A b s t r a c tThe interaction of piles with different lengths and the grillage in the foundations of high-rise buildings is considered. The numerical modeling of the «basefoundation -superstructure» system is performed. The redistribution of the efforts in piles depending on the sizes of a foundation slab and the parameters of piles (length and location) is investigated. Typical zones of a foundation such as central, lateral, and angular ones are separated. The redistribution of efforts between piles and a grillage is revealed.

This paper presents the numerical modelling of the fire exposure on structural element and its static analysis taking account of the material properties reduction due to elevated temperature. The reinforced concrete wall of basement storey was subjected to one side fire exposure. Advanced calculation methods were used to assess the fire exposure on the structural element as most reliable and approximate to fire test results. Thermal analysis was performed with LIRA-CAD software by the simulating of three main heat-transfer ways: thermal conductivity, convection and radiation. As a result of the thermal analysis, temperature distribution in the concrete and reinforcement parts of the structural element cross-section was obtained. The thermal analysis of the wall structural element was performed for 120 minutes in standard fire exposure. Reduction factors for the strength of concrete and reinforcement steel were determined based on the temperature distribution in the wall structural element cross-section. The cross-section is divided into a number of parallel zones of equal thickness where the mean temperature and the corresponding mean compressive strength is assessed according to the Zone method procedure. The fire damaged zone of thickness az at the fire exposed sides and reduced cross-section were obtained. Static analysis of the wall load-bearing capa-city was performed using the procedure applicable for normal temperature design. The Deformation method for normal temperature design taking to account concrete deformation was used. The Deformation method for normal design taken to account concrete deformation at every step of iteration was used. The reduced load-bearing capacity of the fire damaged wall taking into account residual concrete strength was calculated and relevant load-bearing capacity diagrams were determined.

In the paper, the influence of the selected model of the soil environment on the stress-strain state (SSS) of the pile foundation is studied. The following issue sare considered: 1) analysis of the main models of the soil environment, widely used in modeling the interaction of foundations with soil foundations; 2) Numerical modeling of the stress-strain state of the "base – pile foundation" system was performed using foundation models in the form of: variable stiffness coefficients, volume tricelastic and elastic-plastic elements of the soil mass; 3) a comparison of the SSS of a pile foundation obtained by numerical modelingusing various foundation models and verification of the results by comparing with the data of a field experiment of testing a group of piles is given. This study is based on field experiments on testing full-scale piles, conducted by prof. Bartolomey A.A. and colleagues. In the experiment, a groupof 9 piles with a length of 5 m and a section of 30x30 cm was driven into the ground. The piles were combined with a reinforced concrete grillage. Numerical modeling of the stress-strain state of the system "base - pilefoundation" was carried out using the SP "Lira – SAPR 2019". It was revealed that the calculated values of longitud in alforces in piles modeled by rod elements, and the interaction with the base of the base stiffness factors simulated by variables give good convergence with the data of experimental studies. The error for all experimental fields in a wide range of loads is up to 20%. When determining the value of the variable stiffness coefficients, it is necessary to refine the miteratively more than 3 times. The disadvantage of modeling the foundation with variable stiffness factors is the difficulty in obtaining the correct values of bending moments in piles. When using a soil foundation model in the form of volumetric elastic finite elements, the error in determining the longitudinal forces in piles is up to 45%, and the use of elastic-plastic volumetric soil elements increases the accuracy of the calculation. After comparing the calculated and actual values of piles ettlement, we observe an excellent correlation of the results in the variant of the numerical model using volumetric elastic-plastic finite soil elements with the Mohr-Coulomb strength criterion. The error is within 0.8 ... 2%. The use of the model of volumetric elastic elements of the soil massif leads to an under estimation of settlement in piles within the range of up to 8%. The model using variable foundation stiffness factors also under estimates settlement in piles by up to 15%.

The work compares the stress-deformed states of the pile foundations of the house depending on the method of modeling the joints of the wall panels. The use of wall panels is due to the fact that their installation is a relatively fast technological process, but the disadvantage of such buildings is, among other things, the lack of free spatial planning [1]. During the creation of a numerical model, questions arise: what method (type of connection of panel elements to each other) should be used to model the joints of prefabricated reinforced concrete structures and how does this affect the stress-strain state in above-ground structures and foundations? This paper presents the influence of the adopted decision (chosen method of joint modeling) on the stress-strain state of pile foundations. A comparison was made of the stress-strain state of the pile foundation (piles and grid), which were obtained using the following joint modeling options: 1) reinforced concrete elements: monolithic floor, monolithic staircase-elevator shaft and prefabricated wall panels are rigidly connected to each other. 2) the joints between reinforced concrete elements are made using the principle of "combination of movements", i.e., the nodes of the finite elements of the structures are stitched and interact with each other on the basis of certain parameters: horizontal joints - only vertical movements are taken into account (combination movements in the HSC along the Z axis); vertical – take into account movement only in the horizontal plane (along the X and Y axes, in GCS); 3) joints between reinforced concrete elements are made using the functionality of PC "Sapphire". Horizontal joints take into account filling with solution (the so-called platform joint), the behavior of which is described by the elastic law of deformation. Vertical joints take into account embedded details, with the help of which elements are connected to each other in the corresponding places foreseen by the project. It is shown that the choice of modeling option for the joint of reinforced concrete structures affects the VAT not only of the foundation structures, but also of the vertical load-bearing elements of the building (wall panels and monolithic structures of the stair-elevator shaft). When using various joint modeling options, it is possible to obtain quantitative differences in forces from 2 to 20%, and the type of joint practically does not affect the deformation of foundation structures.

A comparison of the calculation results of the foundation pit enclosure made of flexible retaining walls is presented. Calculations were performed by the method of numerical modeling using Plaxis PC software, which is based on the finite element method. This task was implemented in three-dimensional (3D) and flat (2D) formulations of the problem, which provides more opportunities for a comprehensive assessment of the stress-strain state (SSS) of the elements of the "soil massif - anti-landslide structures" system when using complex configurations retaining walls. Calculations were performed within three calculation sections for different stages of construction: 1st stage – the initial faze (formation of the soil massif in its natural state), 2nd stage – excavation of the first layer of the pit, 3rd stage – excavation of the second layer of the pit. Based on the results of the calculations, the SSS analysis of the elements of the "soil massif - anti-landslide structures" system was carried out and the reinforcement of the retaining walls was selected. An assessment of the slope stability was also performed at the stage of full excavation of the foundation pit. It is shown that the advantage of using a plane FEM to assess the stress-strain state in the anti-slide structures is a much smaller amount of time spent on calculations and ease of understanding, but the disadvantage of this method is the lack of the possibility of taking into account the spatial stiffness of structures. It has been demonstrated that the use of spatial FEM allows taking into account the spatial stiffness of structures, which in the future makes it possible to more effectively design the retaining walls structures, however, modeling using this method is quite labor-intensive and requires significant resources of computer equipment for making calculations. According to the results of the calculations, the displacements obtained in the calculation using 2D modeling are on 6-43% more than using 3D modeling, the bending moments are on 12-33% more.