Steel bridge on highway I-75, Georgia, USA の変更点

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*Steel bridge on highway I-75, Georgia, USA : Hole-drilling or re-weld, followed by Ultrasonic Impact Treatment (UIT) [#md8780ec]

**1.Field of application [#qa4087c9]

>Steel bridge on highway I-75, Georgia, USA.

**2.Circumstances of Repair [#pdd2d3ff]

>Numerous cracks at main girder beams were found in September of 1996. Inspection of bridge structures indicated that repair was required.

**3.Types of structure [#k3354f52]

>The structure was a highway bridge at Allatoona Lake, Georgia, USA, which had been in service for 20 years. Each span consisted of 7 steel longitudinal girders and 10 transverse frames between them. A reinforced concrete road-bed was installed at the top. The transverse frames were manufactured from L-bars and welded to the vertical stiffeners of the main longitudinal girders. The girder walls were of 11mm thickness A36 steel ([[Fig.1>#fig1]]).

**4.Details of loading [#l830e91a]

>Fluctuating loads due to highway traffic. This resulted in large cyclic stresses at the top ends of the vertical stiffeners of the main longitudinal girders.
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**5.Description of damage [#jc02c0cd]

>Fatigue cracks were found at the top ends of vertical stiffeners of main longitudinal girders([[Fig.1>#fig1]] and [[Fig.2>#fig2]]). They penetrated through the stiffener end weld. Most of these welds had defects: insufficient welding, undercuts, interrupted weld, etc.

>Causes of fatigue cracks were as follows:
-space between vertical stiffener end and longitudinal girder top flange; 
-tension stresses at stiffener top end caused by irregular bend loads to girder top flange and loss of girder wall stability at this area; 
-tension stresses caused by transverse framework welded at stiffener top end area; 
-stiffener end weld defects as areas of internal and external stress concentration. 
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**6.Repair method applied [#ucb0926b]

>Repair and strengthening of parts subjected to cracking were performed by hole-drilling or welding, followed by Ultrasonic Impact Treatment (UIT) to improve the fatigue performance of the repairs. Some apparently undamaged parts were also subject to UIT to reduce the risk of cracking in future.

>To evaluate the effectiveness of UIT, some parts equivalent in terms of design, position and loading conditions were repaired but not subsequently subjected to UIT. These sites were used as controls.

>Welding equipment used in this work was Lincoln 250 with Hobart E7018 electrodes.
-Case I. Prevention of cracking by applying UIT to weld toes in the main girder wall at stiffener ends and the top flange area according to [[Fig.3>#fig3]].
-Case II. Cracks stopping by drilling, followed by UIT([[Fig.4>#fig4]])performed at: 
--areas around the holes; 
--holes chamfers; 
--areas of existing welds at high stresses concentration zone (space between stiffener and top flange).
-Case III. Cracks repair by welding, followed by UIT([[Fig.5>#fig5]]):
--gauging and welding of cracks with full penetration welding; 
--welding layer-by-layer with UIT followed at each layer; 
--UIT of existing welds in regions of high stress concentration. 
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**7.Outcome [#m359a318]

>It was planned to monitor the performance of the repairs, including comparison of the repairs with and without UIT in intervals of 3,6,9,12,18,30 and 36 months after completion of the repair work.

>Inspection by magnetic particles and visual methods after 3,6,9 months indicated the following:
-no new cracks appeared at UIT treated joints (Case I); 
-for joints with cracks stopped by drilling holes, old cracks did not propagate further in either UIT treated or untreated joints (Case II); 
-no new cracks appeared in joints subjected to weld repair and subsequent UIT. However, new cracks did appear at similar places in control joints subjected to weld repair without UIT (Case III);

>Thus, UIT was effective in improving the fatigue performance of weld repairs.

**8.Observations [#xbfea8ed]

>Due to the circumstances, the UIT operations could not always be performed under optimum conditions. Nevertheless, the obvious benefit in terms of fatigue life improvement indicated that the technique is suited to difficult on-site applications on complex structures.

**Reference [#ka524072]

>William Wright, P.E.,. Post-weld treatment of a welded bridge girder by ultrasonic impact treatment, Preliminary Report, Federal Highway Administration. Turner-Fairbanks Highway Research Center. 6300 Georgetown Pike. McLean, VA22101. September 29, 1996.
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>Acknowledgement: Authors appreciate help, support and constant encouragement of this work by K.K. Verma of Federal HWA in Washington, D.C., and T.Burch of Federal HWA Office in Atlanta, Georgia.

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