By Acres Manitoba
Manitoba Winter Road BridgesEngineering
Manitoba's Winter Road Network is over 2,000 kilometres long in total and serves 25 First Nation communities in remote northern areas. Without the roads, many communities are accessible only by air, w...
Manitoba’s Winter Road Network is over 2,000 kilometres long in total and serves 25 First Nation communities in remote northern areas. Without the roads, many communities are accessible only by air, which is an expensive means of transportation.
Basically consisting of hard-packed snow and ice, the winter roads were originally created by contractors in the 1950s as a means to cross land and lakes. At that time the transportation vehicle was a string of sleighs pulled by track bulldozers. In 1971 the province of Manitoba assumed the responsibility of constructing and maintaining the network.
Each type of terrain presents difficulties, but the stream crossings are particularly critical. If a single crossing becomes impassable, the entire road must be closed and vehicles and their drivers could be trapped until the next season.
The stream crossings have traditionally been constructed using either “corduroy” or ice bridges. Corduroy bridges are built by laying logs and culverts parallel with the stream flow. Successive layers of logs, snow and water eventually create a frozen mass strong enough to support vehicle traffic. Ice bridges are similar but without culverts. Both types of bridges take time to build and then remove, often leaving an unsightly mess behind. As well, their load-carrying capacity can vary substantially over the season.
Manitoba Transportation and Government Services asked Acres to develop an alternate stream crossing design.
Acres produced a generic bridge design for use in multiple locations, in crossing widths of 20 to 40 feet (6 to 12 m). It is designed for HS-25 truck loading, suitable for all vehicle types and loads allowed on the winter road network, where truck weights of up to 37,500 kilograms are allowed. (Bridges on the winter road network are governed by the same codes, safety standards and environmental regulations as conventional bridges.) The bridges sit above the stream’s high water mark, are easy to construct using typical local labour forces and equipment, and are cost effective.
Acres’ team found that some solutions to the design challenge actually lay in the features that make the Winter road network unique. For example, it was difficult to design a generic abutment since the conditions at every crossing were unknown. The team decided that since the bridges are only used when the ground is frozen and capable of providing substantial load carrying capacity, they would use simple timber cribs filled with granular material from local sources for the abutments. Also, since the bridges are for single lane traffic, and since speed and spacing between vehicles is restricted on the road network, the design could allow for reduced vehicle dynamic and braking forces.
In selecting the bridge material, cast-in-place concrete was discounted because of its high cost and the logistical problems of pouring at remote locations in mid-winter. Pre-cast concrete was eliminated because of its weight and associated handling issues.
The selected design is a composite bridge with environmentally friendly pressure-treated timber cribs, steel girders, timber deck panels and a timber wearing surface. Commonly available and reasonably priced 6″ x 6″ timbers combined with steel support beams were chosen as being light and easier to handle on site. Where spans longer than 40 feet are required, multiple shorter spans are combined.
A single size for beams is used whatever the length of the span in order to standardize the drawings and avoid errors. Indeed, standard section sizes and shapes, as well as stock lengths are used for as many components as possible. To ease the installation, the bridge and all its components are symmetrical and use only one size of bolt throughout.
To address the complex problems of large concentrated wheel loads acting on the timber beams, the deck has 3*-thick wearing planks along the wheel paths. The 12* -high timber guardrails are designed to give the drivers visual alignment and not to interfere with snow clearing equipment. The guardrails are “sacrificial” and can be replaced using basic repair tools.
Construction made simple
The components for an entire bridge can be transported on a single flat bed semi-trailer, which reduces shipping costs.
The Ministry wanted complete flexibility in how any project could be executed. Acres responded by separating the timber, steel and assembly drawings into three packages. One set of drawings can be used for supply only, fabricate only, or supply and fabricate contracts.
The fieldwork for an entire bridge can be completed within two working days using a crew of only seven people with a large backhoe and dump truck. By the end of February last year, 14 of the bridges — affectionately nicknamed “Meccano” bridges by the Ministry — had been installed successfully. To date they have been responsible for extending the season where the Winter Road Network is operating by 50-75 per cent in the northwest region.
Client: Manitoba Transportation & Government Services (Don Kuryk, Ron Richardson, P.Eng. and Larry Halayko, P.Eng.)
Prime consultant, project management & engineering: Acres Manitoba (A. Douglas McAndrew, P.Eng., Raj Mannem, P.Eng., Tom Hayhurst, Blair Fraser, P.Eng., Danny Zaborniak, EIT).