Guidelines for Choosing Dimensions and Reinforcement of Beams

                    Guidelines

1. It is economical to select singly reinforced sections with a moderate percentage of tension reinforcement which will result in ductile sections.

2. The minimum percentage of steel is around 0.3 per cent. Choose the depth of the beam such that the percentage of steel required is less than 75 per cent of the balanced steel.

3. At least two rods must be provided as tension steel, and not more than six bars are to be used in one layer. It is preferable to adopt a single size of bars or two sizes at the most. When two sizes of bars are adopted, it is better to choose such that the sizes do not vary much.

4. Often two bars are used as hanger bars, which are placed on the compression side of the beam. Their purpose is to provide support for the stirrups and to hold them in position. The minimum diameter of the main tension bar should be 12 mm and that of the hanger bar 10 mm.

5. The usual diameters of bars adopted in practice are 10, 12, 16, 20, 22, 25, and 32 mm. If two different sizes are used as reinforcement in one layer, the larger diameter bars are placed near the faces of the beam. It is preferable to keep the rods symmetrical about the centre line of the beam.

6. The width of the beam necessary to accommodate the required number of rods is dependent on the side cover and minimum spacing. The cover and arrangement of bars within a beam should be such that there is provision for the following:
(a) Sufficient concrete on all sides of each bar to transfer forces into or out of the bar, that is, to develop sufficient bond
(b) Sufficient space for the fresh concrete to flow around the bar and get compacted
(c) Sufficient space to allow vibrators to reach up to the bottom of the beam.

Read more: Structural Design Considerations


7. In building frames, the width of the beams is often selected based on the lateral dimension of columns into which they frame. These widths should be equal to or less than the dimension of the column into which they frame.

8. When architectural considerations restrict the size of the beam, the required moment of  resistance may be achieved by increasing the strength of concrete or steel or by providing compression steel to make the beam doubly reinforced.

9. Increasing the depth is more advantageous than increasing the width (which is often fixed on architectural considerations) and results in an enhanced moment of resistance and a flexural stiffness with reduced deflections, curvatures, and crack widths, however, very deep beams are not desirable.

10. It is often recommended to have the overall depth to width ratio (D/b) of rectangular beams in the range 1.5–2.0, though it may be higher (up to 3.0) for beams carrying heavy loads or having larger spans. The width and depth may also be governed by the shear force acting on the section.

11. The deflection requirements often control the depth of the beam.

12. It is desirable to limit the number of different sizes of beams in a structure to a few standard modular sizes, as they will reduce the cost of the formwork and permit re-usability of forms.


Read: Thumb Rules for Reinforced Concrete Designers