days or as suggested by the consultant.
36. Deshuttering of the exterior sides of the beam should be accomplished after 24 hours . Deshuttering of the inner sides of beam should be accomplished after 48 hours . Deshuttering of the beam bottoms should be accomplished after 14 days for beam lengths up to 3 M or as per suggestions of the consultant. Deshuttering of the slab should be performed after 7 days for slabs up to 3 M span or as directed by the consultant.
37. Finishing of honeycomb should be accomplished cautiously in front of engineer the next day.
38. Hacking of the beam sides, beam bottoms, slab bottoms should be accomplished within 1 or 2 days from deshuttering.
39. Date of casting and the number of slab should be painted on the front side beam.

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Published By
Rajib Dey
www.constructioncost.co
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Some useful construction tips to organize bar bending schedule manually

Reinforced concrete is a popular structural material that is utilized extensively in construction engineering. Concrete is very powerful to withstand compressive stress but it is weak in tension. In order to combat tensional stresses, steel is required in concrete. The reinforcement in concrete refers to simple bars or rods bend and connected to a specified schedule with stirrups. The minimal diameters of bars which are utilized at site were Y10, Y12, Y16, Y20, Y25 and R6.

Steel is available in two basic types.

1. Mild steel (250 N/mm2)

2. Tor steel (460 N/mm2)

Bar code 

Implication of Reinforcement in Drawings

Engineering drawings is a language to disclose information thoroughly. So, there is a standard like 5Y10- 001- 150 to specify reinforcement in drawing such as, 5Y10- 001- 150:-It signifies 5 Number of Tor steel, 10mm Diameter, Bar mark 001, At 150mm CRS. At bottom face.

Bar location is changeable as below:

Notation for Slab :-

T1 -Top outer layer, T2 -Top second layer

B1 -Bottom outer layer, B2 -Bottom second layer

Cutting and Bending of Bars Reinforcement bars are preserved, sliced and bent in a steel yard in the site. Reinforcement bars are sliced into the desired lengths and bent into requisite shapes demonstrated on the bar schedule either by hand or through machinery.

Under manual processes, laborers applied the bar bending bench upon which robust nails and GI pipes are set with proper lengths to bend the bars for being utilized for smaller diameter bars. The larger diameter bars are bent through bar bending machine. Once the bending process is completed, all reinforcement bars are stacked and perfectly numbered as stated by the bar mark to avoid any issue at the time of fixing them.

To read the complete article, visit basiccivilengineering.com

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Published By

Rajib Dey

www.constructioncost.co

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Categories And Design Of RCC Slabs And RCC Structures

Reinforced Cement Concrete Slab

• A Reinforced Concrete Slab Component plays a vital role in a building. It is considered as a useful structural element of the current buildings. There are various columns and beams which provide support to Columns and Beams.

• The density of RCC Slabs varies from 10 to 50 centimetres which are generally found in developing floors and ceilings.

• Thin concrete slabs are also utilized for the objective of exterior paving.

• In several domestic and industrial buildings, a solid concrete slab, sustained on foundations or directly on the sub soil, is utilized to develop the ground floor of a building.

• Thinner, pre-cast concrete slabs are pushed among the steel frames of tall buildings and skyscrapers to construct the floors and ceilings on each level.

• At the time of creating structural drawings of the reinforced concrete slab, the slabs are shortened to “r.c.slab” or simply “r.c.”.

Design of various types of slabs and their reinforcement

There are lot of designs available for a suspended slab to develop the strength-to-weight ratio. In most cases the top surface stands flat, and the bottom is modulated:

• If the concrete is poured into a corrugated steel tray, generally it is called corrugated. It enhances the strength and avoids the bending of slab under its own weight. The corrugations happen on the short dimension, from side to side.

• A ribbed slab provides significant additional strength on one direction.

• A waffle slab provides extra strength in both directions. Reinforcement design

• A one way slab contains structural strength in shortest direction whereas a two way slab contains structural strength in two directions.

These slabs belong to cantilevered or Simply Supported Slabs.

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Rajib Dey

www.constructioncost.co

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