Manual for Repair of Fatigue Cracks in Steel Bridges

Quiz Question

1. Figure 1 is a
Typical Fatigue crack
Top view of a typical I beam
2. In figure 2, the Chevron marks point to
Shear stress on the flange cross section
Fracture initiation
3. Figure 3. is
General flowchart for fatigue retrofit
General flow chart of building the bridge
4. Figure 4. shows the difference between the stress level near the welded details
5. On Figure 7, the discoloration at the weld toe indicates
Where the crack is
Where the steel has been oxidized
6. The red dye in Fig 8 exposed
Drawing marks on the bridge
7. Which one of the following treatment can be considered “weld improvement”.
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.
Reduction of tensile residual stress or the introduction of compressive residual stress.
All of the above
8. Disc grinding is an effective means to remove metal. However, if the grinder operator is not careful, too much material may be removed. In addition, if the operator loses control of the grinder, the metal may be ____________.
9. Burr grinding has advantages and disadvantages over disc grinding. Burr grinders are easier to handle and have the ability to work in more confined regions than grinding discs. Burr grinders are excellent for grinding the terminations or edges of welds and for enlarging small drilled holes, especially if the hole desired is not exactly circular.
10. Hammer peening is a simple and effective technique for weld improvement. The method uses an air hammer with a blunt tip (commonly used by welders to remove slag from welds, see Figure 15) to plastically deform the weld toe creating a compressive state of stress where tensile residual stress was produced during the welding process. This lowers the tensile stress range the weld toe experiences under live load and extends the fatigue life. The hammer peening process changes the appearance of a weld, giving the weld toe a shinier surface as shown in Figure 16.
Weld improvement
Hammering the material
11. As an alternative to hammer peening, __________ treatment (UIT) is an attractive option. Hammer peening is a noisy and physically enduring task. Ultrasonic impact is relatively quiet, and the equipment is much easier to handle. Ultrasonic impact treatment involves low-amplitude (20-50 microns), high-frequency displacements (27-55 kHz). UIT has proven more effective than hammer peening at improving the fatigue performance of welded joints, so it can be assumed that UIT will increase the fatigue strength of cover plate details and transverse stiffeners by at least one detail category
Ultrasonic impact
12. Hole drilling is perhaps the most widely used repair method for fatigue cracks. The hole drilling method requires placing a hole at the tip of the crack, removing the sharp notch at the crack tip. However, the hole needs to be of sufficient diameter to be successful in arresting the crack. As a rule of thumb, larger holes are better as long as strength and stiffness of the structure or connection is not compromised. Sufficient hole diameters typically in the range of 2 to 4 inches (50.8 to 101.6 mm) have proven successful through field experience, but depending on application a 1 inch (25.4 mm) hole may also be sufficient. Larger holes are not always practical and many owners are uncomfortable placing such a large hole in a member.
13. For fatigue cracks to propagate, the crack needs to open and close. Crack closure concepts can be used to slow down or eliminate fatigue crack growth. Post-tensioning forces can be applied to cracked beam sections, forcing crack faces to close together and push the effective stress range into the compression regime to hinder additional crack growth.
Additional crack growth
Buckling of the structure
14. An attractive retrofit is to loosen bolts that connect the bracing elements to the connection plates.
15. By far the newest, and quite possibly the best alternative for stiffening the web gap is powder-actuated fasteners to attach the angle or tee to the girder flange. Powder-actuated fasteners are high-strength nails that are embedded into other materials, propelled by an explosive discharge. This retrofit offers a positive and permanent connection that can easily be performed under traffic, without the need to excavate the concrete deck.
Steel powder
16. Figure 46 and 47 illustrates large diameter hole retrofit for bridges.
17. Retrofitting web-gap fatigue is not an exact science, and not all retrofits will work in all situations. Therefore, verification of a particular retrofit must be performed before specifying every detail in an entire bridge be retrofitted. Analytical modeling (i.e., finite element analysis) can be used to compare various retrofit strategies and to determine the effectiveness of different retrofits. However, the possibility of modeling assumptions (i.e. connection stiffness assumptions, boundary conditions, etc.) would still make field stress-range verifications prudent. The easiest and simplest plan would be to instrument an uncracked detail, perform a retrofit based on recommendations from this manual, and verify the reduction of stress ranges and/or out-of-plane displacement at locations of potential cracking. When specifying welded retrofits, ensure that the weld retrofit itself has adequate fatigue resistance so as not to create a new problem.
18. Strain gauges are a very effective way to measure stress ranges on steel bridges. They work under the principle that the change in foil grid resistance is proportional to the strain in the grid. The only drawback to using a strain gauge is an external conditioner will be required to complete the Whetstone bridge circuit to measure the resistance change within the gauge, and such equipment may not be readily accessible.
19. Strain gauges can either be glued or spot welded to steel bridges, but weldable gauges are better suited for cold climates because some adhesives do not work well in cold conditions.
Better suited
20. Strain Gauges are offered in a variety of formats, but simple, uniaxial grid patterns with 3-6 mm (0.125-0.25 inch) gauge lengths are all that is needed for verification purposes.