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Placement of Reinforcing Steel 
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1. Why is  Proper Placement important? 4. Lap Length
2. Different aspects of reinforcement placement 5. Development Length
3. Concrete Cover 6. Bar Spacing and Bar Size
  7. Photos showing proper placement of Renforcing bars
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1. Why is  Proper Placement important?
Improper placement of reinforcing steel can greatly reduce the strength and life of a structure requiring premature costly repairs and replacement of structures. It may also to a structural failure.  The failure of a structure does not necessarily mean the collapse of a structure.  A structure fails when it can no longer be used in the manner in which it was intended.
2. Different aspects of reinforcement placement
The important aspects of reinforcement placement are:-
  • Concrete Cover
  • Lap Length
  • Development Length
  • Bar Spacing and Bar Size
3. Concrete Cover
Cover less than the design cover: The life of the reinforcing steel can be shortened due to corrosion from increased exposure to deicing materials and/or the elements. Corrosion of the reinforcing steel will cause an increase in the diameter of the steel, which will cause the concrete outside the steel to sometimes spall off and the concrete around the reinforcing steel to become debonded from the reinforcing steel.  This debonding of the concrete from the reinforcing steel can reduce the strength of the structure by preventing the necessary interaction between the concrete and the reinforcing steel. 

Cover more than the design cover:  will reduce the strength of the structure.  Placing the reinforcing steel with more than the design cover causes the neutral axis to be shifted higher in the section, which reduces the area of concrete that is in compression and increases the cracked area of concrete in tension. This decrease in useful concrete (over Neutral Axis) and increase in useless concrete (below Neutral Axis)  greatly reduces the strength of the structure.

4. Lap Length
It would be ideal to make reinforcing bars in structural members (like slabs, columns, beams etc) one continuous bar, this however would be impractical due to difficulties in transporting and handling the steel.

In order to achieve the same effect as having one continuous bar, shorter bars with minimum (over) lap lengths are used.

The design lap length is usually a minimum length required to transfer stress from one bar to another. If the actual lap length is less than the required lap length, the stress may not be transferred to the other bar, which could cause a failure in the structure at that lap location.

5. Development Length
The development length is often shown on the plans as a minimum embedment length. The purpose of the development length is to anchor the reinforcing bars beyond the area where the strength of the bars is needed.
Without the required development length, the reinforcing bar would pull out of the concrete surrounding it and the structure could fail.

A typical example of this situation would be the top transverse reinforcing bars in the cantilevered section of a bridge slab outside the exterior beam of a bridge. The critical section of the slab is just outside the outside edge of the beam supporting the slab. The reinforcing bars must extend into the slab beyond the critical section for a required length. If the reinforcing bars do not extend beyond the critical section sufficiently, they will be pulled out and the structure will fail.
6. Bar Spacing and Bar Size
Both the spacing and the size of the reinforcing bars control the amount of steel in the tension area. The amount of reinforcing steel in the tension area of the structure  has a large impact on the strength of the structure.
7. Photos showing proper placement of Renforcing bars
placement of reinforcing steel
Template used in a pier footing to assure proper spacing & alignment of column steel
placement of reinforcing steel
Hoop bars in pier diaphragm are properly and adequately supported
placement of reinforcing steel
Two separate mats of reinforcing steel are independently supported in this slab for the RCB
placement of reinforcing steel
Proper spacing of vertical column bars
placement of reinforcing steel
Vertical bars in a culvert wall are properly and adequately supported
placement of reinforcing steel
The vertical wall steel is properly secured to prevent it from drifting inward

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Source : NEITC's Reinforcing Steel Placement Handbook: Contributed by George Melory,  U.S.A . All rights of the article lie with original right holders
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