Discussion: View Thread

  • 1.  TC Bolts - Why Agencies are not a fan. Are we wrong?

    Posted 10-24-2017 03:51 PM
    Edited by Hashim Muhammad 10-25-2017 07:54 AM
    ​Currently in the discussion on the use of TC bolts in our structures. Fatigue and load reversals are typical for our structures so we specify slip critical (SC) connections. Now, theoretically, a TC bolt , if properly installed, provides the clamping action needed for plies to be engaged and ready for SC service.  But then there is the question of whether it was properly installed and trusting that the splined end is all that is needed per RCSC 9.2.3.  So far, my experience, although limited, says no. So far, I've inspected post-installed fully torqued TC bolted SC connections, where:
    * I've been able to stick a crisp $1 bill through the plies - signifying to me that the bolt in the vicinity reached the prescribed tension without the plies being pulled together
    * I've seen bolts slightly twisting under a wrench even though the ends are splined - signifying to me that the torqueing pattern they used on other bolts in the connection, relaxed the pre-tensioning force in the particular bolt. 

    Although those situations were corrected, and thankfully only demand light static service, those things I should not have seen if the connection was properly installed. Obviously the QC during installation wasn't as good as one would hope. And here is my problem, nothing that I did to check the post-installed connection...is prescribed by AISC or other. What if the $1 I had at the time wasn't crisp enough ! :-0  I would have gone ahead and signed off on a slippery slide. And what if, a tool smaller than the thickness of a $1 could fit through. I'm left to guess. Which is, like our Mentors and professors would say, not enough for us to get good sleep at night.      

    So, Am I wrong in recommending: no TC bolts on structures supporting fatigue or load reversals? or is there 
    1.some prescribed method for checking post-installed bolts I can hold on to and communicate as a sure validation of the connection
    2. some decent QC directive that can be specified and recorded during installation.

    Hashim Muhammad P.E., M.ASCE
    Civil Engineer III
    Brooklyn NY

  • 2.  RE: TC Bolts - Why Agencies are not a fan. Are we wrong?

    Posted 10-26-2017 12:29 PM

    I think the commentary right below 9.2.3 may hold a clue as to what happened:

    " The sheared-off splined end of an installed twist-off-type tension-control bolt
    assembly merely signifies that at some time the bolt was subjected to a torque
    that was adequate to cause the shearing. If in fact all fasteners are
    individually pretensioned in a single continuous operation without first properly
    snug-tightening all fasteners, they may give a misleading indication that the
    bolts have been properly pretensioned. Therefore, it is necessary that the
    inspector observe the required pre-installation verification testing of the fastener
    , and the ability to apply partial tension prior to twist-off is
    demonstrated. This is followed by monitoring of the work in progress to ensure

    that the method is routinely and properly applied within the limits on time
    between removal from
    protected storage and final twist-off of the splined end."
    (Emphasis added)

    Tony Shelton A.M.ASCE
    Allen TX

  • 3.  RE: TC Bolts - Why Agencies are not a fan. Are we wrong?

    Posted 10-26-2017 12:29 PM
    The faying surface of each bolt is shown in Figure C-3.1 of the manual.  Does the dollar bill encroach on the area?

    i could see a few reasons why the TC bolt might snap too soon:

    1.  Bolt is too long
    2. connecting plate is too thick
    3.  fit up and erection tolerances have not been considered in detailing

    I don't see any of these problems specifically related to the use of TC bolts or reason to exclude there use.  Once the desired pretension is achieved, even a squirter washer, or turn of the nut method will indicate the same value.  Section 5.1 of the RCSC does not prohibit the use of shims/fillers to ensure the proper faying surface exists.  Shim as required during and after erection.  Check to see if there is only a gap on one edge of the plate or is there no contact at all. 

    Chad Morrison P.E., M.ASCE
    Professional Engineer
    Greenville RI

  • 4.  RE: TC Bolts - Why Agencies are not a fan. Are we wrong?

    Posted 10-26-2017 12:35 PM
    Look at the commentary for that section.  Running the bolts to full torque without bringing them all to snug tight will probably cause the issue you and your dollar bill are seeing.  I would expect two or more passes through the pattern (working inside to outside) to achieve snug tight in all of the bolts.  Then torque them.

    Tony Shelton A.M.ASCE
    KONE, Inc.
    Allen TX

  • 5.  RE: TC Bolts - Why Agencies are not a fan. Are we wrong?

    Posted 10-26-2017 12:38 PM
    Edited by Jane Howell 10-26-2017 12:37 PM
    To address your basic question: Should the use of TC bolts be prohibited on structures supporting fatigue or load reversals? I think the answer is No. I will now provide some rationale for my response. I will break the discussion into three parts.
    1. Prohibiting TC bolts does nothing to address the problem.
    The RCSC Specification lists four pretensioning methods: Turn-of-Nut Pretensioning, Calibrated Wrench Pretensioning, Twist-Off-Type Tension-Control Bolt Pretensioning, and Direct-Tension-Indicator Pretensioning. 
    There are procedures that must be followed to ensure that each method produces a properly pretensioned joint. If the joints are not being properly pretensioned, then the procedures are not being followed. A prohibition on TC bolts does not address this underlying problem. It is just as likely (and in some cases more likely) that the procedures may also not be adhered to relative to another method. Many engineers consider Turn-of-Nut Pretensioning the "gold standard". Arguments can be made that there are fewer variables involved with Turn-of-Nut Pretensioning making it more reliable. However, assuming one of the issues you are seeing is a lack of firm contact between the plies before pretensioning, this is just as likely to happen with Turn-of-Nut Pretensioning. Even when match-marking is performed, if the initial mark is placed before the plies are in firm contact, then some of the turns will be used to establish firm contact instead of to achieve the proper pretension. Problems with "cross talk" or improper tightening sequence can occur with all of the tensioning methods.
    2. The $1 Test
    This brings us to a second issue your $1 test, which may be giving less than one-dollar's worth of insight into the pretension in the joint. RCSC defines firm contact as, "The condition that exists on a faying surface when the plies are solidly seated against each other, but not necessarily in continuous contact." All of the pretensioning methods start with a snug-tight condition defined as, "A joint in which the bolts have been installed in accordance with Section 8.1. The snug tightened condition is the tightness that is attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the plies into firm contact." Firm contact does not mean continuous contact. As stated in the Commentary, "However, in some joints in thick material or in material with large burrs, it may not be possible to reach continuous contact throughout the faying surface area as is commonly achieved in joints of thinner plates. This is generally not detrimental to the performance of the joint." Non-continuous, but firm, contact is not detrimental to achieving and sustaining pretension or to providing slip resistance. I know that there are joints in service that I have designed with oversized holes supporting about 30-stories of high-rise building where I am certain I could easily insert a crisp $1 or even a nickel in places. It does not keep me from getting a good sleep at night.
    3. A path forward
    Franz Kafka once said, "In the fight between yourself and the world, back the world." I am not completely sure what he intended since it was written without context, but I like to apply it to engineering. You have stated a problem. Setting aside my doubts about the conclusiveness of the $1 test, if you can easily turn a nut on a bolt with a wrench, then the joint is not properly pretensioned. No amount of quoting from codes or numbers from technical models can eliminate a physical reality. 
    I have asserted under Item 1 that prohibiting TC bolts does nothing to address the problem. I believe this is accurate. Using F3125 Grade A325 or A490 bolts will eliminate the issues related to Twist-Off-Type Tension-Control Bolt Pretensioning, but these issues will simply be replaced with other issues. None of the methods are foolproof.
    So what can be done? You have indicated that problem may be associated with quality control. This is a possibility, and in my opinion a likely cause. Returning to my example of the high-rise building with the oversize holes, oversized holes were used to provide more tolerance on fit-up during erection. An exchange was made, fewer bolts would have been required and the bolt installation would have been much simpler, if bearing bolts in standard holes were used, but it was decided that the additional hole clearance was worth the expense associated with more bolts to be installed using a more complex method. Having made this choice, extra care was dedicated to ensuring that the faying surfaces were properly prepared and maintained and that the bolts were properly pretensioned. In other words, "There ain't no free lunch."
    The biggest problem I see with slip-critical or pretensioned joints is the over-specifying of these conditions. The vast majority of joints used in structural steel buildings can be installed snug-tight. Snug-tight installation is quick, easy and economical. A snug-tight, bearing joint is virtually foolproof; there is little that can go wrong. My advice to engineers is that they should allow snug-tight installation wherever possible and then dedicate extra care and attention to those relatively few joints that must be slip-critical or pretensioned. In fact your discussion seems to indicate that you may actually be applying unnecessary requirements to your projects, potentially drawing attention and resources away from critical joints. You state, "Although those situations were corrected, and thankfully only demand light static service, those things I should not have seen if the connection was properly installed." If the demand was only "light static service" then as long as the requirements for a snug-tight joint were satisfied the "the connection was properly installed". There was no need for any remediation. By specifying these joints as pretensioned, you are pulling ironworkers' and inspectors' attention and time towards joints where these issues are of little consequence and potentially away from joints where these issues are of great consequence. This is a poor use of resources.
    Tables N5.6-1 through 3 provide the QC and QA task that must be performed to ensure proper bolting. You should ensure that the contractors and the inspectors working on your projects are aware of these requirements and conform to them. 
    I will also address a related issue, and that is loosening. AISC and RCSC take the position that a properly pretensioned joint will not loosen. I have seen nothing that leads me to believe this position is incorrect. However, some engineers involved with industrial structures, especially those containing cranes, recommend a regular program of inspection and maintenance to detect and remedy loose bolts. I recently spoke to an engineer who was struggling with loose bolts in a crane building and he has apparently worked with a researcher to try to ferret out the cause. They believe that within-tolerance geometric imperfections combined with restraint at the joints are resulting in conditions where firm contact is not being achieved even when large forces are applied to the joints. If this is the case, then the problem is that the joints effectively have never been pretensioned. The lack of pretension is allowing the bolts to loosen. One solution might be to provide tapered shims to adjust for the geometric imperfections. Where inspections reveal problems with loosening, steps should be taken to ensure that the joints can be properly pretensioned.
    Another common misconception is that simply having a TC spline break or DTI washer flatten or having match-marks a certain distance apart indicates that the joint is properly pretensioned; it does not. A joint has been properly pretensioned if all of the RCSC requirements are met. The following Steel Interchange was intended to address this misconception head-on:
    Relative to a "prescribed method for checking post-installed bolts" Section 10 of the RCSC Specification provides an arbitration procedure.

  • 6.  RE: TC Bolts - Why Agencies are not a fan. Are we wrong?

    Posted 10-26-2017 12:39 PM
    ​I believe the direct answer to your questions is:

    Yes, you would be wrong in recommending no TC bolts for structures supporting load reversals and fatigue. In fact RCSC bolt specifications require the use of pretensioned or slip critical connections for these types of loads.

    It sounds as if the problems you are experiencing may be related to improper installation procedures or fabrication issues. For very thick plates the faying surfaces may not ever come into contact but this isn't necessarily a fault or failure.

    I believe you'll find most of your answers available through many different articles, reports, and other information available at AISC. Just a quick google search on the topic brought up a whole page of references, many from AISC. Also, go throught the entire RCSC Specification for Structural Joints Using High-Strength Bolts including all the commentaries. There's a lot of info pertinent to this topic there.

    Brett King P.E., M.ASCE
    Senior Structural Engineer
    GHD Inc.
    Lake Oswego OR