Aluminum Bridge Decks

In Canada and the United States aluminum is widely used in highway bridge components like guardrails, luminaires, and signage, while structural applications have been limited to pedestrian bridges, sidewalk or bicycle path additions to existing bridges. But this limitation may soon be a thing from the past.

With the development of greatly improved extrusion alloys, the availability of large structural extrusions and new welding technology, fabricated aluminum bridge decks are emerging as a viable alternative to traditional materials and designs.

The first roadway bridge in Canada incorporating an extruded aluminum deck is under construction in Saint-Ambroise, Quebec, and is expected to open to traffic early in the summer of 2014. This pilot project led by Quebec’s Ministry of Transport may open the way to a greater use of aluminum in similar public infrastructure projects. MMM Group is the consulting engineer on the Saint-Ambroise bridge.

The bridge incorporates an 8″ deck by AlumaBridge, LLC. At 23 lbs./sq.ft., including wear surface, the deck is very lightweight. But it shows excellent structural efficiency (composite behaviour with steel girders), very similar to a monolithic concrete deck, 90% as strong transversely as longitudinally.

Design aspects

Early aluminum bridge structures between the 1930s and into the 1960s were made from Al-Cu (2000 series) alloys, which were strong, but hard to extrude into complex shapes. They were also prone to significant corrosion problems, leading to early failures. The introduction of 6000 series alloys (which rely on Mg₂Si precipitation hardening) made it much easier to extrude large and long beams with complex hollow cross sections. The 6063 and 6061/6082 alloys established themselves as the standard alloys for most applications and are now the workhorses of the extrusion industry. The most recent bridge decks in North America have been made in these alloys.

As current designs are mainly limited by stiffness and fatigue, and as both the elastic modulus as well as fatigue strength of these alloys are similar, the Saint-Ambroise bridge will use the AASHTO fatigue-tested standard 6063 alloy in T6 temper.

The Aluminum Design Manual published by the Aluminum Association of Canada helps engineers with the design and development of aluminum structures. Ideally an extruder should be involved as early as possible to make use of the full capabilities they can offer.

(See also Aluminum Structures, by Kissell & Ferry; Canadian Highway Bridge Design Code, Chapter 17 CAN/CSA S6-06; “Calcul des charpentes d’aluminum – PRAL,” by Denis Beaulieu.)

AASHTO LRFD Section 7 Code incorporates aluminum, but not the latest alloys. An AWS D1.2 standard is pending a vote to allow friction stir welding.

Lightweight and durable

At approximately 19 to 25 lbs. per square foot, depending upon deck depth and the wearing surface applied, an aluminum bridge deck can be 70-80% lighter than concrete, reducing dead load, while offering increased bridge width and capacity without the necessity of strengthening the supporting bridge elements.

This light weight means lower transportation costs and fast installation. Aluminum bridge decks made from modern 6000 series alloys require minimal maintenance, offering more than a 100-year bridge life without paint! Aluminum is able to withstand de-icing chemicals better than traditional materials and is ideal for colder climates, as it eliminates concerns about brittle fracture, even in severe Arctic weather conditions.

Extrusions can be made with high recycling content and aluminum components are easily recyclable and potentially even reusable. In Quebec and British Columbia, the material is manufactured using hydro-generated power.

The Aluminum Association of Canada has long advocated the increased use of aluminum in civil engineering infrastructure. It has now passed this responsibility to the Grappe industrielle de l’aluminium du Québec (GIAQ), Quebec’s industrial cluster of companies and institutional stakeholders with a focus on aluminum. GIAQ has been tasked with enabling the future use of aluminum in bridge structures in Quebec, adjacent Canadian provinces and the Northeastern U.S. The objective is to overcome the remaining technical and commercial hurdles to continue to actively promote the use of aluminum bridge decks, and to establish a supply chain for their manufacture. cce

 Martin Hartlieb is the director of business development with the Grappe industrielle de l’aluminium du Québec (GIAQ), based in Montreal.