Cargo weight is then added in to that section depending on the loading conditions being checked. The total still water bending moment is then calculated by integrating the difference between buoyancy and total weight along the length of the ship. For a ship in motion, additional bending moment is added to that value to account for waves it may encounter. Standard formulas for wave height and length are used, which take ship size into account. The worst possible waves are, as noted above, where either a wave crest or trough is located exactly amidships. Those total bending loads, including still water bending moment and wave loads, are the forces that the overall hull primary beam has to be capable of withstanding. The secondary hull loads, bending, and strength are those loads that happen to the skin structure of the ship (sides, bottom, deck) between major lengthwise subdivisions or bulkheads. For these loads, we are interested in how this shorter section behaves as an integrated beam, under the local forces of displaced water pushing back on the hull, cargo and hull and machinery weights, etc. Unlike primary loads, secondary loads are treated as applying to a complex composite panel, supported at the sides, rather than as a simple beam. Secondary loads, strength, and bending are calculated similarly to primary loads: you determine the point and distributed loads due to displacement and weight, and determine local total forces on each unit area of the panel. Those loads then cause the composite panel to deform, usually bending inwards between bulkheads as most loads are compressive and directed inwards. Stress in the structure is calculated from the loads and bending. Tertiary strength and loads are the forces, strength, and bending response of individual sections of hull plate between stiffeners, and the behaviour of individual stiffener sections. Usually the tertiary loading is simpler to calculate: for most sections, there is a simple, maximum hydrostatic load or hydrostatic plus slamming load to calculate. The plate is supported against those loads at its edges by stiffeners and beams. The deflection of the plate (or stiffener), and additional stresses, are simply calculated from those loads and the theory of plates and shells. This diagram shows the key structural elements of a ship's main hull (excluding the bow, stern, and deckhouse). The depicted hull is a sample small double bottom (but not double hull) oil tanker. The total load on a particular section of a ship's hull is the sum total of all primary, secondary, and tertiary loads imposed on it from all factors.