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
Direct Tension Indicationg washers provide a false sense of security: They cannot indicate whether the joint has been overtightended. Neither do they provide a means of checking load migration after initial installation.
March 22, 2009 at 8:39 amSkidmore-Wilhelm machines provide an accurate indication of torque to load but ONLY on the exact fastener being measured under the the conditions experienced in the device AT THAT TIME. Once the tested wrench is used in the field, myriad friction factors and load interactions mean that all bets are off; one has no way of knowing what the actual load will be – it’s still a “guessing” exercise unless load is verified AFTER the bolt has been tightened and then again, after all of the other bolts have been tightned (to compensate for load transfer)
Galliou » It is hard to say if these calculations would be very accurate for anchor bolts. The biggest uncontrolled variable when determining torque is friction. Since the surface between the bolt assembly and the joint surface in the field can vary greatly it is difficult, if not impossible to accurately estimate torque for anchor bolts. One option would be to consider load indicating washers. This washer will give consistent tension values regardless of environment.
April 7, 2008 at 8:34 amIs the “Bolt Torque Chart” applicable for anchor bolts?
April 7, 2008 at 8:30 amBryan Carr » The formula from Industrial Fastener Institute (IFI) “Design of Bolted Connections” (M-64), used to determine values for our toque chart states it is meant for, “… steel bolts in their as-received condition.” Although, IFI goes on to state there many factors (surface texture, material hardness, thread series) that only have a “modest effect” on the torque-tension relationship. The primary influence on torque calculations is coefficient of friction. Again, the only “correct” way to establish an accurate torque value is to test the fastener in their actual joint application.
January 8, 2008 at 3:43 pmShould you torque a bolt to a recommended torque even when using helical spring lock washers?
January 8, 2008 at 3:22 pm