Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process (2021)

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74 This attachment contains proposed changes to (1) AASHTO LRFD Specifications for Struc- tural Supports for Highway Signs, Luminaires, and Traffic Signals, first edition (2015) and (2) AASHTO LRFD Bridge Design Specifications, eighth edition (2017). These proposed changes are the recommendations of the research team for NCHRP Project 10-94, “Mitigation of Weld- ment Cracking of Highway Steel Structures due to the Galvanizing Process,” at the University of Kansas. These changes have not been approved by NCHRP or any AASHTO committee nor formally accepted for AASHTO specifications. A T T A C H M E N T Proposed Specification Changes

Proposed Specification Changes 75   Suggested Revisions to the AASHTO LRFD Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, First Edition (2015) The specific articles in the specifications to which changes are proposed are included in this attachment. Other articles to which no changes are recommended are not presented. Underlined text indicates proposed revisions to existing specifications and/or commentary. Strikethrough text indicates proposed deletions from existing specifications and/or commentary. The following edits to 5.4 and C5.4, limiting steel to be galvanized to materials with sufficient ductility in air to ensure reasonable performance during galvanizing, are recommended. The limit proposed here is based on a cutoff between the ductilities observed in air for A36/A572-50/A572-65 and HPS100W, and evidence that as variables are cumulatively added to a steel structure (HAZ, stress concentrations, etc.), the structure is more likely to experience greater strains than the steel part has in terms of strain capacity in the galvanizing bath. Additionally, a statement is recommended prohibiting galvanizing HPS50W, HPS70W, and HPS100W based on extremely poor performance in tension tests in zinc for HPS100W and its high level of interaction with other variables studied. Since the chemical compositions of HPS50W and HPS70W are more similar to HPS100W than to A36, A572-50, or A572-65, they have been included in this requirement as well. 5.4 – Material Grades of steel listed in the AASHTO LRFD Bridge Design Specifications (LRFD Design) are applicable for welded structural supports for highway signs, luminaires, and traffic signals. For steel that is to be galvanized, the fabricator shall provide copies of all Material Tests Reports (MTRs) or other documentation of material type and chemical composition for primary members and applicable shop drawings to the galvanizer prior to delivery of assemblies for galvanizing, correlating the MTRs with the primary member piece marks. Steel with elongations less than 15% as represented on the MTR shall not be hot dip galvanized. HPS 50W, HPS 70W, and HPS 100W shall not be galvanized. For steels not generally addressed by LRFD Design, but having a specified yield strength acceptable to the Owner, the LRFD limit state design criteria shall be derived by applying the general equations given in LRFD Design except as indicated by this Section. All steels greater than 0.5 in. in thickness, used for structural supports for highway signs, luminaires, and traffic signals, that are main load carrying tension members shall meet the current Charpy V-Notch impact requirements in LRFD Design. C5.4 Steel other than that listed may be used with permission from the Owner. Typical steel materials used in structural supports for highway signs, luminaires, and traffic signals are: • ASTM A595 Grade A, B, and C • ASTM A572 Grade 42, 50, 55, 60, and 65 • ASTM A1011 • ASTM F1554 Grade 36, 55, and 105 Anchor Bolts Generally, the Specification indicated in this Section applies. Research performed under NCHRP 10-94 indicated that liquid metal assisted cracking affected a wide range of steels commonly used in highway structures. Steel to be galvanized should have sufficient elongation to reduce risks of cracking during galvanizing. HPS 100W exhibited a high propensity for liquid metal assisted cracking as well as increased interaction with other aggravating factors that could be present during galvanizing. Although HPS 70W and HPS 50W were not included in the NCHRP 10-94 study, their chemical compositions are more similar to HPS 100W than to the other grades included in the study (A572-50, A572-65, and A36). Therefore, HPS 70W and HPS 50W are considered to likely also be at increased risk of liquid metal assisted cracking. Although the structural supports addressed by these Specifications are not subjected to high-impact loadings, steel members greater than 0.5 in. in thickness should meet a general notch toughness requirement to avoid brittle fracture. The non-fracture critical values may be used

76 Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process The following edits to C5.6.2 are recommended to more accurately identify the corners of multisided poles as stress concentrations that may increase the risk of cracking during galvanization. 5.6.2 – Minimum Number of Sides Tubular structures shall be of round or multisided cross- section, Cross-sections with concave external surface, such as “fluted” cross-sections, are not covered by the provisions for multisided cross-sections and shall not be used without approval of the owner. Multisided tubular sections shall have a minimum number of sides as stated in the following equation, and minimum internal bend radius of five times the tube wall thickness or 1 in., whichever is larger. ≥ √(5 ) (5.6.2-1) where: D = outside distance from flat side to flat side of multisided tubes (in.) and N = greater than or equal to the square root of 5D or 8, whichever is larger C5.6.2 Fatigue cracking in multisided tube-to-transverse-plate connections initiates at the bend corners and progresses towards the flat face between the corners. Research has demonstrated the existence of high stress concentration at the bend corners, which caused crack initiation during the fatigue tests in eight-sided tubes with sharp bend radii (Roy et al., 2011) Compared to a round tube of similar size, welded connections in multisided tubes with fewer sides and internal bend radii less than 1 in. exhibited significantly less fatigue resistance. Increasing the number of sides and/or increasing the internal bend radius can improve fatigue performance of multisided sections (Roy et al., 2011). The requirement of minimum number of sides for multisided tubes was derived considering a maximum ½ in. radial distance between the multisided tube and its inscribed circle at a corner. The ½ in. distance was chosen based on the performance of specimens that were fatigue tested in the laboratory. For flat-to-flat distance D of multisided cross- sections, the minimum number of sides is conservatively provided as: D up to 13 in. 8 sides (octagonal) D greater than 13 in. and up to 28 in. 12 sides (dodecagonal) D greater than 28 in. and up to 50 in. 16 sides (hexadecagonal) Although there is no clear consensus among the research community, multisided galvanized tubes employing very sharp bend radii can be susceptible to strain age embrittlement, cold-worked embrittlement, and hydrogen embrittlement leading to early fatigue cracking in service. A minimum bend radius of five times the tube wall thickness can mitigate the possibility of such embrittlement. Research performed under NCHRP 10-94 separately indicated that stress concentrations, such as those at the corners of multisided poles, can aggravate susceptibility to liquid metal assisted cracking during galvanizing, leading to early fatigue cracking in service. This susceptibility increases with severity of stress concentration. Therefore, increasing the bend radii in multisided poles can help to mitigate the possibility of such cracking. Round poles have shown considerably less susceptibility to LMAC than multisided poles and should be preferred wherever possible. In cases where multisided poles must be used, it is better to use poles with higher-order polygonal shapes than lower number of sides (e.g., use 12- sided poles rather than 8-sided poles). Square or rectangular sections are susceptible to early fatigue cracking leading to poor fatigue performance. These sections should not be used for highway sign, signal, and high- level luminaire support structures. (Dexter and Ricker, 2002) Fluted cross sections and other complex hollow sections present a variety of issues that may affect the design and performance of structures. Owner approval of poles or mast

Proposed Specification Changes 77   arms using fluted and other complex hollow sections should require verification of the materials, design methods, detailing requirements, fabrication methods, and fatigue performance by analysis and testing of components and systems.

78 Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process The following edits to C5.6.3 are recommended to clarify that increasing transverse plate thickness may be advisable for design for fatigue in ungalvanized structures, but it may increase the risk of liquid metal assisted cracking during galvanization. A limit of 6.0 for the ratio of base plate thickness to pole thickness is recommended for multisided poles. 5.6.3 – Transverse Plate Thickness The base plate thickness shall be considered in the design of tube-to-transverse-plate connections. In addition, for arms or pole bases of supports that are designed according to Section 11, the minimum plate thickness shall be as provided in Table 5.6.3-1. Table 5.6.3.1 – Minimum Transverse Plate Thickness for Fatigue Section Diameter or depth D, in. Minimum Plate Thickness, in. D ≤ 8 1.5 D > 8 2.0 C5.6.3 Experimental and analytical studies (Koenigs et al., 2003; Hall, 2005; Warpinski, 2006; Ocel et al., 2006; Roy et al., 2011; Stam et al., 2011) demonstrated that the fatigue resistance of tube-to-transverse-plate connections is a function of the relative flexibility of the tube and the transverse plate. Transverse plate flexibility has a major impact on stress amplification in the tube wall adjacent to the weld toe. Increasing the transverse plate thickness is the most cost- effective means of reducing the flexibility of the transverse plate and increasing the connection fatigue resistance. In- service fatigue cracking in tube-to-transverse plate connections often occurred where relatively thin plates were used along with a few discrete fasteners. It should also be noted that research performed under NCHRP 10-94 showed a strong trend indicating that increased ratios of transverse plate thickness to tube thickness corresponded with significantly greater stress and strain demands during galvanizing, and an increased risk of liquid metal assisted cracking. Therefore, this competing criterion should be balanced with design for fatigue in structures that are to be hot-dip galvanized. One way in which the transverse plate to tube thickness ratio can be controlled without adversely affecting fatigue performance is to utilize a thicker pole section near the base. This also helps to lower stresses at the handhole detail. It is recommended that multisided poles be designed to have a ratio of base plate thickness to pole thickness equal to or less than 6.0. Reducing the opening size in the transverse plate and or increasing the number of fasteners are other cost-effective means of reducing the flexibility of the transverse plate and increasing the fatigue resistance of tube-to-transverse plate connections. In laboratory tests, groove-welded tube-to- transverse-plate connections exhibited significantly better fatigue resistance compared to fillet-welded connections and identical structures, because a smaller opening could be used in the transverse plate. Fluted cross-sections and other complex hollow sections present a variety of issues that may affect the design and performance of structures. Owner approval of poles or mast arms using fluted and other complex hollow sections should require verification of the materials, design methods, detailing requirements, fabrication methods, and fatigue performance by analysis and testing of components and systems.

Proposed Specification Changes 79   The following edits to 14.4.7.3 are recommended to assist designers and galvanizers in mitigating cracking during galvanization. 14.4.7.3 – Galvanized Structures Hot-dip galvanizing after fabrication shall conform to the requirements of ASTM A123. It is preferable that tubular steel pole shafts to be galvanized have a silicon content equal to or less than 0.06%. Other components, such as base plates, should have silicon content controlled as required to prevent detrimental galvanizing effects. The placement of drainage and vent holes shall not adversely affect the strength requirements of the galvanized member. Damage to the coating shall be repaired after erection by a method approved by the owner. For structural bolts and other steel hardware, hot-dip galvanizing shall conform to the requirements of ASTM A153. Exposed parts of anchor bolts shall be zinc coated or otherwise suitably protected. The zinc coating shall extend a minimum of 4 inches into the concrete. Steel anchorages located below grade and not encased in concrete shall require further corrosion protection in addition to galvanizing. Thermal-cut edges shall be ground back to remove the thermally affected region before galvanizing. To protect against liquid metal assisted cracking, tin (Sn) shall not be added to the galvanizing bath in any quantity. Lead (Pb) and bismuth (Bi) have been found to slightly increase sensitivity to LMAC; their use in galvanizing baths should be minimized. Galvanization shall be performed at the highest practical rate of speed. C14.4.7.3 Drainage and vent holes result in a reduction of the member’s net section and cause stress risers, thereby reducing fatigue resistance. Holes shall be placed at noncritical locations where these reductions will not result in the member’s strength being less than the required strength for maximum design loadings or fatigue. Structural steel commonly used in highway structures can be susceptible to liquid metal assisted cracking (LMAC) during galvanizing, wherein the liquid zinc reacts with the steel base metal resulting in significantly lower strain capacity in the galvanizing bath than is available in an in-air environment. To control against potential cracking during hot dip galvanizing, attention should be paid to the following fabrication and galvanizing aspects shown to be influential in NCHRP Project 10-94: • Minimize stress concentrations in the steel part(s) to be galvanized: The severity of stress concentrations was found to be closely linked with increased susceptibility to LMAC during HDG. This factor is especially important to control, as there is an influence of increased stress at the notch from the stress concentration, but also increased interaction with zinc in the presence of a notch. Therefore, avoid notches and grooves and crack-like details (e.g., partial penetration weldments). • Minimize thermal-cut edges and ensure any thermal-cut edges have been ground to remove the thermally affected region before galvanizing. Heat affected zones associated with thermal cutting were found to be sensitive to LMAC. • Cold-working was not found to be a strong predictor of LMAC, however, cold-working is often associated with the formation of flaws and stress concentrations, particularly for areas deformed by cold-bending. These flaws should be ground smooth to the extent possible to reduce likelihood of LMAC. Avoiding cold-bending also eliminates stress concentrations and is recommended wherever possible. • The presence of some alloys commonly added to the galvanizing bath has been shown to aggravate occurrences of LMAC. In particular, tin (Sn) has been shown in multiple studies to aggravate LMAC in extremely small quantities and thus, should not be added to galvanizing baths in any quantity. Lead (Pb) and bismuth (Bi) are common alloy additions to galvanizing baths. When used in typical quantities in combination (<1% Pb, <0.1% Bi), these alloys corresponded with only a slight increased sensitivity to LMAC. However, it should be noted that galvanizing with pure zinc (e.g., Special High Grade zinc) provided less risk in terms of LMAC. • Hardness measurements > 270 HV were found to be a reasonable predictor for increased susceptibility to LMAC. Therefore, it is recommended that hardness measurements be utilized before galvanizing in areas of concern to determine whether thermal pre-treatments should be performed to reduce hardness before galvanizing. • Caution should be exercised when galvanizing statically indeterminate structures or components where there are

80 Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process significant differences between the stiffness and/or thermal mass of their components or parts because large stresses and permanent deformations may be induced during galvanizing. Large differences in stiffness and/or mass of components worsen this problem. • Increasing the rate of dipping and extraction during galvanizing was found to correspond with decreased stresses during galvanizing. • Preheating components is beneficial because thermal shock is reduced. Additional guidance on design and detailing to facilitate galvanizing may be found in “Design of Products to be Hot- dip Galvanized after Fabrication” available from the American Galvanizers Association.

Proposed Specification Changes 81   Suggested Revisions to the AASHTO LRFD Bridge Design Specifications, Eighth Edition (2017) The specific articles in the specifications to which changes are proposed are included in this chapter. Other articles to which no changes are recommended are not presented. Underlined text indicates proposed revisions to existing specifications and/or commentary. Strikethrough text indicates proposed deletions from existing specifications and/or commentary. The following edits to 6.4 and C6.4.1, limiting steel to be galvanized to materials with sufficient ductility in air to ensure reasonable performance during galvanizing, are recommended. The limit proposed here is based on a cutoff between the ductilities observed in air for A36/A572-50/A572-65 and HPS100W, and evidence that as variables are cumulatively added to a steel structure (HAZ, stress concentrations, etc.), the structure is more likely to experience greater strains than the steel part has in terms of strain capacity in the galvanizing bath. Additionally, a statement is recommended prohibiting galvanizing HPS50W, HPS70W, and HPS100W based on extremely poor performance in tension tests in zinc for HPS100W and its high level of interaction with other variables studied. Since the chemical compositions of HPS50W and HPS70W are more similar to HPS100W than to A36, A572-50, or A572- 65, they have been included in this requirement as well. 6.4 – MATERIALS 6.4.1 – STRUCTURAL STEELS Structural steels shall conform to the requirements specified in Table 6.4.1-1, and the design shall be based on the minimum properties indicated. The modulus of elasticity and the thermal coefficient of expansion of all grades of steel shall be assumed as 29,000 ksi and 6.5x10-6 in/in/F, respectively. AASHTO M 270M/M 270, Grade 36 (ASTM A709/A709M, Grade 36), may be used in thicknesses over 4.0 C6.4.1 The term yield strength is used in these specifications as a generic term to denote either the minimum specified yield point or the minimum specified yield strength. The yield strength in the direction parallel to the direction of rolling is of primary interest in the design of most steel structures. In welded bridges, notch toughness is of equal importance. Other mechanical and physical properties of rolled steel, such as anisotropy, ductility, formability, and corrosion resistance, may also be important to ensure the satisfactory performance of the structure. No specification can anticipate all of the unique or especially demanding applications that may arise. The literature on specific properties of concern and appropriate supplementary material production or quality requirements, provided in the AASHTO and ASTM material specifications and the AASHTO /AWS D1.5M/D1.5 Bridge Welding Code, should be considered, if appropriate. AASHTO M 270M/M 270 (ASTM A709/A709M), Grade HPS70W, has replaced AASHTO M270 M/M 270 (ASTM A709/A709M), Grade 70W, and AASHTO M270M/ M270 (ASTM A709/A709M), Grade HPS100W, has replaced AASHTO M 270M/M 270 (ASTM A709/A709M), Grade 100 and 100W in Table 6.4.1-1. The intent of these replacements is to encourage the use of HPS steel over the older bridge steels of the same strength level due to its enhanced properties. The older steels are still available but are not recommended for use and should be used only with the approval of the owner. The maximum available plate lengths of AASHTO M 270M/M 270 (ASTM A709/A709M), Grade HPS 70W and HPS 100W, are a function of the processing of the plate, with longer lengths of HPS 70W produced as as-rolled plate. The maximum available plate lengths of these steels should be determined in consultation with the material producers.

82 Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process in. for nonstructural applications or bearing assembly components. Quenched and tempered alloy steel structural shapes and seamless mechanical tubing with a specified maximum tensile strength not exceeding 140 ksi for structural shapes or 145 ksi for seamless mechanical tubing may be used, provided that: • the material meets all other mechanical and chemical requirements of AASHTO M 270M/M 270 (ASTM A709/A709M), Grade HPS 100W, and • the design is based upon the minimum properties specified for AASHTO M 270M/M 270 (ASTM A709/A709M), Grade HPS 100W. Except as specified herein, structural tubing shall be either cold formed welded or seamless tubing conforming to ASTM A1085, ASTM A500, Grade B or Grade C, or ASTM A847; or hot-formed welded or seamless tubing conforming to ASTM A501 or ASTM A618. Thickness limitations relative to rolled the shapes and groups shall comply with ASTM A6/A6M. Steel tubing for composite concrete-filled steel tubes (CFSTs) designed according to the provisions of Article 6.9.6 shall conform to the requirements of: • American petroleum Institute (API) standard 5L, minimum Grade X42, PSL1 or PSL2, or • ASTM A 252, Grade 3, with all welds satisfying the requirements of the current version of the AWS D1.1/D1.1M Structural Welding Code – Steel. For steel that is to be galvanized, the fabricator shall provide copies of all Material Tests Reports (MTRs) or other documentation of material type and chemical composition for primary members and applicable shop drawings to the galvanizer prior to delivery of assemblies for galvanizing, correlating the MTRs with the primary member piece marks. Steel with elongations less than 15% as represented on the MTR shall not be hot dip galvanized. HPS 50W, HPS 70W, and HPS 100W shall not be galvanized. ASTM A 500 cautions that structural tubing manufactured to that specification may not be suitable for applications involving dynamically loaded elements in welded structures where low temperature notch toughness properties may be important. As such, the use of this material should be carefully examined with respect to its specific application in consultation with the owner. Where this material is contemplated for use in applications where low temperature notch toughness properties are deemed important, consideration should be given to requiring that the material satisfy the Charpy V notch toughness requirements specified in article 6.6.2. ASTM A1085 is an improved specification for cold formed welded carbon steel hollow structural sections (HSS) that is more suitable for dynamically loaded structures. Research performed under NCHRP 10-94 indicated that liquid metal assisted cracking affected a wide range of steels commonly used in highway structures. Steel to be galvanized should have sufficient elongation to reduce risks of cracking during galvanizing. HPS 100W exhibited a high propensity for liquid metal assisted cracking as well as increased interaction with other aggravating factors that could be present during galvanizing. Although HPS 70W and HPS 50W were not included in the NCHRP 10-94 study, their chemical compositions are more similar to HPS 100W than to the other grades included in the study (A572-50, A572-65, and A36). Therefore, HPS 70W and HPS 50W are considered to also be at increased risk of liquid metal assisted cracking.

Proposed Specification Changes 83   The following additions to 6.7 are recommended to assist designers and galvanizers in mitigating cracking during galvanization. 6.7.9 – Galvanizing Hot-dip galvanizing after fabrication shall conform to the requirements of ASTM A123. Components to be galvanized should have silicon content controlled as required to prevent detrimental galvanizing effects. The placement of drainage and vent holes shall not adversely affect the strength requirements of the galvanized member. Damage to the coating shall be repaired after erection by a method approved by the owner. For structural bolts and other steel hardware, hot-dip galvanizing shall conform to the requirements of ASTM A153. Exposed parts of anchor bolts shall be zinc coated or otherwise suitably protected. The zinc coating shall extend a minimum of 4 inches into the concrete. Steel anchorages located below grade and not encased in concrete shall require further corrosion protection in addition to galvanizing. Thermal-cut edges shall be ground back to remove the thermally affected region before galvanizing. To protect against liquid metal assisted cracking, tin (Sn) shall not be added to the galvanizing bath in any quantity. Lead (Pb) and bismuth (Bi) have been found to slightly increase sensitivity to LMAC; their use in galvanizing baths should be minimized. Galvanization shall be performed at the highest practical rate of speed. C6.7.9 Drainage and vent holes result in a reduction of the member’s net section and cause stress risers, thereby reducing fatigue resistance. Holes shall be placed at noncritical locations where these reductions will not result in the member’s strength being less than the required strength for maximum design loadings or fatigue. Structural steel commonly used in highway structures can be susceptible to liquid metal assisted cracking (LMAC) during galvanizing, wherein the liquid zinc reacts with the steel base metal resulting in significantly lower strain capacity in the galvanizing bath than is available in an in-air environment. To control against potential cracking during hot dip galvanizing, attention should be paid to the following fabrication and galvanizing aspects shown to be influential in NCHRP Project 10-94: • Minimize stress concentrations in the steel part(s) to be galvanized: The severity of stress concentrations was found to be closely linked with increased susceptibility to LMAC during hot-dip galvanizing (HDG). This factor is especially important to control, as there is an influence of increased stress at the notch from the stress concentration, but also increased interaction with zinc in the presence of a notch. Therefore, avoid notches and grooves and crack-like details (e.g., partial penetration weldments). • Minimize thermal-cut edges and ensure any thermal-cut edges have been ground to remove the thermally affected region before galvanizing. Heat affected zones associated with thermal cutting were found to be sensitive to LMAC. • Cold-working was not found to be a strong predictor of LMAC, however, cold-working is often associated with the formation of flaws and stress concentrations, particularly for areas deformed by cold-bending. These flaws should be ground smooth to the extent possible to reduce likelihood of LMAC. Avoiding cold-bending also eliminates stress concentrations and is recommended wherever possible. • The presence of some alloys commonly added to the galvanizing bath has been shown to aggravate occurrences of LMAC. In particular, tin (Sn) has been shown in multiple studies to aggravate LMAC in extremely small quantities and thus should not be added to galvanizing baths in any quantity. Lead (Pb) and bismuth (Bi) are common alloy additions to galvanizing baths. When used in typical quantities in combination (<1% Pb, <0.1% Bi), these alloys corresponded with only a slight increased sensitivity to LMAC. However, it should be noted that galvanizing with pure zinc (e.g., Special High Grade zinc) provided less risk in terms of LMAC. • Hardness measurements > 270 HV were found to be a reasonable predictor for increased susceptibility to LMAC. Therefore, it is recommended that hardness measurements be utilized before galvanizing in areas of concern to determine whether thermal pre-treatments should be performed to reduce hardness before galvanizing. • Caution should be exercised when galvanizing statically indeterminate structures or components where there are

84 Mitigation of Weldment Cracking in Steel Highway Structures Due to the Galvanizing Process significant differences between the stiffness and/or thermal mass of their components or parts because large stresses and permanent deformations may be induced during galvanizing. Large differences in stiffness and/or mass of components worsen this problem. • Increasing the rate of dipping and extraction during galvanizing was found to correspond with decreased stresses during galvanizing. • Preheating components is beneficial because thermal shock is reduced. Additional guidance on design and detailing to facilitate galvanizing may be found in “Design of Products to be Hot- dip Galvanized after Fabrication” available from the American Galvanizers Association.