Thin Beam vs. Thick Beam

Standard beam theory (Euler-Bernoulli bending theory) assumes no deformation by shear. This can be safely applied to thin beams that are long and slender. For short and deep beams however, displacement due to shear becomes hugely important which is included in the formulation of thick beam element. Below is an exaggerated example of shear deformation:

Displacement due to shear that standard beam theory does not take into account

     This effect is the key that distinguish between the Euler-Bernoulli and Timoshenko (thick beam theory) bending theories. See below for a direct comparison between thin and thick beams.

Table illustrating the differences between thin (Euler-Bernoulli) beams and thick (Timoshenk) beams. Diagram in "look" section is adapted from Wikipedia.

Also read
Types of elements in the element library

What are Isoparametric Elements?

The truth is, you are probably using isoparametric elements without noticing it. In classes, we might have been asked to derive stiffness equations for elements of simple shapes such as rectangles or cuboids. This is to favour hand-calculations using simple equations.

In the real world however, most objects take irregular shapes. A Jacobian mapping process is therefore required to accommodate for this shape irregularity. Non-isoparametric elements can only be implemented to regular shapes and use shape functions solely for the purpose of displacement interpolation. Isoparametric elements on the other hand can be used to model irregular shapes. They use shape functions not only for displacement interpolation, but also to represent the irregular element geometry. This means shape functions are now responsible for both the displacement interpolation and element shape. This also means that modelling a curved surface within one single element is now made possible.

A mesh of isoparametric elements

In practice therefore, most elements offered by FEA programs are isoparametric elements. They are so widely used that they are not often stated in the user manuals.

Straight Beam vs. Curved Beam

Table illustrating the differences between straight and curved beams

See also

Types of elements in the element library