Çelik Çerçevelerin LRFD-AISC Şartnamesine Göre Av Arama Optimizasyon Yöntemi Kullanılarak Optimum Boyutlandırılması

Konferans Bildirisi -- Teorik ve Uygulamalı Mekanik Türk Milli Komitesi, 2013Conference Paper -- Theoretical and Applied Mechanical Turkish National Committee, 2013Bu çalışmada çelik çerçeveler için av arama optimizasyon yöntemini temel alan bir optimum tasarım algoritması sunulmaktadır. Aslan, kurt ve yunus gibi hayvanların gurup olarak avlanmalarından esinlenilerek geliştirilmiş olan av arama optimizasyon yöntemi de bu tekniklerin son dönemlerde ön plana çıkanlarından biridir. Bu avcılarıjn her biri avlanmalarını farklı şekillerde gerçekleştirir. Fakat ortak yönleri grup içinde birlikte av aramalarıdır. Avın etrafı çembere alınır ve yakalanana kadar çember daraltılır. Mevcut çalışmada av arama tekniği çerçeve optimizasyon algoritmasının otomasyonu için kullanılmıştır. İki boyutlu çelik çerçevelerin optimum tasarımının gereği olarak; yapı elemanlarının W profiline uygun olması, LRFD-AISC tasarım şartnamelerindeki sınırlayıcıları sağlaması ve çerçevenin minimum ağırlığa sahip olması gerekmektedir. Bu amaçla burkulmayı da hesaba katan dayanım sınırlayıcıları göz önünde bulundurulmuş ve algoritmanın kolon ve kirişler için Amerikan Şartnamesindeki 272 kesit içerisinden seçim yapması sağlanmıştır. Her bir grup içindeki elemanlara aynı kesitin tayin edilebilmesi için eleman gruplandırılması yapılmıştır. Şartnamedeki sınırlayıcıları dikkate alarak minimum çerçeve ağırlığını hesaplayan bu algoritmanın etkinliği örnek problemlerle gösterilmiştir.This study presents a hunting search based optimum design algorithm for moment resisting steel frames. Hunting search algorithm is a numerical optimization method inspired by group hunting of animals such as wolves, lions, and dolphins. Each of these hunters performs hunting in a different way. However, they are common in that all of them look for a prey in a group. Prey is encircled and the ring of siege is tightened gradually until it is caught. Hunting search is employed for the automation of optimum design process, during which size variables are selected from an available W-steel section list in such a way that the weight of frame is minimum and the design constraints are satisfied. Provisions of LRFD-AISC (Load and Resistant Factor Design-American Institute of Steel Construction) are considered to formulate the optimum design problem. Different numerical examples are designed by the presented algorithm to demonstrate its efficiency. Results reveal that hunting search shows good performance in finding optimum solutions for moment resisting steel frames

Inter-trial variability in sensory-evoked cortical hemodynamic responses: the role of the magnitude of pre-stimulus fluctuations

Brain imaging techniques utilize hemodynamic changes that accompany brain activation. However, stimulus-evoked hemodynamic responses display considerable inter-trial variability and the sources of this variability are poorly understood. One of the sources of this response variation could be ongoing spontaneous hemodynamic fluctuations. We recently investigated this issue by measuring cortical hemodynamics in response to sensory stimuli in anesthetized rodents using 2-dimensional optical imaging spectroscopy. We suggested that sensory-evoked cortical hemodynamics displayed distinctive response characteristics and magnitudes depending on the phase of ongoing fluctuations at stimulus onset due to a linear superposition of evoked and ongoing hemodynamics (Saka et al., 2010). However, the previous analysis neglected to examine the possible influence of variability of the size of ongoing fluctuations. Consequently, data were further analyzed to examine whether the size of pre-stimulus hemodynamic fluctuations also influenced the magnitude of subsequent stimulus-evoked responses. Indeed, in the case of all individual trials, a moderate correlation between the size of the pre-stimulus fluctuations and the magnitudes of the subsequent sensory-evoked responses were observed. However, different correlations between the size of the pre-stimulus fluctuations and magnitudes of the subsequent sensory-evoked cortical hemodynamic responses could be observed depending on their phase at stimulus onset. These analyses suggest that both the size and phase of pre-stimulus fluctuations in cortical hemodynamics contribute to inter-trial variability in sensory-evoked responses

Linear superposition of sensory-evoked and ongoing cortical hemodynamics.

Modern non-invasive brain imaging techniques utilize changes in cerebral blood flow, volume and oxygenation that accompany brain activation. However, stimulus-evoked hemodynamic responses display considerable inter-trial variability even when identical stimuli are presented and the sources of this variability are poorly understood. One of the sources of this response variation could be ongoing spontaneous hemodynamic fluctuations. To investigate this issue, 2-dimensional optical imaging spectroscopy was used to measure cortical hemodynamics in response to sensory stimuli in anesthetized rodents. Pre-stimulus cortical hemodynamics displayed spontaneous periodic fluctuations and as such, data from individual stimulus presentation trials were assigned to one of four groups depending on the phase angle of pre-stimulus hemodynamic fluctuations and averaged. This analysis revealed that sensory evoked cortical hemodynamics displayed distinctive response characteristics and magnitudes depending on the phase angle of ongoing fluctuations at stimulus onset. To investigate the origin of this phenomenon, "null-trials" were collected without stimulus presentation. Subtraction of phase averaged "null trials" from their phase averaged stimulus-evoked counterparts resulted in four similar time series that resembled the mean stimulus-evoked response. These analyses suggest that linear superposition of evoked and ongoing cortical hemodynamic changes may be a property of the structure of inter-trial variability

The role of subsidiaries in Global Value Chains (GVCs): an institutional voids perspective on LVC upgrading and integration

We explore the process through which MNE subsidiaries engage and retain a critical mass of small suppliers in Global Value Chains (GVCs) while addressing institutional voids in emerging markets. Using evidence from an interpretive inductive longitudinal case study in agribusiness, we draw on the GVC and institutional voids literatures to: (1) extend the GVC literature by offering a subsidiary-focused view of GVCs; and (2) demonstrate the dynamic process of void engagement through complementary institutional bridging activities. Our temporal sequencing of subsidiary institutional agency in response to different modalities of voids demonstrates a constellation of bridging activities that results from a dynamic interplay between voids and practice

ECB policy and Eurozone fragility: Was De Grauwe right?

Paul De Grauwe's Eurozone fragility hypothesis states that sovereign debt markets in a monetary union without a lender-of-last-resort are vulnerable to self-fulfilling dynamics fuelled by pessimistic investor sentiment that can trigger default. We test this contention by applying an eclectic methodology to a two-year window around Mario Draghi's “whatever-it-takes” pledge that can be understood as the implicit announcement of the Outright Monetary Transactions (OMT) program. A principal components analysis reveals that the perceived commonality in default risk among peripheral and core Eurozone sovereigns increased after the announcement. An event study reveals significant pre-announcement news transmission from Spain to Italy, France, Belgium and Austria that clearly dissipates post-announcement. Country-specific regressions of CDS spreads on systematic risk factors reveal frequent days of large adverse shocks affecting simultaneously those five Eurozone countries, but only during the pre-announcement period. Altogether these findings support the fragility hypothesis and endorse the OMT program