In recent years, structural systems with diagrid skeletons comprising modular configurations have attracted lots of attention due to having structurally efficient system and architecturally aesthetic advantages. In this study, the seismic performance parameters of steel diagrid systems have been evaluated through conducting nonlinear time history analyses subjected to near-field earthquakes. The main subject of this study is to assess the effect of incidence angle of near-fault records containing forward directivity on the seismic behavior of steel diagrid systems. Following this purpose, three 20-story diagrid studied structures with different diagonal angles of 56°, 64° and 76°, having the same sizes and skeletal configurations were designed. Furthermore, a set of notified incidence angles with respect to the main axis including 0, 15, 30 and 45 degrees have also been applied to all the studied structures.
It should be noted that the significant characteristics of strong ground motions are the frequency content, effective duration of strong motions and peak ground motion parameters, i.e. PGV, PGA and PGD as well as corresponding response spectra. According to the aforementioned characteristics, an ensemble of strong earthquake records was selected. The selected ground motions include the main shock of the 1978 Tabas earthquake (Iran, Mw=7.3), the Bam record due to the 2003 Bam earthquake (Iran, Mw = 6.6) as well as two other powerful records entitled Rinaldi Receiving Station (RRS), and Sylmar Olive View (SYL) accelerograms related to the 1994 Northridge earthquake (Mw = 6.7) in California. It is worth mentioning that all the selected ground motions are categorized as strong near-fault records containing forward directivity effects. The specific characteristics of near-fault records containing forward directivity effects are the existence of long-period pulses.
These pulses are the evident result of the effects that manifest due to the occurrence of forward directivity process. The forward directivity process causes the induced fault rupture propagation to move toward the site. It usually happens when a velocity of rupture propagation is appropriately close to the earthquake shear wave velocity.
In this research, a number of engineering demand parameters (EDP) i.e. drift ratio, rotation of joints and plastic hinges nonlinear domain have been computed and assessed through conducting nonlinear time history analyses (NTHA). It is demonstrated that maximum seismic response parameters for all the studied structures do not occur necessarily along the main skeletal axes. Furthermore, it is extremely difficult to define the specific structural behavior trends subjected to a nominated critical incidence angle which can be imposed to earthquake records. It is noticeable that the numerical trends in the structural response parameters would change remarkably under the influence of the different frequency content of earthquake records, as well as the probable non-uniform variation for the EDPs and the diagrid skeletal patterns.
The results of this research indicate that the influences of incidence angle of ground motions on the structural seismic performance have different trends which evidently include mingle effects on the nonlinear rotation of joints as well as a complicated distribution for the lateral drift parameter. Moreover, it was obtained that the embossed structural skin containing the triangular modules with the diagonal angle of 56 degrees has relatively the lowest seismic demands subjected to strong near-field earthquake records.