Access to the shipping channel of the Ohio River must be maintained during construction of the Owensboro Bridge. The cable-stayed spans of the bridge were designed to be constructed by the balanced cantilever segmental method, which ensures that ship traffic lanes will remain clear and free of temporary construction supports.

The Balanced Cantilever Segmental Method

The construction sequence of the balanced cantilever method is as follows:

• The tower is constructed.
• A centrally positioned superstructure segment is erected at the tower and supported by temporary bracing in the area of the towers.
• The remaining superstructure segments are then erected sequentially on alternate sides of the tower until connection is made at the approach pier and finally at midspan.

The dimensions of a typical segment for the Owensboro are the full superstructure width by 13.7 meters in length. A typical segment consists of a steel frame (two edge girder segments with cable connections, three floorbeams, and a center beam) and deck (six precast concrete panels and cast-in-place concrete closure strips placed between deck panels and along the edge girders).

Figure 1: Typical Erection Cycle

Modular Construction

Each superstructure segment is erected in the stages listed below. Typical erection stages are shown in Figure 1.

• The steel frame is lifted into position by a barge-mounted crane and field spliced to the edge girder of the previously installed segment. It is allowed to cantilever freely.
• Two stay cables connecting the steel frame to the tower are installed and tensioned to an initial length.
• The six precast deck panels are installed and the cast-in-place concrete closure strips are placed between them and along the edge girders of the previously installed segment.
• The two cables are tensioned to their final length, and the procedure is repeated on the opposite side of the tower.

At least two-level tensioning of the cables is necessary in order to control both erection and “locked-in” stresses in the deck and edge girders.

Maintaining Balance And Stability

Maintaining a balanced and stable structure during construction cannot be over emphasized. It becomes increasingly difficult to do so as the cantilevered superstructure increases in length. To reduce the bending moments in the tower that are caused by wind loads and construction sequence imbalances, a temporary tiedown is connected to the superstructure of the back span. In addition, counterweights are strategically positioned and repositioned on the deck in order to control stresses in the superstructure.

A 3-D computer model (see “Structural Analysis For The Owensboro Bridge” by Fadi Hamawi and Ruchu Hsu) enabled us to:

• Confirm the constructibility of the structure
• Confirm the adequacy of the superstructure elements to resist the forces induced during erection
• Determine the initial cable tensions.

Utilizing Expertise

The construction sequence was developed partly on the knowledge gathered from studying other cable-stayed bridges in the U.S. and from discussions with steel erectors who have cable-stayed bridge experience. We also profited from the experience of many PB experts in the field.