Master's Thesis in Civil Engineering - Structural Orientation
February 2013
Abstract:
The performance of a building during an earthquake depends on many factors, as a result, the prediction of the seismic performance of structures should be considered either explicitly or implicitly as part of the design or evaluation. Predicting the seismic response of the structure is very complicated, which is not only due to the large number of factors involved in the performance, but also due to the complexity of the physical behaviors. In addition, due to the lack of accuracy to accurately simulate the physical behavior, as well as the lack of knowledge in providing an accurate definition of the structural characteristics and the changing nature of future earthquakes, the prediction of seismic performance itself is subject to significant uncertainties. These inherent uncertainties in predicting possible future loadings and responses are not only specific to seismic behavior, and many of these issues are more or less mentioned in the current regulations through the use of load and resistance factors. In earthquake engineering based on performance, it is necessary to determine the capacity and seismic demand of the structure. According to the recent advances in the field of computer analysis, today it is possible to use nonlinear dynamic analysis to achieve this goal. In this method, the response of the structure is determined by considering the non-linear behavior of the materials and the non-linear geometric behavior of the structure under the effect of a specific earthquake. In recent years, the design method based on capacity and demand has attracted the attention of many researchers, which due to its ability to be expressed in the form of probabilities can be used to determine the level of confidence and improve the performance of structures.
The purpose of this research is to compare the level of confidence of convergent bracing steel frames designed with the criteria of the 10th topic (years 1384 and 1387). According to the past research on the level of confidence for structural performance analysis, at first, the design of the studied frames (5, 8 and 11) was done with Etabs software and the incremental nonlinear dynamic analysis (IDA) was performed on the designed sections in OpenSees software, and then it was evaluated using the instructions of FEMA351 regulations. The obtained results show that the confidence level in steel coaxial braced systems with the criteria of the 10th topic of 1384 is more than the steel coaxial braced systems with the criteria of the 10th topic of 1387.
Key words: structure performance - confidence level - convergent bracing - increasing nonlinear dynamic analysis
Chapter 1
Introduction
1-1- Introduction
Today, the convergent bracing system is the most common structural system to deal with seismic loads in steel constructions, and its use is becoming more popular day by day due to its economic efficiency, easy design and implementation. The desire of engineers to use this system has increased dramatically around the world after the Northridge earthquake (1994) and the unexpected damage caused to steel bending frames. Vibration design criteria of convergent braced frames have changed a lot in the last decade. Pre-UBC (1994) building codes treated convergent braced frames as elastic trusses. In these regulations, an attempt is made to prevent the brace from buckling by limiting its slenderness and reducing the compressive strength of the brace. As a result, the structures that were designed using these regulations had limited plasticity. [1]
In recent years, the method of "earthquake engineering based on performance level" has been greatly developed and great progress has been made in earthquake risk analysis, earthquake behavior simulation and seismic performance evaluation of structures. Therefore, considering the wide application of convergent bracing system in constructions and many concerns about the performance of this system. It seems necessary to evaluate the vibration performance of the convergent bracing system using modern methods. 1-2-Statement of the problem and the need to examine the issue. In the 10th topic of 2014, the criteria for the design of wind braces are according to 1997 UBC [2], in which the allowable compressive stress is modified based on the thinness with the coefficient B. While this coefficient is in the 10th topic of 2017. It does not exist and the effect of thinning has not been included in the seismic parameters of wind braces.Also, the combination of intensified (special) loads in the two versions of 1384 and 1387 of the 10th topic is different, which has an effect on the jointing mechanism of the columns around the brace as well as the arrangement of the joints [3, 4].
In these regulations, it is tried to prevent buckling in the back-and-forth cycles of the earthquake by limiting the slenderness and reducing the allowable compressive stress of the brace, as a result of this The structures had limited plasticity, which was not observed in the seismic regulations of the new regulations and caused an increase in the allowable compressive stress of the braces and an effect on the articulation mechanism of the braced columns, which seems that the confidence level in the first case is higher than the second case, which requires research. The reliability of convergent bracing steel frames designed with the criteria of the 10th topic has been tried by determining the performance of the structure according to the criteria of the 6th topic of the National Building Regulations (loads on the building) [5], the 10th topic of the National Building Regulations (design of steel buildings) of the years 84 and 87 [3, 4], Incremental Dynamic Design and Analysis (IDA) [6] and FEMA 351 regulations [7] and analyzed and according to their criteria, the edition that It has a higher level of confidence in the seismic evaluation of the aforementioned wind brace.
1-4- Research assumptions
The assumptions related to this research can be divided into two parts: assumptions related to the modeling and behavior of the structure and the part related to the considered uncertainty model, so these assumptions can be expressed as follows:
Loading is considered one-way.
The building is considered with a regular plan and restraint.
The connection of beam and column in all frames is detailed.
Dynamic analysis performed is of incremental nonlinear type (IDA).
Nonlinear analysis of the frames has been done in two dimensions.
The response of the structure is determined by considering the nonlinear behavior of the materials and the geometric nonlinear behavior under the effect of a specific earthquake.
The strain hardening in these models is considered equal to 3%[8].
1-5- The research method
This research has been conducted based on the following steps using the incremental dynamic analysis method to obtain the performance level and evaluate the confidence level.
The investigated building is considered for residential use, in an area with high seismic risk and on type II soil. In this research, gravity loading is carried out according to the sixth topic of the National Building Regulations and seismic loading is carried out according to the 2800 bylaws.
Design of three frames with three spans, five, eight and eleven stories with a height of 2.3 floors and a span length of 5 meters, based on the criteria of the tenth topic of 1384 and 1387 using Etabs [9] software.
Extraction of frames Two-dimensional (based on criticality).
Analysis of frames based on incremental nonlinear dynamic analysis using OpenSees [10] software.
Calculation of confidence level according to the functional concept of the structure based on Fema351 guidelines and IDA curve.
1-6- Organization of the thesis content
This thesis is organized in six chapters:
In the first chapter, the introduction is stated.
In the second chapter, the convergent bracing systems (with cross bracing) and the regulations of this type of system are examined.
In the third chapter, the design concepts based on the performance level and dynamic analysis are increasingly described.
In the fourth chapter, the uncertainty and reliability level of the structures are examined.
In the fifth chapter, the modeling and comparison of the reliability level of the convergent bracing steel frames designed with the criteria of the 10th topic (years 1384 and 1387) are examined.