Peculiarities of Soil Structure Interaction in Construction with Artificial Bases

A technique is suggested to increase seismic resistance of constructions by using alternative artificial bases. Two types of artificial bases are considered: a (consolidated) soil pad and a pile foundation (with or without pile grating). Criteria have been established for the foundation parameters required depending upon the foundation versus construction stiffness ratio. The construction of a consolidated soil pad 3 m thick (h~3m) with deformation module E0 \u3e 30MPa on soils, which are referred, according to the All-Union Building Standard Specifications (SNIP, 1982), to soil category III, is shown to reduce the level of surface accelerations by a factor of 1.5-2. The thickness of a pad for massive stiff structures has to be minimum and provide the bearing capacity of the foundation. In many cases an artificial base in the form of a pile foundation is desirable. This footing is most efficient in construction on a layer of strongly compressible soil 10-15 m thick overlying solid rock. The effects of such an artificial base on the dynamic characteristics of structures, as well as the properties of pile operation are discussed in the present paper

DEPENDENCE OF DISTRIBUTION FUNCTION OF COMMERCIAL DAMAGES DUE TO POSSIBLE EARTHQUAKES ON THE CLASS OF SEISMIC RESISTANCE OF A BUILDING

Abstract. Objectives To determine the damage probability of earthquakes of different intensities on the example of a real projected railway station building having a framework design scheme based on the density function of damage distribution. Methods Uncertainty, always existing in nature, invalidates a deterministic approach to the assessment of territorial seismic hazards and, consequently, seismic risk. In this case, seismic risk assessment can be carried out on a probabilistic basis. Thus, the risk will always be there, but it must be minimised. The task of optimising the reinforcement costs is solved by using the density distribution function for seismic effects of varying intensity, taking into account the degree of building responsibility. Results The distribution functions of the expected damage for a building with a reinforced concrete frame located in a highly seismic region with a repetition of 9-point shocks every 500 years and 10-point shocks once every 5000 years are constructed. A significant effect of the seismic resistance class of a building on the form of the distribution functions is shown. For structures of a high seismic resistance class, not only is the seismic risk reduced, but also the variance of the expected damage. From the graphs obtained, it can be seen that the seismic resistance class significantly affects the damage distribution. At a probability of 0.997, the expected damage for a non-reinforced building will exceed 43%; for a reinforced one it is only 10%. It also follows from the graphs that the variance of the damage magnitude decreases with the growth of the seismic resistance class of the building. This fact is an additional incentive for investing in antiseismic reinforcement of buildings. Conclusion The study shows the expediency of working with the damage density distribution function when managing seismic risk. In this case, it becomes possible to strengthen the building with a specified probability of damage exceeding the acceptable level during the operation of the construction. This takes into account not only seismic risk (mathematical expectation of damage), but also the dispersion of the expected magnitude of the damage. With the growth of seismic resistance class of the construction, it is possible to reduce both the risk and dispersion of possible losses

Some principles of generating seismic input for calculating structures

In this paper different models of seismic input are analyzed. The most essential characteristics of seismic effects are peak ground acceleration, peak ground velocity, peak ground displacement, Arias intensity, cumulative absolute velocity, seismic energy density, harmonic coefficient κ, pseudo spectral kinematic characteristics, root-mean-square peak kinematic characteristics, plastic forces work and damage spectrum. The influence of seismic impulse on characteristics of seismic input is studied. A.A. Dolgaya’s and L.N. Dmitrovskaya’s models with seismic impulse are compared. L.N. Dmitrovskaya’s model allows to reach estimated values of energy characteristics of seismic input with the smallest deviation. When generating such processes, it is important to take into account both the properties of real actions and the limiting state of the calculated structure. The considered models of seismic inputs should be applied in the following cases: a) in case of designing mass construction projects when it is not possible to get a package of design accelerograms, b) in typical designing when the design object can be located on sites with different seismic and geological conditions, c) at early stages of designing important objects when the package of design accelerograms is not available yet but it is necessary to make technical solutions

Efficiency of Using Tuned Mass Damper to Reduce Damage after Strong Earthquakes

The efficiency of applying tuned mass damper is substantiated for reducing the damageability of structures under strong earthquakes. Two models of structure damage accumulation are considered. The first model is elastoplastic one, the damage degree of the model being determined by the work of plastic deformation forces. The second model is a model with degrading rigidity the damage degree of the model is connected with the development of cracks and is determined by the maximum displacement of the structure in its loading history. For the first type of nonlinearity, i.e. the first model there is an amplitude-frequency characteristic and the optimum tuning of the mass damper corresponds to the maximum of this characteristic. For the second model of accumulation of damages there is no frequency response, therefore the mass damper tuning obtained with harmonic action on the elastic system was used. Calculations of the system with mass damper and without it using earthquake accelerograms have been carried out. Accelerograms, the most unfavorable in terms of the spectral composition for the structures under consideration, were chosen taking into account peak accelerations and energy characteristics of the seismic action. It has been established for elastoplastic systems that tuned mass damper reduces the time of the structure being in plastic stage and the work of plastic deformation forces. For systems with degrading stiffness tuned mass damper reduces the maximum movement of the system in its loading history

Efficiency of Using Tuned Mass Damper to Reduce Damage after Strong Earthquakes

The efficiency of applying tuned mass damper is substantiated for reducing the damageability of structures under strong earthquakes. Two models of structure damage accumulation are considered. The first model is elastoplastic one, the damage degree of the model being determined by the work of plastic deformation forces. The second model is a model with degrading rigidity the damage degree of the model is connected with the development of cracks and is determined by the maximum displacement of the structure in its loading history. For the first type of nonlinearity, i.e. the first model there is an amplitude-frequency characteristic and the optimum tuning of the mass damper corresponds to the maximum of this characteristic. For the second model of accumulation of damages there is no frequency response, therefore the mass damper tuning obtained with harmonic action on the elastic system was used. Calculations of the system with mass damper and without it using earthquake accelerograms have been carried out. Accelerograms, the most unfavorable in terms of the spectral composition for the structures under consideration, were chosen taking into account peak accelerations and energy characteristics of the seismic action. It has been established for elastoplastic systems that tuned mass damper reduces the time of the structure being in plastic stage and the work of plastic deformation forces. For systems with degrading stiffness tuned mass damper reduces the maximum movement of the system in its loading history

Load combinations in performance-based designing of earthquake-resisting structures

Load combinations for seismic and other loads are considered. To this aim the equiprobable sets of loads with the corresponding probabilities are analyzed. The combination of motor-car and seismic loads is considered in details. The log-normal distribution law was used as a distribution density functions for car load. The distribution of the earthquake event stream was taken according to Poisson's law, which makes it possible to estimate the design seismic intensity. In the frame of this intensity peak ground accelerations were estimated. The dependence of the combination coefficient of the seismic load on the combination coefficient of the motor-road load was obtained. The results obtained show, that combination coefficients in Performance Based Designing should be calculated separately for each input level. For the design earthquake and the maximum design earthquake the combination coefficients vary significantly. The values of the combination coefficients are determined mainly by the frequency of the design actions and to a lesser extent they depend on the seismic activity of the building site

Load combinations in performance-based designing of earthquake-resisting structures

Load combinations for seismic and other loads are considered. To this aim the equiprobable sets of loads with the corresponding probabilities are analyzed. The combination of motor-car and seismic loads is considered in details. The log-normal distribution law was used as a distribution density functions for car load. The distribution of the earthquake event stream was taken according to Poisson's law, which makes it possible to estimate the design seismic intensity. In the frame of this intensity peak ground accelerations were estimated. The dependence of the combination coefficient of the seismic load on the combination coefficient of the motor-road load was obtained. The results obtained show, that combination coefficients in Performance Based Designing should be calculated separately for each input level. For the design earthquake and the maximum design earthquake the combination coefficients vary significantly. The values of the combination coefficients are determined mainly by the frequency of the design actions and to a lesser extent they depend on the seismic activity of the building site

Important feature of calculating bridges under seismic action

The purpose of the research is to show the main features of calculating bridge taking into account the inhomogeneous acceleration field along the structure length. The bridge is considered to be a linear structure with point bearings on the soil base. For long bridges it is typical, that their bearings are located in different seismogeological conditions. This result in inhomogeneity of the acceleration field under the piers and non-synchronous pier excitations. The motion equations of the system under consideration are constructed and their decomposition into vibration modes is performed without the account of external and internal damping in the system and with the account of it. Based on the proposed decomposition, formulas for determining seismic loads taking into account various seismicity under piers are obtained. The result obtained show that the peculiarities considered can be easily taken into account in existing software packages. As an example, the authors analyzed the results of calculating a four-span beam railway bridge. In calculating it was taken into account that the first and second piers are located on sandstone, and the rest of them are on water-saturated loose sand. The analysis showed that the account of the non-synchronous support point excitation of the extended system reduces inertial seismic loads on its elements significantly

On estimating the reduction factor of bridge piers

Estimating the reduction factor for calculating massive reinforced concrete bridge piers was made. For this purpose a quasi-static “force-displacement” diagram was built up using the ANSYS software. This diagram has the form of a bilinear one, and the character of the bilinearity depends on the diameter of the reinforcing bars insignificantly. The percentage of reinforcement affects only the moment when all reinforcement bars begin to flow. The reinforcement flow takes place in the displacement interval from 3 to 5 cm. The collapse will occur when the reaction of the bearing part goes beyond the pier cross-section at pier displacements from 5 to 20 cm. Using “force-displacement” diagram, the behavior of the single-mass model with a bilinear deformation diagram and the limit displacement of 20 cm was analyzed. Then, it became possible to obtain for each accelerogram the limit elastic displacement and the limit position of the point corresponding to the maximum structure displacement during structure oscillations. It was done using real accelerograms of earthquakes with intensity 9 on the MSK scale without normalizing their amplitudes. In this case, long-period accelerograms had smaller peak accelerations, but resulted in greater plastic deformations. As a result, no evident dependence of plastic deformation on the input spectral composition was found and the value of reduction factor K1 turned out to be 0.25-0.27. However, it is shown that this reduction factor cannot be used to make transition from seismic loads obtained on the basis of time-history analysis by accelerograms to design loads

Magnetic exchange force microscopy: theoretical analysis of induced magnetization reversals

In magnetic exchange force microscopy a magnetic tip is scanned over the surface of a solid and an image representing the exchange interaction recorded. Sudden changes in the image corresponding to magnetization switching can be monitored as a function of the tip-surface distance thereby giving important information about the lifetime of metastable magnetic states and how it is affected by the exchange interaction. Here, theoretical calculations are carried out to study the tip-surface interaction and determine the mechanism and rate of transitions in a magnetic exchange force microscopy experiment, and comparison made with reported experimental data on an Fe cluster interacting with an antiferromagnetic Fe overlayer on a W(001) surface. The activation energy was determined from calculations of minimum energy paths and the pre-exponential factor in the Arrhenius rate expression evaluated from harmonic transition state theory, extended to account for zero modes. A noncollinear extension of the Alexander-Anderson model was used to describe the magnetic properties of an atomic scale representation of the system. The calculations reveal how the tip size, tip-surface distance and magnetic field affect the lifetime of the magnetic states.Peer reviewe