How can you determine if nonlinear analysis is necessary for your project?

A non linear analysis is needed when there is nonlinear relationship between the forces and the subsequent displacement of structure.

Most projects do not require nonlinear analysis. The common cases where it is needed include buckling and axial shortening analysis. Nonlinear analysis may also be necessary for super tall buildings where tuned mass dampers are used. Additionally, it should be considered for dams, complex tunnels, and suspension bridges.

For many cases, nonlinear analysis is not necessary. The project requirements such as the inteded use of the structure may be a way other than the engineering judgement itself that may make it necessary to recurr to nonlinear analysis.

The material may allow or require nonlinear properties for proper analysis. For example, steel is a material that is widely known for its ductile capacity prior to reaching its ultimate tensile strength. Most steel codes do not require but allow for nonlinear material modelling of steel in a structural design, in order to optimize material consumption. Glass as a structural material, on the other hand, is a material without intrinsic ductile behavior, therefore being unrealistic to model its material behavior as nonlinear in the structural design.

Determining the need for nonlinear analysis involves assessing material and geometric nonlinearities. Check if materials exhibit nonlinear behavior, evaluate significant deformations or rotations, and consider the impact of extreme loading conditions. Assess P-Delta effects, second-order effects, and the influence of boundary conditions. Additionally, consult building codes for specific nonlinear analysis requirements. Conduct sensitivity analysis to identify critical parameters. If deviations from linear assumptions are significant, nonlinear analysis becomes essential for accurate structural predictions.

The geometric behavior of the structural elements or system is another parameter to be considered when determining if nonlinear analysis is required or not. An example is for laminated glazing panels, where linear behavior is only valid if the deflections are within 0.5*t where t is the thickness of the plate. Thus, independent of code or other parameter, it is required to perform a nonlinear analysis of that panel in order to obtain realistic results.

Considering loading conditions is essential in critical cases such as transfer slabs supporting high-rise buildings, soil-structure interaction, modelling of building and foundation strengthening phases, onshore and offshore structures, and dams. Moreover, it is crucial for buildings subject to large vibrations, like those supporting turbines, and in seismic time history analysis and buildings and structures where performance-based design is adopted. Assessing structures’ performance under seismic time history analyses is also vital. Additionally, villas built offshore need to be designed to withstand the implosive loading resulting from seawater waves.

Loading conditions may impose the need for nonlinear analysis. Nonlinear analysis methods allow to take into account higher order of load effects and depending on the nature or type of load, it is unsafe not to recurr to a nonlinear analysis. An example can be of a certain persistent load that deforms a system or a member in such a way that it further increases the moments or other internal forces in the member up to a stable, higher stress state. In that case, simple linear analysis would lead to sub-dimensioned structural members and thus would fail.

Desired or required analysis complexity will potentially make necessary the use of nonlinear analysis. A simple example can be that of modelling the support of a laminated glass floor panel, or a prestressed concrete beam in simply supported condition. In these cases, the materialization of the support may be by means of an elastomeric bearing pad. Such support condition of simple contact requires nonlinear analysis as those supports react to compression but not to tension.

Determining the necessity of nonlinear analysis hinges on several factors. For complex structures, such as high-rise buildings or intricate towers, especially in seismically active zones, both material and geometrical nonlinear analyses may be essential. Material nonlinear analysis is crucial when the stress-strain relationship of the construction materials deviates significantly from linearity under loads, impacting the building's integrity. Geometrical nonlinear analysis becomes critical when structural deformations under load alter the building's initial geometry, affecting its overall stability. In scenarios involving high-intensity loads like earthquakes, these analyses are vital to accurately predict structural response.