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

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 twisting force 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.

See Estimated Torque Values

Written October 16, 2007 by
Dane McKinnonDane McKinnon

Phone: 503.219.6991
Email:

36 comments

  1. Bryan 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.

  2. 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.

  3. 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.
    Skidmore-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)

    1. @Mike Girard – You are absolutely right. The content of the post has been updated. Thanks for catching that.

  4. Hevii Guy,

    There is no such thing as an overtightened connection, until you break the bolt in tension. RCSC – Sec. 9.2.4, states: “A pretension that is greater than that specified in Table 8.1 shall not be cause for rejection”

  5. The same above mentioned formulae are used for calculating torque for countersunk screws or any other multiplication factor available…???

    1. @Manoj – The torque formula would differ from the one listed because screwing into tapped holes usually creates a longer thread engagement which can affect the friction coefficient. An engineer would be the best person to consult regarding this.

  6. For 5/8″ Step bolts, of SAE grade 2, what torque value should we be specifying during installation. This is for a transmission line tower.

    1. @Lily – That is a difficult question to answer. We have torque values on our website, but every application is a bit different, so there is no one all encompassing torque value that works for all applications. Controlled lab testing is the only sure fire way of determining your specific torque values. /technical/bolt-torque-chart/

  7. P = Desired Clamp Load Tension (lbs), is this the value given in kips on the RCSC code, if so how to convert to lbs

  8. Any guidance for 3/4″ lag bolts in SYP timber? Specifically a railing base plate with lags into stringer below (perpendicular to grain), with concerns about torque loading that twists off the lag and how much to reduce this torque to avoid lag damage? Do you have maximum allowable torques for lags?

    1. @Duncan – Apologies, but we do not have any engineers on staff, so cannot comment on application-specific questions like this. I am not aware of any allowable torque values for lag bolts.

    1. @Jared – These torque charts are calculated for use in steel to steel connections. Anchor bolts are not typically torqued to a specific requirement, as the AISC says that snug tight is acceptable.

  9. I used M10 10.9 allen head screw on the aluminium rail. for assembly of bus seats..under rail there is sheet. ı searched a lot. everybody is saying aluminium reduce the torque value.
    ı found 33.9 Nm and 20Nm. difference is too big.
    whats your idea?

    1. @Ersin – Thank you for your question, but our area of expertise is with carbon and alloy steel structural applications, we don’t have any information regarding aluminum Apologies.

  10. Can you explain to me what will happen if we apply more torque on the installed bolt? Example, the required torque is only 200 ft-lb… then we apply 250 ft- lb…. what will be the effect of this on the structure and on the bolt itself?

    1. @Mel – Without more information I cannot be certain, but the possibilities include snapping the bolt, crushing whatever you are bolting, or maybe nothing. It will depend on the capacity of the bolt in question, what you are bolting, and the amount of friction in the bolted assembly.

  11. Hi,

    I am looking for bolts that have rounded/dome ends. Do you know where I could find some? Specifically, I need 1/4-20 stainless steel bolts with rounded ends. I don’t even know if there is a special name for such, but the closest I’ve come to finding them is this website, and I searched for “round end bolt”

    Thanks!

  12. Due to stocking of hardware and varying applications, we use a majority of ASTM A574 Screws. If we are torquing ASTM A574 Screws to a Grade 5 torque(due to lesser application requirements), is it feasible that we are more likely to have screws working loose? My thought is that we are not putting enough tension into the higher strength screw.

    1. @Jeremy – It would all depend on the application. Static loads may be OK, but if there is movement or vibration, you may have an issue. We don’t have any engineers on staff, so we can’t really make a determination. Apologies.

  13. This question is a bit off the beaten path. I am reinstalling a handhole cover on a tank. The drawing gives a torque spec of 27 +/-3 ft/lbs. Original gasket was Grade 1 (65 durometer) rubber. Only gasket material I can get is Grade 2 at (80 durometer). Does the increased hardness of the gasket material affect my torque value?

  14. Specifying bolts sizes and torque value requires you first, analyse the force interaction at the joint you’re connecting, then calculate the tensile stress area based on 75% of your selected material proof strength. Then specify a bolt size. These days, people just specify over and above what is required without any engineering!

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