Seismic improvement of reinforced concrete frames filled with building materials using fiber reinforced polymers

Dissertation for M.Sc degree

Structural

Abstract

In today's reinforced concrete buildings, it is very common to use interframe separators. Intermediate frames with building materials are the main types of separators that are used in this type of buildings. In previous research, these interframe separators are usually considered as non-structural elements. But recent research in this field has shown that intermediate frames have a significant effect on the natural period, stiffness, resistance and overall behavior of the structure, especially against earthquake loads. Considering the poor performance of intermediate frames of building materials in recent earthquakes in terms of ductility and resistance, various solutions have been proposed to strengthen and strengthen it. In this thesis, carbon fiber reinforced polymers (CFRP) have been used to strengthen the intermediate frame. The one-span and two-story frame is related to a laboratory sample, in which the frames are filled with intermediate frames of building materials. And different arrangements of CFRP layers have been used to strengthen it. For the analysis of the samples, the finite element method has been used using the ABAQUS finite element software, and the type of analysis is explicit dynamic. Loading is a cyclic type with an incremental approach. A number of 7 samples with different CFRP arrangements have been modeled and analyzed in the software, and at the end, the finite element analysis results have been compared with the laboratory results, and suggestions for retrofitting with the approach of affordability and acceptable ductility have been presented. 

Key words: concrete frame, CFRP, finite element analysis, intermediate frame of building materials.

Chapter 1: General

1-1- Introduction

Earthquakes have been identified as the most destructive natural hazard for a long time. No other natural force is capable of such great damage in such a short period of time. Earthquakes occur without prior warning and leave many casualties and damages in just a few seconds. Although it is not possible to prevent earthquakes, new technology in science and engineering provides new tools to reduce its destructive effects. The main danger for the safety and life of people is the damage caused by earthquakes and the collapse of buildings and other buildings that have weaknesses in design or construction. After earthquakes, in addition to loss of lives, national wealth is also wasted and a large financial burden is created on the economy of countries, which has serious and long-term effects on countries with fragile economies.

The common type of buildings in urban centers is unreinforced masonry walls [1] that fill the space between the frames of the structures. For this reason, this type of wall is called intermediate frame [2] [1]. Usually, the term intermediate frame [3] is used when the frame is built first and then the intermediate frame is implemented [2]. Although the intermediate frames are considered as non-structural components, but under vibration stimulation, there is an interaction between the walls of the intermediate frame and the enclosing frame and it leads to the creation of undesirable failure modes in the frame and intermediate frame. Generally, intermediate frames have shown poor performance in moderate earthquakes. Their behavior is usually brittle and they have little or no ductility and they suffer various forms of damage such as invisible cracking, corrosion and finally total destruction. This behavior is the cause of many dangers during an earthquake and this weakness in the performance of earthquakes is a big challenge for designers.  Improving vibrations by adding frames or shear walls is impractical and very expensive, and in some buildings, it faces special limitations. Other strengthening methods such as grout injection, reinforcing steel installation, pre-tensioning, jacketing and different surface strengthening methods cause a significant increase in the mass and stiffness of the structure and as a result impose higher seismic loads on the structure. These methods require skilled labor and disrupt the normal functioning of the building. These methods are called "classic methods" of retrofitting. One of the new methods that has attracted the attention of craftsmen in recent years is the strengthening of existing buildings using composites.. A lot of research has been done in this field and preliminary regulations have been prepared for their use. Composites were first used for military applications and aerospace industries, but with the decrease in price, these materials became the attention of craftsmen and industrialists in many industries due to their characteristics such as low weight and very high tensile strength, resistance to atmospheric conditions, etc. The use of fiber-reinforced polymers[4] is a valid alternative strengthening method due to its low thickness, high strength-to-weight ratio, high hardness, and easy application.

Powerful earthquakes cause large internal and external forces to be applied to masonry walls and provide the possibility of catastrophic destruction in these structures. However, most of the actions in this field have been focused on the out-of-plane behavior of masonry walls reinforced with fiber-reinforced polymers. The intermediate frame wall or a part of it may be pushed out of the surrounding frame due to the lack of sufficient outer plates between the interface between the frame and the intermediate frame, or due to shear or bending failure of the intermediate frame wall. In undamaged midframes, this type of failure can be attributed to inertial forces, especially for midframes of higher floors and large slenderness ratio. After the building materials are separated from the frame, there is a possibility of failure of external plates[1]. One of the goals of this research is to investigate the effect of fiber-reinforced polymer layers in changing failure modes, resistance, deformation, and energy lost by the structure in different layer arrangements. Another goal is to investigate the improvement of the shear and compressive strength of the intermediate frame reinforced with fiber-reinforced polymer. Reinforcement with fiber-reinforced polymer preserves the integrity of the intermediate frame wall and prevents it from brittle failure and crushing, and considering that this type of crushing is a great risk for the residents despite the safety of the entire structure, it is very important to prevent it. increases significantly. Of course, the frame must be sufficient to transform the wall into a middle frame, the conditions of which are mentioned in FIMA 356 [5] [4] and the guidelines for improving buildings against earthquakes [5]. The frame in which the intermediate frame is weakly executed slips from the bed during lateral loading, while in the weak frame with a strong intermediate frame, diagonal cracks and shear failure of the loading column are usually observed. And when the frame and intermediate frame are both strong, the ultimate resistance is associated with corner failure. 1-2-1-Interaction between frame and intermediate frame According to the observations of recent earthquakes, the interaction between intermediate frame and concrete columns causes brittle rupture. The existence of the intermediate frame inside the concrete frame is very important and has a decisive effect on the behavior of concrete structures during earthquakes. In recent earthquakes, significant damage occurred due to the interaction phenomenon between frame and intermediate frame. Smith and Cole [6] presented a design method for intermediate frame based on diagonal braced frame criteria. They proposed a method in which three possible failure modes for the intermediate wall were considered: shear along the wall, diagonal crushing of the intermediate wall, and corner crushing in the intermediate wall. Pauli and Priestley [7] presented a theory about the vibration behavior of the intermediate frame and proposed a method for its design. According to this theory, although the intermediate frame may increase the overall lateral bearing capacity of the structure, it causes a change in the response of the structures and absorbs the force to other and undesirable parts of the structure and asymmetrically. This means that the interframe of building materials may affect the seismic behavior of the structure. Bell and Davidson [8] presented a report on the evaluation of reinforced concrete buildings with intermediate frames of masonry materials. In their assessment, they used an equivalent handle to model the masonry wall. Their results showed that intermediate frames have a significant beneficial effect on the behavior of reinforced concrete buildings if they are placed in a regular order in the building, which was contrary to the strategic regulations of New Zealand, which believed that intermediate frames have a harmful effect on buildings due to their interaction effect. Mohiuddin-Kermani et al. [9] specifically focused on the observations made on concrete buildings with intermediate frames of building materials in the Sichuan earthquake [6] and identified damage and rupture modes with their causes.  These failure modes, like the previous earthquakes, are caused by the interaction between the frame and the intermediate frame.