SUMMARY
Three examples incorporating some innovative approaches used to solve bridge design and rehabilitation problems are described. The first example deals with the rehabilitation of a 168 m steel girder bridge damaged due to undermining of a pier by scour. The stresses were relieved from the superstructure, which was then used to support pile driving equipment. Pile bents were installed immediately upstream and the superstructure was transferred over onto new pile bents. The second example involves modification of a bridge to act as a water retaining structure under flood conditions. The third example involves an interchange with 65 skew. The economical solution was to provide a bridge with square ends and longer spans. Post-tensioned concrete diaphragms were cantilevered at piers to support the exterior steel plate girders.

INTRODUCTION
Consulting bridge engineers in North America, and perhaps worldwide, are usually working under budget and time constraints. They are usually competing with each other to secure new design and construction assignments. The clients, usually public agencies or large corporations, are looking for added value such as decreased initial and life-cycle costs, shorter construction schedule, less adverse effect on the environment and aesthetics. In order to meet or exceed the expectations of the clients, given the time and budget constraints, innovative solutions are called for. The survival and growth of a consulting engineering business depends on the ability of the engineers to provide creative solutions.

This paper presents three examples where innovation has been used to meet the client’s needs within the given constraints.

1. SMOKY RIVER BRIDGE

1.1 Background
Located 40 km south of Grande Prairie, the Smoky River Bridge handles up to 10,000 log-haul vehicles during the harvesting season. In July 1997, a flood rendered the 168 m long bridge unsafe for vehicles. Without the bridge, log-haul traffic would have to make a 130 km detour. The client required restoration of the bridge before harvesting operations started in the fall.

In July 1997, subsequent to a flood estimated at 2300 m³ /s, a deep scour hole was created in the river bed immediately upstream of the west pier. The pier was undermined, became unstable and subsequently tilted and subsided into the scour hole, dragging the superstructure with it. The bearings withstood the force of resistance offered by the superstructure, but in doing so, considerably distorted the deck and severely damaged the plan cross bracing. The pier was still attached and the scour hole has been subsequently infilled. The pier still offered support to the downstream girder but pulled down the upstream girder which now spans from the East pier to the West abutment. The damaged bracing, settlement and induced plan curvature made the bridge unsafe and unusable for use by the design vehicle.

Neither the pier nor bridge superstructure were irrevocably damaged by the movement and it was considered plausible to reinstate the bridge by forcing the deck back to its original level and alignment and replacing the damaged bracing elements.

The pier could be re-used but would be required to be stabilized, or alternatively replaced.

The bridge restoration solution involved using the existing bridge as a work platform to construct new piled foundations, and to raise and move the superstructure onto the new piled piers and abutments. This type of construction had not been previously executed on such a large bridge structure, which challenged the project team to devise an innovative construction plan and solution for temporary works. Adopting a fast-track approach, detailed design was completed concurrently with construction, saving approximately six weeks compared with the traditional design, bid, construction approach.

1.2 Design and Construction
Before starting any construction from the bridge deck, the superstructure was leveled and straightened. Damaged plan bracing was removed, and the girders were leveled above the west pier by jacking the girders off the bearings. To act as a temporary support and sliding surface, a skid-beam was fixed to the west pier. A Teflon sheet was placed on the skid-beam, and the girders were jacked onto new temporary sliding supports. Once this was complete, the girders were jacked laterally to straighten the bridge. The buckled and torn bracing elements were then replaced.

Once the girders were realigned, the bridge was used as a work platform for a crane to install piles for new piers (Figure 2). Piles were driven from the bridge deck, reducing instream work. A steel diaphragm was designed which also acted as a driving template to ensure that the piles were accurately positioned. The first pile driven beside each existing pier acted as the guide-pin for the installation of the steel diaphragm that encased the piles and formed the new pier. Diaphragms were fabricated in sections, transported to site, then bolted together. The modular nature of the templates allowed them to be quickly assembled on site while the piles were being driven. Each pier diaphragm was then lowered over its guide-pile and used as a template for driving other piles. Finally, the steel diaphragms were filled with concrete, eliminating the need to build expensive formwork and cofferdams, and reducing the risk of spilling concrete in the river. (Figure 3).

The use of a precast concrete pile cap allowed the work to proceed at a faster pace than casting an integral cap. The contractor easily positioned the cap from the bridge deck using an 80 ton crane. Holes were left in the pile cap above the piles to allow them to be cleaned and socketted.

The bridge was then raised 25 mm at all locations but the west pier, so that the bearing could be removed, and jacked sideways until the frame column was in position. The connection at the west pier was then completed. The temporary column on the bridge deck was removed, and the bridge was jacked sideways until it reached the new substructure, 10 m upstream. At this stage, the bridge was raised so that it could be moved onto the new foundations. The bridge was adjusted to its final alignment, and lowered onto the new bearings.

1.3 Environmental Impact
To restore the Smoky River Bridge, Canfor elected to construct a new substructure for the bridge. Although not the lowest cost, this alternative provided the best protection against future damage by scour and floods, and had minimized environmental impact. By using the existing bridge as a work platform, no construction other than pile driving was required in the river. In addition, this design increased the waterway opening so that the 100-year return flow will pass through the bridge with sufficient freeboard. Demolition of existing piers has been delayed until a suitable construction window is available, also to minimize the impact on the Smoky River.

2. HEART RIVER BRIDGE AT PEACE RIVER
The Town of Peace River downtown area flooded during April, 1997 during an ice jam in the Peace River. (Figure 5). The Peace River flood backed up the Heart River and caused the high water to spill through an opening in the Town dike at the Heart River bridge. The Town dikes have been raised several times, most recently in 1996. However, the deck elevation of bridge over the Heart River, which was constructed in 1956, is lower than the top of the dike elevation. As an interim measure the Town used to temporarily close the breech whenever a flood is imminent. Closure was not possible during the 1997 flood. As a result, the river spilled out through the opening in the dike left by the bridge.

A more suitable permanent solution was desired.

The Town of Peace River invited proposals to raise the bridge deck elevation to be at the top of the dike. This involves raising the bridge by 1.28 m on the south side and 2.0 m on the north side and construction of a ramp on the north side to accommodate two lanes of traffic. Location and elevation of approaches presented special challenges since the bridge is located in a heavily developed area. The western portion of the approach road at the north side had to remain at the existing elevation. Therefore, retaining walls were required at each side of the ramp. Access from the adjacent east-west road had to be closed.

An alternative to raising the bridge is to provide a "gate" at approach to the bridge on the north and south sides, which will be closed (manually or automatically) when water level exceeds the bride deck elevation. Alternatives for pedestrian and emergency vehicle access are required in case the "gating" option is used. Some of the disadvantages associated with the gating option are:

1. It is not a permanent solution.
2. Needs manual intervention if it is not automatic.
3. Needs regular maintenance and testing.
4. Emergency access measures are expensive.

The author presented an alternative solution to maintain the existing level of the bridge deck. This involved modifying the bridge structure to retain water using barrier walls thus closing the breech in the dike. The details are shown in Figures 6. This alternative involves placing a cast-in-place concrete slab varying in thickness from 500 mm at the north end to 300 mm on the south end, on top of the existing bridge deck. The existing bridge rails will be removed and a barrier wall will be built monolithic with the new deck slab. The barrier walls extend into the dike to close the breech. The new deck slab will be connected to steel girders using shear connectors so that the structure acts compositely. The superstructure thus modified will be able to withstand the uplift during ice jam flood conditions assuming a design high water level at the existing top-of-dike. Uplift restraints should be provided at piers and abutments. The expansion joints at each abutment will be augmented by water retaining seals. Depending on the soil conditions, this alternative may necessitate some additional piles at the piers in order to carry the increased load from the structure. If the dikes are raised in the future, the barrier walls could be raised and the deck could be further thickened to resist the increased uplift forces.

The effect of this alternative on the adjacent properties and roads is small. Local regrading is required at the north and south approach road intersections. The traffic pattern remains the same as existing.

The Town council voted to adopt alternative 2. The bridge modifications were completed in February 1998.

1. A single span bridge with Mechanically Stabilized Earth Walls.
2. A three span bridge with skew ends.
3. A three span bridge with two box girders.

Each of these alternatives presented different problems. The soil conditions at the site were such that MSE walls would induce high settlements and also difficult to construct in winter. The three span bridge with skew ends would require long expansion joints and other maintenance problems. The three span box girder bridge would be an elegant solution but was not amenable for future widening.

After some iterations, an innovative solution was arrived at. This involves constructing a three span bridge with square ends. The dimensions of the pier column had to be restricted so that they do not encroach the required horizontal and vertical clearances. This meant that the exterior steel girders had to be cantilevered off an integral pier diaphragm. (A pier cap beam below the girders would have infringed on the clearance requirement).

An integral steel diaphragm was considered. This would involve complete moment connection details at the site and future widening would be more difficult. A post-tensioned concrete diaphragm offered advantages such as ease of construction and future extension. (Figure 8). Ducts for future post tensioning rods were incorporated in the diaphragm.

This bridge was completed in September 1997 and is functioning well.

4. ACKNOWLEDGMENTS

A team approach was used in arriving at and executing these projects. For the Smoky River Bridge, Martin Jobke, Steve Croxford, Tamer Akkurt and Julien Henley were involved in developing solutions along with the contractor, Surespan Construction Ltd. Lianna Mah contributed to the write up of sections dealing with this bridge. For the Heart River Bridge, Steve Croxford, Helder Afonso, Michel El-Khoury and Dusanka Stevanovic were involved in the detailed design and construction. Larry Bodnaruk performed the hydraulic calculations for both bridges. For the flyover, Bob Ramsey was involved in the original review and concept development.