Olson & Nesvold Engineers, P.S.C. is in the process of designing a rehabilitation scheme for the 15th and 16th Avenue S. bridges over the Midtown Greenway.  Each bridge will primarily have the same rehabilitative tasks performed.  In general, the foundations will be strengthened, minor repairs will be made to the piers and abutments, and substantial repairs will be executed on the superstructure. 

During a review, it was observed that the bridges have undergone both uniform and differential settlement.  This has led to significant cracking in the abutment breastwalls, backwalls, and wingwalls.  The abutments appear to be the primary component moving.  Settlement has also occurred on the bridge approaches.  This movement is most pronounced at stub abutments where undermining of the footing has occurred.  To prevent further damage to the bridges and to ensure that the rehabilitation dollars are well spent, the substructure units will be stabilized.  Helical piles will be used to provide additional load capacity and minimize future settlements.  The larger existing cracks in the abutments will be repaired.  Approach panels will be added to each end of the bridges to limit water infiltration to the backside of the abutment backwall (to reduce footing undermining) and to provide a smooth transition onto and off of the bridge for vehicles. 

The superstructure concrete spalls present a safety hazard to trail users below the bridges.  In addition there is concern that once demolition activities begin with the deck and beams it may be difficult to limit the amount of material that needs to be removed.  Adding precast planks between the stems of the tee-beams will satisfy both concerns.  The voids between the existing concrete and the new planks could then be filled with self-consolidating concrete.  This would effectively create a new deck system that would minimize concerns for falling debris hazards on pedestrians and would provide a substantial structural system to support additional rehabilitation work on the superstructure.  Rehabilitation of the railings will be determined based on capacity and concrete quality assessments.  Railing rehabilitation will likely include removing and replacing the top cap of the rail.  Additional rebar will be added to improve crash performance, while not changing or affecting the original look/detailing.

With the superstructure stabilized, the deck and sidewalks on both bridges can be rehabilitated.  They would be milled and any existing bituminous pavement on the deck of the bridges would be removed.  The milled surface would be repaved with a low slump concrete overlay.  The concrete overlay will greatly reduce the amount of salt laden water that can infiltrate and damage the historic concrete components.

Below is an example of the High Density Laserscan data that we use to facilitate the rehabilitation process.