Source: KAI Design & Build, St. Louis
The Lewis & Clark Confluence Tower in Hartford, Ill., will be dedicated September 23, the date the 19-story structure's namesakes completed an early 1800s expedition for the United States westward expansion. Built at the Missouri River and Mississippi River confluence, about 20 miles outside St. Louis, the slender tower combines stairway and elevator shafts of precast panel fins, connected by three structural, lookout platforms. The 180-ft. shafts represent Lewis & Clark; the platforms, set at 50, 100 and 150 ft., mark their expedition stages.
The original concept called for a cast-in-place concrete core with architectural precast fin panels. Working with High Concrete Group, architect KAI converted the design to all-precast (265 pieces total) based on a performance and cost analysis. Each structure's 21 levels are formed from four panels, enclosing 11.25 ft. square shafts. Fin panel widths increase from 13.5 ft. at the base to 18.25 ft. at the top. The precast members are secured with grouted, horizontal spliced sleeves, plus vertical, welded connections at corners. Panels were fabricated in High Concrete's Springboro, Ohio, plant with self consolidating mixes, netting a fluted, stain-ready surface along the tower sides.
A 13.8 (height to least width) aspect ratio prompted wind-tunnel testing to determine the tower's response characteristics in across-wind loads and dynamic torsional loads caused by vortex shedding, notes High Concrete Engineering Coordinator Lyle Bowman, P.E. Testing also determined lookout-level accelerations during a wind storm for comparison to human-comfort acceptance criteria. Fifty wind load combinations identified the structure's response to various wind directions, gust correlation, and strong, local wind directionality.
The $4.8 million Lewis & Clark tower opened in May following eight years of fund raising and construction phases. The project team is pursuing additional funds to cover site amenities and a 350-seat ampitheater.