Analytical and laboratory study of the collapse of concrete columns under the effect of axial force and cyclic lateral load

Doctoral thesis

Civil engineering – Earthquake engineering

1393

Abstract

The improper performance of columns in concrete buildings designed and built in the past years has caused concern and attention to their vulnerability to collapse. Examining the damage to structures after the earthquake and the research done during these years have caused changes in the design rules to achieve ductile behavior in the columns. Most of the efforts have been made in these years to improve the design criteria, but the effect of the axial force or the loss of the axial capacity of the columns in the existing buildings built in the past years is still unknown. Most of these columns have transverse reinforcements with large intervals, which provide little lateral resistance to the longitudinal reinforcements, as well as little confinement for the concrete in seismic loads. Many of these columns are not accepted according to today's regulations. In these columns, not only determining the shear capacity, but also the ability of the column to withstand the axial force after shear yielding is of great importance. As a result, in order to understand the seismic behavior of such structures, it is necessary to evaluate the behavior of its columns.

For this purpose, a laboratory and analytical research was conducted on non-standard concrete columns. Six examples of concrete columns with a scale of ½ It was prepared in order to evaluate their seismic behavior. The important parameters in this test were the amount of axial force, the percentage of transverse reinforcement and the compressive strength of concrete. The samples were subjected to constant axial load and quasi-static cyclic lateral load until gravitational collapse. The obtained laboratory results are the hysteresis response, the cracking pattern, the amount of drift corresponding to the axial destruction and the amount of cumulative absorbed energy.

Comparison of the response of these samples with ASCE 41-07 and FEMA 356 regulations shows that both of these regulations did not correctly predict the failure mode of the samples. Also, the values ??obtained by these regulations for the initial stiffness and the amount of drift corresponding to the axial destruction were lower than the values ??obtained from the experiment. Next, with an analytical approach and the use of the deformation caused by cutting and bending, an attempt was made to obtain the effective stiffness of the column. Using a parametric study, the effect of different parameters on the effective hardness was investigated. And finally, a simple relationship was presented to calculate the effective stiffness of columns. Then, the efficiency and correctness of the proposed method approach was validated using the data obtained from this research and also the results of studies by other researchers, which showed that the accuracy of the presented relationship is better than other relationships available in the technical literature. Finally, a model was presented to estimate the amount of displacement corresponding to gravitational collapse. This model is based on balance and consistency and an iterative solution method, and according to it, the amount of rotational displacement caused by the opening of a critical shear crack can be determined. By simplifying it, a model was finally presented to estimate the corresponding drift of gravitational collapse. The accuracy of this model was shown by comparing with other models. Also, using this model, the amount of drift corresponding to gravitational collapse was calculated for a number of columns available in the technical literature and compared with its laboratory value. These investigations show the correctness and accuracy of this method in estimating the amount of drift corresponding to axial destruction.

Key words

Concrete column, shear yielding, gravitational collapse, effective stiffness, parametric study, rotational displacement, critical shear crack Chapter 1

1- General

1-1- Introduction

Inappropriate performance of columns in buildings Concrete designed and built in the past years has caused concern and attention to their vulnerability to collapse. Examining the damages caused to the structures after various earthquakes and the research conducted during these years have caused changes in the design rules to achieve the ductile behavior in the columns. Most of the efforts have been made in these years to improve the design criteria, but the effect of the axial force or the loss of the axial capacity of the columns in the existing buildings built in the past years is still unknown.Most of these columns have transverse reinforcements with large intervals, which provide little lateral resistance to the longitudinal reinforcements, as well as little confinement for the concrete in seismic loads. Many of these columns are not accepted according to today's regulations. In these columns, not only determining the shear capacity, but also the ability of the column to withstand the axial force after shear yielding is of great importance. The failure of such columns is due to shear deformation, which leads to shear destruction and then axial destruction. Figure 1-1 - Column shear destruction - 1999 Izmit earthquake in Turkey [1] Many laboratory studies have been conducted on ductile columns in the past years, and as a result, the seismic behavior of such columns has been widely known. While the research done on columns with non-seismic design is very limited. Also, most of the tests performed on the columns are stopped shortly after the reduction of the lateral resistance, and the number of tests performed until the gravitational collapse is very small. As a result, there is very little information about the failure and collapse mechanism of these columns.

In order to better understand the seismic behavior of concrete columns with details and non-seismic design, laboratory and analytical research was conducted at the International Institute of Seismology and Earthquake Engineering. The results of this research in the correct understanding of the mechanism of destruction and collapse of this species. Columns are very helpful.

1-2- Objectives and scope of studies

In this report, the behavior of concrete columns with transverse reinforcement with long distance and non-seismic details is investigated. We call these types of columns abbreviated as non-deformable columns. This report includes the following analytical and laboratory components: 1- Presenting the laboratory results of six concrete column samples until gravity collapse and comparing the results with the evaluation instructions. Presenting a simple model in order to estimate the amount of displacement corresponding to the axial destruction of non-deformable concrete columns. In this review, an attempt is made to collect the analytical and laboratory studies available in the technical literature. These studies are related to the testing of concrete columns until the time of gravitational collapse and the behavior models of the columns until this limit state. Then, according to the collected information and their analysis, a number of columns will be designed and built for the purpose of laboratory study. These columns are designed to represent the condition of columns in old concrete buildings. In other words, the amount of transverse reinforcement and the details of their reinforcement do not meet the standards of today's rituals. As a result, the amount of ductility of these columns will be limited and according to ASCE 41 [2], the failure mode of these columns is expected to be bending and shearing. Then, the designed columns are modeled using Diana finite element software. The purpose of this work will be to examine the behavior of the samples and their responses more precisely. The results of this modeling will be used before making the samples and after performing the tests on the samples to compare the results. Finally, using the results obtained from these laboratory studies and also with the help of the results of other studies,  Analytical investigations will be done. The purpose of the analytical studies will be to find more accurate relationships for the effective stiffness of the columns and also the amount of drift corresponding to the gravitational collapse.

1-4- Different chapters of the report

This report consists of 8 chapters and begins with the first chapter on generalities. In the second chapter, the review of past researches is discussed. The third chapter deals with the discussion of concrete column samples and their loading. The test results are reviewed in the fourth chapter. Chapter five discusses and compares the test results of the samples. The sixth chapter is dedicated to examining and presenting a method to obtain the effective stiffness of columns. In the seventh chapter, a simple model is presented in order to estimate the amount of drift and displacement of the column during axial destruction. Finally, in the eighth chapter, it summarizes and concludes and presents suggestions to continue the work.