DESIGN AND DETAILING OF RCC BEAMS

RCC beams structural elements are designed to carry transverse external loads that cause bending moment, shear forces and in some cases torsion across their length. Concrete is strong in compression and very weak in tension. Steel reinforcement is used to take up tensile stresses in reinforced concrete beams.
Mild steel bars of round section were used in RCC work. But with the introduction of deformed and twisted bars, the use of mild steel bars had declined. Deformed or High yield strength deformed bars (HYSD) have ribs on the surface and this increases the bond strength at least by 40% compared to that of mild steel bar.
Good detailing of reinforcements with proper drawings are essential at the site to provide good construction process. These drawing generally also include a bar bending schedule. The bar bending schedule describes the length and number, position and the shape of the bar.
The detailing of beams is normally associated with:
i) Size and number (or spacing) of bars,
ii) Lap and curtailment (or bending) of bars,
iii) Development length of bars,
iv) Clear cover to the reinforcement and
v) Spacer and chair bars.
Anchorage in steel bars is normally provided in the form of bends and hooks. Twisted steel bars or deformed steel bars are not provided with hooks. The anchorage value of bend of bar is taken as 4 times the diameter of bar for every 45⁰ bend subjected to maximum of 16 times the diameter of bar. Fig.1 shows the standard hooks and bends. Bars are lapped over each other for increasing the length of bars. Minimum lap length should be equal to development length. Development length for bars in different concrete mix is given tables 4.2 to 4.4 of SP34.

Fig.1: Standard Hooks and Bends in Reinforcement

The value of K in above figure depends on type of steel used which is given below:

Sl. No.	Type of steel	Min. value of K
1	Mild steel	2
2	Cold worked steel	4

The beams are classified as:
i) According to shape: Rectangular, T, L, Circular etc.
ii) According to supporting conditions: Simply supported, fixed, continuous and cantilever beams
iii) According to reinforcement: Singly reinforced and doubly reinforced
Depth of the beam is determined based on flexural strength and to satisfy the deflection criteria. Generally the ratio of span to depth ratio is kept as 10 to 15 and the depth to width ratio of rectangular be is taken in the range of 1.5 to 2.
Minimum cover in beams must be 25 mm or shall not be less than the larger diameter of bar for all steel reinforcement including links. Nominal cover specified in Table 16 and 16A of IS456-2000 should be used to satisfy the durability criteria.
Generally a beam consists of following steel reinforcements:
i) Longitudinal reinforcement at tension and compression face (Min of two 12 mm diameter bar is required to be provided in tension) in single or multiple rows are provided.
ii) Shear reinforcements in the form of vertical stirrups and or bent up longitudinal bars are provided. ( The bar bent round the tensile reinforcement and taken into the compression zone of an RCC beams are called stirrups)
iii) Side face reinforcement in the web of the beam is provided when the depth of the web in a beam exceeds 750 mm. (0.1% of the web area and shall be distributed equally on two faces at a spacing not exceeding 300 mm or web thickness whichever is less)
Arrangements of bars in a beam should confirm to the requirements of clause given in 8.1 and 8.2 of SP34. Bars of size 6,8,10,12,16,20,25,32,50 mm are available in market. Fig.2 shows different types bars used in a beam.

Fig.2: Reinforcement details in Beams

While drawing the details of a beam following convention representation of bars are used:
Mild steel bars:
HYSD bars: # or
Main bars are shown by thick single line. Hanger bars are shown by medium thick lines. Stirrups are shown by dotted or thin line. Different types of stirrups used are shown in Fig.3.

Fig.3: Details of Beam Reinforcement

REINFORCEMENT DETAILING IN BEAMS

REINFORCEMENT DETAILING IN BEAMS

Reinforcement are provided to resist tensile stresses due to bending and shear in beams for singly reinforced sections. But when depth of section is restricted due to reasons such as architectural reasons, or basement floors, the beam section is designed as doubly reinforced to resist tensile as well as compressive stresses and are provided with reinforcement in compression face.
Beams when subjected to torsion are provided additional longitudinal and shear reinforcement to resist bending and shear stresses developed due to torsion.
In practice, for singly reinforced beams, two additional bars are provided in compression face of the beam so that stirrups can be tied with bars. These additional reinforcement are of nominal diameter of 8mm or 10mm.
When the width of beam is insufficient for number of bars to be provided, keeping minimum clear distance bars based on maximum size of aggregate as per standard code guidelines, the reinforcement can be provided in layers in both tension and compression face of the beam. These reinforcement are provided as straight bars in both zones.
When a beam is designed with slab, called slab beam or Tee-beam, reinforcements are provided as shown in figure below. The beam is generally designed as simple beam but additional reinforcement are provided on top with slab to make it behave like a Tee-beam.

Fig: Mid-span Details of Tee-Beam.

Fig: Slab-Beam Details

Following are the types of stirrups provided in beams­:

Fig: Types of stirrups in beams

Standard bends and hooks required for reinforcing bars are shown in figure below. The specification for detailing of reinforcement in beams are given in cl.26.5.1 of IS 456 – 2000.

Fig: Standard bends and hooks in beams

Curtailment of Reinforcement in Beams:
Reinforcements are curtailed along its length in beams depending on the bending moment at the section. Anchorage or development length required at support is provided during curtailment of reinforcement. The anchorage length required for main reinforcement in tension and compression is given in cl.26.2 of IS 456 – 2000.
Typical details of curtailment of reinforcement in cantilever and continuous beams are shown in figure below:

Fig: Typical Details of Reinforcement curtailment in beams

Details of anchorage length required for main reinforcement in tension and compression is shown below:

 Fig: Typical Details of anchorage length of reinforcement in beams