Fatigue Crack Formation and Repair Strategies for Steel Cantilever Bracket Tie Plates

Abstract

A regular inspection of a bridge carrying U.S. Route 422 over the Schuylkill River in Berks County, Pennsylvania, revealed cracks in the floor beam–to–cantilever bracket tie plates. Similar fatigue damage of this type has been observed over two decades ago in other bridges. During this period, much advancement has taken place in structural evaluation through finite-element analysis. This paper presents a comprehensive procedure for the examination and retrofitting of tie plates using analysis and field measurement. A field and forensic investigation was conducted to examine the cause of the crack formation and the behavior of the bridge. The results of the field study and a fractographic examination confirmed that the cracks had occurred because of cyclic horizontal bending of the tie plates, which was generated by secondary out-of-plane bending of the top of the floor beam and cantilever bracket. Field measurements and evaluation indicated that some of the as-built tie plate details had exceeded their estimated fatigue life as computed according to specifications. An analysis by the finite-element method predicted locations of likely future fatigue damage. A retrofit consisting of removal of the restraint between the tie plate and the girder by unbolting the connection or through complete replacement of the tie plate resulted in a significantly lower stress in the tie plate. Measurements show that the unbolted condition will allow an infinite fatigue life. The removal of the tie plate restraint, however, elevated stresses in the web region and could produce fatigue damage in the distant future if the magnitudes of these stresses are high. This was examined through monitoring. The comprehensive procedure for investigation of tie plate cracks is recommended for handling possible future cases of similar damage.