The ETR 407 and 403/QEW interchange has had over a dozen bridges built over time, showcasing some of the most common bridge designs in the GTA, such as this cast-in-place post-tensioned superstructure.

The definition of a good bridge from a design and construction perspective is a bridge that is durable, low maintenance, and can be built quickly, safely, economically, and with minimal disruption to the public. Although bridges come in all shapes and sizes, over the years several bridge types have prevailed in engineering circles as the most efficient types. Here’s what they are:

A bridge can be divided into two main components. The superstructure is the portion of the bridge up in the air that directly supports the roadway, known as the bridge deck. And the substructure is the portion of the bridge in contact with the ground that supports the superstructure, such as columns, piers, and abutments, as well as their underground foundations (see Figure 1).

Figure 1: The part of the bridge that flies over the roadway underneath is called the superstructure. The columns in the foreground and the abutments at either ends of the bridges comprise the substructure.

To select the bridge superstructure, the designers need to consider the span of the bridge and the difference in elevation between the two roads (over and under). Different girders and bridge decks have different advantages.

Steel box girders (steel “tub” girders) are typically lighter than concrete and can span longer distances without intermediate supports. All steel bridges built today are made of weathering steel, which is highly corrosion-resistant. It has a special chemical formula (including small amounts of nickel, copper, and chromium) that makes it develop a thin coat of stable adhesive rust that actually protects it from further corrosion. The tub shape (because it’s shaped like a “U”) gives it more stability and a higher capacity than the older style I-beams (see Figures 2 and 3).

Figure 2: QEW overpass on Fairview in Burlington has two steel tub girder bridges spanning about 30 m across, while maintaining a relatively low profile — meaning that the thickness of the bridge deck is fairly thin. This is important if the elevation difference between the overpass and the road below it is low. Other bridge options would require the highway to be raised to accommodate a thicker superstructure.
Figure 3: Steel diaphragms connect the girders for additional stability mainly during construction, until the bridge deck is built overtop and locks the girders together (left). Tub girders have access hatches so that they can be inspected inside regularly for maintenance purposes (right). Note the small ports (holes) in the middle of each tub to allow for drainage of any moisture.

These girders require good welders and require some time to build.  Another challenge is that the price of steel is always subject to global market fluctuations unlike concrete, which is mostly made of aggregate, sand, and water — always locally sourced.

Cast-in-place post-tensioned bridge decks are also very common in southern Ontario.  As the name suggests, these bridges are completely built in-place using concrete then tensioned using steel cables in embedded conduits in the concrete.  This increases the capacity significantly, therefore allowing for longer spans between piers.  Another advantage is that they can be much thinner than bridges made of girders, allowing for smaller elevation differences between the bridge and the road below (see Figures 4 and 5).  Sometimes the road above cannot be raised high enough for a girder-style bridge.

Figure 4: The access bridge to the Mapleview Mall in Burlington is a slender cast-in-place post-tensioned bridge. Without the tensioning cables, the bridge would collapse.
Figure 5: A cast-in-place deck needs to be supported along its entire length by shoring towers and formwork until the concrete hardens and can support itself. The outline of the plywood forms can be seen on the underside of the bridge deck (left). Special bridge bearings allow for slight movements of the bridge during temperature changes throughout the year.

The most common roadway bridge built in North America these days is the precast girder bridge. These girders are much quicker to fabricate than steel girders, and they are made mainly from local materials. Depending on how the precast manufacturer’s facility is set up, girders can be cast two or three at a time in one day! Fabrication is much quicker than their steel counterparts. Their average span is typically less than that of steel girders, but they are very efficient and more cost effective than almost any other type of bridge (for your average local highway type bridge). See Figures 6 and 7.

Figure 6: Nothing beats the cost effectiveness of a single-span precast girder bridge as seen here at the 407/403/QEW interchange (left). The ends of the girders are cast into the abutments, locking them into place (right). Built in 2000, these are the youngest of the bridges at this interchange, representing a newer approach to bridge design.
Figure 7: Note the precast concrete deck panels between the girders, eliminating the need for expensive, labour-intensive formwork to pour the concrete deck (left). Also note the gap between the two bridges held up by precast MSE wall panels (see my article “How Things Work: Mechanically Stabilized Earth Walls”). For multiple spans, concrete piers are easily developed to support the girders (right).

The longer the span that is being supported by the girders, the deeper the girders need to be to increase their ability to carry all that weight. Of course, the bridge always needs to maintain a minimum clearance to the road below; therefore, increasing the depth of the girders requires raising the road above. Another way to increase capacity of the deck is to instead shorten the spacing between the girders, making room for more. This is a very flexible and efficient design approach.

As you drive throughout the Greater Toronto Area, or anywhere in North America for that matter, most of the bridges you drive over or under will fit one of the descriptions above. Slowly, over time, the most efficient bridge designs prevail, leaving us with the best in technologies and construction methodology… until something better comes along. Take notice and be safe! 

Fun facts

The longest precast girders ever manufactured were 205 ft long, installed on the Deerfoot Trail bridge project recently built across the Bow River south of Calgary.

Have you seen an interesting building or piece of infrastructure in or around southern Ontario that you’d like Eric Chiasson, your personal engineer, to write about?

Send us your suggestions, comments, or questions to articles@local-news.ca and we’ll see what Eric can find out!

Sources:

Metal Supermarkets. What is Weathering Steel. Url: https://www.metalsupermarkets.com/what-is-weathering-steel/#:~:text=Weathering%20steel%20is%20a%20family,typical%20low%20carbon%20steel%20grades.  (accessed July 5, 2022).

Canadian Precast Concrete CPCI. Record-Breaking Precast Nu Girders Installed in Alberta. Url: https://www.cpci.ca/en/about_us/project_month/april_2002/#:~:text=The%2064%20m%20(210%20ft,if%20not%20in%20the%20world. (accessed July 5, 2022).

For more information on the author: https://www.linkedin.com/in/eric-chiasson-10601082