Typical Detailing of Reinforcements in Beams and Slabs

Detailing of reinforcements in beams and slabs plays an important role in providing strength, durability and cost optimization. Reinforcement details of concrete beams and slabs should specify clearly about cover to reinforcement, length of reinforcement, curtailment of reinforcement, number and diameter of reinforcement to be provided.

For a simply supported beam and slab, the maximum bending moment occurs at the center of the span and shear force at a distance of d/2 from the face of the support, where d is the effective depth of beam or slab. Considering this, the bending reinforcement is necessary at the center of the span and not required at the support where as shear reinforcements are required at the support.

So, it is not necessary to provide full length bending (tension) reinforcement and 50% of the reinforcement can be curtailed at suitable locations as shown in images below or can be bent upwards to provide as shear reinforcements.

Reinforcement Detailing of Simple Beams and Slabs

1. Reinforcement Details of Simply Supported Beam and Slab

As seen in figure-1 below, for a simply supported beam and slab, 100% reinforcement as per the design is provided as tension reinforcement at the mid span of the beam and slab, and 50% is curtailed at the distance of 0.08L from the center support.

Fig.1: Curtailment of tension steel in simply supported beam and slab

2. Reinforcement Details of Continuous Beams and Slabs

For a continuous beams and slabs, the shear force and bending moment diagrams are drawn and reinforcement details are provided based on the value of the shear force and bending moment.

As can be seen from the figure-2 below, the tension reinforcement are 100% at the mid-span of the beam and slab, while it is curtailed at the distance of 0.1 L from the center of the support at end support and 0.2L at the intermediate support. L is the effective length of the beam and slab.

The shear reinforcements are provided from the support to the distance of 0.1L from the face of support on end support and at a distance of 0.3L from the center of support at intermediate support.

Fig.2: Curtailment of reinforcement in beam and slab construction

3. Typical reinforcement detail for concrete beam

The typical reinforcement details of a concrete beam shall indicate the number of reinforcements, diameter and length of reinforcement for both top and bottom reinforcements.

Fig.3: Typical reinforcement detail for concrete beam

Types of Concrete Beams and their Reinforcement Details

Reinforced concrete beams are structural members that support the transverse load which usually rest on supports at its end. Girder is a type of beam that supports one or more smaller beam.

Types of Concrete Beams

Beams are classified as

1. Simple Beam
2. Continuous Beam
3. Semi-Continuous Beam
4. cantilever beam
5. T- beam

1. Simple Concrete Beams

Simple concrete beam refers to the beam having a single span supported at its end without a restraint at the support. Simple beam is sometimes called as simply supported beam. Restraint means a rigid connection or anchorage at the support.

Fig: Simply supported beam

2. Continuous Beam

It is a beam that rest on more than two supports. It can be a single beam provided for long span between columns or walls with intermediate supports of smallar beams or a single continuous beam for entire length of the structure with intermediate column or wall supports.

Fig: Continuous beam with reinforcement details

3. Semi-Continuous Beam

Refers to a beam with two spans with or without restraint at the two extreme ends.

Fig: Semi-continuous beam

4. Cantilever Beam

Cantilever beams are supported on one end and the other end projecting beyond the support or wall.

Fig: RCC Cantilever beam reinforcement details

5. T – Beam

When floor slabs and beams. are poured simultaneously  producing a monolithic structure where the portion of the slab at both sides of the beam serves as flanges of the T-Beam. The beam below the slab serves as the web member and is sometime called stem.

Fig: RCC T-beam

What is a hidden beam? Purpose, Applications and Design

What is a Hidden Beam?

Hidden beam is a reinforced concrete beam, also called concealed beam provided within the depth of supporting slabs. So, the depth of hidden beam is the same as slab depth as it can be noticed in Figure 2. Concealed beams are popular and form an essential part of modern reinforced concrete framed structures.

Lastly, strict and rigid architectural consideration led to the provision of hidden beams. In this manner, the load which might be brick wall will be dealt with and height of the floor will not be compromised.

In this article, different aspect of reinforced concrete hidden beams will be discussed.

Fig.1: Hidden beams, details of longitudinal reinforcement and stirrup placement

Fig.2: Reinforced concrete hidden beam placed with a slab, its depth is equal to the slab depth

How to Design a Hidden Beam?

The design of hidden beam is the same as conventional beam, but its depth is restricted and should not be greater slab thickness. So, it may be required to increase reinforcement ration and width of the beam to overcome this restriction to a certain degree.

Fig.3: Typical reinforcement and dimension details of hidden beam

Purpose of Hidden Beam

• It is used to help disperse loads imposed on the slab for example weight of brickwork masonry wall
• It allows to use greater span for the slab
• Hidden beams are provided exclusively for architectural aesthetic purposes in the building interior

Advantages of Hidden Beams

Hidden beams are desired structural element because of several advantages which include:

• It saves on floor height clearance
• Hidden beam saves cost of formwork, labor, and materials
• It creates an acceptable aesthetic appearance which permits for efficient interior space partitioning
• Concealed beam clears the way for horizontal electromechanical ductwork

Disadvantages of Hidden Beam

The disadvantage of hidden beam is that it has considerably low ductility due to height reinforcement ratio which is provided to compensate for depth restriction specifically at column connection.

Types of slabs in which hidden beams are introduced

Hidden beams are widely used in three types of slabs which include:

• Waffle slab
• Ribbed slab
• Solid slab

CONCRETE MIX DESIGN AND ITS ADVANTAGES

 CONCRETE MIX DESIGN AND ITS ADVANTAGES

Concrete mix design is of two types:

1. Nominal concrete mix

2. Designed concrete mix

Nominal concrete mixes are those specified by standard codes for common construction works. These mix takes into consideration the margin for quality control, material quality and workmanship in concrete construction.

M10, M15, M20 are the commonly used nominal mixes used in construction. For higher grade of concrete, i.e. M25 and above, it is advised to have designed mix concrete.

Designed mix concrete suggests proportions of cement, sand, aggregates and water (and sometimes admixtures) based on actual material quality, degree of quality control, quality of materials and their moisture content for given concrete compressive strength required for the project. Designed mix concrete are carried out in laboratory and based on various tests and revisions in mix designs, the final mix proportions are suggested.

The concrete mix can be designed from M10 to various grades of concrete such as M50, M80, M100 etc for various workability requirements from no slump to 150mm slump values. These grades of concrete can be achieved by variations in the mix proportions and laboratory tests to ascertain it.

Sometimes admixtures are also required to enhance some properties of concrete such as workability, setting time etc. These admixtures also need to be considered during concrete mix design calculations for its optimum use. Their overdose can affect the properties of concrete and can cause harm to strength and durability.

Concrete mix design is the method of proportioning of ingredients of concrete to enhance its properties during plastic stage as well as during hardened stage, as well as to find economical mix proportions.

Properties desired from concrete in plastic stage: –

• Workability – Suitable workability for proper placement of concrete in structural member.

• Cohesiveness – better cohesiveness between cement and aggregates to prevent segregation of concrete.• Initial set retardation – to control the initial setting time of concrete based on requirements.

Properties desired from concrete in hardened stage:-

• Strength – Strength of concrete is the main objective of the concrete mix design.

• Imperviousness – Better mix proportions to improve imperviousness for protection of reinforcement form corrosion and enhanced durability of concrete.

• Durability – To increase the durability of concrete.

Advantages of Concrete Mix Design:

Concrete mix design is economically proportioning of concrete ingredients for better strength and durability based on construction site. While the nominal concrete mix may have higher amount of cement, when it is designed mix, the cement requirement may be low for the same grade of concrete for a given site. The proportions resulting from concrete mix design are tested for their strength with the help of compressive strength test on concrete cubes and cylinders.

The concrete mix design proves to provide better quality economically.

Following are the advantages of concrete mix designs:

1. Good quality concrete as per requirements – this means the concrete will have required strength, workability, impermeability, durability, density and homogeneity.

2. Nominal mix concrete may suggest more cement than other materials, and concrete mix designs gives the accurate quantity of cement consumption. Thus it is an economical solution for large projects.

It is possible to save up to 15% of cement for M20 grade of concrete with the help of concrete mix design. In fact higher the grade of concrete more are the savings. Lower cement content also results in lower heat of hydration and hence reduces shrinkage cracks.

3. Best use of available materials:

The nominal mix of concrete does not consider the quality of local construction materials. The concrete mix design is based on the quality of available materials locally. Thus it is also an economical solution to reduce the transportation cost of materials from long distance.

4. Desired Concrete Properties:

The designed mix concrete will have desired concrete properties based on project or construction requirements. Requirements such as durability, strength, setting times, workability etc. can be controlled with the type of construction with concrete mix design.

Other requirements such as early de-shuttering, pumpability, flexural strength, lightweight concrete can also be controlled