Effects of Connection Deformation Softening on Behavior of Steel Moment Frames Subjected to Earthquakes

Janise E. Rodgers,Stephen A. Mahin 한국강구조학회 2011 International Journal of Steel Structures Vol.11 No.1

In the years since the 1994 Northridge earthquake, the profession has paid significant attention to the potential effects of various forms of deterioration in connection strength and stiffness that steel moment-resisting frames can experience during severe seismic excitations. The brittle connection fractures that a number of welded steel moment-resisting frame structures experienced during recent earthquakes have been the most extensively studied to date. However, cyclic testing of post-Northridge beam-column connections demonstrates that ductile connections may suffer other forms of deterioration. Negative post-yield tangent stiffness or capping, hereafter referred to as deformation softening, is a behavior of particular interest because it may have significant adverse effects on frame system behavior. The effects of deformation softening on frames subjected to pulse excitations were examined as part of an integrated experimental and analytical investigation of the effect of various forms of hysteretic deterioration on the overall system behavior of moment resisting steel frames. Pulse excitations, and the near-field ground motions they represent, can be highly damaging to structures and are therefore the primary focus of the results presented in this paper. The experimental portion of this study consisted of a series of thirty-two shaking table tests, which were performed on a one-third scale, two-story, one bay, steel moment frame with idealized, mechanical connections. These tests and subsequent analytical studies show that, in general, significant loss of connection strength capacity, whether from deformation softening or other types of deterioration, leads to large residual drifts and, for large pulse excitations with durations longer than the fundamental period of the structure, to collapse. In particular, frames with connections exhibiting negative post-yield stiffness tend to have substantially increased peak and residual displacements when subjected to pulse excitations.

Steel Concentrically Braced Frames using Tubular Structural Sections as Bracing Members: Design, Full-Scale Testing and Numerical Simulation

Jiun-Wei Lai,Stephen A. Mahin 한국강구조학회 2014 International Journal of Steel Structures Vol.14 No.1

This paper presents the results of experimental and analytical studies carried out on two full-scale, one-bay, two-story steelconcentrically braced frames. Square hollow and round hollow structural sections were used for the bracing components. Thespecimens were designed and detailed according to the 2005 AISC Seismic Provisions, and tested cyclically under displacementcontrol but with a fixed lateral load distribution over height. Numerical computational models including the brace components,gusset plate details and frame members were implemented in OpenSees. Numerical simulations were then performed toinvestigate the cyclic behavior of brace components, brace failure mechanisms and overall system response. Satisfactoryagreement was obtained in comparisons of experimental and numerical results. Premature failures observed suggest that beamto-gusset plate connections could be pinned to accommodate large rotational demands at this location without the need to formplastic hinges. Test results also showed that for the braced frames having the same configuration, designed for similar base shearcapacities, and subjected to the same roof level displacement history, the braced frame specimen using round tubular sectionsas diagonal braces was able to sustain larger story drifts without brace fracture than the specimen employing square tubularsections. Fracture of the column base in the second specimen, although inconclusive from a single test, suggests more studyis needed of design requirements for column to base plate connections where large variations of axial, bending and shear loadare expected.

Proposed Approaches to Fracture Damage Detection for Sparsely Instrumented Steel Moment-framedBuildings

Janise. E. Rodgers,Stephen A. Mahin,Mehmet Celebi 한국강구조학회 2007 International Journal of Steel Structures Vol.7 No.1

Performance-based design of seismic isolated buildings considering multiple performance objectives

Morgan, Troy A.,Mahin, Stephen A. Techno-Press 2008 Smart Structures and Systems, An International Jou Vol.4 No.5

In the past 20 years, seismic isolation has see a variety of applications in design of structures to mitigate seismic hazard. In particular, isolation has been seen as a means of achieving enhanced seismic performance objectives, such as those for hospitals, critical emergency response facilities, mass electronic data storage centers, and similar buildings whose functionality following a major seismic event is either critical to the public welfare or the financial solvency of an organization. While achieving these enhanced performance objectives is a natural (and oftentimes requisite) application of seismic isolation, little attention has been given to the extension of current design practice to isolated buildings which may have more conventional performance objectives. The development of a rational design methodology for isolated buildings requires thorough investigation of the behavior of isolated structures subjected to seismic input of various recurrence intervals, and which are designed to remain elastic only under frequent events. This paper summarizes these investigations, and proposed a consistent probabilistic framework within which any combination of performance objectives may be met. Analytical simulations are presented, the results are summarized. The intent of this work is to allow a building owner to make informed decisions regarding tradeoffs between superstructure performance (drifts, accelerations) and isolation system performance. Within this framework, it is possible to realize the benefits of designing isolated buildings for which the design criteria allows consideration of multiple performance goals.

Potentiality of Using Vertical and Three-Dimensional Isolation Systems in Nuclear Structures

Zhiguang Zhou,Jenna Wong,Stephen Mahin 한국원자력학회 2016 Nuclear Engineering and Technology Vol.48 No.5

Although the horizontal component of an earthquake response can be significantly reduced through the use of conventional seismic isolators, the vertical component of excitation is still transmitted directly into the structure. Records from instrumented structures, and some recent tests and analyses have actually seen increases in vertical responses in base isolated structures under the combined effects of horizontal and vertical ground motions. This issue becomes a great concern to facilities such as a Nuclear Power Plants (NPP), with specialized equipment and machinery that is not onlyexpensive, but critical to safe operation. As such, there is considerable interest worldwide in vertical and three-dimensional (3D) isolation systems. This paper examines several vertical and 3D isolation systems that have been proposed and their potential application to modern nuclear facilities. In particular, a series of case study analyses of a modern NPP model are performed to examine the benefits and challenges associated with 3D isolation compared with horizontal isolation. It was found that compared with the general horizontal isolators, isolators that have vertical frequencies of no more than 3 Hz can effectively reduce the vertical in-structure responses for the studied NPP model. Among the studied cases, the case that has a vertical isolation frequency of 3 Hz is the one that can keep the horizontal period of the isolators as the first period while having the most flexible vertical isolator properties. When the vertical frequency of isolators reduces to 1 Hz, the rocking effect is obvious and rocking restraining devices are necessary.