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Design of Soft Story in Reinforced Concrete Structures Due to Removal of Masonry Infill Walls

Hosseini Gelekolai, Mojtaba | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40967 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Golafshani, Ali Akbar; Tabeshpour, Mohammad Reza
  7. Abstract:
  8. In past few decades researchers and engineers have researched extensively on the effects of masonry infill walls in structural frames and the effects of masonry infill walls on the structural response was a concern for them. Since used materials and therefore the strength and stiffness of these walls are extremely depends on construction site and available mate-rials, according to geographical conditions most of countries inserted their own rules for considering effects of masonry infill walls in their codes and standards. Our country, Iran was not excepted from this regularity and in past few years the effect of existence of ma-sonry infill walls in structural response has been considered in some sections of 2800 code. One of the sections that existence of masonry infill walls in buildings affects it is the sec-tion 1-5-6 of 2800 standard that accordance to this section, if nonstructural members affect the motion of structural members during earthquake, effects of these members on the anal-ysis of structure must be included. Section 2-10 of 2800 codes would be influenced by not satisfying this section. Accordance to section 2-10, in order to prevent soft story failure when there is a discontinuity in bracing or shear walls in a certain story, columns of that story must be designed for factored loads. Even though in the contest of 2800 code there is no notification to necessity of increasing the design loads in existence of discontinuity of masonry infill walls, based on experiences of structural response in past earthquakes even in the case of existence of discontinuity in masonry infill walls the columns of the story must be designed for factored loads. The question is that in this case how much the factor is? The main goal of this research is to investigate the appearance of soft story failure in first story in reinforced concrete structures with moment frame system and different ar-rangements of masonry infill walls in upper stories (in different bays and different thick-ness) and also suggesting a factor to strengthen the first story columns in order to prevent the soft story failure. We call this factor, the overstrength factor which is the 2.8 factor in section 2-10 of 2800 code and is equal to 2.8 for all cases while it should be calculated ex-actly so that not results in overdesigning or under designing of structures. If column of soft story be designed basis of this factor the existence capacity will be used optimally and also we will not have terrible events like soft story failures in future earthquakes. In this re-search the overstrength factor in different cases obtained by using nonlinear static analysis (pushover) and a formula for predicting this factor in the design procedure accordance to percentage of masonry infill walls in the plan of the building is peresnted. Then this formu-la was checked by nonlinear dynamic analysis (time history and incremental) and the results showed that by using this factor the behavior of the building improve and the soft story failure did not happen anymore. Also the effects of masonry infill walls on main period of structures were investigated in this research. The results show that if masonry infill walls do not be considered in the design of the structure, the computing of main period of structure and also the lateral load of earthquake in equivalent static procedure could be different from the precise one. Then the effects of masonry infill walls in the modal analysis were investigated and the results show that if we do not consider the masonry infill walls in our modeling, the modal effective masses of structural vibration would be different from the real ones. With the existence of masonry infill walls in upper stories of the structure, it vibrates more affected by its first vibration mode
  9. Keywords:
  10. Soft Story Design ; Masonry Infilled Walls ; Overstrength Column Factor ; Intermediate Concrete Moment Frame

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