What is development length in construction? What is the formula?

Construction is a process that is not done overnight. In fact, it can take months or years to complete. One of the most important factors in this process is development length. This article will discuss what development length is and why it’s so important in construction.

 

What is development length?

The term “development length” signifies the bar length that must be present for the stress to be transferred successfully into the concrete. For concrete and steel, it is the length of the bar that must be implanted into the material to generate enough tension to provide the appropriate bond strength.

See also: Types of building materials

 

Development length: Importance

To ensure the bar surface is adequately protected, a development length is specified at the column footing or column beam junction. The most important reasons why we specify the development length are as follows.

• To secure the bar’s placement in the concrete and prevent it from moving.
• Under the most extreme loads, the bars don’t slide and cause any damage.
• The pressures generated in one area are transferred to neighbouring sections due to the additional bar length specified as the production length at the column. 
• This length, which is specified at the intersection of the beam and the column, is where the stresses that are created are readily transferred by the steel bond.

 

Development length: What happens if we fail to provide it?

The significance of development length may be understood by visualising a situation in which it is not provided. There will be no stress transmission to adjacent sections without developed bars, leading to structural failure from joint slippage before the steel bars can release their ultimate loads. In these kinds of circumstances, the bars won’t break first; rather, the breaking will happen at the joints, and the fortification will disengage from the concrete.

When applied to a Beam-column junction with no specified development length, loads in excess of those permitted will cause the joint’s resisting force to be insufficient to maintain the beam in place. Consequently, the concrete will break.

 

Accurately predicting development length

The following expression is used to describe the development length in accordance with clause 26.2.1 of the Indian Standard IS 456: 2000.

Where,       

 Ø = nominal diameter of the bar

 σs = stress in bars at the part considered during load design

 τbd = Design bond stress

For a given bar diameter, the above formula may be used to get the corresponding development length in millimetres; this holds true for both the limit state approach and the working stress method. The sole difference between the two approaches is the selection of a design bond stress value.

 

Optimising development length for strength

The development length is affected by the following elements, which are listed below:

• Density of concrete

The development length is influenced by the concrete’s density. Either by using concrete with a low density or by using concrete with a high density, the development length will be extended depending on which one is chosen.

• Steel distance from centre to centre

More concrete is required to make steel resistant to horizontal shearing if the distance between the steel members is increased. Beams typically have steel that is separated at a distance of one to two steel diameters apart, while other kinds of members have steel that is spaced at greater distances, reducing the amount of development length required.

• Structural steel encased in concrete

The concrete covering of steel also has an effect on the development length. An increase in concrete cover thickness will shorten the development length and vice versa.

• Compressive resistance of concrete

Compressive strength and development length are inversely related to one another, meaning that a greater magnitude of compressive strength will result in a shorter development length.

• Diameter of steel

The development length is affected by the diameter of the steel. It has come to everyone’s attention that steel with a smaller diameter requires a shorter amount of development length compared to steel with a bigger diameter. The diameter of the steel is exactly proportional to the development length.

• Steel coating

Epoxy-coated steels are utilised in place of ordinary steel in some construction projects since the structure will be exposed to environmental conditions that may cause corrosion. Because of this, the binding strength between the concrete and the steel must be increased, which requires a longer development length.

Maximising development length in concrete

By maximising the development length of reinforcement in concrete structures, you can ensure that the structure is strong, stable and able to resist the stresses that it may be subjected to over its lifetime. Mentioned are some ways to maximize the development length in concrete.

Embedment length increase: The portion of the reinforcing bar that is embedded in the concrete- the embedment length should be increased. With this, you can increase the development length and better the steel and the concrete bond strength.

Increase the diameter of the reinforcing bar: Development length can be increased owing to greater surface area for bonding with the concrete because of larger diameter bars.

Increase the concrete cover: Concrete cover thickness over the reinforcing bar also impacts the development length. By increasing the concrete cover, you can improve the bond strength and increase the development length.

Use of deformed bars: These have greater surface area and provide better bond strength with concrete than smooth bars. This can also result in increasing the development length.

Use epoxy-coated reinforcing bars: Corrosion effects bond strength. Use of epoxy-coated reinforcing bars prevent corrosion as they have a protective coating and thus increase development length.

Development length: Different situations

• Beams

The beam/column intersections serve as extensions of the available development length. Because of this, steel and concrete bonding is excellent at transmitting stress. 

• Columns in foundations 

Compression development length refers to the vertical distance that a column bar penetrates and remains embedded within the footing concrete, while tension development length refers to the same vertical distance that a column bar penetrates and remains embedded within the footing concrete but flexes in tandem with the footing reinforcement.

• Tension and compression

The phrase “development length in compression” refers to bars implanted in concrete during axial compression; for flexural compression, the word “development length in tension” is used instead.

 

FAQs

What if we don't specify a development length?

The structures are vulnerable to failure, owing to slippage of joints if we don't offer it or if we give less development length than is necessary. However, in such a scenario, the reinforcing joints and laps would fail first, before the bars themselves broke.

What exactly is the point of allowing for sufficient development length?

The goal here is to create a secure connection between the surface of the bar and the concrete in order to prevent any failure caused by the bar slipping under the maximum load conditions.

Where do we offer development length?

In order to prevent damage to the bar surface from the concrete, a development length is specified at the column footing or column beam junction. In order to ensure a strong bond between the bar's surface and the concrete, it is important to allow for this development length.

What is the minimum development length?

In accordance with IS 456:2000, the bare minimum for the development length is to be 41Ф.

What should you remember about development length?

Note that the development length is 41 times of nominal bar diameter. So, if the bar diameter is 10 mm, the development length is 10 x 41 = 410 mm.

What impacts a bar's development length?

A bar's development length is impacted by the concrete cover's thickness.

Got any questions or point of view on our article? We would love to hear from you.Write to our Editor-in-Chief Jhumur Ghosh at jhumur.ghosh1@housing.com