PSI - Issue 10

N.G. Pnevmatikos et al. / Procedia Structural Integrity 10 (2018) 195–202 N.G. Pnevmatikos et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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done. Amongst them is the work of Suidan and Eubanks (1973), where they worked on cumulative fatigue damage in seismic structures. Vayas et al. (2003) performed fatigue analysis of moment resisting steel frames subjected to earth quake excitations. According to the work of Campbell et al. (2008), a method for calculating the fatigue damage in steel moment frames is presented. They combine the results of nonlinear analysis with experimentally obtained fatigue damage curves to predict failure or the remaining useful life after an earthquake. In the work of Plumier et al. (2012) the resistance of steel connections to low-cycle fatigue is calculated. Applications of the above methods are presented in the work of Koutsomichali (2012). Castiglioni et al. (2007), work on constant and variable amplitude cyclic be havior of welded steel beam-to-column connections. Lassen and Reecho (2006), propose a fatigue life analysis of welded structures. In the present work a methodology for low cycle fatigue assessment of steel frame subjected to number of earth quake excitations is presented. Furthermore, the phenomenon of the high cycle fatigue to structures which are subjected to a high number of aftershock earthquakes is investigated.

Nomenclature Mw

Richer scale

PGA

Peak Ground Acceleration

Δσ EC

Stress range Eurocode

EN 1993-1-9

Eurocode 3 Part 1-9

Δφ p

fatigue deformability (plastic rotation) Number of rotation range cycles slope constant of the fatigue curve

N m

log a

constant

DI

Damage Index Fatigue Life

T

2. Methodology

Earthquakes are always followed by extensive sequences of aftershocks near to the region of main earthquake. One example is the Great Tohoku earthquake (north-east Honshu, Japan) on 18 March 2011, 9 Richer of magnitude. The total number of aftershocks was 419, i.e., there had been 369 aftershocks with magnitude 5 to 6, 48 with magni tude 6 to 7 and 2 greater than magnitude 7. Fig.1 shows the aftershock locations and magnitude frequency distribution for the earthquakes recorded. Looking at Greek region, last earthquake in Lesvos on June 12 2017 with magnitude 6.3 was followed by more than 100 aftershocks observed. Another quake in Lefkada on 17-11-2015 with magnitude 6 in Richter scale gave 110 aftershocks. The two recent earthquakes and the aftershocks are shown in Fig.2. Figs.1 and 2 are referred here in order to show that the number of earthquake aftershocks is significant and their influence to structural fatigue could be examined. Fatigue due to main earthquake event is the case of low-cycle fatigue, since the steel connection in structures exhibits a small number of applied inelastic deformations cycles. Furthermore, due to a great number of aftershocks, with smaller magnitude, fatigue of steel connection detail, can be influenced by a number of applied cycles of low nominal stress, caused from aftershocks events, this is a high cycle fatigue case. The procedure of the fatigue damage assessment to a steel frame subjected to earthquake excitation consists of four basic steps:  Determine the time history load sequences representing an estimated upper limit of all operating loads expected to occur during design life. Execute linear and nonlinear time-history analysis of the structure subjected to time history loads.  Extract the time history response quantities of interest, such as bending moment, axial forces, tensile stresses, plastic beam rotations, storey drift, etc. Convert the time history response to an equivalent number of loading cycles, for instance, reservoir method.