Reinforcement

Cover to Reinforcement

I.S. : 456-1978- Code of practice for plain and reinforced concrete lays down that reinforcement bars shall have concrete cover (exclusive of plaster or other finish) as

follows :

• At each end of reinforcing bar not less than 25 mm, nor less than twice the diameter of such bar;
• For a longitudinal reinforcing bar in a column, not less than 40 mm, nor less than diameter of such bar, In case of columns of minimum dimension of 200 mm or under, whose reinforcing bars do not exceed 12 mm, a cover of 25 mm may be used;
• For longitudinal reinforcing bar in a beam, not less than 25 mm, nor less than diameter of bar;
• For tensile, compressive, shear or other reinforcement in a slab, not less than 15 mm, nor less than the diameter of such bar; and
• For any other reinforcement, not less than 15 mm, nor less than the diameter of such bar.

The cover may be increased by the RCC designer by inserting notes in the structrual drawings by making stipulations in the specifications to cater for special considerations like proximity of saline water, harmful chemicals etc.

For articles with thin cross section like precast RCC fencing posts, certain government departments like the MES specify concrete cover of 20 mm or twice the diameter of main bars, whichever is greater.

Lap Splices

Length of bars supplied by manufacturers is usually in the range of 8 to 13 m. When length of bars in stock is shorter than the required length the bars are extended by providing lap splices. Allowance to be made for lap splices in the calculation of quantity of reinforcement should be:

• 24 times the bar diameter, for bars in compression.
• 30 times the bar diameter, for bars in tension.

When the estimator is in doubt as to whether a particular bar is in compression or tension he should allow for 30 times the bar diameter.

Normally, bars larger than 36 mm diameter are to be extended by butt welding, but if lap splices are permitted for such bars additional spirals of 6 mm diameter are provided around the lapped portion.

When bars of different diameters are spliced (as in curtailing of reinforcement) the lap length is to be calculated on the basis of the smaller diameter of bar.

Cranked Bars

For cranked bars add 0.4 t to  allow for extra length due to cranking when bars are bent to an inclination of 45⁰, and add 0.3 t to allow for extra length due to cranking when bars are bent to an inclination of 30⁰. The values of 0.4 t and 0.3 t are for each crank and thus if the bar is cranked at both ends twice these values will have to be added. (t = vertical distance through which bar is cranked.)

Allowance for hooks

For the purpose of anchorage, ends of all plain round mild steel reinforcing bars including the spliced ends are provided with hooks. Such hooks are not obligatory when using torsteel/ ribbed/ deformed/ cold twisted bars unless particularly shown or stipulated in drawing or specifications.

When using plain round mild steel bars conforming to IS: 432-1960, or deformed mild steel bars conforming to IS: 1139-1959 the minimum allowance for each hook is 9 times the diameter of bar (but in no case less than 75 mm) rounded off to the nearest 5 mm, and works out as follows :

Diameter of bar	Minimum allowance for each hook	Diameter of bar	Minimum allowance for each hook	Diameter of bar	Minimum allowance for each hook
mm	mm	mm	mm	mm	mm
5	75	16	145	32	290
6	75	20	180	36	325
8	75	22	200	40	360
10	90	25	225	45	405
12	110	28	250	50	450

The same allowance as given above is required for each of the two hooks provided for the purposes of forming binders, stirrups links and the like.

Allowance for Hooks

The minimum allowance for each hook (which in no case shall be less than 75 mm) when using the following categories of steel is given below:

Medium tensile steel (IS:432-1960) or (IS:1139-1959) – 11 times diameter of bar.

Cold twisted steel bars (IS:1786-1961) ………………  - 13 times diameter of bar.

(Reference IS:2502-1963 – Code of practice for bending and fixing of bars for concrete reinforcement)

Reinforcement for Circular Slabs

For finding length of bars required (excluding hooks and laps) in a circular flat slab, find out a square of the same area as the circular slab using formula is:

Side = diameter X 0.88623

Total length of bars required for such a square slab (calculated with the same reinforcement and spacing as that of the circular slab) will be same as total length required for the circular slab. Allowance for hooks and laps can be added later on.

The same principle as above can be applied for finding reinforcement required in flat walls and slabs of curved or irregular shapes.

Two Way Reinforced Slabs

In a two way reinforced slab, distribution or temperature bars that may be indicated in the structural drawings are required only for the portion of the main bars taken to the top of the slab after cranking.

In the sketch shown, main reinforcement bars, in the innermost central rectangle (shown by dotted lines) are at bottom in both the directions. No distribution/ temperature/ tying bars are needed in this area as the main bars in one direction can be tied to the main bars running at right angle to them.

Similarly, in the four rectangles formed at corners of the slab, main reinforcement bars in both the directions will be available at top as well as at bottom for tying to each other at intersections.

Temperature/distribution/tying bars indicated in the schedule of reinforcement of a two way reinforced slab are therefore required to be calculated only for the four rectangles shown shaded in the sketch, at top, as the bent up portion of main bars need lateral support which is provided by tying up with the temperature/distribution/tying bars.

In a two-way reinforced slab, depending on the design requirements, other reinforcement like extra bars over support at top, corner bars etc., may be required. These are not shown in the accompanying sketch.

When working out the number of main reinforcement bars required in a slab, at the given centre to centre spacing, it should be remembered that:

a)  No bars are required to be provided over the support (wall or beam) in the direction parallel to the support. The first  bar in the direction parallel to the support is placed at a distance of half the specified centre to centre spacing of the bars.

b)  The number of bars required are calculated by working out the number of spaces and adding one for the end bar.

Binders/Stirrups/Links

a)  Except where otherwise clearly indicated in the drawings, calculation for the number of binders/stirrups/links in columns should cater for the complete vertical length of the main reinforcement of the column including the portion in footing/pedestal/base and junction with beams.

b) Where a beam is supported on columns, either terminating at, or continuous beyond the column, no stirrups are needed for the portion of main reinforcement of the beam entering or passing through the column.