Outrigger Systems to Mitigate Disproportionate Collapse in Building Structures

Abstract

Outrigger systems are often used to transfer lateral forces to the foundation. They are particularly designed to control drift in tall buildings due to wind and seismic forces. Progressive collapse resistant design focusses on reducing the risk of the disproportionate collapse associated with the loss of primary load-carrying elements. This paper advocates the use of outrigger systems to control lateral drift in a wider class of structures to benefit from the additional advantage of reducing the potential for collapse associated with the loss of one or more perimeter columns. Properly designed outrigger systems are capable of transferring loads associated with the collapse of a perimeter column to other parts of the structural system, even when the loss is not due to a natural phenomenon such as wind or earthquakes. Toppling of buildings, caused in some instances by the existence of a soft-story, is an example of a disproportionate collapse caused by the failure of perimeter columns. The paper discusses how outrigger systems may mitigate toppling of building structures. The loss of any vertical element may result in reversal of internal axial forces in stories above the notionally removed column from compression to tension. When outrigger systems are utilized to resist lateral forces, vertical elements on the perimeter are naturally designed to resist tension under lateral forces, as well as compression. It is therefore intuitive to consider their design to resist the loss of perimeter vertical elements. This uses a case study to demonstrate the advantage of using outriggers to mitigate disproportionate collapse of building structures.