At 800 Fulton Market in Chicago, the design and engineering team replaced the complex sliding mechanism used at 100 Mount St. with a simpler geometric mechanism; the central node of the braced bay is displaced 24 inches out of the plane defined by the four corners of the braced bay. When the braces lengthen and shorten, the central node automatically moves inwards or outwards to reconfigure the geometry without inducing any forces in the bracing.
simply displacing the central node of the braced bay 24 inches out of the plane defined by the four corners of the braced bay. The four diagonal brace members in each bay form the edges of a shallow, horizontal pyramid. When the reinforced concrete columns eventually shorten due to creep, the central node simply moves further out of plane, forming a slightly ‘taller’ pyramid. Similarly, when subjected to temperature variations, the braces lengthen and shorten and the central node automatically moves inwards or outwards to reconfigure the geometry without inducing any forces in the bracing. In this way, the forces acting on the braces are limited to wind, which are equal and opposite axial forces, thereby stabilizing the geometry of the central node. When one diagonal is in compression and tends to buckle outwards, the other diagonal is automatically in tension, holding the central node in position. The detailing of all the connections in the brace system allows this geometric reconfiguration with only elastic deformations of the steel plates. Instead of a pin-jointed hinging mechanism, a high-ductility “hinging plate” is used at areas of maximum deformation demand. The hinge is formed by a simple rectangular steel plate with the stiffness of the hinge designed by optimizing the height, length, and thickness of the plate. The design challenge of the hinge is to minimize its flexural stiffness to reduce stresses induced as the brace geometry reconfigures, while maintaining adequate strength to transfer the required shear between the two halves of the node. The plate also is designed to withstand the expected deformations elastically and at stress levels and cycle counts that mitigate any concerns about fatigue in the plate itself. To alleviate concerns about fatigue in the welds connecting the hinging plate to the remaining parts of the steel node, the hinging plates are machined down to a dog-bone profile from a thicker plate, significantly reducing the stress levels at the welding points. Beyond the hinging plate, the geometry of the nodes is all determined by analytical optimization. A deliberately oversized design space is analyzed and the material in areas of low stress is removed, until only the useful material remains. The sculptural nature of the nodes is an honest expression of the force flow through the nodes. SOM has a long history of designing buildings through an integrated approach, producing engineering solutions that are expressed in the building architecture. These buildings’ designs enable an inherent understanding of how they work, and they tend to be well received by the general public. 800 Fulton Market is a prime example, with its state-of-the-art integrated design and research-driven structural systems that are carefully detailed to optimize performance and artfully expressed in the architecture.■ Aaron Mazeika is a Structural Engineering Principal with SOM. Licensed both as Structural Engineer and Architect, Mazeika focuses on an integrated approach to design that seeks to infuse SOM's architecture with innovative and efficient structural systems.
The functionality of the central nodes is provided primarily by the central hinging plate that flexes allowing the release of temperature and column creep strains. The sculpted form of the nodes was determined based on a stress analysis of an oversized design space.
46 STRUCTURE magazine