Confederation Bridge vibrating more than models predicted

According to the Toronto Star newspaper, engineers in the federal public works department have discovered that the link between P.E.I. and New Brunswick is vibrating more than was predicted by the computer models.
The link, known as Confederation Bridge, was completed between 1993 and 1997, as a fixed crossing measuring 13 kilometres over the choppy waters of the Northumberland Strait. It has won many engineering awards, including a 1998 Canadian Consulting Engineering Schreyer Award given to engineering firm Stanley Consulting Group of Calgary (now Stantec). Some of the project’s innovations included the fast-track delivery process which was achieved partly through the use of precast piers. The structure consists of 190-metre double cantilever main girders fixed to 8-metre octagonal piers founded on bedrock. Girders measuring 60 metres long are dropped in the intervals to make up a 250-metre frame.
The newspaper article reporting on the bridge’s unexpected “wiggle” follows up on a scientific paper issued in the Canadian Journal of Civil Engineering by Moe Cheung. Cheung is an engineer who heads an investigating team attached to Canada’s Public Works department.
The greater than expected vibrations in the bridge are not seen to be a problem, since a three-fold difference between predicted and measured vibration is, according to another expert quoted in the Toronto Star, “a reasonable approximation for such a complicated structure.”
Nonetheless, the experts are curious as to why the extensive computer modelling differed from the actual vibrations. Apparently the bridge is vibrating between one and two times a second, half the width of a pencil over a distance of two soccer fields. The researchers attribute the movement to high winds and heavy traffic. Because the bridge vibrates at higher frequencies, the engineers believe it must be stiffer than expected as well. However they do not believe the new conditions will reduce its projected 100-year life span.