GUIDELINES FOR DEVELOPMENT LENGTH DESIGN | CALCULATION OF DEVELOPMENT LENGTH

FIRST CHOOSE BETWEEN THESE OPTIONS THEN START TO READ THE ARTICLE, WE EXPLAINED ABOUT DEVELOPMENT LENGTH 

WHAT IS DEVELOPMENT LENGTH?

It is Used at

BEAM-COLUMN JOINT

OR

COLUMN-FOOTING JOINT.

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).

 

GUIDELINES FOR DEVELOPMENT LENGTH DESIGN | REQUIREMENTS FOR DEVELOPMENT LENGTH DESIGN | LCETED 

DEVELOP LENGTH CALCULATIONS

For +ve moment, 𝐿𝑑 = 47𝜙 (for M 20 and Fe 415)

For +ve moment, 𝐿𝑑 = 57𝜙 (for M 20 and Fe 500)

For – ve moment, 𝐿𝑑 = 𝐿/4

At least 1/3 of the +ve moment steel in S.S. beam and 1/4 of the +ve moment steel in continuous beam extends to support for a distance of 𝐿_𝑑/3

If total bars = 6, therefore 1/3 × 6 ≥ 2 (means 2 bars will curtail and 4 go to support)

For a cantilever beam, available development length is nearly equal to span length

For continuous beam, available length = 𝑥𝑜 + D or 12𝜙 or 𝐿/16 (whichever is greater); 𝑥𝑜 = length of –ve moment region, i.e., the distance between the point of contra flexure from the centre of support.

In practice, usually, bars are provided for a length = 𝐿/4 from the centre of support which then becomes the available length.

At least 1/3 of the -ve steel should have an embedment length beyond the point of inflexion not less than

D or 12𝜙 or 𝐿/16 (whichever is greater)

Top steel in beams is to be spliced at mid-span (for 𝐿_𝑑) and bottom bars to be spliced near support after (2𝐷 𝑜𝑟 𝐿/4).

Lap of bottom steel in slab is to be done near support (if required)

At the junction of beam/shear wall, 𝐿_𝑑 = 1.5𝐿_𝑑

If bars are provided in 2 layers, it is better to curtail bars of layer 2 only.

For beams of up to 2.5 m, no need of curtailment of top and bottom bars, i.e., all bars are ALTH (all through).

Curtailment:

TPC from centre of support,

𝑁′=no of bars to be curtailed

𝑁𝑚𝑎𝑥 = no of bars at mid-span

The actual point of cut off (APC) from the centre of support= 𝑥1 − 𝑒𝑓𝑓 𝑑𝑒𝑝𝑡ℎ(𝑑)

 

CALCULATION OF DEVELOPMENT LENGTH

 

As per the Indian Standard – IS 456: 2000, clause 26.2.1 the development length Ld is given the following expression;

 

Where,             

Ø = nominal dia of reinforcement bar

σ_s = Stress in the bar at the section considered at design load

τ_bd = Design bond stress

 

The formula given above is used to calculate the required development length in mm for any bar dia, the same formula is used for the limit state mode and the working pressure system. The only change in the calculation in both methods is due to the different value of the design bond stress; The values of the design bond for the limit level and working pressure are as follows;

              

DESIGN BOND STRESS IN LIMIT STATE METHOD
Concrete Grade	M20	M25	M30	M35	M40 & Above
Design Bond Stress (τbd, N/mm2)	1.2	1.4	1.5	1.7	1.9	For Plain Bars in Tension
	1.92	2.24	2.4	2.72	3.04	For deformed bars in tension

 

**Note: For bars in compression 1.25 times the above-given values shall be used.

            

DESIGN BOND STRESS IN WORKING STRESS METHOD
Concrete Grade	M20	M25	M30	M35	M40	M45	M50
Design Bond Stress (τbd, N/mm2)	0.8	0.9	1.0	1.1	1.2	1.3	1.4	For Plain Bars in Tension
	1.28	1.44	1.6	1.76	1.92	2.08	2.24	For deformed bars in tension

**Note: For bars in compression 1.25 times the above given values shall be used.

 

How to calculate the development length for different grades of concrete as per IS 456?

Example - 1

 

Given data

 

Calculate the development length for rebar in tension, by limit state method for the below-given data.

 

Grade of concrete = M25

 

Reinforcement bar = Fe500 (σs = 500 )

 

Diameter of bar = 20mm. (∅ )

 

The development length Ld =   ∅ × σs ÷  4 × τbd

                                                  

Ld = 20 × 500 ÷ 4 × 2.24

 

(τbd = 2.24 N/mm2 for M25 in tension, for the limit state. From the above-given table,)

 

= 10000 ÷ 8.96

 

Ld = 1116.07mm.

 

                                             

Example - 2:

 

Given data

 

Calculate the development length for rebar in compression, by working stress method for the below-given data.

 

Grade of concrete = M30

 

Reinforcement bar = Fe415 (σs = 415 )

 

Diameter of bar = 16mm. (∅ )

 

The development length Ld =  ∅ × σs ÷  4 × τbd

                                                   

Ld = 16 × 415 ÷ 4 × 1.6

 

(τbd = 1.6 N/mm2 for M25 in compression, for the working stress method. From the above-given table,)

 

= 6640 ÷ 6.4

 

Ld = 1037.5mm.

 

 

As per thumb rule development length is 45d-55d.normally we take 50d.which means for 16 mm dia bar development length would be 50*16 = 800mm=2.6 feet

 

FAQ ON DEVELOPMENT LENGTH

What will happen if we don't provide development length?

 

REASONS FOR PROVIDING DEVELOPMENT LENGTH

FACTORS AFFECTING DEVELOPMENT LENGTH?

What will happen if we don't provide development length?

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