Inspection and Management of Bridges with Fracture-Critical Details (2005) / Chapter Skim
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Pages 7-23
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From page 7... ...
These provisions have reshaped industry practices and result in an acceptably low probability of fatigue cracking and brittle fracture in new bridges. However, many older steel bridges built before the implementation of modern fatigue design provisions in the mid1970s possess poor fatigue details, such as cover plates that can develop fatigue cracks (Figure 2)
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From page 8... ...
. In addition, there have been few if any cases where weld defects or low-toughness steel has been an issue for modern steel bridges, owing primarily to improvements in details, fabrication practices, and fracture toughness of the steel and weld metal (12,15)
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From page 9... ...
. A key element was more stringent CVN requirements for base metal and weld metal for FCMs.
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From page 10... ...
In some circumstances, bridge designs with FCMs, such as tied arches, two-girder bridges, and trusses, may be the most efficient and cost-effective structural system. Although the more stringent CVN requirements, the fabrication FCP, and the additional inspection requirements for FCMs are beneficial, if they are overly conservative for modern bridges they can become an obstacle to the savings gained in using more cost-effective designs.
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From page 11... ...
REDUNDANCY AND COLLAPSE OF STEEL BRIDGES Definition of Redundancy and Contrast to Indeterminacy and FCMs The AASHTO LRFD Bridge Design Specifications define redundancy as "the quality of a bridge that enables it to perform its design function in the damaged state." In NCHRP Report 406 (34) , Ghosn and Moses defined superstructure redundancy as "the capability of a bridge superstructure to continue to carry loads after the damage or the failure of one of its members," and this definition is also used in the AASHTO Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR)
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From page 12... ...
This is an important point because in these cases it reflects on the maximum live load the damaged structure is likely to experience in the brief period before closure. Periodic inspection may be more helpful in finding fatigue cracks and fractures because they are often not immediately obvious.)
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From page 13... ...
According to Ghosn and Moses (34) , a redundant superstructure has at least one alternate load path and is capable of safely supporting the specified dead loads and live loads and maintaining temporary serviceability of the deck following failure of a main load-carrying member.
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From page 14... ...
riveted built-up girder and (b) bolted built-up tie girder proposed for Blennerhassett Arch Bridge.
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Truss action between slab and diaphragm FIGURE 12 Schematic of twin composite tub girder superstructure showing internal redundancy provided by bracing system and possible alternative load path provided by slab and diaphragm. (Courtesy: HNTB.)
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From page 16... ...
Note that several diagonals, hangers, and upper chord braces are completely severed, but the truss did not collapse even though a significant portion of the live load remained on the bridge.
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From page 17... ...
This suggests that load redistribution did not occur until the bottom flange was completely severed. They also reported that most of the load was redistributed through the damaged girder and stringer deck system to the interior supports.
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From page 18... ...
used the same approach in which they compared the safety reliability index of a redundant structure considering all failure paths and the safety index of the exact same structure with no alternative load path. A reliability approach was also used by Moses (42)
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From page 19... ...
LF1 is typically on the order of 3.8, depending on the ratio of live load to dead load LFd is the residual capacity of the damaged structure and is calculated by performing a nonlinear analysis of the damaged structure (with the FCM removed) under the effect of the unfactored dead load and incrementing the multiple of side-by-side HS-20 truck loads until the system collapses.
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From page 20... ...
Because the section is a bolted built-up section, it has internal redundancy as discussed previously in this chapter. In this case, a fatigue crack, fracture, corrosion failure, ductile rupture, or other failure of any one of the four plates cannot propagate directly into any of the other three plates.
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From page 21... ...
• With fix hanger suspended span • With suspended span • Welded plate girders • Riveted or bolted plate girders • Suspended span with two girders • Simple span two-girder bridge with welded partial length cover plates on the bottom flange • Continuous span two-girder system with cantilever and suspension link arrangement and welded partial length cover plates • Simple span two-girder system with lateral bracing connected to horizontal gusset plates that are attached to webs Truss system (simple and continuous span) • Eyebar truss • Welded truss • Riveted truss • Three deck truss with pin and hanger assembly Two-truss systems Truss system (simple and continuous spans)
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From page 22... ...
Tie arch Tie Arch Welded tie arches with box shape tie girder TABLE 1 (Continued)
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From page 23... ...
Where cross sections contain more than two box girder sections, all of the tension flanges should be considered non-fracture-critical. Therefore, the redundancy of two box or tub girders is sometimes even in question.
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