Lesson 15 : Reinforced Cement Concrete


A beam is a flexural member and supports the imposed load. The most common shape of beam is a rectangle. In general the width of beam is equal to the width of supporting wall or column. Its depth depends upon the imposed loading. The R.C.C. beams may be singly reinforced or doubly reinforced.

For a wide and long hall, ‘T’ beams can be planned at regular intervals. The beams and the slab in a ‘T’ Beam are cast monolithically.

In a ‘T’ Beam the horizontal part is called the ‘flange’ which is a part of the slab itself whereas the vertical part that protrudes below the bottom level of the slab is called its ‘web’ or ‘rib’. As the flange is only a part of the slab with only some extra reinforcement, no additional concrete is necessary for the flange of the ‘T’ beam and as such, this brings in overall economy. If ‘T’ beam are not provided, the thickness of slab will be more and the overall quantity of concrete will be more.

The depth of the beam from top surface of the flange to the bottom face of the web, the breadth of the rib, thickness of slab, details of reinforcement for the web, the flange and the slab, size number and spacing of bars, the cover of concrete on all sides, details of stirrups to hold the bottom and top reinforcing bars in position are determined by design considerations.

The overall depth of the beam i.e. the depth from the top of the flange to the bottom of the rib is taken as 1/12 of the span for heavy loads, 1/12 to 1/15 for medium and 1/15 to 1/20 for light loads. In a house the depth of the ‘T’ beam is limited by the clearance required from the floor to the bottom of the beam. If the height of a building is low and the span is long, the depth of the ‘T’ beam comes up for prime consideration. The width of the rib should be such as to accommodate the reinforcing brs, allowing sufficient space in between bars and also a minimum of 25 mm on either side for cover of concrete. These conditions are amply satisfied in actual practice because the width of the rib is determined by architectural considerations, and is made equal to the size of the supporting columns or wall. If the beam is spanning from wall to wall as in case of hall, 25 cm width for the rib will suffice. The thickness of the flange which is same as the thickness of the slab is usually 10 cm.

If the depth of a beam exceeds 60 cm in a high building, skin reinforcement on both faces of the web are to be provided. They are to be minimum 12 mm diameter bars placed longitudinally along the faces of the beam at not more than 25 cm vertically apart.

The gross cross sectional area of ‘T’ beam is the product of its overall depth and the width of the rib.

Minimum area of tensile reinforcement for the ‘T’ beam i.e. the bars to be placed in the bottom layer of the rib shall not be less than 0.3% of the gross cross sectional area of the beam. In actual practice However, this comes to 0.8% to 1%.

The area of cross section of each size of bar is given in ‘Technical Information’. Number of bars to be provided in a ‘T’ Beam can be computed for the area of reinforcement required. The bars should not be less than 10 mm diameter. Generally bars of the same category and size are used even if the total required area of reinforcement is slightly exceeded. Such bars can be provided in two or three layers if the width of the rib is less. Additional bars, minimum 2 nos. and of minimum 10 mm diameter are provided at the top of the flange parallel to the main reinforcement in the rib for binding of stirrups. They are called ‘hangers’. Covering of concrete beyond stirrups shall not be less than 25 mm.

The stirrups in beams shall be taken around the outer most bottom reinforcing bars of the rib and the top hangers in the flange. A stirrup is anchored by hooking its end over one of the main bars. The diameter of bars meant for stirrups for beams shall not be less than 8 mm diameter plain bars. Such stirrups are to be provided at the rate of 15 cm centers upto one-third distance from supports at either end and thereafter at the rate of 22 cm centers for the middle third. For larger spans, however , the stirrups are to be spaced closer as per design requirements.

In ‘T’ beam and slab construction, extra top reinforcement for the slab is provided at the top of the ‘T’ beam in the flange upto a distance of ¼ th spacing of ‘T’ beam. This is provided on either side of the rib, by cranking bars of the slab on either sides as explained earlier.

The bearing of a ‘T’ beam shall be given over the entire width of a wall up to its outer edge.

Deflection of a R.C.C. beam or slab not only affects its appearance but also effects the erection of partitions and interior decoration. The deflection is caused by its own dead load, by live load coming upon the structure, and due to effects of temperature , creep and shrinkage occurring after erection. The total deflection due to all causes should not exceed one-two-hundred fiftieth of the span length. The deflection of an R.C.C. beam remains within the above limit if its span to depth ratio is 18 for a simply supported beam and 6 for a cantilever beam. For simply supported slabs the same ratio should not exceed 36. For partially fixed conditions such ratio can be larger excepting for a cantilever beam whose one end is always fixed. For spans spanning in two directions, the shorter length is taken for calculating span to depth ratio.

Last modified: Saturday, 21 April 2012, 11:59 AM