Ch. J. Carter/C. J. Duncan · Recent changes in U.S. connection design practice
added; alternatively, additional bolts can be added to develop the fillers. The filler factor hf is determined as follows: – Where bolts have been added to distribute loads in the fillers: hf = 1.0 – Where bolts have not been added to distribute loads in the fillers: hf = 1.0 for one filler between connected parts, and hf = 0.85 for two or more fillers between connected parts
ways. Provisions in section J2.4 (a) and (c) in the 2010 AISC Specification are based on a load–deformation behaviour that is affected by the weld size [7]. Accordingly, these provisions have been clarified to reflect that they are based on fillet weld groups in which the size of the weld is uniform. When the weld group is not of uniform size, section J2.4 (b) can be used to account for size variations.
It also is worth noting that prior to the 2010 AISC Specification, fillers > ¾ in. (19 mm) thick had to be developed. This is no longer the case. A reduction factor still applies to the bolt shear strength when fillers are not developed, but the 2010 Specification recognizes that the reduction factor need not exceed 0.85 regardless of the thickness of the filler.
7 Prying action formulas improved with simple change
4 Base metal design at welds Table J2.5 in the 2010 AISC Specification summarizes the available strengths for welds and base metal and weld metal in welded joints. Base metal strength at welds is now based on the rupture strength rather than the yield strength. Previously, the design was based on yielding in the base metal, which has come to be viewed as conservative and incorrect since the weld itself adjacent to the base metal is designed for a rupture limit state.
5 Directional strength increase extended to out-of-plane loading Prior to 2010 the AISC Specification included the words “in plane” when provisions were given for the directional strength increase for fillet welds, i.e. the provisions were limited to loading in the plane of the weld or weld group. Common usage of the provisions in practice, however, extended these provisions to out-of-plane loading as well. Research [6] was conducted to evaluate that practice and showed that the restriction (the words “in plane”) could be eliminated. Accordingly, they do not appear in the 2010 AISC Specification.
6 Weld group size uniformity requirements added Fillet welds used in groups are generally all of the same size – but not al-
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design. As a result, eccentricity requirements re-appeared in the single-plate connection design procedures in the 14th edition of the AISC Manual. Table 2 illustrates the eccentricities that are used in the design of single-plate connections. References
Changes to the bolt shear strength values necessitated a change in the 14th edition of the AISC Steel Construction Manual procedures for single-plate connections. In the 13th edition of the Manual, the 20 % bolt shear strength reduction was used as a convenient way of simplifying the design of single-plate connections. That is, we knew the effect of most eccentricities was less than the 20 % reduction, and we also knew that shear connections are not end-loaded and did not need the 20 % reduction. On this basis it was accepted that most eccentricities in these connections could be ignored. The changes to the 2010 AISC Specification cut the margin on bolt strength to a 10 % reduction, which was no longer enough to offset the impact of eccentricity in the connection
[1] AISC: Specification for Structural Steel Buildings (ANSI/AISC 360-10), AISC, Chicago, IL, 2010. [2] AISC: Steel Construction Manual, AISC Chicago, IL, 2011. [3] Borello, D. B., Denavit, M. D., Hajjar, J. F.: Behavior of Bolted Steel Slip-Critical Connections with Fillers. Report No. NSEL-017, Department of Civil & Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 2009. [4] Dusika, P., Iwai, R.: Development of Linked Column Frame Lateral Load Resisting System. 2nd Progress Report for AISC and Oregon Iron Works, Portland State University, Portland, OR, 2007. [5] Grondin, G, Jin, M., Josi, G.: Slip-Critical Bolted Connections – A Reliability Analysis for the Design at the Ultimate Limit State. Preliminary Report prepared for AISC, University of Alberta, Edmonton, Alberta, CA, 2007. [6] Kanvinde, A. M., Grondin, G. Y., Gomez, I. R., Kwan, Y. K.: Experimental Investigation of Fillet Welded Joints Subjected to Out-of-Plane Eccentric Loads. Engineering Journal, American Institute of Steel Construction, 3rd Quarter, 2009. [7] Muir, L. S.: Deformational Compatibility in Weld Groups. ECCS/AISC Workshop Connections in Steel Structures VI. 23–24June 2008, Chicago, IL. [8] Swanson, J. A.: Ultimate Strength Prying Models for Bolted T-Stub Connections. Engineering Journal, AISC, 2002, vol. 39, No. 3, 3rd Quarter, AISC, Chicago, IL, pp. 136–147 [9] Thornton, W. A.: Strength and Serviceability of Hanger Connections. Engineering Journal, AISC, 1992, vol. 29, No. 4, 4th Quarter, AISC, Chicago, IL, pp. 145–149.
Table 2. Bolt strength levels as grouped in the 2010 AISC Specification
Keywords: connections; bolts; welds; prying action; slip critical; AISC
Treatment of prying action in the AISC Manual and other sources has traditionally been based on the use of Fy in the calculations. At the same time, it has long since been known that the resulting predictions of the equations for prying action are significantly conservative [8], [9]. To address this in a simple manner, the AISC Manual now uses Fu in place of Fy for prying action checks.
8 Single-plate connection eccentricity calculations revised
n 2–5 6–12
Hole type
e [in.]
max. tp or tw [in.]
SSLT
a/2
none
STD
a/2
db/2 + 1/16
SSLT
a/2
db/2 + 1/16
STD
a
db/2–1/16
Authors: Charles J. Carter, SE, PE, PhD, Vice-President and Chief Structural Engineer, American Institute of Steel Construction, Chicago, IL, USA, carter@aisc.org Cynthia J. Duncan, Director of Engineering, American Institute of Steel Construction, Chicago, IL, USA, duncan@aisc.org
Steel Construction 6 (2013), No. 1
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