2017 Annual Stability Conference Presentation

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

Efficient GBT Displacement-Based Finite Elements for Non-Linear Problems

In this paper it is shown that the computational efficiency of GBT displacement-based finite elements can be significantly improved by resorting to a “node-based” DOF approach. Although the standard GBT deformation modes are not directly included in the analysis, their participations are straightforwardly recovered in the post-processing stage. Although more cross-section DOFs are involved, the additional cost is offset by the overall efficiency of the formulation. In addition, discrete variations of the wall thickness in the longitudinal direction can be easily handled, including holes. Illustrative numerical examples, involving linear and non-linear problems, are presented and discussed.

Rodrigo de Moura Goncalves, Universidade Nova de Lisboa, Caparica, Portugal; Dinar Camotim, Univeristy of Lisbon, Lisbon, Portugal


2017 Annual Stability Conference Presentation

Session S2 – Seismic Stability of Members and Systems
Wednesday, March 22, 2017
9:45 am

Seismic Stability of Multi-Tiered Ordinary Concentrically-Braced Frames

Multi-tiered braced frames (MT-BFs) are created when a tall single-story braced bay is divided into multiple bracing panels over the height, with no diaphragms or out-of-plane column supports between the base and roof. Due to the unique conditions in MT-BFs, during nonlinear seismic response, they are susceptible to column instability due to combined axial force and bending moment. The present research is using numerical simulations to investigate the seismic response of multi-tiered ordinary concentrically-braced frames (MT-OCBFs), which are designed with a relatively simple procedure and are expected to provide limited inelastic deformation capacity. The baseline for the study is the previous version of the AISC Seismic Provisions (AISC 341-10), which require column design for an amplified axial demand. The newer AISC Seismic Provisions (AISC 341-16), which are based on a limited initial evaluation to develop the multi-tiered OCBF requirements, stipulate that MT-OCBF columns be designed for an additional amplified axial demand to approximately account for moment. This approach is now being more comprehensively studied, and the interaction effects of axial force, in-plane moment and out-of-plane moment are being thoroughly assessed. This paper presents the results from nonlinear static (pushover) analysis of a subset of the prototype frames. Concentration of inelastic deformations and column buckling were observed in some of the baseline designs, while the newer provisions allow for a more even distribution of inelastic demand over the frame height. For OCBFs, a simple but effective design approach is desired so that drift concentration in a single tier is limited and column stability is maintained, even without employing a rigorous capacity-based procedure.

Aradhana Agarwal and Larry A. Fahnestock, University of Illinois, Urbana, IL

2017 Annual Stability Conference Presentation

Session S3 – Advances in Stability Bracing
Wednesday, March 22, 2017
3:15 pm

Partial-Depth Precast Concrete Deck Panels on Curved Bridges

This ongoing research program is focused on the use of partial depth precast prestressed concrete panels (PCPs) as bracing elements in straight and curved bridges during the construction phase. The research team developed a connection detail between the PCPs and the girders with the input from a local precaster and construction experts. The shear stiffness of the PCP/connection assembly was experimentally determined in a full-scale shear test frame.  To investigate the performance of the PCP/connection system, numerous tests were performed on a 72 ft. long full-scale twin I-girder system with and without PCPs connected to the girders. The system was first loaded laterally and then loaded with various combinations of bending and torsion to simulate bridges with varying radii of curvature. The results from the experimental tests are currently being used to validate the finite element models (FEM) that will be used for parametric studies to investigate a wide array of systems.

Colter E. Roskos, John R. Kintz, Paul Biju-Duval, Todd A. Helwig, Michael D. Engelhardt, Patricia Clayton, Eric Williamson and Ozzie Bayrak, University of Texas at Austin, Austin, TX

2017 Annual Stability Conference Presentation

Session S7 – Stability of Columns
Thursday, March 23, 2017
1:15 pm

Design Method for Columns with Intermediate Elastic Torsional Restraint

Cold-formed steel haunched portal frames are popular structures in industrial and housing applications. They are mostly used as sheds, garages, and shelters, and are common in rural areas. Cold-formed steel portal frames with spans of up to 30 m (100 ft) are now being constructed in Australia. Frames used for shelters over large areas have unbraced columns, and for larger spans, a knee brace is required to transfer the large bending moment from the rafters to the columns. The knee brace to column connection creates an intermediate elastic torsional restraint on the column as the column is partially restrained against twist rotation where the knee brace joining the column is connected. Current design guidelines do not directly account for the restraint provided by the knee connection and require the determination of the member effective length. Due to the variations of the column base stiffness and rotational restraint of the knee connection, the column effective length is difficult to quantify. Therefore, a new design method is proposed which eliminates the need to determine the effective length. The design capacity is calculated using the Direct Strength Method with inputs from a column buckling energy analysis. Internal actions are determined using a calibrated beam finite element model with notional horizontal forces, and the interaction equation involving bending and compression is utilized to determine the column strength. A reliability check is completed and the results compared to experimental frame ultimate loads. It is shown that the frame strength determined from the design method presented herein is a suitable method for the design of columns with an intermediate elastic torsional restraint in haunched portal frames.

Hannah B. Blum and Kim J.R. Rasmussen, University of Sydney, Sydney, NSW, Australia

2017 Annual Stability Conference Presentation

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

Stability and Moment-Rotation Behavior of Cold-Formed Steel Purlins with Sleeved Bolted Connection

Long runs of cold-formed steel Z-section purlins are often segmented due to assemblage and transportation issues. The segments are commonly connected by bolts to a short cold-formed steel member similar to the purlin; this short member is typically called a sleeve; from a structural point of view, it does not guarantee a state of full continuity to the purlin. This study reports a series of 15 experiments on cold-formed steel Z-sections purlins with sleeved bolted connections tested in bending. These experiments vary cross-section height, thickness and length of sleeve, and span. This parametric experimental study seeks to better understand the flexural-buckling strength, collapse mechanism, and moment-rotation behavior of purlin-sleeve systems. Since the design of purlins with sleeved bolted connections is often limited by the serviceability limit state, in this case excessive displacement, special attention is given to accurately determining and understanding the moment-rotation behavior of purlin-sleeve systems. Based on the experimental results, an expression is proposed to predict non-linear moment-rotation behavior; the proposed expression is compared to expressions previously proposed in the literature. The proposed moment-rotation expression, when used in a simple rotation spring-beam model, leads to accurate prediction of displacement in the purlin-sleeve system.

Fernando H. Gilio and Maxiliano Malite, University of São Paulo, São Paulo, Brazil; Luiz C. M. Vieira, Jr., University of Campinas, São Paulo, Brazil