Trinational Tower Design Project
Team: Steven Cerri, Yu Zhang, Jassica Duke
This project required each team (3 structural engineers) to design a tower with constraints on minimum height and minimum areas for retail, office, residential and parking. The site was at the corner of Boston Common in Massachusetts, with dimension about 310ft by 160ft. To satisfy the programmatic requirement, our team proposed a combination of a RC tower, a podium and a bundle with more than 60 floors. Structurally, two distinct lateral systems (a dual-core system and a core-outrigger system with belt-truss) were used according to the plan geometry of this building. Based on the building shape and slab spans, a 9-inch post-tensioning system was decided upon in order to reduce the thickness of the slabs at every level not located in the podium. Due to thin slabs, punching shear was a strong factor in our final building design and was thus carefully checked.
The figure above shows the programmatic partitioning of the tower.
The above figures show the core sizing and designs of critical columns.
In the stump direction a dual core system, with reasonable thickness, was sufficient to resist the lateral loads and to satisfy serviceability in this direction. In the sail direction, the lateral forces were larger and there was less structural depth available in the core. Thus a core-outrigger system (with belt truss) was used to increase the inertia in this direction, and to avoid having excessively large columns in the condominium portion of the tower.
There are four typical floorplans shown from the above figures. From the bottom to the top, the retail center starts with 12 elevators and 4 stairwells. For the convenience and efficiency of both types of users, the four elevators at the exterior sides will only be used for the office section and thus only go up to the top floor of office sections. In the condo sections, balconies are designed for several types of apartment fit with different preferences.
The figure above shows the detail of connections in the outrigger system, where the shear force is transferred by embedding a steel w section in the concrete column. The shear is then transferred between the steel section and the column using sear studs which will be welded onto the embedded column. The embedded column will be connected to the outrigger with a simple shear tab since we do not want any moments to be transferred from the outriggers into the columns. Conversely, moments do need to be transferred from the core to the outriggers. It is for this reason that the outrigger will have an x-framing when it reaches the core. This will increase the area for shear transfer between the core and the outriggers. The framing will also have shear studs on it to transfer the shear from the core to the outrigger.