American Institute of Steel Construction

Recent work on a self-centering column base connection with friction dampers is highlighted. This research is a collaborative effort by Senior Researcher Massimo Latour and Professor Gianvittorio Rizzano from the University of Salerno, Italy, and Professors Aldina Santiago and Luis Simões da Silva of the University of Coimbra, Portugal. The Salerno-Coimbra team has developed an alternative self-centering column base solution to minimize initial costs and economic losses. Specific objectives are to limit damage and residual drifts, with connection components that are easy to repair or replace if needed. The self-centering column base has been validated through quasi-static cyclic testing and pseudo-dynamic testing. The self-centering connection has also been investigated through numerical time-history analyses of moment-resisting frames comparing conventional, fixed column bases to self-centering column bases. Some highlights of the research are presented.

American Institute of Steel Construction (AISC) document 360, Specification for Structural Steel Buildings, is the basic reference for the design, fabrication, and erection of structural steel buildings and other “building-like” steel structures in the United States. When applied in conjunction with AISC 360, AISC 341, Seismic Provisions for Structural Steel Buildings, is the standard reference document for the seismic design of steel structures throughout the United States. Balloting is complete to update AISC 341-16 that will be incorporated with ASCE 7-16 and AISC 360-16 into the 2018 International Building Code. The document will have significant technical modifications including new material specifications, use of steel braced diaphragms, new column splice details, requirements for SCBF gusset plate welds, and application of demands on columns that participate in intersecting frames. In addition, significant new provisions related to the seismic design of multi-tier braced frames will be provided. A new composite shear wall system has also been developed. This paper will summarize the changes proposed for AISC 341-16.

While current AISC provisions have greatly improved the seismic behavior of conventional braced frame systems, they still have a tendency to form weak stories. The strongback system was developed as a method of delaying or preventing weak-story behavior. Conceptually, the inclusion of an “essentially elastic” backbone, or strongback, enforces a nearly uniform drift distribution, thereby engaging adjacent stories upon the initiation of inelastic behavior in the opposite braces. Ongoing work on the strongback braced frame is highlighted. The research is led by Dr. Stephen Mahin, professor at the University of California–Berkeley, and Byron L. and Elvira E. Nishkian, professors of structural engineering. Dr. Mahin currently leads the SimCenter as part of the Natural Hazards Engineering Research Infrastructure (NHERI) and was recently awarded an AISC grant to research and develop a possible design method for the strongback system.

When a plate is subjected to applied loads and significant out-of-plane deformation, the demand on the connection may exceed that derived from calculations due to the applied loads only. Where inelastic behavior is acceptable, the intent may be to ensure ductile behavior. For plates with fillet-welded edge connections, this can be accomplished by sizing the fillet welds to develop the strength of the plate. One specific example of this arises when a brace in a special concentrically braced frame (SCBF) is subject to compression and buckles out-of-plane. Bending of the gusset plate may demand more of the gusset plate edge connection than the calculated forces that result on the gusset edge due to the brace force specified in Section F2.6c.2 of AISC 341-16 (AISC, 2016). If not accounted for in the weld size, the uncalculated weak-axis moment on the welds from out-of-plane bending of the gusset plate might cause rupture of the fillet welds to govern the behavior of the system. The method provided in this paper is suitable to determine the minimum size of fillet welds necessary to prevent weld rupture as out-of-plane deformations occur. It can be used for fillet-welded gusset plate edges in SCBFs to satisfy the exception provided in Section F2.6c.4 of AISC 341-16.

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The geometry of a single-plate connection is really quite simple — a rectangular plate is fillet welded to a support, and field-bolted to a beam. But the simple elegance of this connection can often be overshadowed by the complexity of the structural model. The force distribution on this connection is indeterminate, and the connection itself, being relatively stiff, has inelastic features that are important to its performance. Unlike many of the other types of connections, shear tabs rely upon inelastic deformation of the plate to achieve the level of rotation that allows it to be considered a simple connection by the definitions presented in the AISC Specification. The complexity of the inelastic performance of shear tab connections prompted the AISC Committee on Manuals and Textbooks to develop the new, largely prescriptive design procedure now found in the Steel Construction Manual, 13th Edition. The Manual presents two newly developed design procedures — one that is a refinement of older procedures for the design of the classic shear tab connection, now called "conventional" single-plate connections, and an entirely new design procedure for other applications, termed "extended" single-plate connections in the Manual. Like all simple shear connections, single-plate shear connections have some level of restraint and a corresponding eccentricity on the connection. This restraint is largely relieved before the connection reaches its ultimate strength through both elastic deformations, such as flexing of the plate and bending of the supporting member, and inelastic deformations associated with the movement of the bolts as they shift in the holes. As the ultimate load is approached, the bolt holes deform in the plate, providing relief to much of the eccentricity of load.

Preface to the North American Design Comparisons

Technical changes made in the 1994 RCSC LRFD Specification for Structural Joints Using ASTM A325 or A490 Bolts include such areas as shear-tension interaction for high-strength bolts in bearing connections and clear distance- based bolt bearing strength. In this paper, corresponding changes are proposed for the AISC LRFD Specification for Structural Steel Buildings and the supporting background information is summarized. Additionally, a change is proposed for the service-load slip resistance reduction factor for shear-tension interaction in slip-critical connections in both documents. Finally, a change made in 1993 AISC LRFD Specification Section J8.1 covering bearing strength of pins is documented.

An accurate and complete design will result in an economical and safe connection. Yet it is entirely common for the engineer of record to withhold, either intentionally or unintentionally, the information necessary to the fabricator or detailer to perform a design which is both accurate and complete. Specifically, actual reactions are seldom shown on the contract drawings from which the connections must be designed. AISC states, "For economical connections, beam reactions should be shown on the contract drawings. If these reactions are not shown, connections must be selected to support one-half the total uniform load capacity...for the given beam, span, and grade of steel specified." No quantification is given, however, of the actual difference in economy between the two cases. In fact, this difference is somewhat elusive as it may vary greatly among specific examples. For the general case, however, it is possible to determine a reasonable estimate of the economic sacrifice incurred when a larger connection than required is used. The focus of this paper, then, is this economic sacrifice. For simplicity, a standard configuration of the double angle connection will be considered in which only n, the number of bolt rows (and consequently, the length of the angles), varies. Based on values of n from 2 to 10, the cost of these standard connections will be estimated. Ranges of n compatible with each beam size group will be identified and the percent increase in cost which results when a larger connection than required is used will then be determined over these ranges. Note that the practices which routinely result in uneconomical connection designs are not specifically addressed in this paper. For a discussion of these practices, the reader is referred to "Eliminating the Guesswork in Connection Design—Communication of Design Requirements Between Fabricator and Engineer is Crucial for a Safe and Economic Structure" by W.A. Thornton, in the June 1992 issue of Modern Steel Construction. Also not addressed is the effect of standardization on the detailing costs, ease of fabrication, and overall quality of the constructed product. While in general, standardization will reduce detailing costs, increase the ease of fabrication, and lead to improvements in quality because of decreased variability, these considerations are more project related than connection specific. Thus, it would not be feasible to consider their effect in this paper.

In the buckled configuration of a built-up compression member, shear force is developed between individual components due to secondary moments caused by P-deffect. AISC-ASD1 requires that stitches be designed such that they have adequate strength to resist the shear force developed between individual components. AISC-LRFD also has a similar requirement (see Section E4, p. 6-40). However, neither specification gives a procedure to calculate the shear force developed between individual components in a buckled configuration. This paper presents a derivation of analytical equations to calculate the shear force developed between individual components of built-up struts in buckled configuration. The equations are presented for two cases. First, for the case in which only the first buckling load is of interest. Second, for the case in which, in addition to the first buckling load, post-buckling bending is involved such as in seismic-resistant design. The proposed equations are general enough so that they are applicable to any end condition including the two extreme cases of hinged-and fixed-end conditions. The proposed equations are verified analytically and experimentally. For analytical verification, the results from the proposed equations are examined for the extreme cases of end conditions and separation between the components. For experimental verification, test results by the authors are used. The stitch strength required for some test specimens are calculated according to the proposed equations. The results are compared with actual strength provided by the stitch welds of the corresponding specimens. It was found that specimens which suffered unsymmetrical buckling and/or post-buckling behavior did not have adequate stitch strength according to the proposed equations.