We propose a dynamic analysismethod – a refined version of the DEM- that can simulate three-dimensional elastic, failure and collapse behaviors of structures. A structure is modeled as an assembly of rigid elements. Interaction between elements is modeled using multiple springs and multiple dashpots attached to surfaces of the elements. The elements are assumed to be rigid, but the method allows the simulation of structural deformation by permitting penetration between elements. There are two types of springs: one is a restoring spring to simulate elastic behavior before failure and the other is a contact spring for simulating contact and recontact between elements. A contact dashpot is also used to dissipate the energy of contact. Structural failure is modeled by replacing restoring springs with contact springs and dashpots. A method for determining spring constants is also proposed. The validity of the method is confirmed by the numerical simulation of masonry wall models. First, the elastic behavior induced by an impact force is calculated. It is found that the elastic behavior determined using the proposed method is in good agreement with that determined using the finite element method. Second, the seismic behaviors of masonry wall models with different laying patterns and a wall model with reinforcement are analyzed. It is found that the proposed method allows expression of the difference in behavior due to different laying patterns and reinforcement. The validity of the proposed method is thus confirmed. The proposed method is suitable for simulating seismic behavior of masonry structures.
On April 25, 2015, a M w 7.8 earthquake struck the Gorkha district of Kathmandu, Nepal. In Patan, vibrational characteristics of a 300-year-old two-story masonry building near Patan Durbar Square had been measured prior the Gorkha earthquake. In the inspection of the building after the Gorkha earthquake, several new cracks were found. The vibrational characteristics of the building were measured again, and it was found that the natural frequencies after the earthquake were smaller than those before the earthquake, indicating the reduction of the stiffness. Finite element models of the structure representing pre-and post-earthquake conditions are established so that the natural frequencies match the pre-and post-earthquake measurements and the structural damage is identified based on the stiffness reduction. Finally, the dynamic analysis of the finite element model of the building in the pre-earthquake condition using the observed ground motion record during the Gorkha earthquake as the input is conducted, and the structural response of the building during the Gorkha earthquake is discussed.
This study examines the effect of axial force fluctuation in supporting columns of a traditional wooden temple on the seismic response of the structure. The main structure and high wooden stage of the Kiyomizu Temple, a Japanese national treasure located in Kyoto, were reconstructed in 1633 following a fire. The temple was modeled numerically for three-dimensional inelastic earthquake response analysis. An inelastic vertical spring was set at the base of each column to represent uplift during an earthquake, coupled with a horizontal spring set to represent variable friction corresponding to the varying axial force of the column. The results reveal that axial force fluctuation has little effect on the maximum seismic response of the structure. However, this effect does influence the residual displacement of each column, particularly near the perimeter of the structure.
The process of failure propagation of masonry buildings during earthquakes is simulated using a refined version of the distinct element method that simulates three-dimensional elastic, failure, and collapse behaviors of structures. Models with a flat roof and models with a vault roof are considered, and their failure propagation mechanisms are examined. The influence of the direction of the input ground motion on failure propagation is also investigated. Moreover, the effectiveness of three reinforcement measures is compared. One measure is increasing the mortar strength, the second is increasing the thickness of the bearing walls, and the third is introducing wooden columns and beams. Among these measures, increasing the mortar strength is found to be the most effective. Increasing the thickness of the bearing walls and introducing wooden columns and beams are found to be effective only if the roof has sufficient integrity since the collapse of the roof depends on the integrity of the bearing walls and the roof itself.
Saving human lives and cultural heritage from natural disasters is a key to earthquake-disaster mitigation strategies. Culturally valued structures built before earthquake codes and regulations emerged are often vulnerable to earthquake loads, but such structures must be comprehensively studied before applying mitigation measures. Microtremor measurements in Patan Durbar Square area, a World Heritage site, were investigated to determine dynamic properties of the soil at four locations and the predominant ground frequency calculated to be 2.07 Hz. Ambient vibration in an old masonry building was measured and vibration frequencies in different modes were detected using the Fourier spectrum, which found that the building has fundamental period in transverse direction. The building’s damping was estimated to be 5.2-6.4%.
This paper shows activity of CEORKA (The Committee of Earthquake Observation and Research in the Kansai Area) on observing strong ground motion records and providing the data. A quasi-real time data acquisition and broadcasting system of strong motion information is maintained by CEORKA. Members of the committee who supports the CEORKA project can receive seismic intensity map and waveform plots via E-mail, and can access automatically up-loaded digital files on home page (http://www.ceorka.org/).
The cultural property protection field is wide and varied, with the problem of natural disaster alone often being overlooked, especially in seismic hazard measures. Cultural property preservation field experts recognize that fire-prevention measures, for example, having focused on accidental fires and arson within shrine and temple precincts that have been ineffective in preventing fires from spreading to historical buildings during simultaneous fire outbreaks in surroundings of concern during earthquakes. In 2003, the Japanese government recognized the importance of cultural heritage disaster mitigation, and a National Committee was organized whose first report was released in 2004, leading, in turn, to the first national project for protecting cultural assets against natural disasters. The project focused on two 1,500-ton underground water storage tanks near Kiyomizudera and Sanneizaka. With a pressurized water sprinkler system and other fire control facilities, the facility is expected to be used to fight fires during earthquakes and to provide easy-to-use fire hydrants for other fires.
Written history of great earthquakes in excess of magnitude M8 and recently identified 92 small faults around underlying big three fault systems parallel to the Himalayas show a high seismicity in Nepal. However, since faults are so closed that it is difficult to judge which earthquake belongs to which fault and even some of the faults do not hold earthquakes, the usual method of assigning the earthquakes to the nearest fault developing magnitude-frequency relationship is not applicable. Thus, an attempt has been made here to address the problem considering area sources with different densities at each location based upon historical earthquakes and faults which is real evidence of the seismicity of the region. Separate earthquake densities are calculated based upon historical earthquakes and maximum magnitudes of faults using the kernel estimation method which accounts the significance of both the number of earthquakes and size. Since there is no specific attenuation laws developed for the Himalayan region, five attenuation laws developed for seduction zone are selected and used, giving equal weight to all to minimize the uncertainties. Then, the probabilistic spectra for various return periods are calculated, compared with previous estimates and various aspects discussed.
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