The oscillation resistance ratio (ORR) for understanding inelastic response

Single-storey systems with different hysteretic characteristic are subjected to impulse-type short duration and long duration earthquake records to investigate the effects of hysteretic behaviour and ground motion characteristics on the seismic response. EPP, bilinear, Takeda, SINA, and flag-shaped hysteretic models loops are considered and an energy approach is taken to explain the inelastic behaviour. The first part of the work is based on analyses of the single-storey systems without any torsion, however; torsional irregularity is considered in the later analyses. It is shown that structures with the same backbone curve, but different hysteretic characteristics, tend to experience the same maximum response under short duration earthquake records, where there is one major displacement excursion. The likelihood of further displacement in the reverse (i.e. negative) direction is characterized using energy methods and free vibration analyses along with a new proposed “oscillation resistance ratio (ORR)” are employed to improve the understanding of the seismic response. Hysteretic models with low ORR, such as SINA and flag-shaped, are shown to have a greater likelihood of higher absolute displacement response in the negative direction compared with those with fatter hysteretic loops. The understanding of the response in terms of energy reconciles some differences in the ability of initial stiffness versus secant stiffness based methods to predict peak displacement demands with account for different ground motion characteristics. The same peak displacements in the primary direction was also observed for structures with stiffness/strength eccentricities under an impulse-type earthquake record. However, during unloading, the elastic energy stored in the out-of-plane elements is released causing greater displacement on the weak side in the reverse direction.

Characteristics of seismic motions at a concrete gravity dam site and suggestions for setting the engineering bedrock

The earthquake records of a concrete gravity dam with eight seismographs were spectrally analyzed to clarify the characteristics of the seismic motions at each monitoring station. Based on the analysis results, whether it is appropriate to use such earthquake records to generate the input ground motion in assessments of the seismic performance of dams was discussed. It was found that the earthquake records at the dam base show reduced amplitudes at frequencies corresponding to the natural vibration modes of the dam. When these earthquake records are used to generate the input ground motion in an assessment of the seismic performance of a dam, sufficient consideration should be given to this feature. Suggestions on where to install seismographs at dam sites are presented. Based on the numerical simulation of the seismic response of the dam, it is suggested that the engineering bedrock should be considered to be located at a depth of at least 1.5 times the dam height, and the shear wave velocity of the rock at that depth should preferably be 2000 m/s or more. Since this study uses seismic motions recorded across the entire dam site, including the deep rock, which is globally rare, it is considered that the basic conclusions obtained in the article can provide a reference for other concrete dams.

Seismic Behavior of LRB and FPS Type Isolators Considering Torsional Effects

This study aims to investigate seismic behavior of LRB and FPS type base isolated models considering torsional irregularity due to distance between stiffness center of isolators and mass center of superstructure for low- and mid-rise reinforced concrete (RC) frame buildings with no shear walls. Nonlinear behavior of structural members was also considered to be able to observe probable yielding of structural members in superstructure due to the torsional irregularity. 528 different nonlinear time history analyses of 3-dimensional 3, 5, 7 and 9-story models subjected to 11 pairs of earthquake records were performed. The results indicate that FPS type isolators tend higher displacement demands while LRB type isolators are more sensitive to torsional effects. Torsional irregularity coefficient values of LRB models with 20% eccentricity are 47% higher than symmetrical models in terms of averages. Since significant part of the demands is absorbed by the isolator system, the remaining seismic demands for the superstructure is relatively low. Besides, the outcomes underline the careful selection of number of ground motion records in dynamic analysis as mentioned in the literature. While some studies in the literature indicate that torsional effects due to 10% or 20% eccentricity have significant role on building response, they used very limited building models subjected to a few earthquake records. The use of four different RC frame building model with LRB and FPS type isolators subjected to 11 pairs of ground motion records do not strongly support the abovementioned studies in literature. Therefore, more research is needed in this field.

An improvement of direct displacement-based design approach for steel moment-resisting frames controlled by fluid viscous dampers

Direct displacement-based design (DDBD) method is one of the most effective methods for performance-base design of structures that has been also employed to design structures controlled by fluid viscous damper (FVD). In previous studies, a modified DDBD has been developed to apply the higher mode effects as well as difference between spectral velocity and pseudo-spectral velocity on the design velocity of FVD. To this end, two constants were defined to correct the damping coefficient of FVD that these correction constants had been determined in a non-classical and iterative manner. In this study, a new classical method is proposed for determining these constant such that no iteration is required in DDBD. In order to be able to introduce this design approach as a reliable framework, its performance is validated under different sets of earthquake records and this design approach is also developed for structures controlled by nonlinear FVD. Steel moment-resisting frames with different numbers of stories have been designed using this method. For comparison, structures have been also designed based on DDBD proposed in previous researches. The results show that DDBD approach improved in this study is capable to achieve the design performance level under different sets of earthquake records and this design approach has more effective performance than previous design methods. Performance of steel moment-resisting frames equipped with nonlinear FVD also shows excellent performance of this design approach in achievement of desirable performance level. Therefore, DDBD approach proposed in this study can be introduced as a new classical and reliable framework because of its simplicity and excellent performance under different sets of earthquakes.

Influence of the Vertical Earthquake Component on the Shear Vibration of Buildings on Sliding Foundations

The paper studied the vibrations of the buildings on the sliding foundation with dry friction under the action of real earthquakes at the intensity of 8 and 9 on the MSK-64 scale. It was developed a unique algorithm for calculating the displacements, velocities, accelerations, and shear forces resulting from the simultaneous action of the horizontal and vertical components of the seismogram record. It was studied four-story and nine-story buildings under the set of the three earthquake records. It was shown that the use of a sliding foundation does not always lead to a significant reduction in the shear force on the building floors and that the vertical component of the seismic effect has a significant influence on the shear vibration of the building.