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,
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:
Type of steel
Min. value of K
Cold worked steel
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