Structural steel design using advanced analysis with strain limits
Creators: name: Fieber, Andreas Christian
Publisher: name: Imperial College London
Year: 2019

description: Steel structures are affected, to greater or lesser extent, by (i) geometrical nonlinearity associated with the change in geometry of the structure under load and (ii) material nonlinearity related to the onset and spread of plasticity. In traditional design of steel structures, these effects are accounted for partially during the analysis and partially through subsequent cross-section and member checks. Design by advanced analysis avoids some of the issues encountered by traditional design methods (e.g. calculating effective length factors, choice of appropriate type of analysis) and provides a more accurate visualisation of the failure mechanisms, as most of the limit states governing the behaviour of a structure are directly captured in the analysis. The structural analysis of steel frames is typically performed using beam finite elements, which are usually not able to capture local buckling explicitly. Instead, in traditional steel design the assessment of local buckling and rotation capacity is made through the concept of cross-section classification, which places class-specific restrictions on the analysis type (i.e. plastic or elastic) and defines the cross-section resistance based on idealised stress distributions (e.g. the plastic, elastic or effective moment capacity in bending). This approach is however considered to be overly simplistic and creates artificial ‘steps’ in the capacity predictions of structural members. A more consistent approach is proposed herein, whereby a geometrically and materially nonlinear analysis with imperfections (GMNIA) of the structure is performed using beam finite elements, with strain limits employed to mimic the effects of local buckling. The strain limits are obtained from the Continuous Strength Method and effectively control the spread of plasticity, capture the effects of local moment gradients and, ultimately, define the structural resistance in bending dominated cases. Conversely, the structural capacity of stability governed systems is defined by the peak load factor of the advanced analysis. The practical application of the proposed design method is facilitated through the development of explicit functions to predict the elastic local buckling stress and half-wavelength of full cross-sections (i.e. including the effects of element interaction) subjected to pure compression, pure bending and combined compression plus bending. The presented functions have been derived from the results of finite strip analyses and are based on the concept that the full cross-section response lies between the lower and upper bound limits of the critical isolated plates with simply-supported and fixed boundary conditions along the adjoined edges. The developed method of design by advanced analysis is applied to individual columns, beams, beam-columns, continuous beams and planar frames. Capacity predictions are compared to the results obtained from benchmark shell finite element models and conventional steel design. It is found that the proposed method predicts safe-sided ultimate capacities that are consistently more accurate than current design methods, particularly for structures benefiting from strain hardening and for structures composed of non-compact cross-sections that previously were not able to benefit from inelastic redistribution.