I-93 †

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1.Field of application †

The Central Artery of I-93 in Boston, Massachusetts.

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2.Circumstances of repair †

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3.Types of structure †

The bridge is a viaduct structure with bi-level rigid steel frame bents supporting the reinforced concrete decks, two longitudinal I-girder per level ,and multiple longitudinal stringers per level.

Plan view and framing plan is shown in figure 1,2. The columns of the bents are box sections and the transverse beams of the bent are either box section or I-section (Fig.3,4,5). The longitudinal girder top and bottom flanges (an webs) are coped by flame-cutting to permit a bolted web connection, using double angle, to the transverse beams of the bent. The girder webs are bolt connected to the double angles which are bolt connected to the transverse beams of the bent (Fig.6). The structural steel is ASTM A588.


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Fig.1 Plan view of bents


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Fig.2 Typical framing plan for one span.


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Fig.3 Bent 7 elevation (with beam I-section).


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Fig.4


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Fig.5


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Fig.6
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4.Details of loading †

Fluctuating loads due to vehicles live load. This resulted in significant cyclic stress occurred from the longitudinal traction and braking forces.

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5.Description of damage †

Ⅰ. Horizontal cracks were found along the longitudinal girder bottom web-flange weld at the flange termination (Fig.7).

Ⅱ. Vertical cracks were found in the longitudinal girder webs at the reentrant corner of the bottom cope and along the bolt fixity line of the connection angle (Fig.8).

The fatigue cracks occurred at locations where the girder web was bolted full depth to the connection angle.

Other locations where the girder web was not bolted full depth to the connection angles did not contain fatigue cracking in either the cope or the connection angles (Fig.9).

Ⅲ.A crack was found at the intersection of the fillet weld, which connected the closure plate to the column flange, and the groove weld, which connected the top flange of the box beam to the column flange.


Causes of fatigue crack were as follows:


Ⅰ.The lateral displacement of the longitudinal girder bottom flanges originated during fabrication (Fig.8) and the corresponding web gap distortion at the connection between the longitudinal girder and transverse beam.

Ⅱ.End fixity of the double angle connections resulted in large cyclic stresses acting on the web plate.

·Coping by flame-cutting which resulted in high tensile residual stress along the cut edge and a sharp.

Ⅲ.A fabricated lack of fusion plane at the corner fillet welds of the bent transverse beams.

Cause of fatigue crack was as follows:

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6.Repair method applied †

The fatigue cracked girder web and bottom tension flange were bolt spliced (Fig2).

File not found: "i95_ht84.gif" at page "I-93"[ź]

Fig.2 Bolt splice repair of fatigue cracked girder web and bottom flange.

The possibility existed for fatigue crack growth in the butt connection groove welds of other tension longitudinal stiffeners. Therefore, from the exterior of the fascia girder web, small hole was drilled through the girder web at the top of the fillet weld toe centered over the tension groove weld.

A 51mm diameter holes were trepanned from the interior of the fascia girder web centered on the longitudinal stiffener groove weld (Fig.3).The holes extended through the web into the longitudinal stiffener a maximum 3mm past the stiffener fillet weld toe.

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Fig.3 Retrofit of tension longitudinal stiffeners.


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