SSRC 2017 Annual Meeting Presentation

Session SS2A: Technical Presentations: Stability of Thin-Walled Columns
Tuesday, March 21, 2017
3:15 p.m.

Flexural-Torsional Buckling of General Cold-Formed Steel Columns with Unequal Unbraced Lengths

The design of cold-formed steel columns must consider flexural buckling, torsional buckling, and flexural-torsional buckling. The American Iron and Steel Institute incorporated equations for the critical elastic buckling loads corresponding to these failure modes in the North American Specification for the Design of Cold-Formed Steel Members. These equations were originally developed for columns with consistent boundary conditions for all three modes. However it is common in practice to have different unbraced lengths for major axis flexure, minor axis flexure, and torsion. Furthermore, it is common for certain members to be oriented such that intermediate bracing restraint directions do not align with the principal axes. This paper investigates and develops a general formulation of the column buckling equation to incorporate unequal unbraced lengths and non-principal axes.

Robert S. Glauz, RSG Software, Inc., Lee’s Summit, MO

SSRC 2017 Annual Meeting Presentation

Session SS2B: Technical Presentations: Stability at Elevated Temperatures
Tuesday, March 21, 2017
3:15 p.m.

DSM Design of Cold-Formed Steel Columns Failing in Distortional Modes at Elevated Temperatures

This paper extends the scope of previous work by the authors aiming at investigating the structural behavior, strength and Direct Strength Method (DSM) design of cold-formed steel columns failing in distortional modes at elevated temperatures – the temperature-dependent steel constitutive law is based on the EC3-1.2 model. The new results concern pin-ended and fixed-ended columns displaying four cross-section shapes (lipped channels, hats, zeds and racks), with various dimensions, subjected to 7 elevated temperatures (up to 800ºC) and also ambient temperature (for comparison purposes). On the basis of the failure load data obtained in this work, it is first shown that the current DSM distortional design is unable to handle adequately distortional failures at elevated temperatures. Then, a modified DSM design approach is proposed: it consists of incorporating a temperature-dependent reduction factor ratio, based on the EC3-1.2 model, in the existing strength curve – the modified design curves are shown to provide adequate (reliable and mostly safe) failure load predictions for the set of columns under consideration.

Alexandre Landesmann and  Fernanda Cristina Moreira da Silva, Federal University of Rio de Janeiro, Brazil; Dinar Camotim, University of Lisbon, Lisbon, Portugal

2017 Annual Stability Conference Presentation

Session SS1A: Technical Presentations: Stability of Thin-Walled Components and Assemblages
Tuesday, March 21, 2017
1:40 pm

Characterization of Cold-Formed Steel Member Dimensions and Geometric Imperfections Based on 3D Laser Scanning

This paper briefly introduces the full-field laser-based imperfection measurement platform as well as the four-step post-processing toward measurement point clouds from the platform. The first three steps can lead to one application: variation of dimensions and correlation study of cold-formed steel members. The last step of post-processing enables another two applications, i.e., imperfection characterizations and finite shell-element analysis. The imperfections (deviation from perfect) may be characterized in geometric terms: bow, camber, twist, crown of a given flat plate, flare of a given element; or may be characterized in terms of their modal buckling content: fit to flexural modes, torsional mode, local mode, and distortional mode. In addition, the geometric imperfections may be transformed into the frequency domain and power spectrum of the imperfection magnitudes can be obtained. This 1D spectral approach provides a potentially novel means for generating realistic, but random geometric imperfections for use in shell finite element simulations. Shell finite element collapse analyses that compare the sensitivity in response to true, and various simulated imperfections are provided.The simulations indicate how simple modal imperfections are powerful for predicting strength conservatively, but the 1D spectral approach more closely approaches the results from the true (scanned) members. In the future larger Monte Carlo simulations should be performed to assess the reliability of cold-formed steel members using these results.

Xi Zhao and Benjamin W. Schafer, Johns Hopkins University, Baltimore, MD

2017 Annual Stability Conference Presentation

Session SS2A: Technical Presentations: Stability of Thin-Walled Columns
Tuesday, March 21, 2017
3:15 pm

On the Distortional-Global Interaction in Cold-Formed Steel Columns: Relevance, Post-Buckling Behavior, Strength and DSM Design

This work reports results of an ongoing numerical (shell finite element) investigation on the relevance, post-buckling behavior, strength and design of fixed-ended cold-formed steel columns undergoing distortional-global (D-G) interaction. The columns analyzed exhibited three cross-section shapes, in order to study distinct natures of D-G interaction, which may involve distortional and (major-axis) flexural-torsional buckling (plain and web-stiffened lipped channel columns), or distortional and (minor-axis) flexural buckling (Z columns). The occurrence of different types of D-G interaction, namely “true D-G interaction” or “secondary-(distortional or global) bifurcation D-G interaction” are investigated for each of the aforementioned D-G interaction natures. The results presented concern columns with various geometries and yield stresses, ensuring a wide variety of combinations between (i) global-to-distortional critical buckling load ratios and (ii) squash-to-non critical buckling (distortional or global) load ratios. Then, the numerical failure load data obtained are compared with their predictions by (i) the current DSM (Direct Strength Method) column global and distortional design curves, and (when necessary) (ii) proposed DSM-based design approaches, developed to handle D-G interactive failures.

André Dias Martins, Dinar Camotim, and Pedro B. Dinis, University of Lisbon, Lisbon, Portugal