Investigating the effect of soil and structure interaction on the redistribution of forces in the elements of steel structures

Dissertation for Master's Degree

Civil-Structural Field

Abstract

In our country, most of the structures and maybe all the conventional buildings are analyzed and designed independently, which means that these structures are mainly analyzed and designed by conventional software considering a rigid or articulated support, and then the forces obtained at the base of the columns are used to analyze and design the foundation of the structure. The effect of foundation settlement on the distribution of forces in the structural elements is not affected. The interaction between the structure, the foundation and the underlying soil changes the actual behavior of the structure compared to the case where the structure is examined alone. In this thesis, different steel buildings are modeled together with the foundation and the soil under them, then the analysis results of the said structures are compared with the results of the analysis of the same structures that are modeled in the conventional mode to determine the degree of influence of the soil and structure on the redistribution of forces in the steel building elements. The conventional methods used in the country for the analysis and design of the mentioned buildings should be determined. Key words: interaction, soil, structure, metal structures, redistribution, power. The structure is modeled, analyzed and designed on rigid supports or supports. Then, the forces at the foot of the columns are given as input to the specific foundation design software, and the foundation is analyzed and designed independently of the structure.

In fact, despite the fact that the foundation of the structure has a flexible function in principle and undergoes general and non-uniform settlements under the forces applied by the superstructure elements, structural engineers in the analysis and design of buildings actually assume the foundation to be rigid and pay attention to the effect that the settlement of the soil under the foundation and the flexibility of the structure foundation may have on the response of the structure. They do not.

If the interaction between the structure, the foundation and its supporting soil environment changes the actual behavior of the structure significantly compared to the investigation of the behavior of the structure alone, the settlement of the foundation may create new load distribution conditions in the pavement, which will cause its elements to be damaged and cracked. By introducing the phenomenon of soil-structure interaction in the analysis of the superstructure, measures can be taken to prevent possible damage to the building. 1-2 The concept of soil-structure interaction Analysis of the interaction between deformable bodies has a prominent place both in the topics related to applied mathematics and in the topics related to engineering sciences. Apart from mathematical issues, the interaction issue is one of the important issues in the field of engineering. These issues include floating structures, foundations of structures, issues related to fatigue and cracking phenomena, layered and composite materials as well as underground structures. For example, consider the two-dimensional two-span frame that has single foundations [Figure (1-1)]. Considering that the soil under the foundation is soft and flexible, so for the pressure under the foundation of each frame column, we have:

Pressure in the middle of the consolidated layer (for example, at depth h) under the foundation of the side column:

Pressure in the middle of the consolidated layer (for example, at depth h) under the foundation of the middle column:

Since it is , the middle column settles more than the side column. As a result, the axial force of each of the side columns is increased and the axial force of the middle column is decreased. Due to the greater concentration of load on the central support, the soil under its foundation tends to settle more; In other words, the behavior of the frame shell causes the load to be transferred from the middle column to the side column due to the tendency of the central support to settle more compared to the side support. As a result, the final values ??of the force and settlement of the frame members can only be obtained correctly by analyzing the interaction of the soil-structure-foundation system. This example shows the importance of soil-structure interaction.

Soil-structure interaction has been analyzed by researchers in two general ways. A group of researchers have analyzed the soil-structure interaction from a static point of view, and another group has analyzed the category of soil-structure interaction from a dynamic aspect.A group of researchers have analyzed the soil-structure interaction from a static point of view, and another group has analyzed the category of soil-structure interaction from a dynamic aspect.]1[

1-2-1)) Static structure-soil interaction

In this case, it is assumed that the building consists of three phases: soil-foundation-superstructure, and the connection between the members of this complex through the interaction between them creates a single structural system. .

Therefore, the definition of the interaction phenomenon is useful when the additional actions, which are created by changing the foundation locations, do not change the state of the internal forces and the change of the structure locations, which is assumed to be at the level of the foundation. In general, the efficiency of the static interaction phenomenon can be estimated by the hardness ratio of the aforementioned phases. The stiffness of the system is directly controlled by the mechanical, structural and geometric properties of each phase. For example, soil can mechanically change from a nearly rigid state (rock) to a relatively soft state (clay). When the soil is soft and the structure is rigid, the interaction is beneficial. The supporting soil is modeled using one of the two general methods of Winkler and the semi-infinite elastic half-space (Boissenk). [5] In Winkler's method, it is assumed that each point of the soil surface changes its location independently. There have been many terms on this theory and in many cases the modified Winkler theory has shown that it has logical validity and can be a useful design tool. Another method, which is actually a modification of the classical Vanikler method, is the pseudo-conjugate method; In this method, instead of using the matrix module for the soil under the foundation, the module of different substrates is used in different areas of the soil under the foundation. Studies have shown that this method is more accurate than the classical Winkler method and gives favorable results. In the semi-infinite elastic half-space method, the soil under the foundation is modeled as a semi-infinite elastic body in a three-dimensional space; Where the characteristics of the soil under the foundation are very variable and the use of the elastic half-space analytical model leads to the creation of a large space compared to the actual results, numerical methods such as the finite element method are used to represent the soil under the foundation. Using this method, the soil under the foundation is modeled in a three-dimensional space as a finite element network, and for each element, the soil properties related to the element coordinates are considered; This method is not used in conventional structural analysis due to the complexity and calculations.

1-2-2) Soil-Dynamic Structure Interaction

The classic assumption is that the structures are fixed in their foundations, but during the analysis and design of the structure under dynamic loading, considering the real flexibility of the support reduces the stiffness of a structure and increases its natural cycle time, on the other hand it changes It is noticeable in the spectral acceleration with natural periodicity from the negative of the structure's response spectrum. Therefore, the change in natural periodicity significantly changes the seismic response of any structure. In addition, the soil environment underneath the structure transmits the damping property due to its inherent properties.

Studies that have been conducted in the field of increasing the natural period and the presence of high damping in the soil due to the soil-structure interaction in building structures show the important effect of the dynamic soil-structure interaction phenomenon on the natural characteristics of the structure such as stiffness and damping. On the other hand, the relationship between the vibration modes of the structure and the supporting soil of the structure is fundamentally important, regardless of the seismic response of the structure, and currently there are two methods for analyzing the seismic vibration of structures, taking into account the dynamic structure-soil interaction effect: (1) elastic half-space theory (2) concentrated coefficient method with concentrated mass. Based on a general review of articles and researches, the concentrated crime solution is more reliable and basically more general than the first method. In this method, three translation springs and three rotation springs are connected two by two perpendicularly along three axes, and three rotational degrees of freedom are assumed around the mentioned axes under the foundation of each structure. The stiffness of these springs for foundations of any arbitrary shape resting on a possible elastic half-space has been extracted in the form of impact functions in past papers and researches. Using the results of the aforementioned research, it has been observed that the stiffness of the springs will depend on the frequency of the impact function (especially if the foundation is strip and is built on saturated clay. Also, the additional damping effect applied by the soil to the entire system can be calculated in this analytical method. However, in some complex situations, the idealization of the limited components of the elastic half-space that represents the soil under the foundation can be useful.