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Type of Beam Section

There are three type of Beam Section

  • Balanced Beam Section
  • Under-Reinforced Beam Section
  • Over-Reinforced Beam Sections

Beam Section

Balanced Beam Section

  • Reinforced concrete beam sections in which the tension steel also reaches yield strain simultaneously as the concrete reaches the failure strain in bending  call balanced sections.
  • The neutral axis corresponding to this condition is called critical neutral axis.

Beam Section

Terms used in figures

  • b- Width of beam
  • d – Depth of beam
  • Ast– Area of steel
  • Na –  Neutral axis of beam
  • Nc – Critical axis of beam

Under-Reinforced Beam Section

  • Reinforced concrete beam sections in which the steel reaches yield strain at loads lower than the load at which the concrete reaches failure strain are called under-reinforced sections.
  • The failure in under-reinforced beam section is due to the concrete reaching its ultimate failure strain of 0.0035 before the steel reaches its failure strain which is much higher 0.20 to 0.25.
  • Every singly reinforced beam should  design as under-reinforced sections because this section gives enough warning before failure.
  • In an under reinforced section, the position of actual neutral axis lies above the critical axis.

Beam Section

Over-Reinforced Beam Sections

  • Reinforced concrete beam sections in which the failure strain in concrete reach early than the yield strain of steel reach, are called over-reinforced beam sections.
  • If over-reinforced beam design and loaded to full capacity then the steel in tension zone will not yield much before the concrete reaches its ultimate strain of 0.0035. This due to little yielding of steel the deflection and cracking of beam does not occur and does not give enough warning prior to failure.
  • Failures in over-reinforced sections are all of a sudden. This type of design not recommend in practice of beam design.
  • In an over reinforced section, the position of actual neutral axis lies below the critical axis (i.e towards steel ).

Beam Section

 

Points to be remember

  • For a particular c/s—

(A) Area of steel for a balance section can reduce or saved by increasing the grade of steel and by reducing the grade of concrete, keeping one parameter at a time constant.

  • For over R/f section there is no advantage in increasing the  grade of steel because failure is brittle or sudden but is advantageous to improve grade of concrete.
  • For under R/f sections it is advantageous to improve the grade of steel not the grade of concrete because primary reason of failure yielding of steel.

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