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

2017 Annual Stability Conference Presentation

Session S5 – Stability of Members and Connections
Thursday, March 23, 2017
8:00 am

Stability of Extended Beam-to-Girder Shear Tab Connections Under Gravity Induced Shear Force

A coordinated experimental and numerical investigation of the behaviour and stability requirements of full-depth extended shear tabs is described in the paper. Based on the FE analyses, the load transfer mechanism and the buckling capacity of the stiffened portion of the full-depth shear tab were determined. The parameters, which influenced the buckling of the stiffener, were studied; including the depth and thickness of the shear plate, the depth of the girder, the width of the girder flanges, and the flexibility of the girder web. Further FE analyses were completed to determine the buckling capacity of the shear tab with reduced depth of the stiffener. In addition, the load-transfer mechanism and buckling capacity of these shear plates were modeled when they are used in double-sided configurations, i.e. when a beam is placed on both sides of the girder.

Mohammad Motallebi and Colin A. Rogers, McGill University, Montreal, QC; Dimitrios G. Lignos, Swiss Federal Institute of Technology, Lausanne (EPFL), Lausanne, Switzerland

2017 Annual Stability Conference Presentation

Session SS2B – Technical Presentations: Stability at Elevated Temperatures
Tuesday, March 21, 2017
3:15 pm

Effect of Boundary Conditions on the Creep Buckling of Steel Columns in Fire

A computational study using Abaqus was performed to investigate the influence of boundary
conditions on the creep buckling behavior of steel columns at elevated temperatures of fire. W12×120
wide flange columns with the unsupported length of 240 inches are used in the simulations. Thermal
creep of steel is modeled following equations proposed by Fields and Fields for the creep of ASTM A36
steel. Four different classical support conditions and seven imperfection amplitudes are considered to
quantify the effect of boundary conditions on the time-dependent strength of steel columns in fire.
Thermal restraints, both axial and rotational, were ignored in the analyses. Representative results from
creep buckling tests simulated at 500 °C are presented and discussed. Results from creep buckling
simulations presented in this paper indicate that the rotational and translational restraint at the column
ends along with the initial crookedness of the column have a significant impact on the predictions of the
time-dependent strength of steel columns subjected to fire.

Mohammed A. Morovat, Michael D. Engelhardt and Todd A. Helwig, University of Texas at Austin, Austin, TX

2017 Annual Stability Conference Presentation

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

Numerical Study on the Behavior and Design of Screw Connected Built-up CFS Chord Studs

This study presents the development and validation of a finite element modeling protocol in ABAQUS for screw connected, back-to-back built-up cold-formed steel (CFS) columns using results from experiments conducted at Johns Hopkins University. The goal is to examine the buckling, peak, and post-peak behavior (with an example shown in the figure) of built-up CFS columns of different cross section sizes and sheathing conditions, and under monotonic loading to improve existing built-up CFS column design guidelines so that all relevant failure modes are considered. Ongoing working includes cyclic collapse analyses, the characterization of chord stud buckling limit states, and a framework to implement these limit states in seismic simulations of CFS-framed shear walls.

Smail Kechidi and Nouredine Bourahla, University of Blida 1, Blida, Algeria; David C. Fratamico and Benjamin W. Schafer, Johns Hopkins University, Baltimore, MD; José Miguel Castro, University of Porto, Porto, Portugal


2017 Annual Stability Conference Presentation

Session S12 – Advances in Stability Analysis
Friday, March 24, 2017
1:00 pm

Non-Uniform Modal Decomposition of Thin-Walled Members by the Constrained Finite Element Method

In the paper a new method, the constrained finite element method is applied for the modal decomposition of thin-walled members. With the help of this method the thin-walled member can be enforced to deform in accordance with some predefined criteria. In stability analysis, thus, it becomes straightforward to directly study various buckling types, for example flexural buckling, flexural-torsional buckling, distortional buckling, etc., as desired by the user. The main focus of the paper is on the special feature of the method that modal decomposition can be performed for non-uniform members, too, more specifically, for any members built of from prismatic segments. The connecting segments can have different cross-sections: this makes it possible, among others, to build a single realistic shell model with e.g., directly modeling gusset plates or strengthening plates, while still to utilize modal decomposition advantages. Moreover, the segments can be constrained into different deformation modes, that is the enforced deformations may vary from segment to segment: this allows calculating the critical load value directly to a certain type of buckling in a certain part of the member. Thus, unlike in a regular shell finite element buckling analysis, it is not necessary to check hundreds of mostly interacted buckling modes to find a desired mode, but this desired mode can be found in a way which is more direct, more objective, and more efficient.

Sándor Ádány, Budapest University of Technology and Economics, Budapest, Hungary

Rodrigo Gonçalves, Ph.D., Assistant Professor at NOVA University Lisbon is the 2017 recipient of the MAJR Medal.

The McGuire Award for Junior Researchers (MAJR Medal) has been established in honor of the late Professor William “Bill” McGuire, a long-term member of SSRC, who emphasized that state-of-the-art research is instrumental to improve the quality of stability design. The award recognizes the research activity of the candidate in a period not exceeding nine years after obtaining his/her Ph.D. degree.

Gonçalves will receive the award and deliver a presentation on his recent research activity at the 2018 SSRC Annual Stability Conference in Baltimore, Maryland.

The following jurors collaborated to select Gonçalves as the recipient of the 2017 award:

  • Larry A. Fahnestock, University of Illinois at Urbana-Champaign
  • Todd A. Helwig, University of Texas at Austin
  • Daniel G. Linzell, University of Nebraska

2017 Annual Stability Conference Presentation

Session S8 – Stability of Wall Systems
Thursday, March 23, 2017
3:00 pm

SC Wall Compression Behavior: Interaction of Design and Construction Parameters

Modular steel-plate composite (SC) construction involves pre-fabricated steel modules that are transported to the site, assembled and then filled with concrete. The construction parameters (concrete casting height, etc.) and casting sequence for these modular walls may vary, leading to small but permanent stresses and deformations (or imperfections). These geometric imperfections, combined with the variations in SC wall design and detailing parameters (such as steel and concrete grades, tie bar spacing, faceplate slenderness), could influence the compressive behavior and capacity of the SC walls. This paper explores the effects of imperfections and design parameters on the axial compression capacity of the SC walls. The analysis procedure involves simulating the effects of initial imperfections, construction sequence, etc., followed by axial compressive loading up to failure. Parametric studies are conducted to evaluate the effect of variability in steel grades, faceplate slenderness, and height of concrete pour on the compression behavior of SC walls. The analysis results indicate that the compression behavior of SC walls (for nuclear facilities) is dominated by concrete. Faceplates for SC walls meeting the requirements of AISC N690s1 perform adequately (yield in compression before buckling) for concrete pour heights up to 30 ft. However, the concrete pour height and plate slenderness affect the faceplate waviness tolerance, and need to be addressed in the analysis. The performance of specimens with 36 ksi faceplates is acceptable for the current configuration of ties, but needs to be explored for different configurations. Future studies will further evaluate the effects of tie spacing and configuration, and concrete grades.

Saahastaranshu R. Bhardwaj and Amit H. Varma, Purdue University, West Lafayette, IN

2017 Annual Stability Conference Presentation

Session SS2B – Technical Presentations: Stability at Elevated Temperatures
Tuesday, March 21, 2017
3:15 pm

Stability Analysis of Steel Columns under Cascading-Hazard of Earthquake and Fire

In this study, a nonlinear finite element formulation is developed and utilized to perform stability analysis of W-shape steel columns subjected to non-uniform longitudinal temperature profiles in the absence or presence of inter-story drift, representing residual drift following an earthquake. This formulation takes into account the residual stress distribution in steel hot-rolled W-shape sections, initial geometric imperfections in the steel columns, non-uniform longitudinal temperature distribution, and temperature-dependent material properties. The results indicate an excellent agreement with available strength design equations of steel columns at ambient and elevated temperatures. A set of equations is then proposed to predict the critical buckling stress in steel columns under fire and fire following earthquake considering residual drifts and non-uniform longitudinal temperature distributions. The proposed equations can be implemented to investigate the performance of steel structures under fire and fire following earthquake considering stability as engineering demand parameter.

Mehrdad Memari and Hussam Mahmoud, Colorado State University, Fort Collins, CO