Precast Concrete High Rise Building Designed in Seismic Zone
San Francisco is located in one of the most active seismic areas in the world posing a great challenge to the construction industry.
The 39-story, 133-m tall tower being built at 680 Mission in San Francisco is a residence and commercial building featuring about 500 luxury apartments, retail stores at the lower level and numerous levels of underground parking.
Traditionally steel moment frames are used for high-rise buildings in seismic zones, but for this project the designers ingeniously utilized a combination of precast and cast in place concrete frame with two special connection systems.
First of them is the contractor’s Hybrid Moment Frame (HMRF) which uses a combination of DYWIDAG unbonded multistrand post-tensioning system and grouted reinforcing steel to connect precast beams and columns. High strength strands are inserted into the precast ducts and anchorages and stressed at the column face. Post-tensioning tendons with straight-line configuration are connecting as many as seven bays in a row, or around the corner. The cornering aspect is unique to this project. This layout makes it possible to stress two perpendicular frames simultaneously. Mild steel reinforcing bars are installed above and below post-tensioning tendons in grouted ducts. The reinforcing steel is ductile and will deform plastically during an earthquake, but the post-tensioning tendons will behave elastically and restore the frame to its original position.
The second system used is DYWIDAG Ductile Connector (DDC®) that in this case connects precast columns to cast-in-place beam. This system was also tested for high seismic zones and proved particularly economical for shorter bays.