2017 Annual Stability Conference Presentation

Session S10 – Stability of Plates
Friday, March 24, 2017
8:00 am

Postbuckling Mechanics of Square Slender Steel Plates in Pure Shear: Examining the Role of Second Order Effects

Slender steel plates possess strength beyond the elastic buckling load, which is commonly referred to as the postbuckling capacity.  In current practice, postbuckling mechanics of slender web plates under pure shear loading in bridge girders are characterized by semi-empirical approaches whose origins date back to experimental tests in the 1960’s.  To date, however, the postbuckling mechanics of these plates are still not fully understood, and this paper therefore explores such fundamental behavior.  Using finite element analyses (which are validated against available results of previous tests), outputs such as von Mises stresses, principal stresses, and principal stress directions are examined on both surfaces of a buckled slender plate acting in pure shear. The internal bending, shear, and axial stresses in the plate’s finite elements are evaluated for a simply-supported plate with aspect ratio equal to 1.0 and slenderness equal to 134 – future work will examine a wider range of plate parameters. Results show that localized bending in the plates due to the out-of-plane postbuckling deformations (right contour plot, MPa) are significantly larger than the in-plane membrane stresses (left contour plot, MPa).  Bending thus appears to be a significant factor in the ultimate shear buckling capacity of the plate. Also, the compressive stresses continue to increase beyond elastic buckling in some regions of the plate, contrary to current design assumptions.

Maria E. Moreyra Garlock, Princeton University, Princeton, NJ; Spencer Quiel, Lehigh University, Bethlehem PA;  José Alós Moya, Universitat Politècnica de València, Valencia, Spain; Jonathan Glassman, Exponent Failure Associates, Los Angeles, CA

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Session S6 – Stability of Assemblages and Systems
Thursday, March 23, 2017
10:15 am

Loss-of-Stability vs Yielding-Type Collapse Mode in 3D Steel Structures Under a Column Removal Scenario: an Analytical Method of Assessing the Collapse Mode

Progressive collapse of structures is the phenomenon of an initial failure mushrooming into global level, resulting in total or partial damage of the structure. Aiming to reduce the potential for progressive collapse, current guidelines have proposed a variety of design procedures. Among these, the threat-independent “alternate load path method” is most commonly employed, according to which a key-component is removed from the structural model and the capability of the remaining structure to withstand this loss is assessed. The current paper develops a detailed 3D numerical model of a prototype 10-story steel framed composite building and investigates its response as interior gravity columns are removed along the height of the structure at each floor individually in-turn. The primary focus of the study is the investigation of the correlation between the column removal location and the corresponding collapse mechanism. Two failure modes are taken into consideration, namely the “yielding-type” and the “stability” one. The former is a ductile mode associated with excessive vertical deformations of the beams and slabs above the column removal, while the latter is a brittle mode which manifests through column buckling. Numerical results clearly indicate that column removal scenarios at the lower part of the structure trigger the stability collapse mechanism, which is considered highly undesired. The importance of accurate connection modeling was highlighted and it was shown that even with very conservative connection modeling assumptions the stability mode is dominating the collapse after the removal of the ground floor column. The ultimate aim of the authors is the development of an analytical solution to describe the progressive collapse behavior of a 3D structure and ongoing research is focusing towards this direction.

Panos Pantidis and Simos Gerasimidis, University of Massachusetts, Amherst, MA

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Session SS2b- Technical Presentations: Stability at Elevated Temperatures
Tuesday, March 21, 2017
3:15 pm

Analysis of Rectangular CFT Columns Subjected to Elevated Temperature

This paper used a fiber-based analytical approach, which was developed and benchmarked previously by the authors, to investigate the effects of various parameters on the behavior and strength of rectangular CFT columns subjected to elevated temperature. The parameters considered were the steel tube yield stress (Fy), concrete compressive strength (f’c), tube width-to-thickness ratio (B/t), column slenderness ratio (KL/r), and magnitude of temperature. Results from the parametric studies indicated that the axial compressive strength of rectangular CFT columns decreased significantly with increasing temperature. The effect of temperature was more significant for columns with intermediate length and less significant for short and slender columns. Increasing the concrete compressive strength (f’c) effectively improved the axial compressive strength of rectangular CFT columns subjected to elevated temperature. While changing the steel tube yield stress (Fy) or the steel tube width-to-thickness ratio (B/t) had smaller influences.

Zhichao Lai and Amit H. Varma, Purdue University, West Lafayette, IN; Anil Agarwal, Indian Institute of Technology, Hyderabad, Telangana, India

 

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Session SS2a – Technical Presentations: Stability of Thin-walled Columns
Tuesday, March 21, 2017
3:15 pm

Spherically-Hinged Short-to-Intermediate Angle Columns: Stability, Non-Linear Behavior and DSM Design

This paper reports a numerical investigation on the structural behavior, strength and Direct Strength Method (DSM) design of spherically-hinged (simply supported) short-to-intermediate equal-leg angle columns, thus extending the scope of similar studies recently carried out by the authors for fixed and cylindrically-hinged (simply supported) columns with the same characteristics – hot-rolled (stocky legs – b/t < 20) and cold-formed (slender legs – b/t ³ 20) are dealt with. A modified/proposed DSM design approach, involving new (length-dependent) flexural-torsional strength and reduction factor curves, is shown to lead to safe and reliable failure load predictions for both hot-rolled and cold-formed columns buckling in flexural-torsional modes – the failure load estimates lead to LRFD resistance factors higher than fc=0.85.

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