AWARD OF EXCELLENCE – TRANSPORTATION Petite-Nation River Bridge

The “lifeline bridge” concept was first developed by the California Department of Transportation to guarantee that certain roads are safe and remain accessible to emergency vehicles during a major earthquake and in the aftermath.

In this project CIMA+ engineers designed the first “lifeline bridge” of its size built in Quebec, where earthquake engineering for bridges has only recently been implemented. The bridge spans a broad river valley as part of the extension to Autoroute 50 near Lochaber, between Ottawa and Montreal.

The challenge was designing a seismically-sound bridge that spans 304 metres, 35 metres high across a valley, while minimizing any disturbance to the environment and without building in the river below.

The completed bridge has three piers that are each at varying heights caused by the site’s rugged topography and complex and varying geotechnical conditions. The 24.5-metre high Pier 2, which is seated directly on bedrock, is much shorter than the 32.5-metre high Pier 3, which rests on drilled shafts that pass through 35 metres of ground before bearing on bedrock, for a total height of approximately 70 metres. Such a difference in pier heights and foundations, and therefore in their rigidity, results in a concentration of seismic loads in the shorter piers, placing them at a very high risk.

To meet this challenge, the engineers designed a hollow structure for the piers in order to obtain the required rigidity while reducing their mass. Each pier has a pier cap beam supported by four ductile rectangular columns of equal height, which are supported by a multi-cellular truncated pyramid at the base. Although the ductile columns and pier cap beams are identical for all the piers, the pyramids consist of three cells for each pier, with walls of varying thickness. The engineers made the rigidity of the piers uniform by adjusting the wall thickness of the cell according to the height of the pier. Yet from the exterior the piers look identical.

Drilled shafts were used to transmit seismic loads to the bedrock through 35 metres of ground, and also to install active rock anchors to avoid any rotation of footings.

For the seismic resistant system, CIMA+ designed a longitudinal bridge restraint system that consists of three fixed supports (one for each pier). There are also steel shear keys for increasing resistance to transverse loads. The structure’s protected components (pier caps, bearings and foundations) were designed to withstand the maximum probable forces that are likely to develop in the plastic hinges of the ductile columns.

The light, steel superstructure is perfectly symmetrical, which benefits its seismic performance and behaviour in service. The forces in each span are perfectly balanced, minimizing the mass of steel required for the girders and limiting the adverse effects of structural fatigue.

The length of the central spans was 85 metres in order to avoid the need for major work in the riverbed. Originally, the bridge was intended to be 425 metres long, but CIMA+’s studies revealed that the length could be reduced by more than 100 metres. This represented substantial savings in terms of the total cost.

The project was completed on time and on budget in October 2010. cce

 Project name:

Petite-Nation River Bridge, Lochaber, Québec

Prof. Robert Tremblay, ing., École Polytechnique de Montréal (seismic design); Prof. Omar Chaallal, ing., École de Technologie Supérieure (soil-structure interaction); Qualitas (geotechnical).