Design of steel members according to ANSI/AISC 360-10

 

2013.1

Summary

The more recent version of the AISC Specification for Structural Steel Buildings (ANSI/AISC 360-10) has been implemented in SCIA Engineer v2013.1. Furthermore, the available functionalities in the software enable a rigorous structural analysis according to the Direct Analysis Method, including considerations of 2nd order deformations, initial imperfections, and plasticity.

By performing the 2nd order analysis in SCIA Engineer, all required deformations are taken into account:

  • both P-δ and P-Δ effects may be included in the non-linear solution while their effect can also be quantified;
  • before starting the analysis, it is possible to evaluate the ratio of second to first-order drift by performing a simple linear stability analysis.

Related to initial imperfections, a number of approaches have been enabled in order to fit the preferences of the designer.

Related to global imperfections:

  • Notional loads (calculated as a fraction of the gravity load) may be applied on the structural nodes to take out-of-plumbness into account;
  • As an alternative, the user may opt for use of actual global imperfections* obtained from the linear stability analysis.

Related to member imperfections:

  • An option in SCIA Engineer allows the user to explicitly define member imperfections** for all members in a steel building. This allows for P-δ imperfections to also be considered explicitly, although this is not necessary according to the standard, as member imperfections are accounted for in the provisions of Chapter E of AISC 360-10.

The Direct Analysis Method in the AISC 360-10 standard explicitly requires that the member stiffness be reduced to take into account member imperfections, residual stresses and plasticity, and the stiffness/strength uncertainty in the structural response. In addition, the flexural stiffness of axially-loaded members whose stiffness contributes to the structural stability has to be often reduced even further, if large normal forces are present in the member.

  • SCIA Engineer includes the possibility to use property modifiers since version 2013; these allow for the axial and bending stiffness of the modelled members to be reduced when the Direct Analysis Method is used;
  • τb = 1.0 may be used if additional lateral notional loads or lateral global imperfections are added in all nodes of the structure for all load combinations, or as an alternative, flexural stiffness could be modified manually.

In SCIA Engineer 2013.1, it is also possible to perform the structural analysis according to the Effective Length Method or according to the First-order Analysis Method.

  • The K-factor for effective length may be obtained by analysis, as in previous versions of the software, or it may be defined manually.
  • The first-order analysis may be performed according to Appendix 8 using the B1 and B2 coefficients. To calculate the B2 factor, additional linear stability analysis should be performed. The K-factor may be set to 1.0 for all members.

* For such cases, AISC 360-10 recommends out-of-plumbness amplitude per storey equal to 1/500 of the story height.

** AISC 360-10 recommends the out-of-straightness to be taken as 1/1000 of the member length between bracing points (these values are related to and larger than fabrication and erection tolerances).

Checklist of the standard update:

All modifications related to member design since version 2005 of the AISC 360 standard have been taken into account, including the new checks for local buckling for T-stems in flexural compression and for torsional-flexural buckling for single angles. Since version 2013.1, cross-section classification is performed separately for flexure and compression. Other important changes compared to the older version were found in the following chapters of the standard:

  • Chapter B – changes in classification rules and added rules for T-sections, changes in naming for reports;
  • Chapter E – torsional-flexural buckling for single angles with slender webs;
  • Chapter F – lateral-torsional buckling modifications for I- and U-sections, cb coefficient for flexure, LB of T-stems in flexural compression;
  • Chapter G – shear check modification for single angles;
  • Chapter H – changes in notation;

The main improvement in the AISC 360-10 standard compared to the AISC 360-05 version is related to the recommended analysis paths. SCIA Engineer 2013.1 enables the user to perform a rigorous structural analysis following the Direct Analysis Method, including considerations of the 2nd order deformations, initial imperfections, and plasticity.

11/11/2013