U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000


Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration

Bridges & Structures

High Strength Bolts

Structural Fastener Questions & Answers

Q1: Why do we typically use A325 bolts rather than A490 bolts? If A490 bolts are used what test procedures are followed?

A: A490 bolts are heat-treated steel structural bolts with 150 ksi minimum tensile strength and 170 ksi maximum tensile strength. Quoting from the "Guide to Design Criteria for Bolted and Riveted Joints" Second Edition, by G. Kulak, J. Fisher And J. Struik, " The higher the strength of the steel, the more sensitive the material becomes to both stress corrosion and hydrogen stress cracking. A490 bolts are more susceptible to brittle fracture than A325 bolts which are 120/105 minimum tensile strength steel. To minimize the risk of brittle fracture A490 bolts have both a minimum and a maximum tensile strength specified. FHWA recommends that A325 bolts be used wherever possible. The use of A490 bolts should be thoroughly evaluated prior to being included in a structure.

When A490 bolts are included in a design the following items should be included in the project specifications or inspection requirements.

  1. The certifications for the A490 bolts should be reviewed when the fasteners are received to assure the tensile strength is within the limits in the specification.
  2. The Rotational Capacity (RC) test shall be performed on all combinations of fastener assembly lots by the manufacturer or distributor and by the fabricator or contractor at the time of installation. The RC Test should follow the procedure used for A325 bolts with the appropriate A490 tension values in the tables of the procedure. During the performance of the RC Test the maximum tension values should be observed to assure they do not exceed the upper limits of the specification. In addition to the requirements of the RC test, if the bolt tensions are outside of the specified limits the fasteners shall be rejected.

In addition to the above notes, the fasteners shall be properly stored, the installation verification tests shall be performed and the standard installation procedures shall be followed.

Q2: How is the length of a structural bolt measured?

A: The length of the bolt is measured from the thread end of the bolt to the washer face at the head. This is the length of bolt used to determine the rotation required for the rotational capacity test and the turn-of-nut installation method.

Q3: What is the grip length of the bolt?

A: The grip length is the distance between the washer face of the bolt head and the washer face of the nut.

Q4: What are the required rotations for the Rotational Capacity Test?

A: The Rotational Capacity test in the AASHTO Materials Specification M164 is being changed to the same rotation requirements as in ASTM A325. The rotations required are: 2/3 turn for bolt lengths 4 X the bolt diameter or less; 1 turn for bolt lengths greater than 4 X the bolt diameter but no more than 8 X bolt diameter; and 1 1/6 turn for bolt lengths greater than 8 diameters but not more than 12 X the bolt diameter. These rotation requirements apply to inch series bolts only.

Q5: What are Turn-of-the-Nut requirements for metric bolts, A325M and A490M?

A: The installation rotation requirements are based on research performed at the University of Texas by Michael J. Gilroy and Karl H. Frank, "Tightening of High-Strength Metric Bolts" Research Report 2958-1F. The results of this research indicated that the required rotations are 1/2 turn for bolt lengths 8 X the bolt diameter or less with no minus tolerance and 2/3 turn for bolt lengths greater than 8 X the bolt diameter. There is no minus tolerance with the metric bolt installation rotations as there is with the inch series bolts.

Q6: What are the metric bolt Rotational Capacity Test rotation requirements?

A: I am not aware that this has been researched, however based on the requirements of the Rotational Capacity Test for inch series bolts I recommend 1 turn for bolt lengths 8 X the bolt diameter or less and 1 1/6 turn for bolt lengths greater than 8 X the bolt diameter.

Q7: When DTIs are used to verify installation tension, are the DTIs included in the fastener assembly during the rotational capacity test?

A: No, the bolt, nut and washer are tested as an assembly during the rotational capacity test to assure the assembly functions satisfactorily. The DTI is tested on the assembly during the installation verification tests.

Q8: Can the DTI installation verification test be performed using an impact wrench or is a manual torque wrench required to tension the bolt during the test?

A: A manual torque wrench is required as the tension approaches 1.05 X the minimum required installation tension and for the rest of the required rotation. An impact wrench may me used to bring the tension to near the above noted tension.

Q9: What is the required stick-out for structural bolts?

A: The recommended stick-out is flush to three threads. The stick-out is based on having three to five threads within the grip.

Q10: Where do I find high-strength fastener installation, installation verification and inspection procedures?

A: The procedures are in the FHWA Report No. FHWA-SA-91-031 "High Strength Bolts for Bridges". They are in the following Appendices: A2 - Turn-of-Nut Method; A3 - Calibrated Wrench Method; A4 - Alternate Design Fasteners; and A6 - High Strength Bolts with Direct Tension Indicators (Revised April 1992). Appendix A5 is a "Procedure for Inspection of High-Strength Fasteners

Q11: Where do I find the procedures for performing the Rotational Capacity (RC) tests in tension calibrators and steel plates?

A: The procedures for the RC tests are in Appendix A1 of the FHWA Report No. FHWA-SA-91-031 "High Strength Bolts for Bridges". There are two procedures, one for bolts that are long enough to be tested in a tension calibrator and a second for bolts too short to fit in the tension calibrator. All bolts, that are long enough to fit in tension calibrators, shall be tested in the tension calibrator.

Q12: What can be learned from the bolt stick-out?

A: Traditionally the bolt stick-out through the nut has been an indicator that the correct length of bolt has been selected for a connection and to verify that the nut will not bottom out at the end of the threads during tensioning. The stick-out can be used as an inspection tool. We look for a uniform stick-out of all bolts with in a connection. Prior to tensioning, this indicates that all the bolts used are the same length. I have seen connections with bolts of different lengths used together and other connections where washers were added under the bolt head when they were not needed, both causing non-uniform stick-out. After tensioning the stick-out should still be uniform. We occasionally will see a bolt in a connection with excess stick-out. Excess stick-out after tensioning typically indicates over tensioning. The bolts with excess stick-out after tensioning will typically neck down in the threads within the grip length of the bolt. The bolts, with excess stick-out, may be providing the required minimum tension, however the standard installation procedure has not been followed. My recommendation is that any bolt with excess stick-out after tensioning be removed and replaced.

Q13: When relubrication is required in the field what lubricant should be used?

A: When relubricating fasteners, we have found through experience that wax based lubricants provide lower torque values than oils and grease products. There are a number of wax based lubricants that work well for lubricating fasteners. They include water/wax emulsions such as Johnson Wax "JON-COTE 639" and MacDermid "TORQUE'N TENSION CONTROL FLUID". There are stick waxes, bees wax and even toilet bowl ring wax that work well. To provide best results the nuts are dipped in the water/wax emulsions and then laid out on a clean surface to dry. When dry they are placed in protected storage until they're used. When using the firmer waxes, best results are obtained by melting the wax and dipping the nuts, allowing them to warm to the wax temperature then removing them allowing excess wax to drain and cooling on a clean surface. These should also be placed in protected storage until used.

Q14: What is the recommended cleaning method for removing lubricant for installed bolts?

A: The wax on the nuts will weather after installation of the fasteners, however all of the wax may not be removed by weathering. A number of States require pressure washing of the steel prior to completion of steel bridge projects for both painted and weathering steel. Soaps such as TSP used with the pressure washing should remove all or most of the lubricants from the fastener assemblies. There are several cautions. The more wax or lubricant used, the more effort that will be needed to remove it. Observe environmental regulations when cleaning the steel.

Q15: What is the proper procedure to remove excess galvanizing from anchor bolt? How can this problem be minimized or eliminated?

A: Anchor bolt manufacturers or the company that galvanizes the anchor bolts is required to test the finished product to assure that the nuts turn freely on the anchor bolts, however this does not always happen. To remove excess zinc, warm the threaded portion of the anchor bolt with a torch and brush the threads with a wire brush. A solution that has worked for a number of States, is to require that all anchor bolts be shipped with the nuts assembled on the anchor bolts, thus assuring that the nuts will turn freely on the anchor bolt threads.

Updated: 09/03/2013
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000