AISC

4.8. Other General Information

4.8.1. When a stiffening element is attached to a plate, what width of plate is effective in resisting load with the stiffening element?

A general rule of thumb that is used in American Petroleum Institute and American Iron and Steel Institute standards for liquid storage tanks is that 16 times the plate thickness is effective on either side of a stiffening element, but not greater than:

  1. The actual projecting width. 
  2. One-half the distance to the next stiffening element. 
  3. One-quarter of the stiffener span. 
  4. The width based upon the limiting width-thickness ratios in the 2005 AISC Specification Table B4.1 for compression elements.

4.8.2. How are stepped columns designed?

This subject is addressed in several references:

Fisher, J.M., 2005, AISC Design Guide No. 7, 2nd Edition, Industrial Buildings -- Roofs to Column Anchorage, AISC, Chicago, IL, pg. 67.

Anderson, J.P. and J.H. Woodward, 1972, "Calculation of Effective Lengths and Effective Slenderness Ratios of Stepped Columns," Engineering Journal, Vol. 9, No. 4, (October), AISC, Chicago, IL.

Barnes, D.W. and C.P. Mangeldorf, 1979, "Allowable Axial Stresses in Segmented Columns," Engineering Journal, Vol. 16, No. 1 (1st Quarter), AISC, Chicago, IL.

Agrawal, K.M. and A.P. Stafiej, 1980, "Calculation of Effective Lengths of Stepped Columns," Engineering Journal, Vol. 17, No. 4 (4th Quarter), AISC, Chicago, IL.

Errata to Agrawal and Stafiej paper

Castiglioni, C.A., 1986, "Stepped Columns: A Simplified Design Method," Engineering Journal, Vol. 23, No. 1 (1st Quarter), AISC, Chicago, IL.

Moore, W.E., 1986, "A Programmable Solution for Stepped Crane Columns," Engineering Journal, Vol. 23, No. 2 (2ndQuarter), AISC, Chicago, IL.

Fraser, D.J., 1990, "The In-Plane Stability of a Frame Containing Pin-Based Stepped Columns," Engineering Journal, Vol. 27, No. 2 (2ndQuarter), AISC, Chicago, IL.

4.8.3. Should weep holes be used at the bottoms of Hollow Structural Section [HSS] columns?

If the HSS assembly is fabricated as an airtight enclosure, weep holes need not be provided because any moisture in the contained air will quickly be used and corrosion cannot progress. When non-airtight HSS columns are exposed to the weather or to temperature changes that can cause interior condensation, weep holes should be provided. If, however, a column is protected from the elements and is neither subject to drastic temperature change nor a humid environment, weep holes may not be necessary. Note that HSS members need weep holes if they are to be galvanized.

4.8.4. What special considerations should be noted in crane-girder design?

The following special design considerations should be noted: 

  1. Adequate vertical and lateral stiffness are of primary importance.

  2. Simple-span construction should be used (no knee braces).

  3. The cross-section should be proportioned on the basis of its elastic flexural strength.

  4. Lateral stability should be provided at the bearing ends without inhibiting end rotations.

  5. Vertical and lateral impact loads should be considered.

  6. Lateral forces due to trolley acceleration and braking, runway misalignment, crane skew, and other sources should be considered.

  7. Fatigue should be considered along with the anticipated number of loading cycles.

  8. The class of service to which the crane will be subject should be considered.

  9. The local strength of the web under crane wheel loads should be checked.

  10. Biaxial bending of the top flange should be checked.

  11. Bottom-flange bracing should be provided when required; bottom-flange bracing is recommended for spans over 36 ft in AIST1.

  12. For built-up crane-girders, the weld between the top flange and the web should be a CJP groove weld.

  13. Suitable provision should be made for the crane-rail attachment system.

  14. Suitable provision should be made for the electrification of the crane system.

  15. End stop configuration and design.

  16. Spacing of wheels on the end truck.

  17. Provisions for maintaining center-to-center of crane rails.

  18. Location of expansion joints, if required.

  19. Crane column bracing.

Further information is available in the following references: AIST1Fisher2, and Ricker3.

[1] Association for Iron and Steel Technology, 2003, AIST Technical Report No. 13: Guide for the Design and Construction of Mill Buildings, Warrendale, PA.

[2] Fisher, J.M., 2005, AISC Design Guide No. 7, 2nd Edition, Industrial Buildings - Roofs to Anchor Rods, AISC, Chicago, IL.

[3] Ricker, D.T., 1982, “Tips for Avoiding Crane Runway Problems,” Engineering Journal, Vol. 19, No. 4, (4th Qtr.), pp. 181-205, AISC, Chicago, IL.

4.8.5. Does AISC have any information on stainless steel?

No. Two excellent resources for information on stainless steel are the Specialty Steel Industry of North America, www.ssina.com, and the Nickel Development Institute, www.nidi.org.

4.8.6. Why are some of the cross-sectional dimensions for wide flange shapes different between the new 13th Edition Manual and previous ASD and early LRFD Manuals?

In 2001, increased fillet radii for wide flange shapes were recognized per the AISC Advisory dated February 1, 2001, which affected the Tk and k1 dimensions. Since tee shapes are made from wide flange shapes, tees were affected as well.

Essentially, now we have a design value kdes (in decimal form), which is used for calculating local web yielding strength and calculations involving clear web depths, and a detailing value kdet (in fractional form) used to ensure that transverse stiffeners, continuity plates and other such details will fit. It follows that the design value kdes is smaller than the detailing value kdet, as each represents the minimum and maximum of k values for any particular wide flange being produced. See FAQ 1.4.5 for more information.

Note that there may have been changes in the ASTM A6 shape dimension tables as well over the years, and this may have resulted in some dimensional changes for certain shapes, but this is in addition to the fillet change issue.