You must account for thermal expansion & contraction. Depending on your seismic zone, there may also be requirements for allowing relative movement between pieces of equipment during an earthquake. A third use may be to allow for easy isolation of equipment such as transformers during routine testing.
Equations for thermal expansion lengths & forces can be found in IEEE 605 or probably on Google.
IEEE P1527 discusses the use of flexible bus to mitigate some seismic risks at transmission voltages.