The twin towers of the Lewis and Clark Confluence Tower work together to achieve a unique design and a capacity for withstanding severe seismic stress and wind loads.
By Deborah R. Huso
In 2000, with the bicentennial celebration of the 1803-1806 expedition of Meriwether Lewis and William Clark approaching, the village of Hartford, Ill., wanted to mark the event in a meaningful and enduring way. It was there, at Camp River Dubois, that Lewis and Clark launched their “Corps of Discovery” exploration of the Louisiana Purchase within sight of the confluence of the Mississippi and Missouri rivers, the third-largest river confluence in the world.
The village held a series of ad hoc community meetings to design a tower that would offer views of the rivers’ confluence and also memorialize the historic partnership between the two explorers.
The project was funded entirely by grants and private donations, but because of the challenges of acquiring the funding, the confluence tower, associated park area and museum, took many years to complete. Even though the village of Hartford broke ground on the project in 2002, it was not officially completed until 2010. The towers stood, fully erected, for several years without handrails on the stairs while the village of Hartford worked to acquire more funding to complete the entire park project.
But this was to be no ordinary tower. It needed to reflect the symbiotic relationship of the two explorers it sought to memorialize. The friendship and partnership between the two explorers was the basis for the tower’s design.
Consisting of two shafts of equal size and height, one representing Lewis, the other representing Clark, the towers rise 19 stories or 190 ft with one shaft containing an elevator, the other a stairwell. Three viewing platforms join the three towers at 50, 100 and 150 ft, symbolizing the working partnership of the two explorers.
Apart from serving as lookouts, the viewing platforms also serve a structural purpose: They allow the tower shafts to work interdependently much like the explorers they symbolize.
In an unusual design, the precast concrete panel fins that achieve the tower’s fluted look increase in size as tower elevation increases, establishing a sense of openness at the top of the structure. To create the fluted look, High Concrete Group LLC of Denver, Pa., the precaster for the project, cast the panels using self-consolidating concrete (SCC).
There were only a few different panel shapes and sizes in the project, with the structural panels that make up the towers’ sides accounting for more than 170 of the pieces. These panels were more than 1 ft thick and consisted of varying dimensions of the same shape that increased slightly in size to create the fluted expansion of the towers as they grew higher.
Connecting all of the panels offered a small challenge. The design-build team had to consider how to connect the rebar between the precast panels. The idea was to vertically align the connecting precast panels by aligning the splice sleeves at the base of each panel with the projecting vertical reinforcing bars at the top of the precast panel below. The structural engineers considered the joining of the panels critical to the ability of the tower to withstand seismic stress given its location within the New Madrid seismic zone.
Like the towers, the viewing platforms increase in size as the tower height increases. The lowest one is 32 ft across and the highest, at 150 ft, is 36 ft across. The platforms were pivotal to the tower’s structural design, particularly for maintaining the square shape of the elevator tower. Unlike the stair tower, it did not have internal diaphragms. Because it is so difficult to attain a moment connection at the inside corners with vertical precast panels, the connection of the three observation decks to the tower wall panels helped to maintain the square shape. The decks also keep the towers from rotating in high winds, removing the potential for structural twisting.
The decks themselves presented problems because of their size. As one precast unit, each deck would have been too large to transport by truck, so each one was cast as two pieces with support from a single center connector beam that spanned the space between the two towers. Both platforms are semi-cantilevered as well.
The tower’s tall and narrow design, however, promised challenges with regard to wind-load carrying capacity as well as response to seismic activity. Before tower construction even began, designers subjected the structure’s plans to wind testing to make sure the design could withstand severe weather and geologic events and to establish the human comfort level at the various viewing platforms in the event of high winds.
The completed structure consists of 256 precast panels weighing in at about 4 million lbs, and comprising 24,700 sq ft. The tower portion of the project cost $4.8 million with the cost of the precast accounting for about $1.35 million.
The Lewis and Clark Confluence Tower held its dedication ceremony Sept. 23, 2010, on the anniversary Lewis and Clark returned to the site at the close of their journey to the Pacific Coast.
Deborah R. Huso is a freelance writer who covers home design and restoration, sustainable building and design, and home construction.