6.5.1 Shape of the element

Elements used in finite element analysis should avoid irregular shapes.It is better to be as regular as possible，because elements in irregular shapes will cause numerical instability or inaccuracy of numerical analysis.Whether an element is in a good shape can be evaluated by its aspect ratio.The aspect ratio is the ratio of the length to the width of an element（L／B），as shown in Figure 6.13.The closer to 1.0 is the aspect ratio，the better is the shape.That is，the square or an equilateral triangle is the best choice.Elements with angles of 90°（quadrangles）or 60°（triangles）are also good elements.Since neither squares nor equilateral triangles are easily found，elements with an aspect ratio within the range 1.0≤≤2.0－2.5 can be viewed as good ones.

Figure 6.13 Definition of the aspect ratio

The shape of an element will influence the analytical accuracy of the element and the surrounding elements.It is therefore necessary to place good elements in crucial areas.In less crucial areas，some elements not so good can be placed.For example，if the retaining wall is an important object of analysis，good elements should be placed in its surroundings.On the other hand，if the boundary areas are not important areas，some elements not that good can be placed there.