Secondary bracing systems for diagrid structures in tall buildings

• We analyze the global and local behaviour of megadiagonal structures for tall buildings.
• We present a methodology for establishing the need for a specific secondary bracing system (SBS).
• A design procedure for secondary bracing systems (SBS) is proposed.
• The structural responses of diagrid structures without and with SBS are compared for some 90 story building models.

In this paper the authors define a framework for assessing the “local” structural issues in the design of diagrid tall buildings, and present a methodology for establishing the need for a specific secondary bracing system (SBS) as a function of the diagrid geometry. Further, design criteria for secondary bracing systems are worked out and applied to some 90 story building models, characterized by perimeter diagrid structures with different module height and diagonal cross sections. The outcomes of the proposed simplified procedures, both for assessing SBS necessity and for the consequent SBS member design, have been compared to the structural response of the diagrid building models, obtained without and with SBS, demonstrating both the accuracy of the proposed formulations and the primary importance of the discussed local questions. In fact, all analyzed diagrid models exhibited problems concerning stability of interior columns (i.e. multi-storey buckling modes) and/or local flexibility (excessive interstory drift); the above local problems are completely solved after the introduction of a SBS at the central core location, and, against a modest increase of structural weight (about 3%), any flexural engagements in the diagrid member is eliminated.