1. What is the development length of steel bars?
The length of the reinforcement bar that is extended in the concrete, at the junction point of the two RCC members, so that the member won't get slipped away is called the development length.
This length is provided to transfer the maximum stress to the surrounding concrete to such a length, where the holding stress becomes zero at the discontinuous end.
2. What are the factors that affect the development length?
The factors that affect the development length are,
1. Grade of concrete:
The higher the grade of concrete, the lesser will be the development length.
2. Diameter of the reinforcement bar:
The development length of the rebar increases with the increase in the diameter of a bar.
3. Yield strength & stress on the different grades of steel rebars:
The higher the stress in a bar at the section considered at design load, the more will be the development length.
4. Type of structural member:
The development length will be 25% more in the compression member than in the tension member.
5. Calculation method for designing:
In the limit state method of designing, the development length will be less when compared to the designing of the same structure by using the working stress method.
3. What happens if we don't provide sufficient development length?
If we provide insufficient development length, the structure may slip away at the joints or junction point of two RCC members. The lesser development length ensures a lack of grip or holding property to safely transfer the designed stress to the succeeding structural member, thus resulting in a failure of an RCC member at the structural joints.
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4. What is the common mistake while calculating the development length?
As you can observe in the above drawing, the development length is provided in the footings for the reinforcement bars coming from the column. Here, the grade of concrete used in the column is M30, and the M25 grade is used in casting the footings.
The value of design bond stress (τbd ) changes, according to the grade of concrete used in casting the concrete members.
In the above-given example, you have to consider the concrete grade (M25) of the footing, to evaluate the development length. The reason is, that the development length of the bar is anchored in the footings, although the bar is extended from the column into the footings.
Considering the M30 grade of the concrete used in a column for the calculation purpose is a common mistake, that should be avoided to get an accurate development length.
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