Guidelines for rebar detailing of RCC structures

Reinforcement Detailing or Rebar detailing is a detailed construction engineering process normally accomplished by the Rebar fabricators, structural engineering consultants or the contractors for generating ‘shop/placing’ drawings or shop drawings and bar bending schedule of steel reinforcement for construction. Architect/Engineers(A/E) produce ‘Design Drawings’ with the purpose of adding strengths with rebar size, spacing, location, and lap of steel.

Rebar Detailing is also known as Rebar scheduling, RC Detailing and Bar Bending schedule predation, RC Drafting, etc in different countries.

Objective of Rebar Detailing - The rebar detailing is done for the following purposes :

a) To produce an error-free Bar bending schedule, when fabricated should be accommodated in the concrete formwork devoid of any issue. The similar Bar Bending Schedule should be utilized for accounting and invoicing.

b) To develop a detailed Rebar placing drawing (known as Rebar Shop drawings). This Rebar placing drawing assists an Ironworker to place rebar perfectly in the site efficiently.

c) To allow the structural engineer to verify and approve when the structural design intent is precisely transformed into the Rebar Placing drawings and Bar bending schedules.

d) To perform a Rebar wastage analytics and minimize probable scrap existing in the Drawing level.

Standard Hooks: The term “standard hook” is defined as follow -

1. 180o bend together with an extension of minimum 4 bar diameters, but not below 65 mm at the free end of the bar.
2. 90o bend together with an extension of minimum 12 bar diameters at the free end of the bar.
3. For stirrup and tie anchorage.

For 16 mm φ bar and smaller, a 90o bend along with an extension of minimum 6 bar diameters at the free end of the bar,

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Published By
Rajib Dey
www.constructioncost.co
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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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Rajib Dey

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Some vital guidelines for measuring staircase dimensions and designs

A staircase mainly includes a series of steps which involve a tread (the horizontal portion, where the foot stands) and a riser (the vertical portion).

In every step, there are one or more landings, handrails, and a small nosing. The latter obtrudes from the tread over the lower step, facilitating to raise its size devoid of inclusion of centimeters to the overall dimensions of the staircase.

By using the following formula, find out the exact dimensions of a convenient and efficient staircase in accordance with its use.

2 Risers + 1 Tread = 63-65 cm

The required space to attain these optimal dimensions is unavailable sometimes, but it's suggested to approach them as much as possible.

A schematic illustration of a steep and low-transit staircase.

(2 x 21) + (1 x 21) = 63 cm

A schematic illustration of an optimal staircase.
(2 x 18) + (1 x 28) = 64 cm

A schematic illustration of a loose staircase, desirably for exterior application.
(2 x 13) + (1 x 39) = 65 cm

Sample measurement of a staircase that should be 2.60 meters high.

1. Workout the required number of steps - Assume an ideal riser of 18 cm, the height of the space is divided with the height of each step. The result should always be rounded up:
260/18 = 14.44 = 15 steps

2. Workout the height of every riser - The height of the space is divided with the number of steps already acquired:
260/15 = 17.33 cm height for each riser.

3. Workout the width of the tread - It can be calculated with the following formula:
(2 x 17.33 cm) + (1 x tread) = 64

Each tread will be computed as 29.34 cm

The consequential staircase will contain 15 steps of 29.34 cm of tread and 17.33 cm of riser.

Based on the use and local regulations, there should be a minimum width of 80 cm for stairs in single-family homes, and more than 1.00 meters in public buildings.

Preferably, a stairway shouldn't contain in excess of 15 steps in a row. After 15 steps, a landing should be arranged. It's suggested that a landing is calculated minimum the same as 3 treads.

The height among the steps and the ceiling should remain 2.15 meters at minimum. The height of the handrail differs among 80 and 90 cm from each step.

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Guidelines to provide concrete cover for reinforcement in slab, footing, beam & column

Concrete cover: Concrete Cover is arranged for the reinforcement in Reinforced Cement Concrete. Cover means the spacing among the exterior surface of the concrete to the inserted reinforcement.

Benefits of arranging Concrete Cover: The purpose of covering is to provide protection against erosion. Reinforcement is susceptible to erosion and fire for atmospheric conditions. In case of improper cover erosion and cracks may occur in hardened RCC.

Covering is arranged for each and every component of the building (Slabs, Beams, footings) where the reinforcement is applied. The covering blocks are utilized to retain the reinforcement in exact position as well as providing a covering for reinforcement.

Several Types of Concrete Cover Block: Depending on the type of materials applied, the following types of covering blocks are commonly found -

1. Wooden concrete cover Block
2. Steel concrete cover block
3. PVC Block
4. Cement Masonry concrete cover block
5. Aluminium Block
6. Stones

Conditions for Concrete Cover: Covering differs based on the dimensions of the components (Slab, beam, column, footings, etc.) The conditions for arranging covering in RCC are provided below -

Condition - Covering

When the length of the item is ≤ 0.3 1 - 1" or 25mm or 0.025mWhen the length of the item remains among 0.4m to 0.5m then - 2" or 50mm or 0.050m
When the length of the item remains ≥ - 0.6m then 4" or 100mm or 0.1m

From above, the maximum concrete cover remains 0.1m or 100cm

1. Concrete Cover in Columns / Beams: The length and width of the column should be 0.5m and 0.45m. The covering for reinforcement in the column should be 0.050m from all sides and similar reinforcement should be designed accordingly. The Dimensions of Reinforcement in the column should be 0.40m and 0.35m.

Suppose the length and width of the column are 0.40 and 0.25. Covering should be equal. Consider the minimum dimension from the two dimensions i.e. 0.25. For 0.25m the covering of 0.025m should be provided. So, the covering of 0.025m is arranged in all the sides. Therefore, dimensions of reinforcement is 0.35m and 0.20m.

Total Length of Stirrup is 2x [0.35+0.20]+ 9D x 2 (hook length)

2. Concrete Cover for Slabs: Suppose, the length and width of the slab are 1.3m and 1.0m. The covering of 0.1m is arranged when the length of the bar is in excess of 0.6m. Use the same condition as mentioned. The covering of 0.1m is arranged from all the sides of the slab.

3. Concrete cover for footings: Suppose, the dimensions of Footing are 0.7m and 0.6m. To length and width of Mesh (reinforcement) utilized in footings are acquired by subtracting the cover. Use the similar principle as above. As per the condition, a concrete cover of 0.1m is subtracted from all the sides. Therefore, the dimensions of reinforcement are 0.5m and 0.4m.

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Guidelines to set up reinforced concrete pipe (RCP)

Reinforced concrete pipes alias RCPs, are suitable for storm sewer systems and water sewer systems. The set up of these pipes is complicated since they are weighty and heavy equipments are required to shift and place them. Given below, the details about RCP handling and installation.

Handling Reinforced Concrete Pipe: Reinforced concrete pipes should be dealt with cautiously so that the bell is not damaged (the wide or flared end of the RCP) and spigot (the narrow end that is inserted into the bell of an adjacent pipe).

RCPs should not be pulled to the site. It is suggested that initially, the pipes should be unloaded with a nylon sling or other certified material to support the weight of the pipe, and make balance properly in the sling.

Excavating Reinforced Concrete Pipe: Prior to start the installation process, make sure to arrange the trench to adapt minimum two pipes so that it becomes possible to check that the installation will retain the necessary slope and the consequent trenching will not affect the pipe being installed as well as endanger the security of workers.

Once each RCP is installed, the line and grade levels should be examined properly. Keep in mind that the trench should be adequately wide to facilitate the workers installing and controlling the pipe securely.

It is recommended not to change the pipe alignment or grade with the pipe in the home position. Keep in mind that all through the installation process the pipes should not be supported on bells since it could damage them. The bedding material should not contain any debris and should retain a consistently level surface.

How to arrange joining surfaces of RCP: Prior to placing the RC pipe, it is required to cleanse all impurities from the joining surfaces of the bell of the pipe cautiously to provide perfect homing of the pipe.

Provide lubricant to the bell portion of the RCP with a brush or gloves. Check with the RCP manufacturer for suggested lubricants that should be used. Lubricant should be sufficient to prevent the gasket from rolling away and damaging the bell end.

Cleanse the spigot or tongue end of the pipe to seal the gasket properly. Lubricate the tongue end of the pipe along with the gasket recess. If the lubricating grease is inadequate, the gasket may twist out of recess. The gasket should be lubricated prior to arrange it on the tongue of the RCP.

How to set up RCP: To set up the RCP, the pipe should be managed with few workers. It is necessary to lubricate pipe bell and gasket not to utilize extra force.

Set up the gasket and apply a round object to level the gasket stretch. Pass the object frequently along the circumference to keep everything in exact location. When the gasket is not expanded, leaks may occur at the joint or the bell may crack. Align bell and spigot, and ensure that the gasket gets in touch with the entry taper.

Ensure the pipe is aligned with surveying or leveling instruments. If the pipe going to be installed contains a small diameter, a wood block should be arranged across the bell end of the pipe and pushed with a wedge bar to shift the pipe gradually into place. Continue pushing unless the pipe is entirely installed. When the pipe diameter remains somewhat bigger and heavier, the pipe pullers should be applied to install the pipe.

Backfilling Reinforced Concrete Pipe: Backfill material should be set cautiously along the pipe and compacted methodically. Backfill material should be arranged consistently in lifts on both sides of the pipe and fill the trench up one foot over the top of the pipe.

The material should not be bulldozed into the trench or provided directly on top of the pipe. The backfill material with large boulders should not be utilized since they will not be consolidated and may damage the pipe.

The material with roots or other organic material should not be used. Backfill should be arranged based on the geotechnical recommendations on particular backfill material.

Lastly, heavy construction equipment should not be operated over the pipe unless sufficient backfill is prepared or the pipe is sufficiently deep in order that it is not damaged.

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Guidelines for making perfect structural design

This civil engineering article focuses on the least standards which should be maintained for the design of various RCC structural elements like the columns, beams, slab and foundation as well as the least safe standards for the reinforcing bars to be applied for making the design of the above mentioned structural elements.

Minimum cross-sectional dimension for a Column should be 9″x 12″ (225 MM x 300 MM). It is the minimum approved size.

It is always recommended to utilize M20 grade concrete for construction as per IS 456:2000. The least steel in a 9″ x 9″ column should be 4 bars of 12 MM with stirrups of 8 MM steel rings at a spacing of 150 MM centre to centre. In a 9″ x 12″ column, more bars (6 bars with 12 mm diameter) should be added to sustain the total efficiently.

Least RCC beam size should not be lower than 9″x 9″ (225MM X 225MM), with an supplementary slab thickness of 125 MM.

Normally, there should be minimum of 4 bars, with 2 bars having 12 MM thickness in the bottom of the beam, and 2 bars having 10 MM at the top of the beam.

A concrete cover of 40 MM should also be provided. It is suggested to utilize M20 grade of concrete (1 part cement : 1.5 parts sand : 3 parts aggregate : 0.5 parts water).

Minimum thickness of RCC slab should be 5″ (125MM) since a slab may comprise of electrical pipes which are implanted into them which could be 0.5″ or more for internal wiring and as a result the depth of slab is decreased at specific places that lead to cracking, weakening and water leakage throughout rains. Therefore, a least thickness of 5″ should be retained.

Minimum size of foundation for a single storey of G+1 building should be 1m x 1m, where safe bearing strength of soil is 30 tonnes per square meter, and the anticipated load on the column does not surpass 30 tonnes.

The depth of footing should be minimum 4′under ground level. It is suggested to get to depths up to had strata.

Minimum Reinforcing bar details:

1. Columns: 4 bars of 12mm steel rods FE 500.
2. Beams: 2 bars of 12 mm in the bottom and 2 bars of 10 mm on the top.
3. Slab

a) One Way Slab: Main Steel 8 MM bars @ 6″ C/C and Distribution Steel of 6 mm bars @ 6″ C/C
b) Two Way Slab: Main Steel 8 MM bars @ 5″ C/C and Distribution Steel of 8 mm bars @ 7″ C/C

4. Foundation: Initially, there should be 6″ of PCC layer. Over it, a tapered or rectangular footing with minimum 12″ thickness should be arranged. Steel mesh of 8 mm bars @ 6″ C/C should be placed. In a 1m X 1m footing, there should be 6 bars of 8 mm on both segments of the steel mesh.

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