Modern Steel Construction

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

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 about the AISC Steel Construction Manual and 2016 AISC Specification for Structural Steel Buildings.

It seems that full-depth transverse stiffeners prevent relative movement of a beam’s flanges. Can the location of transverse stiffeners in a beam be considered a brace point?

We receive this question on a regular basis. The answer is no. Transverse stiffeners are simply along for the ride as the section rotates and provide no resistance to lateral-torsional buckling on their own. As a result, they do not affect the unbraced length of the beam, Lb. Stiffeners can be used to reduce the web deformation and improve the efficiency of torsional braces. However, used alone, web stiffeners are ineffective in enhancing the stability of members.

Please confirm that when third party special inspections are waived by the authority having jurisdiction over the project, the NDT requirements in Chapter N of the Specification are also waived.

This is not correct. The Commentary for Section N6 clarifies the intent, stating: “Granting a waiver of QA inspections in a fabrication shop does not eliminate the required NDT of welds; instead of being performed by QA, such inspections are instead performed by the fabricator’s QC.” NDT must be performed even when the QA inspections are waived.

The article “A Slightly Longer Look at Prying” states that the effective width, p, for prying action can be conservatively taken as 3.5b but cannot exceed the spacing between the bolts. This conflicts with Part 9 of the 14th Edition Manual, which indicates that p is limited to twice b. Can the effective width exceed 2 times b?

Yes. The statement in the article is based on the 15th Edition Manual, which revised the default effective width from p = 2b to p = 3.5b. The new default value is based on guidance provided by the South African Institute of Steel Construction that was evaluated by the AISC Manual Committee and deemed to be adequate. The new assumed distribution angle is 60°, which is conservative but not as conservative as the assumed 45° angle used in the 14th Edition Manual. It should be noted that the 2p limit was not intended to be a requirement. Even though it is not stated, it was only a recommendation. The recommendation was established because it was brought to the attention of the Manual Committee that there was a wide range of assumed tributary lengths being used in practice. It was felt that the Manual should provide guidance. As is often the case when engineers are forced to provide guidance, the first pass was conservative. Given the lack of data available at the time, the committee felt that the 2b guidance was a safe lower bound. With a closer look at the South African Institute of Steel Construction data the 3.5b limit was adopted. Note that the Manual also allows that a larger tributary length may be justified based upon testing or rational analysis.

What are the differences between slip-critical Class A bolts and slip-critical Class B bolts, and how should they be indicated in shop and erection drawings?

There is no such thing as a slip-critical bolt, a Class A bolt, a Class B bolt or a bearing bolt. The same bolt can be used in slip-critical joints with either Class A or Class B faying surfaces. In fact, the same bolt can be used with either slip-critical or bearing-type joints. The difference between a slip-critical joint and a bearing-type joint is that a slip-critical joint resists movement of the plies through friction, and a bearing-type joint resists movement between the plies through bolt shear and bearing at the plies. The Class A and B designations refer to the surface preparation required. Section J3.8 of the AISC Specification for Structural Steel Buildings (ANSI/AISC 360-16, available at defines Class A surfaces as “unpainted clean mill scale steel surfaces or surfaces with Class A coatings on blast-cleaned steel or hot-dip galvanized and roughened surfaces” and Class B surfaces as “unpainted blast-cleaned steel surfaces or surfaces with Class B coatings on blast-cleaned steel.” The detailer must properly indicate on the shop drawings the required surface preparation at the slip-critical joints, as this affects the strength of these joints. Slip-critical joints also need to be pretensioned, and this must be conveyed in some manner in the documents related to the bolt installation, either the shop or erection drawings. Indicating that the joints are slip-critical is sufficient to ensure pretensioning.  As a sideline, please note that all pretensioned joints are not slip-critical.

The following questions all relate to the requirements of Chapter N of the Specification: 1. Must special inspections be performed when AISC Certified fabricators perform the work? 2. Are AISC Certified fabricators allowed to perform special inspections with their own personnel? 3. Are AISC Certified fabricators allowed to perform nondestructive testing (NDT) with their own personnel? 4. If fabricators are allowed to perform NDT with their own personnel, are the qualifications for these personnel any different than those of a typical third party special inspector? 5. Do AISC Certified fabricators typically retain people in-house to perform NDT, or is this work typically done by a third party retained by the owner?

The following answers are provided:

  1. This is not a yes or no question. The AISC Specification for Structural Steel Buildings (ANSI/AISC 360), available at, does not require special inspections. Requirements related to special inspections are defined in the building codes, such as the International Building Code. The IBC permits authorities having jurisdiction to approve fabricators, and some or all third party special inspection requirements can be waived when the work is performed by approved fabricators.

  2. The fabricator can, at the discretion of the authority having jurisdiction, use their own personnel to ensure the quality of the project without outside inspections. Quality control (QC) is defined as “controls and inspections implemented by the fabricator or erector, as applicable, to ensure that the material provided and work performed meet the requirements of the approved construction documents and referenced standards.” Quality assurance (QA) is defined as “monitoring and inspection tasks to ensure that the material provided and work performed by the fabricator and erector meet the requirements of the approved construction documents and referenced standards. Quality assurance includes those tasks designated ‘special inspection’ by the applicable building code.” Both QC and QA are intended to ensure conformance with the approved construction documents and referenced standards. QC is performed and documented by the fabricator. Section N5.3 allows coordinated inspection meaning QC tasks need not be repeated as QA tasks. The waiving of special inspections represents the approval of the engineer of record and the authority having jurisdiction for coordinated inspection. If special inspections are not waived, a third party will perform special inspections. Both QC and QA tasks can be seen by examining the tables provided in Chapter N.

  3. Yes. Section N6 of the AISC Specification specifically allows approved fabricators to perform NDT. It also indicates that when NDT is performed by the fabricator, a QA agency shall review the fabricator’s NDT reports.

  4. No. Section N4.3 defines NDT Personnel Qualifications. The requirements do not vary based on the party performing the work.

  5. Practice varies. My understanding is that most fabricators do not employ personnel trained to perform NDT. If the fabricator cannot perform the NDT, then I believe it is more common for the owner or fabricator to contract the NDT tasks.

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 Two of the FAQs

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