4.4. Special Design Considerations
Toughness is the ability of a material to deform inelastically in the presence of flaws and under conditions of high stress concentration. A tough material will resist the initiation of a crack at points of stress concentration. Although, under certain abnormal conditions, the stress may rise locally to such levels that a crack is initiated, a tough material will better resist the propagation of the crack. The consideration of fracture mechanics principles may be necessary in some instances.
Some examples of when notch toughness should be specified include:
- dynamic or impact loading
- fatigue loading
- service temperatures below 25 degrees F.
Additionally, selected welding details require notch toughness, such as welding of heavy shapes (see the 2005 Specification Sections A3.1c, A3.1d, and J1.5) and CJP groove welds in high-seismic systems (see the 2005 AISC Seismic Provisions). The use of bolted alternatives will often relieve or eliminate the need for special notch-toughness properties.
Notch toughness properties should generally be specified for service temperatures below 25 degrees F. As noted in Part 2 (Fatigue and Fracture Control, Page 2-33) of the 13th Edition Manual, Section S83 of the ASTM A709 Specification (available from ASTM at www.astm.org ) may be useful in determining appropriate notch toughness levels to specify, even though this specification is primarily used in bridge applications. See also 4.4.1 .
The reduction in yield strength as temperature is increased must be considered in design for service at elevated temperature. An excellent discussion of the effect of elevated temperature on mechanical properties as well as creep and other effects can be found in Brockenbrough and Johnston (1981). Another excellent resource is Tide (1998).
Alternatively, the steel can be shielded from the source of heat. For example, a 1/4-in. steel plate shield with a 2-in. air space is sometimes provided for crane girders that span above furnaces in mill buildings.
Brockenbrough, R.L. and B.G. Johnston, 1981, USS Steel Design Manual, United States Steel Corporation, Pittsburgh, PA.
Tide, R.H.R., "Integrity of Structural Steel After Exposure to Fire," Engineering Journal, Vol. 35, No. 1, (1st Qtr. 1998), pp. 26-38, AISC, Chicago, IL.
4.4.4. When a beam with a web penetration requires reinforcement, what is the most efficient location for this reinforcement?
As indicated in AISC Design Guide #2 Steel and Composite Beams with Web Openings, horizontal reinforcement above and below the web penetration as illustrated in Figure 4.4.4-1 is the most efficient and effective, when reinforcement is required.
4.4.5. The 2005 AISC Specification Appendix 3 covers constant amplitude fatigue. What about variable amplitude fatigue loading?
An effective stress range can be calculated using Miner’s rule (Schilling and Kleppstein, 1978). For additional information, see Appendix 3 of the 2005 AISC Specification.
4.4.6. The ASTM A709-05 Specification lists different zones in giving recommended notch-toughness levels. Where are these zones defined?
These various zones are defined by AASHTO. Zone 1 is for areas where the lowest ambient service temperature is expected to be above 0 degrees F; zone 2 is for -30 to 0 degrees F and zone 3 is for -60 to -30 degrees F.
AASHTO has a guide that would be useful: AASHTO Guide Specifications for Horizontally Curved Steel Girder Highway Bridges with Design Examples for I-Girder and Box-Girder Bridges, 2003. Another good textbook is the Design of Modern Steel Highway Bridges, Conrad P. Heins and D. Allan Firmage, John Wiley and Sons, 1979.
Depending on the specific application, the following articles from Engineering Journal may also be useful:
- "Straight Element Grid Analysis of Horizontally Curved Beam Systems." Herbert A Weissman, April 1970.
- "Analysis of Curved Girder Bridges." Charles Culver et.al., Jan. 1970.
- "Approx. Torsional Analysis of Curved Box Girders by the M/R-Method." David H.H. Tung, July 1970.
- "Box Girder Bridge Design - State of the Art." C.P. Heins, 4th Quarter, 1978.
- "The Application of Flexural Methods to Torsional Analysis of Thin-Walled Open Sections." Thomas E. Boothby, 4th Quarter, 1984.
- "Horizontally Curved Steel Girders—Fabrication and Design." W. M. Thatcher, July 1967.
4.4.8. AISC's Design Guide 15: AISC Rehabilitation and Retrofit Guide has such information. The design guide, which includes a historical study of both ASTM and AISC Specifications from the early 1900’s, also includes the AISC Historical Shapes Database, which is available on CD. Further information on this Design Guide
AISC's Design Guide 15: AISC Rehabilitation and Retrofit Guide has such information. The design guide, which includes a historical study of both ASTM and AISC Specifications from the early 1900’s, also includes the AISC Historical Shapes Database, which is available on CD. Further information on this Design Guide is available on AISC’s web site bookstore, http://www.aisc.org/bookstore.
Design Guide 15 replaces AISC's now out-of-print "Iron and Steel Beams – 1873 to 1952", which can periodically be found in online auctions and in used bookstores.
4.4.9. Why are the safe working loads tabulated for clevises and turnbuckles in the 13th Edition Manual based on such a high factor of safety (5)?
As noted in the footnote to the turnbuckle table in the 13th Edition Manual, the large factor of safety is because clevises and turnbuckles are often used for rigging and may be subject to cyclic and impact loading. Fatigue may become a factor.
When used in permanent applications where these considerations do not apply, the design strengths for clevises and turnbuckles in the 3rd Edition LRFD Manual are based on a phi factor of 0.5. This phi factor corresponds with a factor of safety of 3 for the safe working load when using the maximum pin diameter.