# What is EI in Civil Engineering | What is meant by EI constant in a given structure

**What is EI in Civil Engineering**

**EI** is the product of the materials **modulas of elasticity** (E) and the elements second **moment of area** (I).

- E is a function of how
**stiff the material**is. - I is a function of the
**elements shape**.

### What is meant by EI constant in a given structure

The EI value defines how stiff a structure is.

For example in an EI of the simple concrete beam with width b and depth d, the value of I is equal to **b*d^3/12**.

The deflection can be calculated as **5/384 * w*length^4 / EI**.

In this example, the variables are

- The span of the beam (length),
- The weight that the beam carries (w),
- The beam properties (EI).

The EI constant is often an expression in the engineering world to explain the **stiffness of a structural element**. E is the Modulus of Elasticity, and I is the second moment of inertia.

EI is most often used in FEM (Finite Element Modeling). This expression is a fundamental building block for analyzing complex structures and is a property for many FEM elements (Beams, Bars, Rods, etc.)

Also Read: **Why are I beams shaped like an I?**