The option for Large displacements analysis is to include the 3rd Order Theory; this means, including in the non-linear calculations the large displacement theory, also known as the large deflections method.

This calculation uses the rules of large displacement and relates the results to the deformed system line. The equilibrium between the load and the internal forces is built on the deformed system line. Also, this calculation does not take into account the deformation of the load cases from the loads in the summation load case “SumLc”. This calculation type has to be used in the case that the deflection of the structure is considerably big related to the measurement of the structure itself. For example: The deflection of the structure is greater than L/100 of the span length, or in the case of cable sagging on the cable structure.

This calculation type must be used on structures where the stability of the structure will be reached only by membrane forces, or over cable structures. 3rd Order is recommended to be used where these elements have a very small moment of inertia because the transversal deflections are really important.

In the case of construction stages, with changes in the structural system, to be considered together with large deflection users have to switch on also the calculation type Accumulate permanent loads. The calculation type accumulated load could be taken into account in combination with large deflection for the calculation of steel bridges.