FASTENER MATERIALS AND GRADES

U.S. Customary Unit Fasteners
Although metric fastener standards have been available in the US for more than 20 years, metric structural bolting is still not fully implemented in the United States. The Industrial Fasteners Institute publishes an excellent summary of all metric fastener specifications in its Metric Fastener Standards.⁽¹⁾ The specific standards from the American Society for Testing Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO) contain a great deal more information about the geometry, material, manufacturing and marking of these fasteners. Additional information can be found in the AISC Metric Conversion: Load and Resistance Factor Design Specification for Structural Steel Buildings.⁽²⁾ The following sections describe metric structural bolting but the student should also have some familiarity with the US Customary Unit (USCU) fasteners that have been used throughout the past fifty years. Both USCU and metric fasteners are marked so that iron workers and inspectors can quickly and easily verify that they are using the correct type of fastener (e.g., strength of bolt). The sections that follow show the metric head markings and the figure below shows the markings for the most common types of USCU structural bolts. The most commonly used bolts in the USCU system were A307 bolts (also called "common" or "unfinished" bolts) and A325 bolts although A499 and A490 bolts are also sometimes used. Some common head markings are shown in the figure below. Every bolt head must be marked with the bolt type (e.g., A307-A or A325), an indication of the material (e.g., high-strength material is indicated by three radial lines) and the manufacturers mark (e.g., MFG in the figure).

ASTM F568

ASTM F568 defines materials used for manufacturing a broad category of fasteners with a wide variety of materials strengths. All metric fasteners are made using materials that are specified in ASTM F568. Some classes of bolts in the F568 family that are essentially equivalent to some common types of structural bolts are the following:

• Class 4.6 mechanical properties are essentially the same as A307 Grade A,
• Class 8.8 mechanical properties are essentially the same as A449 and A325 Types 1 and 2 (note: the material is similar to A325 but the geometry is different),
• Class 8.8.3 mechanical properties are essentially the same as A325 Type 3, a weathering steel,
• Class 10.9 mechanical properties are essentially the same as A490 Types 1 and 2,
• Class 10.9.3 mechanical properties are essentially the same as A490 Type 3, a weathering steel.

Except for bolts smaller than 5 mm, all bolt heads are marked such that the property class is easily determined as shown in the figure below. The bolt head are also marked with a manufacturers mark (e.g., MFG in the figure). Metric bolts are identified by their nominal diameter and the pitch. A 24-mm diameter bolt with a 3-mm pitch is designated as an M24x3 bolt, the M indicating that this is a metric bolt. Since there is only one pitch per diameter, it is common to refer to metric bolts only with the nominal diameter, for example M24 or M16. The nuts selected for a particular application must match the bolt for both material type and geometry.

Unfinished Bolts

ASTM F568 Class 4.6 fasteners are essentially equivalent to USCU ASTM A307 fasteners. These bolts are often referred to as "common" bolts, "unfinished" bolts or "rough" bolts. Technically they are not really structural bolts at all but are included here because they are often used in bearing connections where the loading is not large or is not critical to the performance or safety of the structure. Such bolts might, for example, be used in a roof truss made up of angle sections in an industrial building or to connect brace members to main structural members.

USCU A307 bolts are not proof tested but F568 Class 4.6 bolts are. The proof loads and tensile strengths of Class 4.6 bolts are shown in the table on the following page. Class 4.6 bolts are available in 19 different sizes ranging from as small as a 5 mm diameter to as large as a 100 mm diameter but unfinished bolts smaller than 12 mm or larger than 24 mm would be unusual for structural applications.

ASTM A325M

ASTM A325M bolts are probably the most commonly used fastener for steel construction (ASTM A325M bolts are also sometimes specified as AASHTO M164M bolts in designing transportation structures like bridges). A325 bolts are intended to be tightened to nearly their proof load so that they develop significant bolt tension. Although A325M bolts are made using A568 Class 8.8 material, the geometry of the bolts are larger than common bolts because A325M bolts are always subjected to high tensile loads. A325M bolts have thicker and wider heads to more effectively distribute the head load to the piece and shorter shanks to keep the threads out of the load bearing part of the bolt. These of larger dimensions are referred to as "heavy hex" geometry. A325M bolts are manufactured using material with a minimum yield strength of 660 MPa and a minimum ultimate strength of 830 MPa.

There are three types of A325M bolts:

• Type 1 -- a general-purpose medium carbon steel materials,
• Type 2 -- a low-carbon martensite steel for atmospheric temperature applications, and
• Type 3 -- a weathering steel suitable for use with sections or plate made using ASTM A588 weathering steel.

ASTM A325M specifies that high strength bolts must be tightened such that the bolt tension is 70 percent of the minimum tensile strength of the bolt (see also LRFD Table J3.1). The minimum tensile strength and the minimum bolt installation tension are shown in the table below. The values in the minimum installation strength column are 70 percent of the values in the tensile strength column (e.g., for an M16x2 bolt the minimum installation bolt tension is 91 which is 70 percent of the 130 kN minimum tensile strength).

ASTM A490M

ASTM A490M bolts (also AASHTO M253M) are similar to A325 bolts in that they are heavy-hex geometry but A490 bolts use even higher strength material conforming to F568 Class 10.9. A490 have a minimum yield strength of 940 MPa and a minimum ultimate strength of 1040 MPa.

There are three types of A490M bolts:

• Type 1 -- a general-purpose medium carbon steel materials,
• Type 2 -- a low-carbon martensite steel for atmospheric temperature applications, and
• Type 3 -- a weathering steel suitable for use with sections or plate made using ASTM A588 weathering steel.

ASTM A325M specifies that high strength bolts must be tightened such that the bolt tension is 70 percent of the minimum tensile strength of the bolt (see also LRFD Table J3.1). The minimum tensile strength and the minimum bolt installation tension are shown in the table below. The values in the minimum installation strength column are 70 percent of the values in the tensile strength column (e.g., for an M16x2 bolt the minimum installation bolt tension is 91 which is 70 percent of the 130 kN minimum tensile strength).

METHODS OF TENSIONING BOLTS

When designing slip-critical connections it is very important that the fastener be tightened such that the bolt is in a slightly yielded condition. If a bolt is not tight enough, the bolt tension may not be enough to generate the required friction between the joined parts. If the bolt tension is too large, the bolt may break. The ironworker installing the bolts must, therefore, get the bolt and nut tight enough to develop good friction in the joined pieces while not breaking the bolt. There are four basic methods for making sure a bolt have the proper amount of tension: (1) the turn-of-the-nut method, (2) calibrated wrenches, (3) tension indicating bolts and (4) direct tension indicators.

Turn-of-the-Nut Method

The turn-of-the-nut method is the easiest and least expensive method for installing fasteners with the proper bolt tension. The procedure generally works as follows. An iron worker tightens the bolt and nut as tight as possible using a spud⁽³⁾ wrench or a pneumatic impact wrench. A chalk mark or paint is then made on the bolt and nut. The bolt is tightened further by either hammering on the spud wrench or using a pneumatic impact wrench until the rotating part has rotated the required amount. The paint or chalk mark shows how far the part has rotated and the rotation is always measured relative to the rotation of the bolt.

The number of turns past snug is dependent on the length of the bolt relative to its diameter and the geometry of the two surfaces. The previous table shows the number of rotations for a variety of bolt lengths for flat horizontal surfaces.

Calibrated Wrench Method

Most iron workers use pneumatic power wrenches, also called impact wrenches, to tighten bolts. Calibrated wrenches can be set to stall at a specified torque thereby achieving the correct amount of bolt tension. Unfortunately, calibrated wrenches need to be calibrated at least every day since they tend to lose their calibration relatively quickly.

Tension Indicating Bolts
An indirect way to verify that a bolt has been properly tightened is to use tension indicating bolts like the one shown at the right. This type of bolt is tightened with a special pneumatic wrench -- one face of the wrench tightens the nut and the other holds the torque pattern head shown at the right side in the figure. When the bolt is tightened to the proper load, the right end of the bolt shank snaps off. An ironworker would simply keep tightening the bolt until the tip breaks off and an inspector could easily verify that the bolt was properly tighten by ensuring that the tip is missing.

Direct Tension Indicator Method

Direct tension indicators are special washers with grooves on one face. As the bolt is tightened the groves on the washer flatten out. An iron worker or inspector knows that the correct tension has been achieved when the gap between the washer and the piece will no long accept a gauge of a specified thickness. The washers are said to directly indicate the tension since they have a know crush load.

With the exception of the calibrated wrench method where the stall speed is set, the tension in the bolt is not directly measured. In fact, it is not necessary to be too careful about achieving some specific amount of tension as long as the bolt is in the yielded condition. If the bolt is over tightened it will probably break in which case the iron worker simply needs to remove the broken bolt and install a new one.

References

1. IFI, Metric Fastener Standards, The Industrial Fastener Institute, 1505 East Ohio Building, Cleveland, OH (1983).

2. AISC, Metric Conversion: Load and Resistance Factor Design Specification for Structural Steel Buildings (DRAFT), American Institute of Steel Construction, Chicago (1992).

3. The term "spud" wrench is a contribution of Irish-American iron workers. A spud wrench is a hand wrench with the grip at one end and a long tapered handle at the other. The tapered handle is similar to a sailor's marlin spike and is used to jamb into a series of holes to help align the holes before inserting the bolt. The name "spud" comes from the Gaelic word for a digging stick, "spaid." The potato is called a spud because the tool used to dig them out was a simple stick.