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Top 10 FAQs about the Manual & 2016 Specification-Part Two

We’re always improving our standards (and our Standards) at AISC and it only makes sense that we would be inspired by our resourceful users’ questions. We’ve compiled and answered the top 10 most frequently asked questions that our Steel Solutions Center has received.

Many of the fabricators we work with regularly use twist-off-type tension-control (TC) bolts regardless of the connection type. Based on this, would it make sense to specify all of our connections as pretensioned, since we are presumably getting pretensioned connections anyway?

No. We are not surprised that some fabricators prefer TC bolts. TC bolts can be used in slip-critical, snug-tight and pretensioned connections. The bolts also have other potential advantages that have made them attractive to fabricators and erectors. The 2014 RCSC Specification for Structural joints Using High-Strength Bolts (available at www.aisc.org/publications) does not limit the amount of pretension in a bolt. Therefore, a TC bolt can be tensioned up to the point where the spline is severed, even in a connection specified as snug-tight and designed as a bearing connection. However, simply severing the spline does not ensure that the joint has been properly pretensioned.

The installation of pretensioned joints involves the following considerations beyond those for a snug-tight joint:

Pre-installation verification. Section 8.2 of the RCSC Specification states: “Pre-installation testing shall be performed for each fastener assembly lot prior to the use of that assembly lot in the work. The testing shall be done at the start of the work.” This is an extra step that must be performed during the bolt installation for pretensioned connections but is not required for snug-tight conditions. Though pre-installation verification is a relatively straightforward process, we do occasionally hear of issues. At the very least, it involves having a properly calibrated tension calibrator (though not all fabricators, erectors and inspectors have one on hand), scheduling the testing and ordering bolts in sufficient quantities to accommodate both the installation and the required testing.

Bolt storage and installation conditions. Section 2.2 of the RCSC Specification addresses the storage of fastener components. These requirements apply equally to snug-tight and pretensioned installations. However, the condition of the bolt and the lubricant is more of a concern for pretensioned joints. TC bolts can be particularly sensitive to the condition of the lubricant. If the bolts are not properly stored or the final tensioning is delayed, then the bolts may need to be cleaned and relubricated. For heavy hex head bolts (Grade A325 and A490) this can be done by the user in the field. TC bolts “shall not be re-lubricated, except by the manufacturer.”

Installation procedures. You cannot simply put a TC bolt in a hole and engage the wrench until the spline breaks and expect to have a properly pretensioned joint. First, the bolts must be installed in accordance with the requirements in Section 8.1 of the RCSC Specification, which lists the installation requirements for snug-tightened joints. For large, heavy joints, you may actually end up breaking bolts or the splice before you bring the plies into firm contact, and the RCSC Specification addresses this, stating: “If a splined end is severed during this operation, the fastener assembly shall be removed and replaced.” Once firm contact is achieved, the installation must progress “systematically from the most rigid part of the joint in a manner that will minimize relaxation of previously pretensioned bolts.”

Inspection. There are inspection tasks required for pretensioned connections that are not required for snug-tight connections. Section N5.6.(a) of the AISC Specification states: “For snug-tight joints, pre-installation verification testing as specified in Table N5.6-1 and monitoring of the installation procedures as specified in Table N5.6-2 are not applicable.

Arbitration. Arbitration is addressed in Section 10 of the RCSC Specification and ideally, will rarely be required. Allowing snug-tight installation eliminates the possibility that it will be required. Each of the above items involves logistical considerations and present potential impacts to the cost and schedule of the project. Some of these items are less of a concern in the shop than they are in field. However, many bolted joints are installed in the field. Therefore, the preferences of the erector must also be considered.

How long can engineers continue to use the 14th Edition of the Manual now that the 15th Edition is available?

The Manual is never referenced in building codes. There is no requirement to use any edition of the Manual. Which edition of the Manual to use is up to the engineer to decide based on the requirements for their project. The Specification is referenced in the building codes, but it is likely that few jurisdictions have adopted a building code that references the 2016 Specification yet. The 14th Edition Manual can be viewed as a tool to make using the 2010 Specification easier in practice. The 15th Edition Manual is an updated reference that reflects the contents of and changes in the 2016 Specification.

The footnote to Specification Table J3.1 indicates that the required minimum bolt pretension is “equal to 0.70 times the minimum tensile strength of bolts as specified in ASTM F3125/F3125M for Grade A325 and Grade A490 bolts with UNC threads, rounded off to nearest kip.” However, the Specification applies a factor of safety, Ω, of two to the nominal strength, allowing only 0.50 times the minimum tensile strength of bolts to be applied to the bolt in service. If the bolt is already stressed to 0.70 times its minimum tensile strength, has the bolt not already “failed” (exceeded its design strength) before any additional load has been applied?

No. The bolt has not failed due to pretension. We recently received a similar question. The pretension does not reduce the available tensile strength of the bolt.

People often feel that a bolt pretensioned to 70% of its tensile strength has already violated the tension limit state for the bolt and any additional applied tension cannot be allowed. This is an incorrect interpretation and is addressed in a couple of places. The RCSC Specification for Structural Joints Using High-Strength Bolts addresses this in the Commentary for Section 5.1: “If pretensioned bolts are used in a joint that loads the bolts in tension, the question arises as to whether the pretension and the applied tension are additive. Because the compressed parts are being unloaded during the application of the external tensile force, the increase in bolt tension is minimal until the parts separate (Kulak et al., 1987; pp. 263-266). Thus, there will be little increase in bolt force above the pretension load under service loads. After the parts separate, the bolt acts as a tension member, as expected.”

AISC Design Guide 17: High Strength Bolts—A Primer for Structural Engineers (available at www.aisc.org/dg) discusses this in more detail in Sections 4.2 and 6.2. You will find that the pretension in the bolt places the material within the bolt grip into compression. As an external tension load is applied to the bolt, this reduces the clamping force and slightly increases the tension in the bolt. This increase is negligible, as the Design Guide states: “Both theory and tests [6] show that the increase in bolt pretension up to the load level at which the connected parts separate is in the order of only 5 to 10%. This increase is small enough that it is neglected in practice. Thus, the assumption is that under service loads that apply tension to the connected parts a pretensioned bolt will not have any significant increase in internal load.” The discussion in Chapter 6 includes more detail and free body diagrams to help illustrate this issue.

Given that ASTM F3125 has replaced the standards for A325, A325M, A490, A490M, F1852 and F2280, what is the proper way to specify high-strength bolts on contract drawings?

In order to minimize confusion, the “old” designations (A325, A490, etc.) in F3125 are used as grades. In order to ease the transition, the common designations (A325, A490, etc.) may be used when the more formal designation is not necessary. For example, Section J3.1 in the 2016 Specification refers to ASTM F3125/F3125M Grade A325, but Table J3.1 simply refers to A325 bolts. The Version 15.0 AISC Design Examples document (www.aisc.org/manualresources) simply refers to Group A or Group B bolts in figures. Section 3.1 of the Code of Standard Practice for Steel Buildings and Bridges (ANSI/AISC 303) states: “The structural design documents shall clearly show or note the work that is to be performed and shall give the following information with sufficient dimensions to accurately convey the quantity and complexity of the structural steel to be fabricated.”

The way in which you specify the bolts should reflect your intent. If your goal is to minimize the cost, then there are advantages to not over-specifying materials or means and methods. If you specifically want the contractor to use twist-off-type tension-control (TC) bolts, then you need to specify ASTM F3125/F3125M Grade F1852 (or F2280). If you specifically do not want the contractor to use TC bolts, then you need to specify ASTM F3125/F3125M Grade A325 (or A490). If you want to give the bidding contractors the freedom to provide bolts and installation methods that provide the least cost to the project, then you might simply specify “Group A bolts as defined in Section J3.1 of the AISC Specification.”

Section J10 of the Specification for Structural Steel Buildings (ANSI/AISC 360) states: “This section applies to single- and double-concentrated forces applied normal to the flange(s) of wide-flange sections and similar built-up shapes.” Does this mean that the provisions of the section cannot be applied to sections other than wide-flange sections?

No. The statement is intended to indicate that the provisions were written to address concentrated forces applied to wide flange sections. Most of the research has also involved only rolled wide-flange sections. However, the provisions can be adapted to address other conditions with some judgment.

For example, Chapter K: Additional Requirements for HSS and Box-Section Connections (K2) states: “The available strength of connections to rectangular HSS with concentrated loads shall be determined based on the applicable limit states from Chapter J.”

In fact, the Commentary to Section J10 has been revised in the 2016 Specification to provide more guidance when checking sections other than wide-flange shapes. An example of this would be the use of Section J10 to check concentrated forces on HSS walls, which was previously addressed in Chapter K. The Commentary for Section J10 states: “The provisions in J10 have been developed for use with wide-flange sections and similar built-up shapes. With some judgment, they can also be applied to other shapes. The Commentary related to the individual subsections provides further detail relative to testing and assumptions.” You will ultimately need to rely on your engineering judgment.

Flange local bending (J10.1) is based on a yield line analysis, which assumes a flange on each side of the web. It might make sense to assume only half the yield lines can be developed at a channel flange. Web local yielding (J10.2) assumes a distribution of 2.5:1 through the flange thickness and the fillet (k-distance). Since there is only a fillet on one side of the web at a channel, it might make sense to discount the strength accordingly. The stability checks, web local crippling (J10.3), web sidesway buckling (J10.4) and web compression buckling (J10.5) are more complex. You may want to look at the original research (listed at the end of the Commentary) and/or use a conservative model (possibly neglecting restraint from the flange) for these checks. The Commentary provides references for all of these checks, except web compression buckling equation, which is derived by Chen and Newlin; see “Column Web Strength in Steel Beam-to-Column Connections” from the 1971 ASCE Annual and National Environmental Engineering Meeting. There are plans to add a reference to this equation to the Commentary of the 2022 Specification.

If you’re interested in learning more, check out our half-day 15th Ed. Manual & 2016 Specification workshops across the country to get in-depth details on both publications.

View Part One of the FAQs.


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