5.3. Other General Information
5.3.1. Are shop assembly requirements, such as subpunching and reaming or reaming to a template, necessary in contract documents?
The use of modern punching and drilling equipment consistently produces and duplicates hole patterns with excellent dimensional accuracy. Some specifications fail to recognize this capability and still require that matching hole patterns be produced by drilling or reaming through a steel template.
In lieu of a template or assembly drilling or reaming, the fabricator should be allowed to demonstrate the capability to fabricate component structural members to the tolerance and accuracy specified so that further shop assembly to assure proper fit can be eliminated. In some cases, such as large trusses or plate girders, shop assembly may be advisable to reduce the occurrence of field fit-up problems. In any case, responsibility for final fit still rests with the fabricator.
Projecting elements of bolted connection attachments, such as clip-angles or end-plates, often are not flat in the plane of the connection because of profile variations due to as-rolled mill tolerances or welding distortions. In double-angle connections, for example, the outstanding legs tend to bend back toward the centerline of the span. Any resulting gaps are usually drawn together when the bolts are installed, except in relatively thick material. The additional tension in the bolts produced by pulling the plies together is not a concern. High-strength bolts must comply with AISC, RCSC and ASTM requirements, which ensures proper matching of the nut and bolt. One reason for this is to ensure that, if the bolt fails in the tightening operation, the failure will be a torque-tension fracture in the bolt shank—not a thread-stripping failure. When this happens, the bolt fractures completely and must be replaced. If the bolt is pretensioned to a higher value than the specified pretension, tests have shown that there are no negative effects on the bolt during service.
Neither bearing nor slip-critical connections require continuous contact between the plies. Therefore the RCSC Specifcaiton 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.”
When firm contact exists between the connected elements, bolts in shear, or shear and tension, will not be subjected to additional bending stresses.
The slip resistance of slip-critical connections is not dependent on the contact area. It is only a function of the pretension and the slip coefficient of the faying surface. Whether the pretension results in a low clamping stress over a large area, or a higher clamping stress over a smaller area is immaterial.
AISC Design Guide 21 states: “The incidence of lamellar tearing today is significantly reduced as compared to the past, due mostly to proper joint selection and better steel chemistry. Current steelmaking practices have helped to minimize lamellar tearing tendencies. With continuously cast steel, the degree of rolling after casting is diminished. The reduction in the amount of rolling has directly affected the degree to which these laminations are flattened, and has correspondingly reduced lamellar tearing tendencies.”
Research (Melendrez and Dexter ) demonstrates that W-shapes are not susceptible to lamellar tearing or other through-thickness failures when welded tee joints are made to the flanges at locations away from member ends. Special production practices can be specified for steel plates to enhance through-thickness ductility and assist in reducing the incidence of lamellar tearing. For further information, refer to ASTM A770. However, it must be recognized that the specification of premium-quality steel does not itself eliminate the potential for lamellar tearing—or the need for careful design, detailing and fabrication of highly restrained joints.
Melendrez, M.I. and Dexter, R.J. (2000), “ThroughThickness Properties of Column Flanges in Welded Moment Connections,” Journal of Structural Engineering, Vol. 126, No. 1, pp. 24-31, ASCE, Reston, VA.
Shear lag describes behavior at an end connection of a tension member where some but not all of the cross-sectional elements are connected; the area that is effective in resisting tension may be less than the full calculated net area. Procedures for treatment of shear lag and determination of the effective net area in bolted and welded connections are provided in the 2010 AISC Specification Section D3.3. Alternatively, shear lag concerns can be addressed by selecting a connection length that mobilizes the entire load-transmitting capability.
Column stiffening requirements are covered in the AISC Specification Section J10 for concentrated flange forces and panel zone shear. Generally, the use of larger columns to eliminate column stiffening, particularly web doubler plates, is recommended. For seismic applications, see the AISC Seismic Provisions.
5.3.6. In many design examples in the Manual of Steel Construction, yielding and buckling in a gusset plate or similar fitting are checked on a Whitmore section. What is a Whitmore section?
A Whitmore section identifies a theoretically effective crosssectional area at the end of a connection resisting tension or compression, such as that from a brace-to-gusset-plate connection or similar fitting. As illustrated in Figure 5.3.7-1 for a WT hanger connection, the effective length for the Whitmore section Lw is determined by using a spread-out angle of 30° along both sides of the connection, beginning at the start of the connection. It is applicable to both welded and bolted connections.
5.3.7. How can adequate flexibility be maintained in double-angle simple shear connections subjected to combined shear and tension load?
As the tensile force component increases in a double-angle shear connection subjected to combined shear and tension, prying action and/or bending require that the fitting thickness be increased or the bolt gage be decreased, thereby decreasing the available flexibility. Thornton (1995) assesses the ductility of bolts in the outstanding legs of double-angle and similar simple-shear connections.
This study validates the long-standing AISC Manual recommendation that maximum angle thickness be limited to 5∕8 in. for usual gages (4½ in. to 6½ in.) in double-angle simple-shear connections. For welded connections, a parallel examination can be made as illustrated in Thornton (1996). It should be noted that an alternative connection detail, such as a single-plate connection, may be more feasible for shear-tension applications.
It is important to realize that Section B1.6a in the AISC Specification requires only that a simple connection have sufficient rotation capacity to accommodate the required rotation determined by the analysis of the structure. This may not dictate that bolts must be stronger than the angles. In some instances the beam may be deep relative to its length or lightly loaded in the vertical direction. In either case the required rotation will be small.
Thornton, W.A (1995), “Treatment of Simple Shear Connections Subject to Combined Shear and Axial Forces,” Modern Steel Construction, September, pp. 9-10, AISC, Chicago, IL.
Thornton, W.A (1996), “A Rational Approach to the Design of Tee Shear Connections,” Engineering Journal, Vol. 33, No. 1, (1st Qtr.), pp. 34-37, AISC, Chicago, IL.
Parts 7-14 of the AISC Steel Construction Manual provide a
wealth of information related to connection design. Additional information can be found here:
➤ Design Guides 4 and 16 address the design of end-plate
➤ Design Guide 8 addresses one type of partially-restrained
➤ Design Guide 13 addresses the stiffening of wide-flange
columns at moment connections.
➤ Design Guide 24 addresses HSS connections.
5.3.9. What are some AISC resources for steel
Detailing for Steel Construction (3rd Edition, 2009) is an excellent reference that discusses some common detailing practices and has many sample detail drawings. Among other things, the reference has a section on drafting, structural steel, detailing and fabricating of steel, some structural engineering fundamentals (stress and strain), bolted connections, welded connections, columns and framing for industrial buildings.
AISC also has a web-based Detailer Training Series. Originally developed by AISC and the National Institute of Steel Detailing, it is now being made available as a free web-based service thanks to funding from IMPACT (the Ironworker Management Progressive Action Cooperative Trust). See it online at www.aisc.org/dts. Note that it also is a great introduction to steel construction for anyone with an interest in steel construction, not just steel detailers.