Tree Beam Continuum Element: Two Case Histories

The development of a unique lateral load resistant system for two tall and slender office towers is described. System design development is documented for a 52‐level downtown office tower with a frame aspect ratio (height/least width) in excess of 6.85 and a 45‐level suburban office tower with a frame aspect ratio in excess of 5.50. Due to limitations imposed by the buildings' configuration, conventional structural systems proved ineffective in resisting the applied hurricane loading. The tree beam continuum element was developed whereby a rigid girder with intermediate vertical cross arms above or below, or both, was connected between the braced core and the perimeter columns. The cross arms introduced a point of reverse bending in the wind girders, thus greatly augmenting their strength and stiffness. The tree beam system and a variation known as the staggered tree beam system provided a continuum element that elastically constrained the perimeter columns to resist overturning forces. The tree beam concept common to the two structures is explained in detail. Critical design aspects to be considered in the detailing of the “tree beam modules” are presented.