DESIGN OF RC BEAMS

Practical Rules

          1. The over-all depth of a singly reinforced rectangular beam shall be not less than 1/20 of the span unless shear and other consideration prevail. The greater depth the less in the steel required and more economical is the beam, but there is a limit to it.

          For the adequate safety against deformation and cracking, international code of reinforcement concrete (1970) recommends a minimum beam depth for both rectangular and T-Beam to be equal to or greater than 1/12 of the span length. For designing the beam depth may be assumed as 1/10 to 1/12 of the span for simply supported beams and 1/12 to 1/18 for continuous beams.

          2. The breath of a beam shall normally be 2/3 to 1/2 of the depth, but not less than 1/3 of the depth. A good rule fot the breath is to take 3/5th of the depth of the beam.

          Where the span/breath ratio exceeds 30 (beam whose length between adequate lateral restraints exceeds 30 times the breath of its compression flange) and it is not practicable to support the compression flange laterally, the permissble compressive stress in the concrete due to bending shall be reduced by a factor (1.75-L/40B), where L is the length of the beam lateral restraints, i.e.-free span and B is the breath of the compression flange. The permissible stress in the compression reinforcement (where provided) shall also be reduced in the stress ratio.

          Slenderness Limits for the Beam to Ensure Lateral Stability : A Simply supported or continuous beam shall be so proportioned that the clear distance between the lateral restraints does not exceed 60b or 250 b*b/d, whichever is less; where d is the effective depth of the beam and b the breadth of the compresssion face midway between the lateral restraints. For a cantilever, the clear distance from the free end of the cantilever to the lateral restraints shall not exceed 25b or 100 b*b/d, which ever is less. (IS :456-1978).

          Beams are often used to supported slabs whcih are mostly cast with beams as a monolithic construction and designed as T-beams. (See also under "Lareral Stability of Beams" in section 10). This condition of lateral support can be deemed to be satisfied by a slab monolithic with beam near its compression flange provided the slab thickness is not less than 1/10 of the beam depth and adequate top and bottom reinforcement, suitably anchored, has been provided at the beam slab junction.

          The clear distance from the corner of a beam or rib to the nearest longitudinal bar should  be not less than 8 cm.

          If depth of the beam exceeds 60 cm, skin reinforcement, on both faces of web, in the form of longitudinal bars (minimum 12 bar dia and spacede not more than 20 cm) should be provided. Such skin reinforcement on each face should be at least 0.05 percent of gross web area.

          The top surface of centering should be givien a camber of 5 mm for every meter of span subject to a maximum of 35 mm, to allow for the initial deflection settlement.

Reinforcement

          Minimum tensile reinforcement in beams shall be not less than 0.30 percent where plain bars are used and 0.20 percent where high-yield strength deformed bars are used of the gross cross-sectional area of the beam. The maximum area of tension reinforcement shall not exceed 4 percent. The area of beam being calculated as total cross-sectional area for rectangular beams and as area equal to overall depth multiplied by the width if the web in the case of T or L beam. At least one-fourth of the tension steel should be carried straight into the support so as to provide anchorage.

Spacing of Reinforcement Bars

          The horizontal clear space between two parallel main reinforcement bars shall be not less than the greatest of the following:

          (a) The diameter of the bar if the diameter are equal, (b) the diameter of the larger bar if the diameters are unequal, (c) 5 mm more than the nominal maximum size of the coarse aggregate used in the concrete. Greater horizontal spacing than the minimum specified shall be provided where provisios.

          This does not preclude the use of larger size of aggregate beyond the congested reinforcement in the same member; the size of the coarse aggregates may be reduced around the congested reinforcement to comply with the above provisions.

          Where needle or immersion vibrators are intended to be used, the horizontal distance between bars of a group may be reduced to two-thirds of the nominal maximum size of the coarse aggregate provided that sufficient space is left between groups of bars to enable the vibrator to be immersed.

          The clear vertical space between two horizontal main reinforceing bars shall normally be 15 mm, the maximum size of the coarse aggregate or the maximum size of the bar, whichever is tha largest. Bars can also be placed one above the other without any space in-between. Steel space-bars may be introduced to maintain correct horizontal and vertical distance apart of the bars.

          Main tensile reinforcement bars in beams shall be not less than 12 mm in dia. Use as few different dia as possible. Additional bars at the top corners have usually to be provided in beams for bending the stirrups for shear. Diameters of these bars may be 10 mm when not required to take any bending moments.