For a three-storey residential building, the plinth beam must be designed to tie the foundation together and carry the brick masonry along with the loads from above. Its dimensions, steel bars, stirrups, concrete grade, and cover all follow fixed structural requirements.
The beam sits directly above the damp-proof course (DPC), which rests on the brick masonry at plinth level. Because a three-storey structure imposes heavy dead load from its own weight plus live load, the selected section must deliver enough strength and rigidity.
Size: Use a cross-section of 18 inches in depth and 12 inches in width for three storeys. A two-storey residential building may use 9 inches by 12 inches, but at three storeys the depth cannot drop below 18 inches and the width cannot drop below 12 inches.
Longitudinal bars: Provide at least four longitudinal steel bars, each with a minimum diameter of 12 mm. A typical section places four 12 mm bars at the bottom and two 12 mm bars at the top.
Stirrups: Transverse reinforcement, or stirrups, resists shear and holds the main cage in position. Fabricate them from steel rods of at least 8 mm diameter, spaced no more than 150 mm centre-to-centre along the beam. This maximum spacing keeps the longitudinal bars confined and improves shear performance.
Hooks: Each stirrup hook must be at least 3 inches long and bent to 135 degrees instead of a standard 90-degree bend. The 135-degree shape locks the hook inside the concrete core under load and avoids structural problems.
Cover: Maintain a minimum concrete cover of 1.5 inches (40 mm) around both stirrups and longitudinal bars. This protects the steel from environmental exposure and ensures proper bond with the concrete.
Concrete: Cast the beam with M20 grade concrete at minimum. Here M means mix and 20 is the characteristic compressive strength after 28 days of curing, equal to 20 N/mm² (Newtons per square millimetre).
Crank bars: Negative bending moments near supports are handled with crank bars (bent-up bars) or extra straight bars. Two bars are bent upward at one-fourth of the span (L/4) from the inner support face. With a clear span of 20 feet, that point is 5 feet (20 feet divided by 4) from the support. Extra top bars across the support region also work, but crank bars are the more economical solution while managing bending demands over the span.
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