Strengthening of fatigue critical welds of a steel box girder

The steel box girder of an artwork pendulum that is mounted at the atrium ceiling of an office building is subjected to alternate bending. After 19 months of service severe fatigue cracks have developed in the welds of the box girder. Several repair options have been investigated by using the Finite Element Method (FEM) and fatigue analyses based on the FKM-Guideline. The most efficient and cost-effective way of increasing the static and dynamic strength turned out to be the bonding of prestressed CFRP laminates onto the tension and compression flanges of the steel box girder. The performance of the strengthening method was proved by a laboratory fatigue test on a comparable strengthened, smaller sized welded box girder. After 5 Million of alternate bending cycles fatigue cracks were initiated but reached lengths of not more than 20 mm. Based on this promising result the steel box girder of the artwork pendulum was dismounted and strengthened by bonding three 6 m CFRP laminates on both girder flanges having an initial prestress of 900 MPa each. Regular inspections of the strengthened box girder have shown, that after 7 years of service since strengthening (2.58 Mio of cycles), no visible fatigue cracks were initiated and no prestress loss was monitored.