C-A3A3 Material

A3.1 Structural Steel Materials

A3.1a Listed Materials

There are hundreds of steel materials and products. This Specification lists those products/materials that are commonly useful to structural engineers and those that have a history of satisfactory performance as anticipated in the other provisions of the Specification. Other materials may be suitable for specific applications, but the evaluation of those materials is the responsibility of the engineer specifying them. In addition to typical strength properties, considerations for materials may include, but are not limited to, strength properties in transverse directions, ductility, formability, soundness, weldability including sensitivity to thermal cycles, notch toughness, and other forms of crack sensitivity, coatings, and corrosivity. Consideration for product form may include material considerations in addition to effects of production, tolerances, testing, reporting, and surface profiles.

Table A3.1 includes information concerning permitted grades and strengths and other limitations that are beyond what has historically been presented. ASTM A572/A572M Type 5 is not included due to potential problems associated with welding. HSS are limited to electric-resistance welding (ERW) or seamless to prohibit the use of furnace welding that may result in seams that do not achieve full strength of the base metal. Materials used in the production of ASTM A1065/A1065M are limited because other materials listed in ASTM A1065/A1065M do not have a history of satisfactory performance when used in buildings. Welding quenched and tempered steels such as ASTM A514/A514M can be challenging due to their higher strength and the need to control welding procedures more carefully. If proper procedures are not followed, high hardness in the heat affected zone is possible. This can lead to premature or even immediate failure. While this should not prohibit its use, the engineer should consult the steel producer if they choose to use the material. ASTM A709/A709M Grade 50CR (345CR) is a stainless steel and is not approved for use with the Specification.

Hot-Rolled Shapes. The commentary herein contains information relevant to hot-rolled shapes, as well as plates. Particularly, in segments of this commentary where the term “rolled steel” or “hot-rolled steel” is used, the information may be applicable to both shapes and plates. The grades of steel approved for use under this Specification, covered by ASTM Specifications, extend to a yield stress of 80 ksi (550 MPa) for rolled shapes and 100 ksi (690 MPa) for plates. Some of the ASTM Specifications specify a minimum yield point, while others specify a minimum yield strength. The term “yield stress” is used in this Specification as a generic term to denote either the yield point or the yield strength.

It is important to be aware of limitations of availability that may exist for some combinations of strength and size. Not all structural section sizes are included in the various material specifications. For example, the 60 ksi (415 MPa) yield stress steel in ASTM A572/A572M includes plate only up to $2 \frac{1}{2}$ in. (63 mm) in thickness. Another limitation on availability is that even when a product is included in this Specification, it may be infrequently produced by the mills. Specifying these products may result in procurement delays or require ordering large quantities directly from the producing mills. Consequently, it is prudent to check availability before completing the details of a design. The AISC web site provides this information (www.aisc.org).

Properties in the direction of rolling are of principal interest in the design of steel structures. Hence, yield stress, as determined by the standard tensile test, is the principal mechanical property recognized in the selection of the steels approved for use under this Specification. It must be recognized that other mechanical and physical properties of rolled steel, such as anisotropy, ductility, notch toughness, formability, corrosion resistance, etc., may also be important to the satisfactory performance of a structure.

For rotary-straightened W-shapes, an area of reduced notch toughness has been documented in a limited region of the web immediately adjacent to the flange. Considerations in design and detailing that recognize this situation are presented in Chapter J.

It is not possible to incorporate in the Commentary adequate information to impart full understanding of all factors that might merit consideration in the selection and specification of materials for unique or especially demanding applications. In such a situation, the user of this Specification is advised to make use of reference material contained in the literature on the specific properties of concern and to specify supplementary material production or quality requirements as provided for in ASTM material specifications.

The following special cases should be considered:

Highly Restrained Welded Connections and Lamellar Tearing. Rolled steel is anisotropic, especially insofar as ductility is concerned; therefore, weld contraction strains in the region of highly restrained welded connections may exceed the strength of the material if special attention is not given to material selection, details, workmanship, and inspection (AISC, 1973).

Potential through-thickness structural problems of hot-rolled steel, including lamellar tearing and delamination, result from an interaction of several factors: connection design and detailing; fabrication and erection procedures; and base metal properties, including fracture toughness and ductility in the through-thickness direction. Highly restrained connections of any hot-rolled members may be susceptible, regardless of thickness, though this phenomenon is more likely with material that exceeds $\frac{3}{4}$ in. (19 mm). See “Through Thickness Properties of Structural Steels” (Barsom and Korvink, 1997) for a complete discussion of the material considerations. As early in the project as possible, the engineer should consult the steel producer regarding if and where improved and consistent through-thickness properties are required. The

steel producer can supply steel with improved through-thickness properties by implementing additional metallurgical or heat treatment processing.

The geometry of the welded connections should be designed to avoid through-thickness tensile stresses and configure the joint to best limit restraint and reduce weld metal volume. See recommendations in AWS D1.1/D1.1M Commentary C-4.7.3 (AWS, 2020) and “Commentary on Highly Restrained Welded Connections” (AISC, 1973).

Other measures the engineer can specify to mitigate through-thickness challenges are listed below:

• (1) Expand use of ultrasonic testing (UT) to inspect the steel at the location of the weld and surrounding area before and after welding. AISC Seismic Provisions for Structural Steel Buildings (AISC, 2022c) Section J7.2c requires the UT of base metal thicker than 1½ in. (38 mm) after joint (weld) completion behind and adjacent to the fusion line, and refers to AWS D1.1/D1.1M (AWS, 2020), Table 8.2, for acceptance criteria.
• (2) Specify nondestructive examination (NDE) during the fabrication process. Visual examination, liquid penetrant, and magnetic particle testing can be used to identify surface discontinuities. UT can be used to detect internal discontinuities. Plate can be mill ordered with supplementary requirement S8, “Ultrasonic Examination,” of ASTM A6/A6M, specifically ASTM A435/A435M (ASTM, 2017b) and ASTM A578/A578M (ASTM, 2017c). Radiography is not recommended because the relevant orientation of the discontinuities of interest are difficult to detect with this method. Results of NDE will enable the fabricator to locate steel members such that particularly susceptible material is not subjected to excessive through-thickness strains.
• (3) Specify through-thickness tensile testing. For steel plates this is achieved in accordance with ASTM A770/A770M (ASTM, 2018b). This test evaluates susceptibility to lamellar tearing, where a minimum area reduction value of 20% has generally been shown to provide adequate ductility to help mitigate the problem, though a minimum value of 15% may be acceptable. CEN (2005c) gives a procedure for specifying area reductions of 15% and greater.

Including any of these measures should be carefully considered with input from the contractor and steel supplier regarding the cost, ease of implementation, and relative effectiveness.

Fracture Control Design for Service Conditions. For especially demanding service conditions, such as structures exposed to low temperatures, particularly those with cyclic or impact loading, the specification of steels with superior notch toughness may be warranted. However, for most buildings, the steel is relatively warm, strain rates are essentially static, and the stress intensity and number of cycles of full design stress are low. Accordingly, the probability of fracture in most building structures is low. Good workmanship and good design details incorporating joint geometry that avoids severe stress concentrations are generally the most effective means of providing fracture-resistant construction (AASHTO, 2014).

TABLE C-A3.1 Recommended CVN Requirements for Exposure Below –30°F (–34°C)

The Charpy V-notch (CVN) test allows for the measurement of the amount of energy absorbed during the fracture of standardized test specimens at a given temperature. The absorbed energy is the measure of the material’s notch toughness. The CVN testing temperature can be significantly different from the material’s service temperature to account for the difference in strain rate between the Charpy impact test (on the order of $10 \mathrm{sec}^{-1} \mathrm{in} . / \mathrm{in} .$ ) and the designed strain rate of the structure (typically $10^{-3}$ $\mathrm{sec}^{-1} \mathrm{in} . / \mathrm{in} .$ for static, wind, and seismic loadings) (Barsom and Rolfe, 1999).

Shapes produced with no minimum specified CVN requirements will generally provide some level of toughness, although no minimum values are guaranteed. In applications where increased fracture resistance is required, specified minimum CVN properties may offer improved reliability. For structures that are exposed to temperatures below –30°F (–34°C), contract documents should specify steel that meets the minimum average absorbed energy recommended in Table C-A3.1 for main members that are statically loaded and subjected to bending or tension.

Plates. Portions of the Commentary for hot-rolled shapes are also applicable for plates.

Hollow Structural Sections (HSS). Specified minimum tensile properties are summarized in Table C-A3.2 for various HSS material specifications and grades. ASTM A53/A53M Grade B is a pipe specification included as an approved HSS material specification because it is the most readily available round product in the United States. Other North American HSS products that have properties and characteristics that are similar to the approved ASTM products are produced in Canada under the General Requirements for Rolled or Welded Structural Quality Steel (CSA, 2013). In addition, pipe is produced to other specifications that meet the strength, ductility, and weldability requirements of the materials in Section A3, but may have additional requirements for notch toughness or pressure testing. As stated in Section A3.1b,

TABLE C-A3.2 Minimum Tensile Properties of HSS Steels

materials not specifically listed in Table A3.1 are permitted when suitability is determined by the engineer of record (EOR).

Round HSS can be readily obtained in ASTM A53/A53M material and ASTM A500/A500M Grade C is also common. For rectangular HSS, ASTM A500/A500M Grade C is the most commonly available material and a special order would be required for any other material. Depending upon size, either welded or seamless round HSS can be obtained. In North America, however, all ASTM A500/A500M rectangular HSS for structural purposes are welded. Rectangular HSS differ from box sections in that they have uniform thickness, except for some thickening in the rounded corners.

Nominal strengths of directly welded T-, Y-, and K-connections of HSS have been developed analytically and empirically. Connection deformation is anticipated and is an acceptance limit for connection tests. Ductility is necessary to achieve the expected deformations. The ratio of the specified minimum yield strength to the specified minimum tensile strength (yield/tensile ratio) is one measure of material ductility. Materials in HSS used in connection tests have had a yield/tensile ratio of up to 0.80 and therefore that ratio has been adopted as a limit of applicability for directly welded HSS connections. ASTM A500/A500M Grade A material does not meet this ductility “limit of applicability” for direct connections in Chapter K. ASTM A500/A500M Grade C has a yield/tensile ratio of 0.807 but it is reasonable to use the rounding method described in ASTM E29 (ASTM, 2019c) and find this material acceptable for use.

Even though ASTM A501/A501M includes rectangular HSS, hot-formed rectangular HSS are not currently produced in the United States. The General Requirements for Rolled or Welded Structural Quality Steel (CSA, 2013) includes Class C (cold-formed) and Class H (cold-formed and stress relieved) HSS. Class H HSS have relatively low levels of residual stress, which enhances their performance in compression and may provide better ductility in the corners of rectangular HSS.

API 5L (API, 2012) is a line pipe specification that has some requirements for mechanical characteristics that make it advantageous for use in specific structural applications, such as in long span roofs with long unbraced lengths or large composite columns in heavy unbraced frames. Note, however, that Section A3.1b states, for materials not specifically listed in Table A3.1, suitability must be determined by the EOR. The specified minimum yield strength of API 5L ranges from 25 to 80 ksi (170 to 550 MPa) and the specified minimum tensile strength ranges from 45 to 90 ksi (310 to 620 MPa), depending on product specification level and material grade. For Grades X42 and higher, additional elements may be used upon agreement between the purchaser and the manufacturer; however, care should be exercised in determining the alloying content for any given size and wall thickness of pipe, because the addition of such otherwise desirable elements may affect the weldability of the pipe. PSL2 pipe is a common structural choice and Grade X52 is probably the most common grade for structural purposes. Some pertinent mechanical and geometric properties for PSL2 X52N are $F_{y}=52 \mathrm{ksi}(360 \mathrm{MPa}) ; F_{u}=66 \mathrm{ksi}$ $(460 \mathrm{MPa})$; toughness $=20 \mathrm{ft}-\mathrm{lbf} @ 32^{\circ} \mathrm{F}(27 \mathrm{~J} @ 0^{\circ} \mathrm{C})$ for $D \leq 30 \mathrm{in} .(750 \mathrm{~mm})$; a wall thickness lower tolerance of $-10 \%$ for ${ }^{3} /{ }_{16} \mathrm{in} .<t<{ }^{19} /{ }_{32} \mathrm{in} .(5 \mathrm{~mm}<t<15 \mathrm{~mm})$, and $-0.02 \mathrm{in} .(-0.5 \mathrm{~mm})$ for $t<{ }^{3} /{ }_{16} \mathrm{in} .(t<5 \mathrm{~mm})$; and a mass or area tolerance of $-3.5 \%$ for regular plain-ended pipe. With a diameter range from ${ }^{13} /{ }_{32} \mathrm{in}$. to 84 in. (10 mm to 2100 mm), this high-quality pipe material addresses a frequent need for either large diameter or thick-walled round hollow sections. Other special features of PSL2 pipe are an upper bound on the yield strength [for example, for X52 the minimum and maximum yield strengths are 52 ksi (360 MPa) and 76 ksi (520 MPa), respectively], and a maximum yield-to-tensile stress ratio of 0.93 in the as-delivered pipe [for $D>12.75 \mathrm{in} .(319 \mathrm{~mm})$ ].

A3.1d Rolled Heavy Shapes

The web-to-flange intersection and the web center of heavy hot-rolled shapes, as well as the interior portions of heavy plates, may contain a more coarse grain structure and/or lower notch toughness material than other areas of these products (Cattan, 1995; Jaquess and Frank, 1999). This is probably caused by ingot segregation, the somewhat lesser deformation during hot rolling, higher finishing temperature, and the slower cooling rate after rolling for these heavy sections. This characteristic is not detrimental to suitability for compression members or for nonwelded members. However, when heavy cross sections are joined by splices or connections using complete-joint-penetration groove welds that extend through the coarser and/or lower notch-tough interior portions, tensile stresses induced by weld shrinkage may result in cracking. An example is a complete-joint-penetration groove welded connection of a heavy cross-section beam to any column section. When members of lesser thickness are joined by complete-joint-penetration groove welds, which induce smaller

weld shrinkage strains, to the finer grained and/or more notch-tough surface material of ASTM A6/A6M shapes and heavy built-up cross sections, the potential for cracking is significantly lower. An example is a complete-joint-penetration groove welded connection of a nonheavy cross-section beam to a heavy cross-section column.

For critical applications, such as primary tension members, material should be specified to provide adequate notch toughness at service temperatures. Because of differences in the strain rate between the Charpy V-notch (CVN) impact test and the strain rate experienced in actual structures, the CVN test is conducted at a temperature higher than the anticipated service temperature for the structure. The location of the CVN test specimens (“alternate core location”) is specified in ASTM A6/A6M, Supplemental Requirement S30.

The notch toughness requirements of Sections A3.1d and A3.1e are intended only to provide structural steel material with a nominal yield strength of 50 ksi with reasonable notch toughness for ordinary service applications [enclosed structures or lowest anticipated service temperature of 50°F (10°C) or higher]. For unusual applications, low temperature service, and/or higher strength material, more restrictive requirements and/or notch toughness requirements for other section sizes and thicknesses may be appropriate (Barsom and Rolfe, 1999). To minimize the potential for fracture, the notch toughness requirements of Sections A3.1d and A3.1e must be used in conjunction with good design and fabrication procedures. Specific requirements are given in Sections J1.5, J1.6, J2.6, and J2.7.

A3.1e Built-Up Heavy Shapes

Portions of Commentary Section A3.1d for rolled heavy shapes are also applicable for built-up heavy shapes.

A3.2 Steel Castings and Forgings

Design and fabrication of cast and forged steel components are not covered in this Specification.

Steel Castings. There are a number of ASTM Specifications for steel castings. The Steel Founders' Society of America (SFSA) Steel Castings Handbook (SFSA, 1995) discusses a number of standards useful for steel structures. In addition to the requirements of this Specification, SFSA recommends that various other requirements be considered for cast steel products. Continued quality assurance is critical to make certain there is confidence in the cast product. This includes testing of first article components as well as production testing. It may be appropriate to inspect the first piece cast using magnetic particle inspection (MPI) in accordance with ASTM E125, degree 1a, b, or c (ASTM, 2013). Radiographic inspection level III may be desirable for the first piece cast. Ultrasonic testing (UT) in compliance with ASTM A609/ A609M (ASTM, 2012a) may be appropriate for the first cast piece over 6 in. (150 mm) thick. UT and MPI of production castings are also advisable. Design approval, sample approval, periodic nondestructive testing, chemical testing, and selection of the correct welding specification should be among the issues defined in the selection and procurement of cast steel products. Refer to SFSA (1995) for design information about cast steel products. For visual examination, refer to ASTM A802 (ASTM,

2015a); for magnetic particle and liquid penetrant surface and subsurface examination, refer to ASTM A903/A903M (ASTM, 2012b); for radiographic examination, refer to ASTM E1030/E1030M (ASTM, 2015b); and for ultrasonic examination, refer to ASTM A609/A609M (ASTM, 2012a). ASTM A958/A958M (ASTM, 2021b) is a cast steel used in the Kaiser Bolted Bracket Moment Connection, a prequalified moment connection in ANSI/AISC 358 (AISC, 2022b), but it may also be specified in some nonseismic applications. Additional information about cast steels can be found in the Steel Castings Handbook, Supplement 2 (SFSA, 2009).

Steel Forgings. There are a number of ASTM specifications for steel forgings. The Forging Industry Association’s Forging Industry Handbook (FIA, 1985) discusses some typical forging issues, but more detailed information can be obtained at www.forging.org. Steel forgings should conform to ASTM A668/A668M (ASTM, 2021a) and the related ASTM testing requirements. UT should be in compliance with ASTM A388/A388M (ASTM, 2019b) and MPI in accordance with ASTM A275/ A275M (ASTM, 2018a). Many of the frequently used structural forgings are catalog items for which the testing has been established. For custom forgings, the frequency and type of testing required should be established to conform to ASTM requirements.

A3.3 Bolts, Washers, and Nuts

ASTM F3125/F3125M is an umbrella specification, first referenced in the 2016 AISC Specification for Structural Steel Buildings (AISC, 2016), that covers fasteners that were previously referred to as ASTM A325/A325M, ASTM A490/A490M, ASTM F1852, and ASTM F2280 fasteners. These previously separate standards have been unified, coordinated, and made consistent with each other, turning them into Grades of ASTM F3125/F3125M. From the user perspective, not much has changed, as the head marks remain the same, and handling and installation remain the same. Nevertheless, the specifier should be aware that ASTM F3125/F3125M now contains Grades A325, A325M, A490, A490M, F1852, and F2280 fasteners. One change of note is that under ASTM F3125/F3125M, Grades A325 and A325M fasteners are uniformly 120 ksi (830 MPa); Grades A325 and A325M had a drop in strength to 105 ksi (725 MPa) for diameters over 1 in. (25 mm) in previous standards.

The ASTM standard specification for ASTM A307 bolts covers two grades of fasteners. Either grade may be used under this Specification; however, it should be noted that Grade B is intended for pipe-flange bolting and Grade A is the grade long in use for structural applications.

A3.4 Anchor Rods and Threaded Rods

ASTM F1554 is the primary specification for anchor rods. Because there is a limit on the maximum available length of structural bolts, the attempt to use these bolts for anchor rods with design lengths longer than the maximum available lengths has presented problems in the past. The inclusion of ASTM A449 and ASTM A354 materials in this Specification allows the use of higher strength material for bolts longer than structural bolts.

The EOR should specify the required strength for threaded rods used as load-carrying members.

A3.5 Consumables for Welding

The AWS filler metal specifications listed in Section A3.5 are general specifications that include filler metal classifications suitable for building construction, as well as classifications that may not be suitable for building construction. AWS D1.1/ D1.1M, Structural Welding Code—Steel (AWS, 2020) Table 5.4 lists the various filler metals that may be used for prequalified welding procedure specifications, for the various steels that are to be joined. This list specifically does not include various classifications of filler metals that are not suitable for structural steel applications. Filler metals listed under the various AWS A5 filler metal specifications may or may not have specified notch toughness properties, depending on the specific electrode classification. Section J2.6 identifies certain welded joints where notch toughness of filler metal is needed in building construction. There may be other situations where the EOR may elect to specify the use of filler metals with specified notch toughness properties, such as for structures subjected to high loading rate, cyclic loading, or seismic loading. Because AWS D1.1/D1.1M does not automatically require that the filler metal used have specified notch toughness properties, it is important that filler metals used for such applications be of an AWS classification, where such properties are required. This information can be found in the various AWS filler metal specifications and is often contained on the filler metal manufacturer’s certificate of conformance or product specification sheets.

When specifying filler metal and/or flux by AWS designation, the applicable standard specifications should be carefully reviewed to assure a complete understanding of the designation reference. This is necessary because the AWS designation systems are not consistent. For example, in the case of electrodes for shielded metal arc welding, AWS A5.1/A5.1M, the first two or three digits indicate the nominal tensile strength classification, in ksi, of the filler metal and the final two digits indicate the type of coating. For metric designations, the first two digits times 10 indicate the nominal tensile strength classification in MPa. In the case of mild steel electrodes for submerged arc welding, AWS A5.17/A5.17M, the first one or two digits times 10 indicate the nominal tensile strength classification for both U.S. customary and metric units, while the final digit or digits times 10 indicate the testing temperature in °F for filler metal impact tests. In the case of low-alloy steel covered arc welding electrodes, AWS A5.5/A5.5M, certain portions of the designation indicate a requirement for stress relief, while others indicate no stress relief requirement.

Engineers do not, in general, specify the exact filler metal to be employed on a particular structure. Rather, the decision as to which welding process and which filler metal is to be utilized is usually left with the fabricator or erector. Codes restrict the usage of certain filler metals or impose qualification testing to prove the suitability of the specific electrode, so as to make certain that the proper filler metals are used.