Lateral Torsional Buckling Strength of Single Angle Members

Discussion in 'Design, Development and Standards Discussions' started by Derek Hartzell, Dec 8, 2008.

  1. Derek Hartzell

    Derek Hartzell Friend of the Community

    On page 79, Section of ANSI/TIA 222-G, an equation is given: L.t=Eb^3t/(3FyIw). When you run the equation with various angles with 3/16? thickness and legs from 1.5? to 3?, you get dimensionless values in the 1400 to 1600 range. What is wrong with the equation? For example, with a L1.5x1.5x.1875 you get 29000000*1.5^3*0.1875/(3*36000*0.110) = 1545. This can?t be compared with the Lb unbraced dimension in inches to determine which F``y to use. Nt is not dimensionless (as it should be) if Lb is in inches and Lt is dimensionless and therefore you can?t use the F``y calculations anyway.

    I checked the LRFD manual. However, it uses somewhat different formulations in the angle calcs, based on moment rather than stress, and I wasn?t able to decipher the correlation and make the correction.

    Any help would be appreciated.

    Derek Hartzell
  2. Simon Weisman

    Simon Weisman Industry Observer

    I am not intimately familiar with this part, but it is possible that there is an error in the formula on page 79 that has not yet been identified.

    If you look into (Galambos), referenced as 70 in Appendix E3 of the LRFD Specification, you will see that the common sub-expression is (bt^3)/3, not (tb^3)/3. It makes a big difference, which one you use.

    Of course, this question is best posed to the TIA TR14.7 committee for the authoritative response.

    Torsional-flexural buckling is not likely to govern in most elements used in towers, because they tend to be long, so that they are likely to fail in a bending mode before torsional effects become significant.
  3. Wireless Estimator

    Wireless Estimator Administrator Staff Member

    According to the Editorial Committee, it was a typo that dropped the ?square?. They will be correcting it in an addendum to the standard.

    The equations came from AISC but were placed in an unbraced length format. There is an error in the TIA equation for Lt. The t term (thickness) is supposed to be squared. The results will exactly match AISC. The 2 exponent was omitted.

Share This Page