Inspection and Management of Bridges with Fracture-Critical Details (2005) / Chapter Skim
Currently Skimming:
Appendix A - Background Discussion on Fatigue, Fracture, Nondestructive Evaluation, and Repair and Retrofit
Appendix A - Background Discussion on Fatigue, Fracture, Nondestructive Evaluation, and Repair and Retrofit
Pages 39-56
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 39... ...
. As explained in chapter three, toughness is specified so that the structure is resistant to brittle fracture despite manufacturing defects, fatigue cracks, and/or unanticipated loading.
|
From page 40... ...
(The reason that the dead load has little effect is that, locally, there are very high residual stresses.) It is worth noting that when information about a specific crack is available, a fracture mechanics crack growth rate analysis should be used to calculate remaining life (A13– A16)
|
From page 41... ...
Fatigue Life Prediction Methodology for Existing Bridges The 1990 AASHTO Guide Specifications for the Fatigue Evaluation of Existing Steel Bridges (A25) can be used for the fatigue evaluation of steel bridges.
|
From page 42... ...
. However, the occurrence of fatigue cracks does not necessarily mean that the structure is in danger (A13,A27)
|
From page 43... ...
Large variability in toughness at constant temperature and large changes of temperature are typical of transition-range fractures. AASHTO specifications for bridge steel and weld filler metal require minimum CVN values at specific temperatures FIGURE A2 Brittle fracture of flange of tub girder of Bryte Bend Bridge in Sacramento, California, while under construction.
|
From page 44... ...
is −15°C must have the minimum impact energy for Zone 1, which is 34 J at 21°C. History of Development of the Fracture Critical Plan The fracture toughness requirements are based on a correlation between the fracture toughness in terms of the stressTEST TEMPERATURE, °F A588 19mm Plate Charpy Data 0 50 100 150 200 250-50-100-150 0 10 30 50 70 20 40 60 90 25 50 75 100-25-50-75 TEST TEMPERATURE, °C CV N, J ou le s CV N, ft -lb FIGURE A4 Charpy energy transition curve for A709 Grade 50 (350 MPa yield strength)
|
From page 45... ...
A few brittle fractures have still occurred without noticeable fatigue crack growth in previously designed bridges, however, although the steel met the modern CVN toughness requirements (i.e., those required by the fracture control plan or general AASHTO toughness requirements)
|
From page 46... ...
If there was no temperature shift, extremely large cracks, greater than 350 mm, can be tolerated in mild steel. If the notch toughness requirements were increased, the increased toughness could also be used to offset the decrease in reliability associated with no longer decreasing the stress range for FCMs in the LRFD Specifications (A2)
|
From page 47... ...
Although higher toughness makes bridges more tolerant to longer cracks, it does not significantly increase fatigue life. Fatigue cracks grow according to a power law; therefore, most of the fatigue life is spent growing the crack while it is very small.
|
From page 48... ...
FIGURE A6 Magnetic particle testing -- Placement of magnetic field. FIGURE A7 Magnetic particle testing -- Application of magnetic particles.
|
From page 49... ...
With any of these treatments, the improvement in fatigue strength can be attributed to one or a combination of the following: • Improvements in the weld geometry and corresponding reduction in the stress concentration, • Elimination of some of the more severe discontinuities from which the fatigue cracks propagate, or • Reduction of tensile residual stress or the introduction of compressive residual stress (A49,A50)
|
From page 50... ...
For instance, if a fatigue crack grows across the full depth of the bridge girder, there are two ways that it can be repaired. First, a vee and weld repair can be specified, but the base metal that is weld repaired will have at best a Category D fatigue resistance (A54–A56)
|
From page 51... ...
The strands are then preloaded and a portion of the dead load transferred to the newly added strands. Thus, in the event the existing tension flange fractures, there are additional tension members (strands)
|
From page 52... ...
The primary function is to provide an additional component(s) so that if the tension flange of an existing member were to fail at some location along the length, the added component would assume the full dead and live loads of the tension flange.
|
From page 53... ...
Figure A17 illustrates a two-girder bridge where the pin and hanger were removed and replaced. DEVELOPMENT OF FRACTURE CONTROL PLAN This section reviews the history of the development of the fracture toughness requirements for steel and weld filler metal, which are a central part of the bridge fracture control plan contained in D1.5.
|
From page 54... ...
McNamee, NCHRP Report 147: Fatigue Strength of Steel Beams with Welded Stiffeners and Attachments, Transportation Research Board, National Research Council, Washington, D.C., 1974, 85 pp.
|
From page 55... ...
Schilling, and K.S. Raju, NCHRP Report 299: Fatigue Evaluation Procedures for Steel Bridges, Transportation Research Board, National Research Council, Washington, D.C., 1987, 100 pp.
|
From page 56... ...
Edinger, NCHRP Report 227: Fatigue Behavior of Full-Scale Welded Bridge Attachments, Transportation Research Board, National Research Council, Washington, D.C., 1980, 47 pp.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.