Angus L. Macdonald Bridge Expansion

O’HALLORAN CAMPBELL CONSULTANTS

The 1950s-era Angus L. Macdonald Bridge is the oldest of two bridges spanning Halifax Harbour across to Dartmouth in Nova Scotia. The suspension bridge measures 762 metres, plus approach spans of 437 and 148 metres. It was designed by Dr. P.L. Pratley, P.Eng., whose son, Hugh Pratley, P.Eng., was associated with the prime consultant, O’Halloran Campbell Consultants of Halifax, for the recent expansion project.

Though the A. Murray MacKay Bridge had opened in 1970 as a second crossing to Dartmouth, by the late 1980s traffic volumes had increased so much that both bridges were clogged during rush hours. O’Halloran Campbell’s studies showed that the most cost-effective solution to ease the congestion was to widen the Macdonald bridge to accommodate a third centre lane on which the direction of traffic could be reversed depending on the peak traffic flow.

The existing bridge deck had two 4.1 metre lanes, plus a 1.5 metre sidewalk on one side, and a 1.5 metre ductway on the other. To make room for three traffic lanes, the engineers designed the road deck to expand over the areas previously occupied by the ductway and sidewalk. The roadway is now 18 metres wide, about 50 per cent wider than the original. The designers then cantilevered a new sidewalk and bikeway off the deck’s main carrying members, and installed the services in a carrying tray on the underside of the deck. Testing at the University of Western Ontario Boundary Layer Wind Tunnel Laboratory had found that there was very little change in torsional flutter wind speed between the original cross section and the proposed symmetrical section with the added sidewalk and bikeway.

The engineers replaced the concrete bridge deck with an orthotropic steel plate deck (OSPD). It consists of a steel plate that spans simultaneously in two directions, laterally between the webs of stiffening ribs, and longitudinally as the top flange of the beam system formed by the deck plate and underlying ribs.

Two chief factors favoured the orthotropic steel plate deck. First, it is relatively lightweight, and thus meant that the engineers could keep to a minimum the amount of strengthening that had to be done to the main carrying members. Secondly, the OSPD is a prefabricated panel, which simplifies the field installation.

It was also important that the upgrade program did not increase the total loads — both dead and live — on the suspended spans. To further control the weight loads, the project adopted high technology products such as lightweight concrete, fibre-optic cables, which are lighter than copper, and a lightweight epoxy-based wearing surface. Jacques Whitford consulting engineers helped to do the extensive materials testing and evaluations. Fortunately, the concerns with weight loads were eased by the fact that the bridge was originally designed for a 750-mm watermain, but a 600 mm diameter line had been installed. As well, since the Macdonald Bridge was no longer on a truck route, the live load had been reduced from the original design. Overall, after the expansion, the total design load on the suspended spans was within 2.5% of the original design load.

Night work

Traffic — up to 40,000 vehicles a day — had to keep flowing over the bridge during the weekdays through the two and a half-year construction period. By using the orthotropic steel plate deck system, the contractor, Walter/Cherubini, was able to work at night, removing sections of the concrete deck piece by piece and replacing them with the orthotropic steel plate deck panels, which were prefabricated off-site. Preparations for replacing the deck were made during the day, such as installing staging, busting rivets and making connections to the underlying support girders and trusses. The five-metre panel was inserted overnight, and by 5.30 a.m. traffic could move over it. During the following days, the contractors did final bolting and removed the redundant top lateral bracing, etc. Over seven months 128 deck panels were installed.

The overall scope of the $62 million project went far beyond the work done to the bridge and approach spans. It included major improvements to the bridge approach roads, modifying the Dartmouth toll plaza, building a new ramp and overpass to access the bridge, and reconstructing a bridgehead in Dartmouth. The 45-year old crossing was also fitted with electronic tolling, variable message traffic management signs, surveillance cameras, and a computerized ice-detection and weather monitoring system.

The team completed the work in October 1999, one month ahead of schedule and $700,000 under budget. The bridge now carries 1,000 more vehicles per hour than it did previously and rush-hour commuters between Halifax and Dartmouth have cut 15-20 minutes off their travel times.–BLCCE

Client: Halifax-Dartmouth Bridge Commission

Prime consultant/engineering/environmental/landscape: O’Halloran Campbell Consultants, Halifax. Project team leaders: Dan O’Halloran, P.Eng., Marcel Deveau, P.Eng., Jon Eppell, P.Eng., Mark Garnett, P.Eng., Steve Eisan, P.Eng.

Mechanical & electrical: Morris & Richard

Materials testing and quality control: Jacques Whitford & Assoc.

Specialist consultant: H.H.L. Pratley, P.Eng.

Contractor: Walter/Cherubini Joint Venture