Tests of Large Girders Treated To Enhance Fatigue Strength

Abstract

Two large-scale fatigue test studies of plate girders are reported on. One study involved a series of coverplated bridge girders with small fatigue cracks that were retrofitted in 1976 as described in NCHRP Report 206. The second study involved plate girders fabricated from HPS-485W (HPS-70W) steel with welded attachments. The Category E’ coverplated beams that were removed from the I-95 Yellow Mill Pond Bridge in 1997 had been retrofitted in 1976 by either air hammer peening or gas tungsten arc (GTA) remelting. All details had small fatigue cracks at the time of retrofit. No further fatigue cracking was observed at the coverplate ends after 20 years of service and an estimated 56 million truck passages. The beams were tested at a stress range of 69 MPa (10 ksi). Cracks developed from the root of the transverse end weld and propagated through the weld throat. The fatigue resistance of the treated weld toe details improved to Category C except for one GTA-remelted detail, which exceeded Category D. Another study was carried out on large-scale HPS-485W plate girders with as-welded and ultrasonic-impact treatment (UIT) details. UIT was applied to the weld toe of transverse stiffeners welded to the web and flanges (Category C) and to coverplated ends (Category E’). The as-welded details cracked at their expected fatigue resistance. The UIT transverse stiffeners improved to Category B fatigue resistance, whereas the UIT coverplated details improved to Category C fatigue resistance.