# Under Reinforced Sections

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Under Reinforced Sections:

Prior to describing these sections, let’s establish some fundamental concepts. It is commonly understood that Concrete is robust in compression but feeble in tension, whereas Steel has equivalent strength in both tension and compression. However, when referring to compression in steel sections, there is a concern regarding Buckling.

The tension capacity is the maximum tensile force that the section can resist before yielding or rupturing, concrete possesses little or no tensile strength and can be ignored, thus all the tensile forces are handled by the reinforcement. The compression capacity is the maximum compressive force that the section can resist before buckling or crushing, which is handled by concrete.

Let’s take a singly reinforced section as an example. The external moment acting on this section is 200kNm. To resist this moment, a compressive force (C) and a tensile force (T) are required, which act as a couple (Internal Forces). In order to maintain the equilibrium of the section, the compressive force (C) and tensile force (T) must be equal (C = T).

In addition, the section possesses its own capacity for Bending moment, as well as Compression and Tension Capacity.

The Under reinforced Section:

If the Compression Capacity is greater than the Tension Capacity and both are greater than the Compressive or Tensile Force (Internal) i.e.,

[“Compression Capacity>Tension Capacity>Compressive/Tensile Force (Internal)”]

then the section is classified as an “Under reinforced” section. Since the Compression Capacity is greater than the Tension Capacity, the Reinforcement failure occurs prior to the concrete failure, resulting in a ductile failure and the appearance of cracks in the tension zone of the section, which serves as an indication or warning before the failure.

This type of failure is also referred to as the Secondary compression failure, where the Concrete strength in compression exceeds the Reinforcement strength in tension, and the failure is initiated by the yielding