Location of Reinforcement

Concrete cracks due to tension and (as a result) reinforcement is required where flexure, axial loads, or shrinkage effects cause tensile stresses.An uniformly loaded, simply supported beam deflects as shown in Fig. -7(a) and has the moment diagram shown in Fig. 7(b). Because this beam is in positive moment throughout, tensile flexural stresses and cracks are developed along the bottom of the beam. Longitudinal reinforcement is required to resist these tensilestresses and is placed close to the bottom side of the beam, and because the moments are greatest at midspan, more reinforcement is required at the midspan than at the ends.as shown in Fig. 7(c)

Figure -7 : Simply supported beam.

Figure -8: Cantilever beam.

A cantilever beam develops negative moment throughout and deflects as shown in Fig. -8 with the concave surface downward, so that flexural tensions and cracks developon on the top surface

Commonly, reinforced concrete beams are continuous over several supports, and under gravity loads, they develop the moment diagram and deflected shape shown in Fig.-9. Again, reinforcement is needed on the tensile face of the beam, which is at the top of the beam in the negative-moment regions near the supports and at the bottom in the positive-moment regions near the midspans. Two possible arrangements of this reinforcement are shown in Figs.-9(c) and Figs.-9(d).

It is necessary to have cover (concrete between the surface of the slab or beam and the reinforcing bars) for four primary reasons :1. To bond the reinforcement to the concrete so that the two

elements act together.The efficiency of the bond increases as the cover increases. A cover of at least one bardiameter is required for this purpose in beams and columns. (See Chapter 8.)

2. To protect the reinforcement against corrosion. Depending on the environment and the type of member, varying amounts of cover ranging from to 3 in. are required (ACI Code Section 7.7). In highly corrosive environments, such as slabs or bridges exposed to deicing salts or ocean spray, the cover should be increased.

3. To protect the reinforcement from strength loss due to overheating in the case of fire. The cover for fire protection is specified in the local building code.

4. Additional cover sometimes is provided on the top of slabs, particularly in garages and factories, so that abrasion and wear due to traffic

Concrete Cover and Bar Spacing

Calculation of Effective Depth and Minimum Web Width for a Given Bar ArrangementThe effective depth, d, of a beam is defined as the distance from the extreme compression fiber to the centroid of the longitudinal tensile reinforcement.

Concrete cover, 1.5 in (ACI 7.7.1) normal exposure

Larger of :• bar diameter db (ACI 761) • 1 in (ACI 761)• 1.33 diameter of coarse aggregate (ACI 332)

Larger of :• 1 in (ACI 762)• 1.33 diameter of coarse aggregate ACI 332)