Quick Points:

• All-precast twin tower rises 190 ft. above grade with three observation platforms
• Self-consolidating concrete with stain
• Elevator in one tower and stairs in the other

Contents:

Summary
Legacy memorial project in self-consolidating precast

Structure
Wind tunnel testing to determine performance

Challenges
Unique geometry required inventive solutions

Legacy memorial project in self-consolidating precast

Commemorating the 200th anniversary of the Lewis & Clark Expedition, the $4.8 million Lewis & Clark Confluence Tower consists of twin observation towers, a gift shop, restrooms, fountains and picnic areas. Visitors are treated to views of the confluence of the Mississippi and Missouri Rivers where the historic expedition began.

The towers rise to 19 stories, with precast panel fins that increase in size to make the towers wider at the top for a sense of openness. Observation platforms are located at 50, 100 and 150 feet above grade, and each increases in area with elevation.

The original concept for the towers called for a cast-in-place concrete core structure using architectural precast panels for cladding to create the fins. Working with High Concrete Group, KAI Design Build converted the design to an all-precast structure based on analysis of performance and cost.

The structure was cast with self-consolidating concrete, a highly flowable material that facilitated the fluted look on the tower sides. The sides were then stained to the desired color.

Wind tunnel testing to determine performance

Because of the structure's aspect ratio (each tower has a height to least width ratio of 13.8), the design team decided that wind tunnel testing was needed to determine response characteristics in across-wind loads and dynamic torsional loads caused by vortex shedding. The testing also determined the accelerations at each deck level during a wind storm for comparison to acceptance criteria for human comfort. Fifty wind load combinations identified the structure's response to various wind directions, modal coupling, correlation of wind gusts and the directionality of strong winds in the local wind climate.

Unique geometry required inventive solutions

While the stair tower had internal diaphragms that maintained their square shape, the elevator tower had to have special connections between the observation platforms and the wall panels. The platforms were made in two pieces that were joined and supported from a single center connector beam.

To connect electrical conduits across the horizontal panel joints, flexible tubing enabling electrical splices to be made allowing for adjustment for obtaining proper alignment.