Embedment length and Development of Length of Rienforcement

Embedment length
The length of embedded steel reireinforcement, 1 provided beyond a critical section.
The fibre length also defines the embedment length. This is especially important for thick, short hooked end steel fibre and low strength concrete.
A slippage of the reinforcement at the beam-column interface is observed because of the accumulation of the rebar strain along the embedment length in the beam-column connection.
Therefore, it is important to understand the proper embedment length of the longitudinal reinforcement considering the interface behavior between the infilled concrete and the steel casing in the Cast In Steel Shell pile.
Development Length
A development length can be defined as the amount of reinforcement(bar) length needed to be embedded or projected into the column to establish the desired bond strength between the concrete and steel (or any other two types of material)

Fig 1: Development length in Footing
Reason for providing Development length

• To develop a safe bond between the bar surface & the concrete so that no failure due to slippage of bar occurs during the ultimate load conditions.
• Also, the extra length of the bar provided as development length is responsible for transferring the stresses developed in any section to the adjoining sections (such as at column beam junction the extra length of bars provided from beam to column).
• where less development length against the required is provided the structures will be prone to encounter failure due to slippage of joints, bonds, anchors & Laps, in such cases the bars will not yield first but the failure will happen at joints & laps prior to yielding of reinforcement bars.

Fig 2: Development length as per IS 1786
Calculation of Development Length

Where
Ø = nominal dia of reinforcement bar
s= Stress in bar at the section considered at design load
bd= Design bond stress
The above given formula is used to calculate the required development length in mm for any given dia of bar, same formula is used for limit state method as well as working stress method.
The only change in calculation in both methods is due to the different value of design bond stress; the values of design bond for Limit State & working stress are as follows;
Table No 1: Design Bond Stress in Limit State Method
Design Bond Stress in Limit State Method
Concrete Grade
M20- 1.2
M25-1.4
M30-1.5
M35-1.7
M40 and above-1.9
(For Plain Bars in Tension
Design Bond Stress )
bd,N/mm2)
M20- 1.92
M25-2.24
M30-2.4
M35-2.72
M40 and above-3.04
(For deformed bars in tension)
(Design Bond Stress bd,N/mm2)
Footnotes-
Development Length of Reinforcement Bars