Wood shims should not be used to level beams and girders because they are?

Wood shims are inherently compressible, which means that they can compress under load or pressure. This characteristic makes them unsuitable for leveling beams and girders that need a stable and uniform support. When used in structural applications, such as leveling, wood shims can lead to uneven settling and potentially compromise the integrity of the structure over time as they compress and lose their effective height.

In contrast, materials that maintain consistent dimensions under load are preferred in these applications to ensure a stable and solid structure. This is why using a compressible material like wood shims for such critical tasks is not recommended, as it can undermine the strength and safety of the construction.