Jensen Construction Company, a long-time client of Willett, Hofmann & Associates, Inc., has often called upon WHA to design structural falsework and erection sequences for the construction of bridges. The design and analysis for the Lewisville Lake Arch Bridge was an especially interesting and challenging project.

WHA’s task was to determine the construction sequence and to design supports to aid in the erection of the tied arch bridge. The 362 foot span tied arch is the signature element of the bridge.

The analysis consisted of an eleven step procedure to construct the arch piece by piece. Each step was analyzed individually to ensure stability of the system and structural adequacy of the falsework supports. The main elements of the structural falsework included 100’ truss towers setting on 10’ diameter steel shafts, arch rib and beam saddles to hold the arch and tie beam in place until erection was completed, and lifting devices capable of rotating the arch elements to provide proper fit in the field.

Each of the four 100’ truss towers consisted of three segments stacked upon each other: two 40’ towers and one 20’ tower. There are two reasons for the unique, segmented towers. The first reason was to aid in the erection of the towers themselves; a single element 100 feet in length would be difficult to lift and set with barge mounted cranes. The second reason was to provide reuse of the towers for future projects. With an eye on both the economic and the environmental benefits of reusing the towers, they were designed so they could be stacked in numerous configurations to obtain any size tower in multiples of 20’.

The truss towers were setting on 10’ diameter, 11/16 inch thick wall steel shafts. The normal water depth at Lewisville Lake is approximately 45 feet, so the length of the steel shafts were required to be over 110’ long to reach the solid foundation located deep in the lake bed.

At the top of the truss towers are fully adjustable arch rib saddles. One interesting challenge to the design and fabrication of the saddles was that the arch rib was rotated at 10° from the vertical. Not only did the saddles have to be fabricated at the tangent to the arch at the location of the truss towers, but they also had to be fabricated at a 10° pitch from the vertical.

To aid in the placing of the arch segments the saddles could be adjusted up to 10 ½” vertically with the use of four 70-ton screw jacks.

The arch rib saddles could also be adjusted horizontally due to slotted holes provided to bolt the saddle to the truss tower. Near the center of the truss towers the tie beam was also supported by fully adjustable beam saddles.

The arch itself was rotated 10° from the vertical, which provided for a unique situation when it came to picking up the arch segments. Traditional rigging does not accommodate picking up objects that need to be rotated in order to set them in their proper locations. In order to accommodate this issue, a picking saddle was designed to not only lift the arch segment, but to allow the segment to be rotated by adding a “rotation arm” and utilizing the crane’s secondary line.

The design and analysis of the arch erection sequence and construction falsework at Lewisville Lake was an interesting and challenging project, however it was the unique challenges that made this project so intriguing to the WHA design team.

The project was nominated for an Excellence in Structural Engineering Award by a member of the Structural Engineers Association of Illinois in 2010.