Design for a 61-Level, Linked, Twin-Tower Structure Built Above Underground Rail Tunnels

Barry Roben
Irwinconsult Pty, Melbourne

The structural design for 308 Exhibition Street produces some unique challenges. The twin towers of 61 and 59 levels, respectively, house a hotel and residential apartments and are connected at levels 45 and 46 by a two-level skybridge. The constraints on foundation loads are significant, due to the presence of four underground rail tunnels under the site and an historic easement agreement that limits the depth permitted for foundations, as well as a peak allowable bearing pressure of 800kPa. In order to meet these limitations, a lightweight structure is required, and overturning moments on the core footings need to be minimized.

Structural design solutions have been prepared for a composite steel/thin-slab concrete floor and for a lightweight concrete, post-tensioned banded slab. These are typically supported on tubular steel/concrete composite columns.

Core moments are being managed by linking to outrigger columns and by rigidly connecting the skybridge to both towers. This creates significant challenges and would normally not be desirable, but wind-tunnel testing has shown that it is essential to manage foundation pressures within the permissible levels.