С 3 по 7 декабря 2018 г. в столице Армении городе Ереване состоялась Международная конференция «Spitak-30.Спитакское землетрясение 30 лет спустя: опыт и перспективы», в работе которой приняли участие более 100 разнопрофильных ученых геологов-тектонистов, геофизиков-сейсмологов, специалистов по сейсмостойкому строительству.
Determination of Bearing Capacity and Stress-Deformation State of Prefabricated Reinforced Concrete Slab after Influence of Freeze-Thaw Cycles
Authors: Minasyan A.
Rubric: Theoretical and experimental studies
In the absence of conservation, reinforced concrete slabs are most exposed to atmospheric influences, affecting the strength and deformation characteristics of concrete and bearing capacity of structures. The impact of field conditions on the properties of concrete
depends on many factors that are almost impossible to simulate in accelerated laboratory con-ditions. Therefore, the General approach to the characteristic of the change of the bearing capacity at corrosion damages of reinforced concrete slabs in the conditions of cyclic freezing-thawing, is to assess the main all possible effects of natural conditions on the change of the bearing capacity. This article discusses the numerical calculations to determine the bearing capacity of corrosion-damaged concrete slabs. For an accurate assessment of the residual life of the bearing capacity of the floors in the conditions of long-term cyclic freezing-thawing, were selected typical schemes of damage to the plates and performed a series of tests of corrosion-damaged concrete slabs.
The work of structures in the inelastic stage, as well as the gradual increase in the load, defined deflections, defor-mations and bearing capacity of the plates in the stages of cracking. The comparative analysis of results of numerical calculations and full-scale tests
of plates is given.
Calculation of the Bearing Capacity of the Ground Base for Baretta and Definition of the Effective Depth of the Foundation
Authors: Utkin V.S. Korepina I.
Rubric: Theoretical and experimental studies
Deep-hole slotted foundations are relatively new constructive parts of buildings and structures. Slot-hole foundations are considered to be foundations of deep embedding, if the vertical loads on it are perceived not only by the soil in the base of the foundation, but also by friction-adhesion forces along its lateral surfaces. Improving the methods of their calculation increases the safety of their operation and is an actual problem. A new description of the operation of the foundation in the soil of the foundation is proposed on the basis of the occurrence of friction-adhesion forces along its lateral surface as a result of microdisplacements (deformations) of the surface layers of the foundation material in the soil. On the basis of a new approach to the description of the operation of the slotted foundation of a deep foundation in the foundation soil, a calculation formula is given for the bearing capacity of the foundation soil and the formula for determining the effective foundation depth.
The combination coefficients of seismic and wind loads for multilevel designing buildings and structures in high seismic regions are considered. For design and maximum design earthquakes equiprobable pairs of seismic and wind loads are determined. Correspondingly, these pairs are iven coupling coefficients for the loads under consideration, which determine the dependence of one combination coefficient on the other. In constructing the equiprobable pairs, the wind load is described by Weibull law and seismic load is described by Poisson’s law. It is shown that for the seventh wind zone with high seismic intensity in calculating structures under the action of the esign earthquake, the combination coefficients are close to 1, and in calculating structures under the action of the maximum design earthquake they are less than 1, and therefore it becomes necessary to choose the most dangerous pair of seismic and wind loads.
The stations of the engineering seismometric service of KazNIISA JSC recorded more than 15 lo-cal earthquakes with the foci in Almaty and Almaty region in 2018. Accelerograms recorded on rigid and flexible buildings during 3 sensible earthquakes were selected from the obtained instru-mental records. These are accelerograms recorded in the basement parts of a four-storeyed frame building and an 11-storeyed steel-framed building. In one instance, the instrumental rec-ords registered in the basement of a 17-storeyed building are used. Flexible buildings are locat-ed near tectonic faults. The spectral curves characterizing the frequency content of the seismic effect are built. It has been established that the frequency response characteristics of the foundation composed of soils of the 2nd category by seismic properties are high-frequency with prevailing periods of 0.1-0.20 sec. Resonance phenomena for flexible buildings under the influence of local earthquakes are not observed. Instrumental records are included into the accelerogram database of KazNIISA JSC.
Updated accurate estimates of seismic effects from scenario earthquakes are obtained for the areas of Kolsky peninsula and Karelia; calculations were carried out with regional characteristics of radiation and propagation of seismic waves that were estimated based on records of local earthquakes. Seismic intensity, peak ground accelerations and velocities, response spectra, and other characteristics are estimated from acceleration time histories of motions on the surface calculated by stochastic method. Higher estimates of seismic intensity and peak ground accelerations and velocities were obtained, as compared to the estimates obtained earlier and based on world-average characteristics or from areas with similar geological structure. The main cause of the obtained differences and high new estimates of seismic effects is neglecting in past studies high values of the stress drop, ??~bars, typical for this region.
Combined Asymptotic Method (CAM) has been developed and used for SSI analysis of stiff and heavy NPP structures. However, some of civil structures nowadays are comparable with NPP structures in terms of size and mass. The question is whether SSI is significant for such structures and whether it should be accounted for in the design process. In the previous two papers CAM was applied to the seismic analysis of the high-rise structure resting on a sample homogeneous half-space modeling soil. The overall impact of SSI or, in other words, the applicability of the fixed-base seismic analysis was studied. In the present paper two specific features are investigated which make horizontal/rocking seismic response of the high-rise building completely different from the vertical response of the same building and from the total response of the conventional NPP buildings. Maximal integral horizontal forces under the base are several times less than vertical force though maximal horizontal base accelerations are greater than vertical one. Transfer functions to the horizontal base response show little peaks at the lowest natural frequencies corresponding to the dominant modal masses. Rocking stiffness of the soil proves to be more important than the horizontal one. Due to the low first natural
frequencies, the response along the first modes is overcritical and out of phase – as a result, the corresponding modal mass is subtracted from the total “rigid” mass. Base rocking is also out of phase and leads to the decrease in deformations in the lower part
In dynamic process the behavior of construction depends on their surface of fluidity. In article analytical decisions for a rigid plasticity system with two degrees of freedom in case of dynamic problem definition when external influence has impulsive character (initial
speed of a system is set) are received and analysed. The mathematical model of a rigid plasticity system dynamic behavior at account and not accounting of interaction projections of plastic reactions effect are provided in article. The interrelation between these approaches is found. An assessment of construction operability is given and also limit cases of the received decision are considered. Comparison with the simplified problem definition upon transition from elliptic to rectangular curve fluidity is carried separately out.