East River Bridge Demolition

 

Project Description

Opened to traffic in 1956, the East River Bridge is located in Sheet Harbour Nova Scotia along Highway 107. The bridge consists of a 139.9m long main, solid ribbed, two –pin, steel, arch span and built-up steel girder approach spans. The structure is the town of Sheet Harbour’s only crossing across the mouth of the East River, with tidal waters having an elevation differential of 1.80 meters. The crossing is essential to local workers, families, industrial transportation and tourists travelling through Sheet Harbour along the Eastern Shore of Nova Scotia. Following a condition assessment, structural analysis and life-cycle cost analysis completed by Harbourside Engineering Consultants (HEC) in 2010, the owner, Nova Scotia Transportation Infrastructure Renewal (NSTIR) chose to proceed with a demolition and full replacement of the structure as opposed to a rehabilitation of the existing structure.

During tendering, HEC was engaged by Dexter Construction to develop a demolition concept and complete a preliminary design followed by a full detailed analysis and demolition phasing after the project had been awarded. HEC’s demolition experience and knowledge of the existing steel structure played a large role in developing a cost effective and efficient design that ultimately won Dexter the contract. Demolition of the cast-in-place concrete deck, steel deck grillage and steel hangers on the main span was completed from the deck by two excavators, starting at midspan and working outwards in a balanced condition. The demolition of both east and west approaches were completed from land.

The demolition of the remaining steel of the main arch span was more problematic as the original “parent” arches built in 1956 - rectangular box sections constructed from riveted plate and angles, were reinforced with steel plates and channels connected with high strength bolts in 1988. The member cross-section depth essentially doubled on account of the reinforcing and as result, the overall weight doubled that of the original parent arches. Traditionally, removal of this type of a water crossing would be completed using multiple bents or a trestle to support the arches along their length as it is de-stressed, cut and dismantled. This approach was not possible since disturbance of the silty channel bottom was not allowed (environmental concerns) and resulting exorbitant costs. In addition, following the removal of the concrete deck and steel deck grillage on the main span, the remaining arches were still too heavy to install (multiple) reasonably sized steel bents in the river for temporary support during the demolition process. Installing steel bents in the river would also result in the requirement of a costly trestle for access to demolish the arches and an unreasonably large crane to lift the heavy arch sections as they were removed.

As a result of the site constraints, HEC developed an alternative demolition concept that involved using modular, connectible flexi-float barges owned by Dexter to demolish the majority of the main arch span. 18 meter tall steel towers were designed to be supported by two - 18.8 x 24.4 meter barge arrangements, essentially placed at third points along the length of the main arch span to support / remove the majority of the main arch span. Because the stability of the barges was the limiting factor for the design, only the center 72\% of the length of the arch span (530 Tonnes) could be lifted by the barge frames. Additional end bents supported on piles driven to bedrock were also required to support the remaining sections of the arches that could not be supported by the barge frames.

Under low wind conditions, four hydraulic strand jacks (two supported on each barge tower) connected to the arch were used to pre-load the barge frames to the full 530 Tonnes prior to flame cutting the center arch portion for release. Once the load was imparted to the barge frames, the pressure in the jacks and the freeboard in the barges had to be monitored constantly (approximately every 5 minutes) to account for the raising and lowering tides and to ensure the barge frames were holding the correct load. In the event the tide raised and the strand jacks were not adjusted, the risk of overloading the barge and frames could have led to a compromise of strength or stability of the barge frames. If the tide lowered and the strand jacks were not adjusted, the risk of not having the proper load in the barge frames could have led to a compromise of stability following flame cuts and a sudden transfer of load to the to the barges. Once all flame cuts were completed, the center cut portion of the arch was immediately lowered and bolted to steel supports near the deck level of the barge to increase stability of the arch and barge frames.

Following the cut and lowering of the center portion of the arches, HEC was involved in developing custom lift connections and detailed phasing removal procedures for the de-construction of the remaining end segments of the arch span. The lift connections were further used by HEC in the de-construction of the lowered center portion of the main span, which was completed by crane from a nearby wharf trestle. The demolition of the center portion of the arches was also completed by HEC and required the design of a third - 12.2 x 18.3 meter barge that supported a steel frame and was placed between the two existing barge support frames. A detailed analysis of the phasing procedure was required for the de-construction of the center portion of the arches on account of the varying weight the barges were supporting and varying barge freeboard experienced during the de-construction.

Client: Dexter Construction

Year: 2015/2016

Role: Demolition Engineer, Quality Control / Quality Assurance, Services during Construction,