There were concerns that the asymmetry of the transverse cross-section of the bridge could cause excessive torsion in the towers and that torsional rotation in the towers might create difficulties during construction. In addition, we had to examine whether the cantilever portion of the main span deck should be erected concurrently with or subsequent to the erection of the mainline. We performed a structural analysis to address these two concerns specifically.

Torsion in Tower and Deck

The first order of business was to examine the layout of the cables in the tower top. The cables on the east side of the main span, which has the cantilevered ramp, are approximately twice the size of the west side cables, carry double the loads and are designed to be balanced by only a single back span cable. To minimize torsion in the tower top, the cable anchorage points were strategically shifted transversely, reducing the dead-load torsion in the tower top to negligible levels. The tower top would still experience live load torsion, however, and the tower legs would still be subject to both dead and live load torsion.

Figure 1: Tower Rotation Due to Dead Load.

Nonlinear dead load analyses alleviated concerns that torsion in the tower will be problematic. It showed that the torsion can be accommodated in the design with normal reinforcement. We also concluded that the torsion in the tower due to the unsymmetrical cable forces will not cause any practical concerns during construction.

The rotations in the tower for the hybrid alternative are very small (Figure 1) and well within construction tolerances. Results for the steel and concrete alternatives were similar. (LARSA was used to analyze the steel and hybrid alternatives and GTSTRUDL was used for the concrete alternative.)

Constructibility Issue: Erecting the Cantilevered Ramp

We analyzed two sequences for erecting the cantilevered ramp:

• Concurrently with the main deck
• After the main deck erection is complete.

Ramp erected concurrently with main deck. The computer model used for the nonlinear dead load analysis was modified to analyze the north half of the structure just before closure at mid span. Supports were added to cable connection points at the deck level in the back span to simulate shoring during construction. Superimposed dead loads were removed and cable self-weights were reduced to account for the absence of grout.

The analysis showed vertical displacements of the east and west sides of the deck at mid-main span to be about equal and the deck remains essentially level. With the hybrid alternative, for example, the west side deflected up 426 millimeters and the east side deflected up 427 millimeters.

Ramp erected after main deck. The ramp structure was deleted from the previous model to examine the behavior of the structure without the ramp just before closure. Analysis of the hybrid alternative showed that the mid-span deflection of the west side will be up 483 millimeters and the east side will be up 717 millimeters.

Concurrent Construction Preferred

We concluded that concurrent construction of the cantilevered ramp would be the preferred method of the two, although construction could be done either way. Concurrent construction would allow for more efficient use of equipment and provide better control over stay forces and deflections. If the ramp is erected after the main structure, the proximity of the existing locks will greatly reduce the working space of the barge-mounted cranes required for erecting the ramp structure.