Geometry helps structural engineers determine the amount of materials required for a given project. For example, if I-beams of steel are used, important questions might be: How much steel is required? How much does that cost? How much will it weigh? In order to answer either of the latter two questions, a structural engineer must first compute the volume of the I-beam, using geometry to do so.
Sometimes, instead of the volume of a material, structural engineers might need to know the surface area, as when installing glass panels of various shapes. Then, the structural engineer must employ formulas for two-dimensional geometric shapes, such as trapezoids and rhomboids.
Structural engineers must compute angles using geometry to ensure that the building is safe and will fit together properly. For example, if a handicapped access ramp is to have an angle of ascent no greater than 10 degrees, the structural engineer would manipulate the height and length of the ramp until, by trigonometry (an extension of geometry), the ramp was of the appropriate angle.
To present a design to colleagues, supervisors, and eventually, the construction workers who will implement it, a structural engineer must use scale drawings. These shrink down the size of a structure so that it is manageable to look at the plan, keeping all the shapes in the same proportion and relation to one another. The idea of scale, and of similar shapes (meaning shapes that have the same angles, but are different sizes) comes from geometry.