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Construction material: Steel

Steel is probably the most versatile comnonly used structural material. Not only is its versatility apparent in great varlety of structures or which it is used but also in many different of structural steel of interest to the designer can be described by the behavior of steel during a simple tension test. The essential elements in steel are metallic iron element non-metallic carbon, with small quantities of other elements such as slicon nickel, manganese chromium and copper. It is thus an alloys.  Though steel is usually more than 98% iron, with other element present in same quantities, these other element have pronounced effect on the properties of steel. Various iron - carbon alloys used as structural material are of three types (I) Cast iron (I) Wrought iron (Ill) Steel cast iron Steel cast iron a low carbon content, while wrought iron has high carbon content. In many ways steels are intermediate in carbon content, between cast iron and wrought iron. The approx...
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Development length


All the questions related to the development length are answered below.

What?


  • Length of the steel bar needed to be embedded into the column to establish the desired bond strength between concrete and steel.
  • In simple language it holds concrete members together. 
  • Like beam, column, footing etc

Where?

  • Development length is provided at column-beam and column-footing joints.

Why?


  • Development length creates a safe bond between bar surface and concrete
  • It also ensures during ultimate loading condition reinforced bars shouldnot slip through the concert
  • It transfers the stresses or load from beam to column safely

If not provided?


  • The structure will prompt to encounter failure due to slippage of joints
  • In such conditions bars will not break first, but the failure will happen at joints and laps promoting the breaking of reinforcement bars
  • For example
  • Lets take the case of beam-column joints in which development length is not provided then,
  • If we apply force maximum than permissible limit, then the restraining force between beam-column will not be sufficient to hold the position.
  • Eventually Beam will come out of the concrete column

Development length acts as a supporting member for the reinforced beam in concrete column

How to calculate?


The basic formula for calculating development length is as below

The formula is same for both limit state method and working stress method

Bundled bars?


  • When the number of bars provided in the beam are more,it may not be possible to place the bars with necessary clearance
  • In such cases there are two options

1. Increasing the size of concrete members
2. Bundled the bars in group of two,three or four bars

1st option is more expensive than other which will affect the overall budget
So, 2nd option is better.

  • When the bars are bundled then this nars will have low contact area with the surrounding concrete when compared to the bars placed seperately
  • It also affect the bonding between concrete and steel bars
  • To compensate this development length is increased suitably

Diameter?

It is observed that small diameter bars required lower development length than the larger diameter bars




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