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ANSI / AISC 360-16 - Steel design

XFEM4U has a fully integrated ANSI/AISC 360-16 steel design module, with steel check and design rules according to the latest American standards.

All design checks are based on the Direct Analysis Method of Design according to AISC 360-16 Section C1.1, using Design for Strength Using Load and Resistance Factor Design (LRFD).

The direct analysis method used is based on an elastic P-Δ-only second-order analysis. Imperfections (initial system imperfections) are automatically included when calculating load combinations.

This is a very accurate calculation method and is applicable to every type of frame, sway as well as braced/non-sway. There is also no need to calculate error-prone buckling lengths (effective lengths) any more: the geometrically non-linear analysis (second-order analysis) ensures the global buckling stability check.

How the steel check according to ANSI/AISC 360-16 is implemented

  • P-Delta (P-Δ) is taken into account in the elastic second-order analysis.
  • P-delta (P-δ), the second-order effect associated with member curvature, is not taken into account.

According to AISC 360-16 Section C2.1 (b), because the analysis is a P-Δ-only second-order analysis, the requirement for considering P-δ effects in the evaluation of individual members can be satisfied by applying the B1 multiplier defined in Appendix 8 to the required flexural strength of the member. All bending moments in each member subject to compression and flexure are multiplied by B1 automatically, according to formulas (A-8-1) and (A-8-3) in Appendix 8.2.

This means the individual stability of the members must be checked according to Chapter E, whereby the buckling length (effective length) may be set equal to the unbraced length (K = 1.00) according to AISC 360-16 Section C3, Calculation of Available Strengths. By applying a geometrically non-linear analysis, the moments and forces of course already include all second-order effects.

In XFEM4U the global initial sway imperfections (initial system imperfections) are taken into account automatically, by direct modelling of these imperfections in the analysis as specified in AISC 360-16 Section C2.2a, Direct Modeling of Imperfections. The default out-of-plumbness ratio is 1/500.

Advantages of this approach

  • There is no need to calculate error-prone buckling lengths (effective lengths) any more. Buckling lengths are not calculated correctly in all cases, and are certainly not load-independent.
  • There is no need to choose a frame type, sway or non-sway. When the frame is sensitive to second-order effects, this follows directly from the analysis.
  • Because global initial sway imperfections are applied to the frame, the calculation process (the iteration) also works well for a symmetrical structure under symmetrical load.
  • Adjacent loads can easily be taken into account.
  • Lateral stability is also included in the stability check according to Chapter H.

See also

The Eurocode equivalent of this approach is described in Buckling.