Abstract
The Grondal Bridge, is a large freivorbau bridge (pre-stressed concrete box bridge), approximately 400 meters in length with a free span of 120 m. It was opened to tram traffic in 2000. Just after opening cracks were noticed in the webs, these cracks have then increased, the size of the largest cracks exceeded 0.5 mm, and at the end of 2001 the bridge was temporarily strengthened. This was carried out with externally placed prestressed steel stays. The reason for cracking is still debated and will be further discussed in this paper. Nevertheless, it was clear that the bridge needed to be strengthened. The strengthening methods used were CFRP laminates in the Service Limit State (SLS) and prestressed dywidag stays in the Ultimate Limit State (ULS). The strengthening was carried out during 2002. At the same time monitoring of the bridge commenced, using LVDT crack gauges as well as optical fibre sensors.
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