Spectra of seismic force reduction factors of MDOF systems normalized by two characteristic periods

Seismic force reduction factor(SFRF) spectra of shear-type multi-degree-of-freedom(MDOF) structures are investigated. The modified Clough model, capable of considering the strength-degradation/hardening and stiffnessdegradation, is adopted. The SFRF mean spectra using 102 earthquake records on a typical site soil type(type C) are constructed with the period abscissa being divided into three period ranges to maintain the peak features at the two sitespecific characteristic periods. Based on a large number of results, it is found that the peak value of SFRF spectra may also exist for MDOF, induced by large high-mode contributions to elastic base shear, besides the mentioned two peak values. The variations of the stiffness ratio λk and the strength ratio λF of the top to bottom story are both considered. It is found that the SFRFs for λF ≤λk are smaller than those for λF > λk. A SFRF modification factor for MDOF systems is proposed with respect to SDOF. It is found that this factor is significantly affected by the story number and ductility. With a specific λF(= λk0.75), SFRF mean spectra are constructed and simple solutions are presented for MDOF systems. For frames satisfying the strong column/weak beam requirement, an approximate treatment in the MDOF shear-beam model is to assign a post-limit stiffness 15%-35% of the initial stiffness to the hysteretic curve. SFRF spectra for MDOF systems with 0.2 and 0.3 times the post-limit stiffness are remarkably larger than those without post-limit stiffness. Thus, the findings that frames with beam hinges have smaller ductility demand are explained through the large post-limit stiffness.

The Shielding Effect of Multi-Pile Structures on Ice Force

The shielding effect of the front pile-row on the ice force acting on the back pile-row is studied by ice force model tests. In the tests, the front pile-row is designed to model jacket legs and the back pile-row to model the water resisting pipe-phalanx within the jacket. The shielding factor for ice force corresponding to different conditions are given in this paper. The research indicates that there are many factors, including the longitudinal and lateral spacing between the front and back pile-row, ice attacking angle and the ratio of pile diameter to ice thickness, that influence the shielding effect on ice force.

Static and dynamic inelastic P-Δ effect for seismic design

Seismic influence of P-Δ effect is the subject of this study.First,it is pointed out that the elastic static amplification factor shall be isolated in formulating the dynamic inelastic second order effect.An amplification factor for the static inelastic P-Δ effect is derived.Seismic force reduction factors(SFRF)for given ductility and stability coefficients are computed for one-story,one-span frames.The P-Δ amplification factors for seismic base shears are obtained by dividing SFRFs with and without P-Δ effect.Both P-Δ amplification factors and SFRFs are presented separately with two kinds of period abscissas.The P-Δ amplification factors are dependent on periods with the maximum occurring at about 0.75 s for site type C and approach to the static inelastic counterpart at long periods.Post-yield stiffness cannot fully counteract the adverse impact of the P-Δ effect.Formulas for seismic P-Δ amplification factors are proposed and compared to results of others.Collapse capacity spectra(CCS)are reviewed and their application in codes discussed.Available CCSs are compared with SFRFs with finite ductility computed for two ensembles of seismic records.A comparison reveals that the SFRFs are affected by seismic records,and available CCSs do not always provide upper limits for the SFRFs when stability coefficients are greater than 0.1 for frame models.

Interaction analysis of back-to-back mechanically stabilized earth walls

Back-to-back mechanically stabilized earth walls （BBMSEWs） are encountered in bridge approaches, ramp ways, rockfall protection systems, earth dams, levees and noise barriers. However, available design guidelines for BBMSEWs are limited and not applicable to numerical modeling when back-to-back walls interact with each other. The objective of this paper is to investigate, using PLAXIS code, the effects of the reduction in the distance between BBMSEW, the reinforcement length, the quality of backfill material and the connection of reinforcements in the middle, when the back-to-back walls are close. The results indicate that each of the BBMSEWs behaves independently if the width of the embankment between mechanically stabilized earth walls is greater than that of the active zone. This is in good agreement with the result of FHWA design guideline. However, the results show that the FHWA design guideline underestimates the lateral earth pressure when back-to-back walls interact with each other. Moreover, for closer BBMSEWs, FHWA design guideline strongly overestimates the maximum tensile force in the reinforcement. The investigation of the quality of backfill material shows that the minor increase in embankment cohesion can lead to significant reductions in both the lateral earth pressure and the maximum tensile force in geosynthetic. When the distance between the two earth walls is close to zero, the connection of reinforcement between back-to-back walls significantly improves the factor of safety.

A Study on Propagation Mechanism of Fracture Systems in Rock Masses by Discontinuity Displacement Method

The main task of fracture mechanics of rock masses is the study on the propagating mechanism of fractures in rock masses , which can be efficiently conducted by discontinuty displacement (DD) numerical evaluation . Firstly ,the element stress and displacement are analysed and the principle and steps of the numerical calculation of stress intensity factor and fracture extension force are introduced .The numerical results of parallel and echelon fracture systems ,which are compared with real field fractures .are presented. Finally . a simple engineering application example is presented .

The Sustainable Development of Resource Environment and Economy

At first, this article gives the calculating formulas about the mutual-motive mechanism of the economy development and its resource, environmental influence. And then it gives out relevant ideas about the sustainable development＇s realizing means, motive force and economic model.

Why Is the East Asian Summer Monsoon Extremely Strong in 2018?——Collaborative Effects of SST and Snow Cover Anomalies

In 2018,summer precipitation was above normal in North and Northwest China and below normal around the Yangtze River valley,due to an extremely strong East Asian summer monsoon(EASM).The atmospheric circulation anomalies in East Asia and key external forcing factors that influence the EASM in 2018 are explored in this paper.The results show that there existed an anomalous cyclonic circulation near the Philippines,while the western Pacific subtropical high was located more northward than its normal position.In the mid–high latitudes,a negative geopotential height anomaly center was found near the Ural Mountains,suppressing the blocking activity.Under such a circulation pattern,precipitation near the Yangtze River valley decreased because local divergence and subsidence intensified,whereas precipitation in northern China increased due to large amounts of water vapor transport by anomalously strong southerly winds.Further analyses reveal that the strong EASM circulation in 2018 might result from the joint influences of several external forcing factors.The weak La Ni?a event that started from October 2017,the positive North Atlantic Tripolar mode(NAT)in spring 2018,and the reduced snow cover over the Tibetan Plateau in winter 2017/18 all collaboratively contributed to formation of the cyclonic circulation anomaly near the Philippines,leading to the extremely strong EASM.Especially,the positive NAT and the reduced Tibetan snow cover may have caused the negative geopotential height anomaly near the Ural Mountains,in favor of a strong EASM.The above external factors and their reinforcing impacts on the EASM are further verified by two groups of similar historical cases.

Paleogeography control of Indian monsoon intensification and expansion at 41 Ma

As a crucial part of the Asian monsoon stretching from tropical India to temperate East Asia, the Indian monsoon(IM) contributes predominant precipitation over Asian continent. However, our understanding of IM’s onset, development and the underlying driving mechanisms is limited. Increasing evidence indicates that the IM began in the Eocene or even the Paleocene and was unexceptionally linked to the early rise of the Tibetan Plateau(TP). These were challenged by the heterogeneous and diachronous uplift of the TP and all the reported records were confined to tropical zone under tropical monsoon driven by the Intertropical Convergence Zone(ITCZ) that is irrelevant to the TP. Therefore, reliable paleoclimatic records from the extra-tropical IM region is crucial to reveal how the tropical IM expanded to subtropical and temperate zones and what driving factors might be related to it. Here we present robust Eocene paleoenvironmental records from central Yunnan(~26°N) in subtropical East Asia. The multiproxy results of two sites demonstrate a consistent sudden switch from a dry environment in the early Eocene to a seasonally wet one at 41 Ma, suggesting a jump of the tropical IM to the southern subtropical zone at 41 Ma.The full collision of India with Asia, and the resulting changes in paleogeography at 41 Ma(closure of the Neotethys sea, retreat of the Paratethys seas, fast northward movement of the southern margin of the TP and rise of the central TP), aided by synchronous Antarctic cooling, might have worked together to drive the IM enhancement and northward expansion.

Analysis and prediction of loudspeaker large-signal symptoms

Nonlinear lumped-parameter force factor Bl(x), stiffness Kms(x) and inductance Le(x) of electrodynamic loudspeakers change frequency responses and generate some nonlinear effects for large stimulus: harmonic and intermodulation distortion, DC component in diaphragm displacement, instability of vibration and jumping effects. By modeling the nonlinear system under large-signal conditions, relationship between the nonlinear parameters and large-signal behavior can be revealed and help to provide guidance to diagnose loudspeakers. Agreement between the measured and predicted responses of a real loudspeaker validates the modeling and enables new methods for loudspeaker diagnosis.