Optimisation of steel multi-tiered X-braced frames under seismic design constraints

Steel multi-tiered X-braced frames (MT-XBFs) are widely used in tall single-storey industrial structures, where achieving minimum structural weight while satisfying seismic design requirements remains a major challenge. This study presents a genetic-algorithm-based optimisation framework for MT-XBFs that minimises structural weight while satisfying inter-tier drift and member strength constraints. Seismic actions are evaluated using a linear elastic analysis based on the equivalent lateral force procedure in accordance with the American Institute of Steel Construction’s 341-22 standard. The framework is verified using a benchmark industrial frame and applied to three representative case-study frames with heights of 15 m, 20 m and 24 m. Verification is performed through comparisons of global drift, inter-tier drift distribution and member utilisation ratios. Three optimisation scenarios are investigated: fixed tier geometry, equal tier heights and variable tier heights. The results demonstrate that systematic optimisation significantly improves structural efficiency without compromising code-compliant seismic performance, achieving maximum weight reductions of up to 43.8% in the most general optimisation scenario. Although the study is computational, the results are validated against code-based drift and member strength limits, ensuring engineering consistency and practical applicability. The proposed framework provides an efficient tool for the preliminary, code-compliant optimisation of MT-XBFs in seismic regions within a linear elastic design framework.