Design Guide for Bridges for Service Life (2013) / Chapter Skim
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Pages 318-344
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316 7.1 introduction This chapter introduces basic principles related to fatigue and fracture in steel bridges and discusses factors that cause fatigue and fracture. Various available options for repairing observed cracking in steel bridges are also presented.
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317 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES conditions.
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318 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE deformation. Microcleavage is a fracture along specific crystallographic planes and tends to be a brittle fatigue mode (Stephens et al.
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319 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES Source: AASHTO (2012)
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320 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 7.2.2 Hot-Spot Stress Approach The hot-spot stress approach is similar to the nominal stress approach, except that the S-N curves are based on the geometric stress ranges, also known as hot-spot stresses. This process is beneficial in instances when the nominal stress approach breaks down, such as offshore tubular structures in which the fatigue resistance is heavily dependent on the geometry of the tubes.
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321 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.3 crAck detection techniQueS Cracks are not always obvious to the human eye and can be difficult to locate at times.
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322 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 7.4.2 Reshaping by grinding Grinding can be used as an effective measure for increasing the fatigue life of the weld toe by removing portions of the weld that contain small cracks. Grinding has proven more effective on larger welds in structures such as offshore structures with large tubular joints.
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323 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.4.4 impact treatments Compressive residual stresses can be induced around the weld toe by using impact treatments.
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324 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 7.4.5 Hole Drilling Hole drilling is the most widely used method for the repair of fatigue cracks. The process involves drilling a hole at the tip of the crack (propagating end)
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325 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.4.8 Posttensioning Posttensioning methods that are applied to cracked sections can prolong the fatigue life of the structure.
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326 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE to properly repair the fatigue cracks the out-of-plane bending needs to be reduced or eliminated. It is important to note that web-gap fatigue retrofits need to maintain symmetry.
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327 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES are effective to some extent in distributing the applied traffic loads (Brakke 2002; Flemming 2002)
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328 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 7.5.1.1.5a Welded Attachments The all-welded retrofit for connection plates can be difficult to implement. For instance, the welded connection itself can cause fatigue cracks (Keating et al.
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329 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.5.1.1.5d Adhesives Adhesives become attractive when short-term positive attachments are needed.
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330 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Leave a minimum 6.4 mm (0.25 inch) gap between angle/tee and longitudinal weld of girder to prevent a fretting fatigue problem Leave a minimum 6.4 mm (0.25 inch)
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331 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.5.1.1.6 Web-Gap Softening Web-gap softening entails the removal of portions of material to make the web gap more flexible.
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332 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 7.5.2 tie girder–floor Beam Connection Tied arch bridges exhibit a specific type of web-gap fatigue in the connections between the floor beams and the tie girders. This fatigue arises from the displacement incompatibility between the floor beams that are composite with the bridge deck and the noncomposite tie girder.
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333 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES Figure 7.15.
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334 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 7.16. Typical cross section of a two-girder bridge with cantilever bracket outriggers.
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335 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.6 retroFit vALidAtion oF SecondAry StreSS FAtigue Because of the unknown nature of retrofitting web-gap fatigue, it is necessary to validate particular retrofits before retrofitting an entire bridge.
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336 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 7.7 LoAd-controLLed FAtigue crAck rePAir 7.7.1 Coverplates Several methods have been explored in the retrofitting of coverplates, including grinding, air hammer peening, gas tungsten arc, and bolted splice plates. Grinding has proven ineffective and is not recommended.
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337 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.7.2 Eyebars and Hangers Eyebars are long slender bars or rods with forged eyes at the ends that are commonly used as tension members in truss bridges.
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338 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Assessing the integrity of eyebars, especially in old truss bridges, is very difficult. An example of an eyebar with extensive corrosion is shown in Figure 7.22.
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339 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES 7.7.3 temporary tack Welds Tack welds used to temporarily hold members in place during construction can be sources of concern for creating fatigue cracks.
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340 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE t g L 12 2 ≤ (7.3) where t = angle thickness, g = bolt gauge, and L = distance between girders.
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341 Chapter 7. FATiGUE AND FRACTURE OF STEEL STRUCTURES Fatigue cracks have also been found to initiate from the ends of the gusset plates and propagate into the girder webs.
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342 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 7.7.6 Longitudinal Stiffeners Fatigue cracks can form at the butt welds of longitudinal stiffeners, primarily as a result of poor workmanship and inherent defects of the welds. Drilling a large-diameter hole in the longitudinal stiffener located next to the girder has proven to keep the crack from developing into the girder web.
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