Click for next page ( 32

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 31
31 CHAPTER FOUR DISCUSSION RESULTS OF FRACTURE CONTROL Interestingly, extreme loadings of substructure elements such PLAN--25 YEARS LATER as piers have led to most of the spectacular collapses of both steel and concrete bridges. It has been approximately 25 years since the introduction of the FCP, as previously found in the AASHTO/AWS-D1.5 (2). In addition to prevention of collapse in the event of fracture, Subsequent to the FCP, there have been relatively few FCBs redundancy of the superstructure is important for several other designed (e.g., two-girder bridges and two-truss systems), reasons. The first is the need to more easily redeck the bridge. when compared with before the plan. This is primarily because Also, events other than fracture can damage and completely of the added costs placed on fracture-critical structures in destroy members of the superstructure. These are compelling terms of materials, fabrication, and in-service inspection. reasons for redundancy. These reasons for redundancy (other Nevertheless, there have been some notable long-span FCBs than fracture) should not be used to justify unnecessary re- built. Some states continue to use steel cross girders and quirements for FCMs, a subset of nonredundant members. The twin tub girders. For example, Texas has built approximately two sets of bridge members, nonredundant members and the 190 FCBs in the last 20 years. Among bridges designed after subset FCMs, should be addressed separately as appropriate. the FCP, the fracture toughness, detailing, and weld qual- ity has been much better than before, resulting in members These reasons for redundancy (other than fracture) should with greater reliability than non-FCMs. Approximately be used to encourage redundancy outright instead of indi- 11% of steel bridges have FCMs; however, 76% of these rectly by penalizing FCMs. For example, in Sections 1.3.2 were built before the FCP. Most of these, 83%, are two- and 1.3.4 of the LRFD Specifications, redundancy is encour- girder bridges and two-line trusses, and 43% of the FCMs aged. Load factors are modified based on the level of redun- are riveted. dancy and it is stated that multiple-load-path and continuous structures should be used unless there are compelling reasons Based on the results of the survey and personal commu- to do otherwise. nications with industry and academic experts, there do not appear to have been any failures (certainly none where there In the AASHTO Manual for Condition Evaluation and was loss of life) of any structures built after the implementa- Load Resistance Factor Rating (LRFR) of Highway Bridges, tion of the FCP. However, it must also be noted that there redundancy is reflected in system factors that reduce the have been only two such catastrophic bridge failures identi- capacity of each member in nonredundant systems. The sys- fied in the past 40 years that are attributed to fracture. Hence, tem factors are calibrated so that nonredundant systems are it is difficult to measure the success of the FCP, and the NBIS rated more conservatively at approximately the level of re- requirements for hands-on inspection introduced in 1988, in liability associated with new bridges designed by the LRFD preventing catastrophic failure of bridge structures, because Specifications, called the "inventory" level in former rating at least one of the two (Mianus River) could have been pre- procedures. Redundant systems are rated at a reduced re- vented through routine inspection and better maintenance of liability level corresponding approximately to the traditional drainage. Fabricators question the need for much of the test- "operating" level. ing, noting that the procedures become routine and automatic after building FCMs for years. Redundancy is related to system behavior rather than indi- vidual component behavior and is often discussed in terms of Nonredundant is a broader term than FCM because non- the following three types: redundant also includes: Internal redundancy, also called member redundancy, Substructures; exists when a member is comprised of multiple ele- Members that may be inherently not susceptible to frac- ments and a fracture that formed in one element cannot ture, such as compression members, but still could lead propagate directly into adjacent elements. to collapse if damaged by overloading, earthquakes, fire, Structural redundancy is external static indeterminacy terrorism, ship or vehicle collisions, and so forth; and and can occur in a two-or-more-span continuous girder Members made of materials other than steel. or truss.