C-M2M2 Fabrication

M2.1 Cambering, Curving, and Straightening

In addition to mechanical means, local application of heat is permitted for curving, cambering, and straightening. Maximum temperatures are specified in Section M2 for hot-rolled structural shapes, hollow structural sections (HSS), plates, and bars to avoid metallurgical damage and inadvertent alteration of mechanical properties. In general, these should not be viewed as absolute maximums; they include an allowance for a variation of about 100°F (38°C), which is a common range achieved by experienced fabricators (FHWA, 1999).

For steels that are heat-treated through quenching and tempering and for steel products not addressed in Section M2, it is advisable to perform heat-assisted processing within temperature limitations established through recommendation by the producer of the material or the manufacturer of the steel product.

ASTM standards may include different grades within a standard. Instances exist where the heat shrinking limits are different for one grade than all other grades within the standard.

Temperatures should be measured by appropriate means, such as temperature-indicating crayons and steel color. Precise temperature measurements are seldom called for. Also, surface temperature measurements should not be made immediately after

removing the heating torch because it takes a few seconds for the heat to soak into the steel.

Local application of heat has long been used as a means of straightening or cambering beams and girders. With this method, selected zones are rapidly heated and tend to expand. But the expansion is resisted by the restraint provided by the surrounding unheated areas. Thus, the heated areas are “upset” (increase in thickness) and, upon cooling, they shorten to effect a change in curvature. In the case of trusses and girders, cambering can be built in during assembly of the component parts.

Although the desired curvature or camber can be obtained by these various methods, including at room temperature (cold cambering) (Bjorhovde, 2006), it must be realized that some deviation due to workmanship considerations, as well as some permanent change due to handling, is inevitable. Camber is usually defined by one mid-ordinate, because control of more than one point is difficult and not normally needed. Reverse cambers are difficult to achieve and are discouraged. Long cantilevers are sensitive to camber and may deserve closer control.

M2.2 Thermal Cutting

Thermal cutting is preferably done by machine. The requirement in Section M2.2 for preheat before thermal cutting is to minimize the creation of a hard surface layer and the formation of cracks. This requirement for preheat for thermal cutting does not apply when the radius portion of the access hole or cope is drilled and the thermally cut portion is essentially linear. Such thermally cut surfaces are required to be ground in accordance with Section J1.6. After welding, the weld access hole surface is to be visually inspected in accordance with Table N5.4-3. The surface resulting from two straight torch cuts meeting at a point is not considered to be a curve.

M2.4 Welded Construction

To avoid weld contamination, the light oil coating that is generally present after manufacturing an HSS should be removed with a suitable solvent in locations where welding will be performed. In cases where an external coating has been applied at the mill, the coating should be removed at the location of welding, or the manufacturer should be consulted regarding the suitability of welding in the presence of the coating.

M2.5 Bolted Construction

In most connections made with high-strength bolts, it is only required to install the bolts to the snug-tight condition. This includes bearing-type connections where slip is permitted and, for ASTM F3125/F3125M Grade A325 or A325M bolts only, ten-sion or combined shear and tension applications where loosening or fatigue due to vibration or load fluctuations are not design considerations.

It is suggested that snug-tight bearing-type connections with ASTM F3125/F3125M Grade A325 or A325M or ASTM F3125/F3125M Grade A490 or A490M bolts be used in applications where ASTM A307 bolts are permitted.

This section provides rules for the use of oversized and slotted bolt holes parallel- ing the provisions that have been in the RCSC Specification for High-Strength Bolts

since 1972 (RCSC, 2020), extended to include ASTM A307 bolts, which are outside the scope of the RCSC Specification.

The Specification previously limited the methods used to form bolt holes, based on common practice and equipment capabilities. Fabrication methods have changed and will continue to do so. To reflect these changes, this Specification has been revised to define acceptable quality instead of specifying the method used to form the bolt holes, and specifically to permit thermally cut and water jet cut bolt holes. AWS C4.1, Sample 3, is useful as an indication of the thermally cut profile that is acceptable (AWS, 1977). The use of numerically controlled or mechanically guided equipment is anticipated for the forming of thermally cut bolt holes. To the extent that the previous limitations may have related to safe operation in the fabrication shop, fabricators are referred to equipment manufacturers for equipment and tool operating limits.

M2.10 Drain Holes

Because the interior of an HSS is difficult to inspect, concern is sometimes expressed regarding internal corrosion. However, good design practice can eliminate the concern and the need for expensive protection. Corrosion occurs in the presence of oxygen and water. In an enclosed building, it is improbable that there would be sufficient reintroduction of moisture to cause severe corrosion. Therefore, internal corrosion protection is a consideration only in HSS that are exposed to weather. In a sealed HSS, internal corrosion cannot progress beyond the point where the oxygen or moisture necessary for chemical oxidation is consumed (AISI, 1970). The oxidation depth is insignificant when the corrosion process must stop, even when a corrosive atmosphere exists at the time of sealing. If fine openings exist at connections, mois- ture and air can enter the HSS through capillary action or by aspiration due to the partial vacuum that is created if the HSS is cooled rapidly (Blodgett, 1967). This can be prevented by providing pressure-equalizing holes in locations that make it impos- sible for water to flow into the HSS by gravity.

Situations where an internal protective coating may be required include (1) open HSS where changes in the air volume by ventilation or direct flow of water is possible, and (2) open HSS subjected to a temperature gradient that causes condensation. In such instances, it may also be prudent to use a minimum 5/16 in. (8 mm) wall thickness.

HSS that are filled or partially filled with concrete should not be sealed. In the event of fire, water in the concrete will vaporize and may create pressure sufficient to burst a sealed HSS. Care should be taken so that water does not remain in the HSS during or after construction, because the expansion for HSS exposed to freezing can create pressure that is sufficient to burst an HSS. Galvanized HSS assemblies should not be completely sealed because rapid pressure changes during the galvanizing process tend to burst sealed assemblies.

M2.11 Requirements for Galvanized Members

Cracking during hot-dip galvanizing has been observed in steel members. The occurrence of these cracks has been correlated to several characteristics including, but not limited to, the following:

• Abrupt geometric changes, such as copes, weld access holes, re-entrant corners with small radii, half depth end plates, and weld terminations—the severity of stress concentration has been linked with increased susceptibility to cracking during galvanizing.
• • Cold-worked steel that has imperfections induced in the material from processing; grinding such details to reduce imperfections may improve performance.
• • Thermally cut edges that are not ground.
• • Welded assemblies involving significant differences in thickness, such as thick base plates welded to thin tubes for signs and light poles.
• • Galvanizing practices, including care for cleaning acids, dipping speeds, and temperatures. Speeds faster than 18 ft/min (5.5 m/min) are preferred, as are shorter holding times.
• Zinc bath composition (Sn ≤ 0.1% is preferred, Pb + Bi ≤ 1.5% is preferred).
• • Steel strength level (lower strength levels are less susceptible).
• • Steel composition (Si, P, and B have been identified as undesirable).
• • Residual stresses from welding or thermal cutting (additional preheat and welding sequence may be used to mitigate residual stresses).
• • Hardness greater than 270 HV may indicate increased susceptibility to cracking during hardening. Hardness measurements may be utilized to determine whether thermal pre-treatments should be applied before galvanizing.

The Specification requirement to grind thermally cut surfaces before galvanizing beam copes may not prevent all cope cracks from occurring during galvanizing; however, it has been shown to be an effective means to reduce the occurrence of this phenomenon. It can be useful to develop an inspection plan that concentrates on susceptible details. ASTM publishes the following standards related to galvanized structural steel.

• • ASTM A123 (ASTM, 2017a) provides a standard for the galvanized coating and its measurement and includes provisions for the materials and fabrication of the products to be galvanized.
• • ASTM A143/A143M (ASTM, 2020a) is a practice standard covering procedures that can be followed to safeguard against the possible embrittlement of steel hot-dip galvanized after fabrication and outlines test procedures for detecting embrittlement.
• • ASTM A384/384M (ASTM, 2019a) is a practice standard that includes information on factors that contribute to warpage and distortion as well as suggestions for correction for fabricated assemblies.
• • ASTM A385/385M (ASTM, 2020b) is a practice standard that includes information on base materials, venting, treatment of contacting surfaces, and cleaning. Many of these provisions should be indicated on the design and detail documents.
• • ASTM A780/A780M (ASTM, 2020c) provides for repair of damaged and uncoated areas of hot-dip galvanized coatings.
• • ASTM F2329/F2329M (ASTM, 2015c) covers the requirements for hot-dip zinc coating applied to carbon and alloy steel bolts, screws, washers, nuts, and special threaded fasteners.