Tension vs. Torque Explained (sort of…)

Question: Can someone explain how tension and torque relates to bolted connections?

Answer: Sure, we’ll try our best. The relationship between tension and torque should be looked at cautiously, since it is very difficult to indicate the range of conditions expected to be experienced by a fastener. Torque is simply a measure of the amount of energy required to spin the nut up along the threads of a bolt, whereas tension is the stretch or elongation of a bolt that provides the clamping force of a joint. Bolts are designed to stretch just a tiny bit, and this elongation is what clamps the joint together. Torque is a very indirect indication of tension, as many factors can affect this relationship, such as surface texture, rust, oil, debris, thread series and material type just to name a few. Virtually all the torque/tension tables that have been developed, including ours, are based on the following formula:

T = (K D P)/12

• T = Torque (ft-lbs)
• D = Nominal Diameter (inches)
• P = Desired Clamp Load Tension (lbs)
• K = Torque Coefficient (dimensionless)

The value of K is a dimensionless torque coefficient that encompasses variables such as those listed above, as well as the most significant variable, friction. The value of K can range from 0.10 for a well lubricated/waxed assembly, to over 0.30 for one that is dirty or rusty. The values we used when calculating our values are:

• 0.10 = Waxed/Lubricated
• 0.20 = Plain, as received condition, slightly oily
• 0.25 = Hot-Dip Galvanized

The appropriate torque value to use in a specific application is best obtained by using a calibrated torque wrench and a Skidmore-Wilhelm load indicating device to equate actual torque to the desired tension. For ASTM A325 and A490 structural bolts The Research Council on Structural Connections (RCSC) recommends:

The pre-installation verification procedures specified in Section 7 shall be performed daily for the calibration of the installation wrench. Torque values determined from tables or from equations that claim to relate torque to pretension without verification shall not be used.
(RCSC Specifications, June 2004, pg. 62, 8.2.2)

An alternative and more accurate method for assuring proper tension would be to use a direct tension indicator or DTI. These are available for use with ASTM A325 and A490 structural bolts and are engineered to compress at the proper tension, assuring the installer that the proper clamp load is achieved. Hopefully, this short introduction to bolt connections helps address some of the confusion surrounding this issue.

Download Estimated Torque Values

Fastener Identification Markings and Bolt Strength Requirements

Download a print version

Grade Marking	Specification	Material and Treatment	Nominal Size (In.)	Mechanical Properties			Hardness Rockwell
				Proof Load Min (ksi)	Yield Strength Min (ksi)	Tensile Strength Min (ksi)	Min	Max
	SAE J429 Grade 1	Low or Medium Carbon Steel	¼" - 1½"	33	36	60	B70	B100
	A307 Grade A		¼" - 4"	-	-	60	B69	B100
	A307 Grade B			-	-	60 min 100 max	B69	B95
One End Green	A307 Grade C *Replaced see note 8.			-	36	58 min 80 max	-	-
One End Blue	F1554 Grade 36			-	36	58 min 80 max	-	-
One End Yellow	F1554 Grade 55			-	55	75 min 95 max	-	-
	SAE J429 Grade 2		¼" - ¾" ⅞" - 1½"	55 33	57 36	74 60	B80 B70	B100 B100
	A193/A320 Grade B8	AISI SS304 Stainless	No Restrictions	-	30	75	-	B96
	A193/A320 Grade B8M	AISI SS316 Stainless Steel
	A325 Type 1	Medium Carbon Steel, Q & T	½" - 1" 1⅛" - 1½"	85 74	92 81	120 105	C24 C19	C35 C31
	A325 Type 3	Atmospheric Corrosion Resistant Steel, Q & T
	SAE J429 Grade 5	Medium Carbon Steel, Q & T	¼" - 1" 1⅛" - 1½"	85 74	92 81	120 105	C25 C19	C34 C30
	A449 Type 1		¼" - 1" 1⅛" - 1½" 1⅝" - 3"	85 74 55	92 81 58	120 105 90	C25 C19 B91	C34 C30 B100
	A193 Grade B7	Medium Carbon Alloy Steel, Q & T	¼" - 2½" 2⅝" - 4" 4⅛" - 7"	-	105 95 75	125 115 100	-	C35 C35 C35
	A354 Grade BC		¼" - 2½" 2⅝" - 4"	105 95	109 94	125 115	C26 C22	C36 C33
One End Red	F1554 Grade 105		¼" - 3"	-	105	125 min 150 max	-	-
	A320 Grade L7	AISI 4140, 4142, or 4145	¼" - 2½"	-	105	125	-	-
	A320 Grade L43	AISI 4340	¼" - 4"	-	105	125	-	-
	A490 Type 1	Medium Carbon Alloy Steel, Q & T	½" - 1½"	120	130	150 min 173 max	C33	C38
	A490 Type 3	Atmospheric Corrosion Resistant Steel, Q & T
	SAE J429 Grade 8	Medium Carbon Alloy Steel, Q & T	¼" - 1½"	120	130	150	C33	C39
	A354 Grade BD		¼" - 2½" 2⅝" - 4"	120 105	130 115	150 140	C33 C31	C39 C39

1. All specifications are ASTM unless otherwise noted.
2. All specifications shall be marked by the manufacturer with a unique identifier to identify the manufacturer or private label distributor, as appropriate.
3. Q & T - Quenched and Tempered.
4. Stamping of F1554 and A307 grade C bolts is a supplemental requirement while color coding is required.
5. Although markings are shown on hex heads, grade markings apply equally to products with other head configurations.
6. All Grade BD products shall be marked "BD". In addition to the "BD" marking, the product may be marked with 6 radial lines 60° apart.
7. Hex cap screws and bolts and one end of studs shall be marked "A449". In addition, fasteners shall be permitted to be marked with three radial lines 120° apart.
8. ASTM A307 grade C has been replaced by ASTM F1554 grade 36.

ASTM A563

The ASTM A563 specification covers the chemical and mechanical requirements for carbon and alloy steel nuts used on bolts, studs, and externally threaded fasteners. The charts below address over tapping allowances for hot-dip galvanized nuts, grade marking requirements, and mechanical requirements. For information on suitable nuts for various grades of bolts, visit the Nut Compatibility Chart.
According the A563 specification, “The requirements for any grade of nut may, at the supplier’s option, and with notice to the purchaser, be fulfilled by furnishing nuts of one of the stronger grades specified herein unless such a substitution is barred in the inquiry and purchase order”. This is important because some nut grades are not readily available in certain sizes and finishes. Additionally, the specification allows for the substitution of ASTM A194 grade 2H nuts in lieu of A563 grade DH nuts due to the lack of availability of grade DH nuts in nominal sizes ¾” and larger.
Hot-dip galvanized nuts must be tapped oversize to allow for the added thickness of the zinc on the threads of the externally threaded fastener.
Various nut styles exist and to some degree are determined by their grade. These styles include hex, heavy hex, square, jam, coupling, and sleeve.

Scope

Standard specification for carbon and alloy steel nuts.

Grades

A	Low carbon steel, hex or heavy hex
B	Low carbon steel, hex or heavy hex
C	Carbon steel, quenched and tempered, heavy hex
D	Carbon steel, quenched and tempered, heavy hex
DH	Carbon steel, quenched and tempered, heavy hex
C3	Weathering steel, quenched and tempered, heavy hex
DH3	Weathering steel, quenched and tempered, heavy hex

MECHANICAL PROPERTIES

Grade	Style	Size, in.	Proof Load, ksi		Hardness, HBN
			Black	Galvanized
A	Hex	1/4 - 1-1/2	90	68	116 - 302
	Heavy Hex	1/4 - 4	100	75	116 - 302
B	Heavy Hex	1/4 - 1	133	100	121 - 302
	Heavy Hex	1-1/8 - 1-1/2	116	87	121 - 302
C / C3	Heavy Hex	1/4 - 4	144	144	143 - 352
D	Heavy Hex	1/4 - 4	150	150	248 - 352
DH / DH3	Heavy Hex	1/4 - 4	175	150	248 - 352
For UNC, 8UN, 6UN, and Coarse Pitch Threads ASTM A563 Nut Compatibility Chart Download a print version Grade of Bolt Surface Finish Nominal Size Inches A563 Grade and ANSI Nut Style Recommended Suitable Hex Heavy Hex Hex Heavy Hex A307 Grade A & C Any ¼ to 1½ A B,D,DH A,B,C,D,DH,DH3 >1½ to 2 A A C,D,DH,DH3 >2 to 4 A C,D,DH,DH3 A307 Grade B Any ¼ to 1½ A B,D,DH A,B,C,D,DH,DH3 >1½ to 2 A A >2 to 4 A A325 Type 1 Plain ½ - 1½ C C3,D,DH,DH3 Galvanized ½ - 1½ DH A325 Type 3 Plain ½ - 1½ C3 DH3 A354 Grade BC Plain ¼ to 1½ C D,DH C3,D,DH,DH3 >1½ to 4 C C3,D,DH,DH3 Galvanized ¼ to 1½ DH >1½ to 4 DH A354 Grade BD Plain ¼ to 1½ DH DH D,DH,DH3 >1½ to 4 DH DH3 A449 Plain ¼ to 1½ B D,DH B,C,C3,D,DH,DH3 >1½ to 3 A C,C3,D,DH,DH3 Galvanized ¼ to 1½ DH D,DH D >1½ to 3 DH D A490 Type 1 Plain ½ - 1½ DH DH3 A490 Type 3 Plain ½ - 1½ DH3 A687 Plain ⅝ - 3 D DH,DH3 Galvanized ⅝ - 3 DH F1554 Grade 36 Any ¼ to 1½ A B,D,DH A,B,C,D,DH,DH3 >1½ to 4 A C,D,DH,DH3 F1554 Grade 55 Plain ¼ to 1½ A B,D,DH A,B,C,D,DH,DH3 >1½ to 4 A C,D,DH,DH3 Galvanized ¼ to 4 A C,D,DH,DH3 F1554 Grade 105 Plain ¼ to 1½ D DH,DH3 >1½ to 3 DH DH3 Galvanized ¼ to 3 DH DH3 1. The availability of A563 grade DH nuts in nominal sizes 3/4" and larger is very limited and generally available only on special orders of 50,000 or more. For smaller quantities, ASTM A194 grade 2H should be considered. Source: http://www.portlandbolt.com

ASTM A193

Scope

Originally approved in 1936, this specification is heavily utilized in petroleum and chemical construction applications. The ASTM standard covers alloy steel and stainless steel bolting materials for high temperature service. This specification includes fasteners intended for use in pressure vessels, valves, flanges, and fittings. Although, this material is often available in national coarse (UNC) thread pitches, if being used in traditional applications, threads are specified 8 threads per inch (tpi) for diameters above one inch.
Below is a basic summary of a few of the common grades. ASTM A193 covers a number of other standard specifications not covered in this description including B5, B6, and B16.

Grades

B7	Alloy steel, AISI 4140/4142 quenched and tempered
B8	Class 1 Stainless steel, AISI 304, carbide solution treated.
B8M	Class 1 Stainless steel, AISI 316, carbide solution treated.
B8	Class 2 Stainless steel, AISI 304, carbide solution treated, strain hardened
B8M	Class 2 Stainless steel, AISI 316, carbide solution treated, strain hardened

Mechanical Properties

Grade	Size	Tensile ksi, min	Yield, ksi, min	Elong, %, min	RA % min
B7	Up to 2-1/2	125	105	16	50
	2-5/8 - 4	115	95	16	50
	4-1/8 - 7	100	75	18	50
B8 Class 1	All	75	30	30	50
B8M Class 1	All	75	30	30	50
B8 Class 2	Up to 3/4	125	100	12	35
	7/8 - 1	115	80	15	35
	1-1/8 - 1-1/4	105	65	20	35
	1-3/8 - 1-1/2	100	50	28	45
B8M Class 2	Up to 3/4	110	95	15	45
	7/8 - 1	100	80	20	45
	1-1/8 - 1-1/4	95	65	25	45
	1-3/8 - 1-1/2	90	50	30	45

Recommended Nuts and Washers

Grade	Nuts	Washers
B7	A194 Grade 2H	F436
B8 Class 1	A194 Grade 8	SS304
B8M Class 1	A194 Grade 8M	SS316
B8 Class 2	A194 Grade 8, strain hardened	SS304
B8M Class 2	A194 Grade 8M, strain hardened	SS316

Bolt Materials

Bolting is a very common method of fastening steel members. Bolting is particularly cost-effective in the field.
The precursor to bolting was riveting. You will probably have occasion to assess connections made with rivets sometime in your career, particularly if you work on restoration projects. Riveting was a very dangerous and time-consuming process. It involved heating the rivets to make them malleable then inserting them in the hole and flattening the heads on both sides of the connection. The process required an intense heat source and a crew of three or more workers. Figure .1 shows a riveted connection in a bridge structure.

Figure 1. A Riveted Connection

In the mid-1900s, high-strength bolts were introduced and quickly replaced rivets as the preferred method for connecting members together in the field because of their ease of installation and more consistent strengths. High strength is necessary since most bolts are highly tensioned in order to create large clamping forces between the connected elements. They also need lots of bearing and shear strength so as to reduce the number of fasteners needed.

There are two basic ASTM high-strength bolt specifications and one non-high-strength ASTM bolt specification that we will be using. The high-strength bolts are ASTM A325 and ASTM A490. The non-high strength bolt is ASTM A307.

The ultimate tensile strengths for A325 and A490 bolts are 120 ksi and 150 ksi respectively. These values are rarely needed in applying the equations in the AISC specification, but are useful to know when using theoretical equations for special circumstances. We will also be obtaining bolt strength information for use in the AISC equations from SCM table J3.2 on page 16.1-104. We'll cover that table in more detail later.

The ASTM bolt specifications require that the bolts and their associated nuts and washers be clearly identified with their specification number. Figure 2 shows the identifiers for A325 and A490 bolts.

Figure 2. Bolt Identifiers

A Beginner's Guide to the Steel Construction Manual, 13th edition https://www.bgstructuralengineering.com/