|Dados being cut by hand in shelf supports for adjustable shelving|
Choices depend on what the shelves are intended to hold. In the above photograph trenches (dados) are being cut in supporting uprights for 1/4" thick shelves to carry paper files horizontally. These shelves are thin, so the weight they can take without bending is limited, but there can be as many shelves as there are trenches and spaces can be arranged to take different thicknesses of file. With uprights that are 3/4" thick to start with, cutting 1/4" deep trenches reduces the effective upright thickness to 1/2".
Adjustable shelving for heavier items such as books uses thicker shelves, say 3/4", to take the weight. Usually books are stored vertically, so the space between shelves will be greater. To support the weight holes are drilled for pegs that carry the shelves. These peg holes make little effective reduction to the strength of uprights.
Equipment shelving depends on the size and weight of the equipment. Because of the pace of change to equipment, particularly computer and audio visual elements, modular construction helps to make later changes easier than if a support is built in permanently.