What are the Differences between Overlapping and Development Length?
Most civil engineering students’ gets confused when asked about the questions related to the differences between overlapping and development length. If you want to know about their differences, this article explains it in detail with definitions and comparison chart.
Before going into the list of differences between overlapping and development length, we are going to discuss what is mean by overlapping and development length and then we will discuss what differentiates them from each other.
Table of Contents
- 1 Overlapping:
- 2 Development length:
- 3 Differences between overlapping and development length:
- 4 Share this:
It is used for lapping two steels in order to transfer the load from one steel to another steel.
Steel present in market in a specific length. If the length of steel is used in construction more than that limit then sufficient lap length will be provided to transfer the load safely.
Overlapping depends on:
Overlapping is depending on these factor that is given below:
- Tension Zone
- Compression Zone
- Grade of concrete
- Rebar size
- Concrete cover
In tension zone the overlapping is provided at the ends where shear force is zero. The overlapping is provided in minimum shear force area.
In compression zone the overlapping should be provided at the center where shear force is zero.
Grade of Concrete:
If the grade of concrete is good, then length of overlapping is less for example in M15 the overlapping is 55d while in M20 the overlapping is 46d.
The overlapping is depending on the rebar size it means if the dia of steel is more than the overlapping length will be more while if the size of steel is less than the length of overlapping will less. For example, the overlapping for 12mm dia steel will be if we assume the overlapping formula 50d the overlapping length will be 50 x 12 = 600 mm. While for greater dia of steel 16 mm the overlapping length will be 50 x 16 = 800 mm
Overlapping is also depending on concrete cover
Overlapping is different according to tension, compression zones and also depends upon the grade of concrete, rebar size, concrete cover.
In construction of RCC structure if the required length of steel is not available during placing of steel then overlapping is done. Usually overlapping length is between 40 to 60d where “d” is the dia of the steel. but if overlap length is not given 50d value is assumed for example if dia of bar is 16 mm then overlapping length= 50x 16 = 800mm
Location of overlapping:
The location of overlapping is different in different members of structure that is given below:
Beam is flexural member it means there is compression and tension the upper side of simply supported, fixed beam or continues is compressive while lower side have tension.
In upper side of simply supported, fixed or continues the overlapping should be provided at the center while in lower side the overlapping should be provided at the end.
Figure of simply, fixed and continues Beam
In cantilever the overlapping should be provided at the ends in upper side of beam while in lower side the overlapping should be provided at the center. It means the method of overlapping in cantilever beam is opposite to other beams.
Figure of Cantilever Beam
It is use to transfer the load from rebar to concrete. Development Length is also known as anchorage length.
According to IS 456:2000, the tension or compression is developed in any type of steel at any section to resist these forces desirable development length will be provided. So that because of development length the structural members (beam, column, slab etc) won’t be slipped away from the support.
Development length will be used in a continuous beam, cantilever slabs and other serious joints such as beam to column joint. This length will be used as a bend where the holding member is thin.
To calculate the development length of the formula is given below:
Ld = Development length
Ø = diameter of steel
σs = Stress in steel.
τbd = Design bond stress
Development length = (diameter of steel × stress in steel) ÷ (4 × design bond stress)
Development length in Tension zone:
To find the development length in tension zone the formula of development length will be used for example:
D =12 mm
stress in bar = 415 N/mm2
design bond stress for M25 CONCRETE = 1.4 N/mm2
Ld = 12 x (415 ÷ 4) x 1.4
Ld = 889 mm say 1m
Development length is 1 m in tension zone for M25 concrete Mix
Development length in compression zone:
To find the development length in compression zone 25 % of value will be deduct from the value of Development length in tension zone
For example, in compression zone development length for same detail will be
Ld = 1- 25% x 1
Ld = 1- (25 ÷ 100) x1
Ld = 1- 0.25
Ld = 0.75m or 750mm
Development length is 0.75 m in compression zone for M25 concrete Mix
Differences between overlapping and development length:
The difference between overlapping and development length are shown in the table which is given below:
|S. No||Overlapping||Development length|
|1||Overlapping is provided for lapping two steels in order to transfer the load from one steel to another steel.||It is provided to transfer the load from rebar to concrete|
|2||overlapping is also known as Lapping length||Development Length is also known as anchorage length|
|3||Overlapping is denoted by “L”||Development length is denoted by “Ld”|
|4||Overlapping is depends on the size of steel if the dia of bar is more than 36 mm than overlapping should not be provided.||Development length is not depends on the size of steel.|
|5||overlapping can be defined as the lapping of two steels side by side to up to the design length.||It can be defined as the quantity of steel length needed to be place into the column to create the bond strength between the concrete and steel.|
The main objective of this article “Differences between overlapping and development length” is that the Overlapping is provided for lapping two steels in order to transfer the load from one steel to another steel while development length is used to transfer the load from rebar to concrete
So friends I have written all the difference between overlapping and development length as much as I know. If this article is helpful do share it with your friends and leave a comment
Thanks for reading the article.
You may also read:
- Dowel bar its purposes, advantages and uses
- Overlapping of Steel Bar in Different Concrete Mix
- Development Length