How to design and detail of RCC Beams
RCC beams structural elements are useful for bearing transverse external loads which can produce bending moment, shear forces and sometimes torsion across their length. It is known that concrete has good strength against compression and but weak strength in tension. Steel reinforcement has the capacity to withstand tensile stresses in reinforced concrete beams.
In RCC work, mild steel bars of round section are utilized. But deformed and twisted bars have taken place of mild steel bars. Deformed or High yield strength deformed bars (HYSD) contain ribs on the surface and thus strength of bond is enhanced by minimum 40% with regards to that of mild steel bar.
Good detailing of reinforcements that includes perfect drawings can be used at the jobsite to initiate good construction process. These drawing usually also contain a bar bending schedule. The bar bending schedule sheds light on the length and number, position and the shape of the bar.
The detailing of beams is generally related to the following:
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 comes in the form of bends and hooks. Twisted steel bars or deformed steel bars do not include hooks. The anchorage value of bend of bar is assumed as 4 times the diameter of bar for each 450 bend dependent on maximum of 16 times the diameter of bar. Bars are enclosed over each other to enhance the length of bars. Lowest lap length should have been identical to development length. Development length for bars in diverse concrete mix is specified in tables 4.2 to 4.4 of SP34.
The value of K in above figure is based on type of steel applied that is provided below - The beams are categorized as:
i) In relation to shape: Rectangular, T, L, Circular etc.
ii) In relation to supporting conditions: Simply supported, fixed, continuous and cantilever beams
iii) In relation to reinforcement: Singly reinforced and doubly reinforced
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Depth of the beam is obtained on the basis of flexural strength and to fulfill the deflection criteria. Usually, the ratio of span to depth ratio is retained as 10 to 15 and the depth to width ratio of rectangular is retained in the range of 1.5 to 2.
Lowest cover in beams should be 25 mm or shall not be below the larger diameter of bar for all steel reinforcement together with links. Minimal cover given in Table 16 and 16A of IS456-2000 should be applied to meet the strength conditions.
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