Rocking damage-free steel column base with friction devices: development of advanced 3D finite element models in ABAQUS

Column bases are critical components of a seismic-resistant steel building as they transfer gravity and lateral forces to the foundations. This paper focuses on the finite element (FE) modelling of a rocking damage-free steel column base, which uses post-tensioned high strength steel bars to achieve self-centering behavior and friction devices to provide energy dissipation capacity. Contrary to conventional steel column bases, the monotonic and cyclic moment-rotation behavior of the column base can be easily described using simple analytical equations. An advanced three-dimensional FE model for the column base is developed in ABAQUS. The techniques used to overcome convergence issues in numerical simulations as well as the constitutive laws for the nonlinear behavior of materials and friction interfaces are described in detail. The FE model provides results that are in very good agreement with the results from the analytical moment-rotation equations. The FE model results are also used to validate a simplified numerical model in OpenSees. Moreover, the FE model provides results that help to assess the level of stress concentration in critical areas of the column base and to evaluate a step-by-step design procedure that ensures damage-free behavior, self-centering capability, and adequate energy dissipation capacity

Rocking damage-free steel column base with friction devices

Earthquake resilient steel frames, such as frames with self-centering connections or frames with passive energy dissipation devices, have been extensively studied during the past decade but little attention has been paid to their column bases. The paper presents a rocking damage-free steel column base, which uses post-tensioned (PT) high strength steel bars to control rocking behavior and friction devices to dissipate seismic energy. Contrary to conventional steel column bases, the rocking column base exhibits monotonic and cyclic moment-rotation behaviors that are easily described using simple analytical equations. Analytical equations are provided for different cases including yielding or loss of posttensioning in the PT bars and their efficiency is compared with numerical results from a three-dimensional non-linear finite element (FE) model in ABAQUS. Moreover, a simplified model is developed in OpenSees to evaluate how the use of the rocking column base affects the global behavior of a self-centering moment-resisting frame. Nonlinear dynamic analyses show that the rocking column base fully protects the first story columns from yielding and eliminate the first story residual story drift

Seismic analysis of spherical tanks including fluid-structure-soil interaction

The accurate and efficient, from a practical point of view, analysis of spherical tanks is the primary motivation for this work. The proposed methodology implements: (a) the Boundary Element Method (BEM) for the simulation of the soil-structure interaction, (b) a discrete 2-DOF mechanical system for the approximate computation of the impedances of a massive, rigid, ring foundation, a usual feature of a spherical tank, and (c) a FEM simulation of the structure and fluid regions. The above modules are coupled via the pre-and post-processing capabilities of the widely used general purpose program ANSYS. The entire analysis is performed directly in time domain assuming linear elastic structure and soil regio

Probabilistic economic seismic loss estimation in steel buildings using post-tensioned moment-resisting frames and viscous dampers

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the economic seismic losses in steel buildings is evaluated. The evaluation is based on a prototype steel building designed using four different seismicresistant frames: (a) conventional moment resisting frames (MRFs); (b) MRFs with viscous dampers; (c) SC-MRFs; or (d) SC-MRFs with viscous dampers. All frames are designed according to Eurocode 8, and have the same column/beam cross-sections and similar periods of vibration. Viscous dampers are designed to reduce the peak story drift under the design basis earthquake (DBE) from 1.8% to 1.2%. Losses are estimated by developing vulnerability functions according to the FEMA P-58 methodology, which considers uncertainties in earthquake ground motion, structural response, and repair costs. Both the probability of collapse and the probability of demolition due to excessive residual story drifts are taken into account. Incremental dynamic analyses are conducted using models capable to simulate all limit states up to collapse. A parametric study on the effect of the residual story drift threshold beyond which is less expensive to rebuild a structure than to repair is also conducted. It is shown that viscous dampers are more effective than post-tensioning for seismic intensities equal or lower than the maximum considered earthquake (MCE). Posttensioning is effective in reducing repair costs only for seismic intensities higher than the DBE. The paper also highlights the effectiveness of combining post-tensioning and supplemental viscous damping by showing that the SC-MRF with viscous dampers achieves significant repair cost reductions compared to the conventional MRF

Design and modeling of a novel damage-free steel column base

Column bases are fundamental components of a steel frame. However their design has not yet received appropriate attention. Conventional steel column bases cannot be easily repaired if damaged and exhibit difficult-to-predict and simulate stiffness, strength and hysteretic behaviour. This paper proposes a novel demountable and fully repairable column base for resilient steel buildings. The new column base isolates damage in easy-to-replace structural elements with the goal of minimizing repair time and disruption of the building service in the aftermath of a strong earthquake. Moreover, it can be easily constructed and deconstructed to enable sustainable steel frame designs. It provides significant flexibility in the design, with rotational stiffness and moment resistance that can be independently tuned. It has self-centering capability for reducing residual drifts. The paper presents design rules, an analytical hysteretic model and a 3D finite element model for the new column base

Work satisfaction or burnout and their impact on innovative work behavior of Greek teachers

Innovation is a lever for growth and prosperity in business and society as a whole. Innovations lead to the industrial revolutions which are transforming our world. Innovation is the solution to many of the side effects of industrial revolutions. We can prepare the future world citizens to face the challenges of the new world only by education. The purpose of this study was to investigate work satisfaction, work engagement, burnout and innovative work behavior of Greek teachers. From the investigation the relationships between the above concepts emerged and the demographic elements associated with them were identified. The applied statistical survey of this study was conducted from December 2018 to January 2019 and 324 primary and secondary school teachers participated in it. Survey data were collected using an online questionnaire that included demographic questions, the Oldenburg Burnout Inventory (OLBI) and a Kleysen & Street innovative behavioral questionnaire adapted to Greek reality. The results of the survey showed a very low rate of burned-out and a high rate of work-engaged teachers. Because previous surveys have reported high rates of burnout for Greek teachers, we conclude that work engagement, as well as burnout, are time dependent on environmental factors related to the socio-economic changes of recent decades. Our research results also showed that establishing and supporting innovative work behavior by leadership and co-workers has the effect of demonstrating a high level of innovative work behavior by Greek educators. In addition, it was found a fluctuation of work engagement with age and an enhancement of innovative work behavior due to post-graduate studies. Moreover, it emerged that work engagement positively correlates with innovative work behavior, creating a virtuous circle, where one feeds the other

Optimistic Competitive Analysis: Stochastic Models, Predictions, and Revocable Decisions

In conventional online competitive analysis, it is assumed that online algorithms are dealing with anadversary who controls the input items and their arrival sequence, while the algorithms are making irrevocable decisions along the way. This approach is often pessimistic and results in worst-case performance guarantees that are rarely reflected in practice. In order to measure performance in more realistic settings, we consider various “relaxations” of the online model that assist the algorithm in three general ways: 1. Allowing the algorithm to alter some earlier decisions that seem to not be good in hindsight. 2. Restricting the adversary’s power to control the arrival sequence. 3. Removing some of the uncertainty of the input by having some information about the offline instance. We focus on two classical online problems: bipartite matching and interval selection. In the case ofbipartite matching, we introduce a new input model and analyze the performance of a natural greedy algorithm [27]. We also conduct an experimental analysis comparing state-of-the-art algorithms in the known i.i.d. setting and show that simple greedy algorithms might be very competitive in practice [32]. We consider the problem of interval selection with revocable acceptances in the adversarial model [30], the random-order model [31], as well as in a predictions setting [83]. We give improved bounds on the performance of some existing algorithms, and introduce new algorithms tailored to these specialized models. Moreover, most of the algorithms we study are conceptually simple, a desirable characteristic that often correlates with inherent efficiency and ease of implementation.Ph.D

Collapse risk and residual drift performance of steel buildings using post-tensioned MRFs and viscous dampers in near-fault regions

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the collapse risk and improve the residual drift performance of steel buildings in near-fault regions is evaluated. For this purpose, a prototype steel building is designed using different seismic-resistant frames, i.e.: moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as stiffness and strength deterioration. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is used to conduct incremental dynamic analysis (IDA), in which each ground motion is scaled until collapse occurs. The probability of collapse and the probability of exceeding different residual story drift threshold values are calculated as a function of the ground motion intensity and the period of the velocity pulse. The results of IDA are then combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess and compare the collapse risk and the residual drift performance of the frames. The paper highlights the benefit of combining the post-tensioning and supplemental viscous damping technologies in the near-source. In particular, the SC-MRF with viscous dampers is found to achieve significant reductions in collapse risk and probability of exceedance of residual story drift threshold values compared to the MRF. © 2016 Springer Science+Business Media Dordrech

Any-Order Online Interval Selection

We consider the problem of online interval scheduling on a single machine, where intervals arrive online in an order chosen by an adversary, and the algorithm must output a set of non-conflicting intervals. Traditionally in scheduling theory, it is assumed that intervals arrive in order of increasing start times. We drop that assumption and allow for intervals to arrive in any possible order. We call this variant any-order interval selection (AOIS). We assume that some online acceptances can be revoked, but a feasible solution must always be maintained. For unweighted intervals and deterministic algorithms, this problem is unbounded. Under the assumption that there are at most $k$ different interval lengths, we give a simple algorithm that achieves a competitive ratio of $2k$ and show that it is optimal amongst deterministic algorithms, and a restricted class of randomized algorithms we call memoryless, contributing to an open question by Adler and Azar 2003; namely whether a randomized algorithm without access to history can achieve a constant competitive ratio. We connect our model to the problem of call control on the line, and show how the algorithms of Garay et al. 1997 can be applied to our setting, resulting in an optimal algorithm for the case of proportional weights. We also discuss the case of intervals with arbitrary weights, and show how to convert the single-length algorithm of Fung et al. 2014 into a classify and randomly select algorithm that achieves a competitive ratio of 2k. Finally, we consider the case of intervals arriving in a random order, and show that for single-lengthed instances, a one-directional algorithm (i.e. replacing intervals in one direction), is the only deterministic memoryless algorithm that can possibly benefit from random arrivals. Finally, we briefly discuss the case of intervals with arbitrary weights.Comment: 19 pages, 11 figure