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1Â Â Structural supports for signs, luminaires, and traffic signals and other steel highway struc- tures are generally galvanized to prevent corrosion and provide a long service life. How- ever, recent investigations have revealed incidents of cracking in weldments of galvanized structures that appear to be induced during the galvanizing process. When placed in service, structures with such flaws will exhibit short service lives and pose safety concerns. There was a need to identify the factors contributing to the occurrence of weldment cracking during the galvanizing process in steel highway structures and to propose changes to current design, materials, and construction specifications to help mitigate such cracking. This information will ensure that galvanized highway structures provide the intended service life and elimi- nate related safety concerns. The objective of this research, conducted under NCHRP Project 10-94, was to propose improved design, materials, and construction specifications of galvanized steel highway structures to mitigate weldment cracking caused by the galvanizing process. This research was concerned with structural supports for signs, luminaires, traffic signals, and bridge superstructures (other than decks). The project was carried out in two phases. In Phase I, a comprehensive literature review was performed to identify factors shown to contribute to cracking of weldments in galva- nized steel structures. The relevance of these factors to weldment cracking in steel highway structures was assessed and considered in the context of existing U.S. specifications and guidance documents. Identified factors included thermal effects, galvanizing bath chemis- try, steel material, geometric effects, level of cold-working, magnitude of hardness (which can be influenced by cooling rate after welding or cutting processes), and magnitude of residual stresses. To investigate the impact of each variable and their potential interactions, an experimental investigation and a computational study were performed in Phase II to identify the relative level of impact of each variable and form the basis for proposed specifi- cations to mitigate weldment cracking caused by the galvanizing process. In the experimental program, hundreds of tension coupon specimens were tested in air (room temperature [RT] and elevated temperature) and in various galvanizing baths. Also, steel wedge-opening loaded specimens were tested in the galvanizing bath. The strains at failure of the tension coupons were measured to quantify the impact of each factor on the susceptibility to cracking of steel due to galvanizing. The main findings from the experimental program are the following: 1. All steels included in the test program (A36, A572-50, A572-65, and HPS100W) were found to be consistently susceptible to the phenomenon of liquid metal assisted cracking (LMAC) as measured by the strain at failure. A36, A572-50, and A572-65 exhibited similar strains at failure during galvanizing but HPS100W exhibited a smaller strain at failure. S U M M A R Y Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process
2 Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process 2. Four different bath chemistries were considered in the test program; these were special high grade zinc (SHG), SHG + 0.1% Bi, SHG + 1.0% Pb, and SHG + 0.1% Bi + 1.0% Pb. There was no evident influence of bath chemistry on steel strain failure when no other factors were present. However, there was tendency for lead and bismuth to reduce steel ductility when used in combination, as indicated by strain reduction during galvanizing when other factors (stress concentrations, heat-affected zone [HAZ], cold-working) were present. 3. Two different levels of stress concentration, achieved by notching tensile specimens with either stress concentration factor (SCF) = 1.5 or SCF = 2.5, were investigated. A sig- nificant influence of stress concentration on LMAC was observed, suggesting that more severe stress concentrations will increase susceptibility to LMAC in steel structures during galvanizing. 4. Three different HAZ microstructures were considered as indicated by different cooling rates: slow cooling rate = 12°C/sec (degrees Celsius per second, which corresponds to 60 kilojoules per inch [kJ/in.] of heat input), medium cooling rate = 30°C/sec (corre- sponds to 30 kJ/in. of heat input), and fast cooling rate (FCR) = 60°C/sec (corresponds to oxyfuel cutting on a 2 in. plate). HAZ-simulated specimens were also tested in air at RT after acid pickling to investigate whether there was any hydrogen embrittlement phenomenon interacting with other variables. All HAZ-simulated specimens experi- enced extremely low levels of elongation in zinc. The microstructures corresponding with oxyfuel flame-cutting processes (FCR) were particularly susceptible to LMAC. The FCR created rough surfaces and flaws that, when combined with stress concentration, increased susceptibility to cracking. 5. Three different levels of cold-working were considered, two of which were obtained through direct tension pre-straining to 6% and 8%; the third level was obtained by cold- bending A572-50 plate 90° and extracting tension specimens along the bend line. Most cold-worked specimens were pre-strained in tension to 6% or 8%, and the elongations at failure for those specimens in zinc were compared against non-cold-worked specimens tested in zinc, and against cold-worked specimens tested in air. The test results suggest that cold-working has less effect on LMAC than do flaws that may be produced through practical cold-working processes. A computational study was performed on high mast illumination pole (HMIP) structures undergoing the full galvanizing process to provide insight into how geometry and thermal load interact during galvanizing. Thermal-mechanical analysis of HMIPs was performed to evaluate the effect of structural configuration on nonlinear deformation demands during the hot-dip galvanizing process. The HMIP models evaluated in this study consisted of a thin pole with thick steel plates welded at both ends. A total of 19 models of poles with round and multisided (8, 10, and 12 sides) cross-sections were analyzed. The following are the main findings from this study: 1. Poles tended to experience the largest strains upon contact with liquid zinc during the dipping part of the cycle; they experienced high strain values upon extraction from the zinc and contacting air. The pole cross-section did not deform uniformly during galva- nizing; the deformed locations around the pole circumference were found to be most susceptible to LMAC during the galvanizing process for different pole geometries. 2. As the ratio of base plate thickness to pole thickness increased, stress and inelastic strain at the base plate connection detail also increased, indicating an increased susceptibility to LMAC. 3. Inelastic stress and strain concentrated significantly at bend lines in multisided poles. The level of concentration was greater for larger ratios of base plate thickness to pole thickness.
Summary 3Â Â 4. Multisided poles consistently experienced greater inelastic stresses and strains during the galvanizing process than round poles. Based on the findings of this research, changes to the AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, first edition (2015) and the AASHTO LRFD Bridge Design Specifications, eighth edition (2017) were proposed; these are presented in the attachment.
