نشریه سازه و فولاد
سال هفدهم شماره 40 (تابستان 1402)

Soil type of building site is one the most effective parameters on the buildings seismic behavior. Site effect, which occurs mostly is softer soils fortifies the influence of earthquakes and apply greater seismic forces to the buildings. Therefore, the buildings in soft soils should be designed for greater lateral seismic forces, based on the codes such as Iranian Standard 2800. In the present research, performance of viscous dampers in buildings is investigated in all soil types from very stiff, type I to very soft soil, type IV. The main purpose of this paper is to find where viscous dampers have the most performance regarding different soil types. For this, some building frames having 5, 10 or 15 stories are designed on different soil types. It is assumed that the buildings have steel moment resisting structures. Each building on every soil type is designed twice: with and without viscous dampers, and their seismic behaviors are calculated under some earthquake records and compared. ASCE7-16 is based for designing the buildings having viscous dampers. The assumed buildings were analyzed for earthquake acceleration records of the corresponding soil type, with return period of 475 years, having PGA of 0.35g.. Nonlinear time history analyses of the buildings under 24 far-field records and 10 near-field records show that: regular buildings, without viscous dampers, on softer soils are more vulnerable, and equipping them with viscous dampers is a good method to protect them in earthquakes and decreasing their responses and damages. Viscouse dampers are more effective in buildings on softer soils. In summary, viscous dampers are always effective in decreasing the structural seismic responses and damages, and they are more effective in buildings on soft soils.

In a core-wall structure with buckling restrained braces (BRB) outrigger, locations of the plastic hinges are influenced by the outrigger action. Therefore, the designer should consider the issue and use suitable details in the plastic hinge area. The essential questions that arise here are the plastic hinge location and the design moment demand used for design of this kind of structure. In this paper, responses of the core-wall buildings with BRB outrigger designed by using the traditional response spectrum analysis (RSA) procedure, are assessed by implementing the nonlinear time history analysis (NLTHA). The result demonstrates that the plasticity can extend over anywhere within the core-walls specially, at the base, and above or below the outrigger levels. Formation of three plastic hinges in the core-wall is recognized suitable for the system.  To control the plasticity extension in the core-wall, it is recommended that a new modal combination method be applied to calculate the moment strength of the three plastic hinges over the height. A capacity design concept is used to design other regions of the core-wall where the plasticity does not extend to. The proposed procedure improves behavior of the system by restricting the plasticity extension to the predefined plastic hinge regions.

Various bracing systems can be used to withstand lateral displacements caused by earthquakes and wind. One of these systems is Buckling Restrained Braces (BRBs), which consist of a steel core known as a "core plate" and a buckling restraining mechanism (BRM) made of full steel, concrete-steel, or other materials. This study presents a novel method to enhance the performance of all-steel BRBs. In this new method, a lubricated perforated steel plate with roller bearings (small solid steel cylinders) is attached to both sides of the core plate within the BRM. During plastic elongation and contraction of the core plate due to tension or compression loads, these movements occur on the roller bearings, which are housed in a grease-filled space. As a result, the friction between the core plate and the BRM is reduced, preventing the transmission of axial forces to the BRM. Additionally, the reciprocating movement of the core plate inside the BRM is smoothed, eliminating the risk of premature core plate failure. Finite element analysis has been conducted to analyze various types of all-steel BRBs with and without the new method, demonstrating the advantages of the proposed approach. The inclusion of roller bearings in the samples reduces frictional forces on the sheath by at least 45% and up to 63% compared to similar samples without roller bearings.

The present paper aimed to evaluate the effects of soil-structure interaction (SSI) on the seismic behavior of base-isolated geometrically irregular steel sliding structures equipped with lead rubber bearing (LRB) isolators.To this aim, the behavior of the irregular system under nonlinear time-history dynamic analysis was assessed using numerical modeling based on the finite element method in ABAQUS. The study considered various factors including irregularity, number of structure floors, soil properties, isolation, and foundation stiffness to analyze their impact on the response of the structures. 3-, 5-, and 7-story structures with 20% and 40% irregularity were modeled to investigate the effects of irregularity and the number of floors. Additionally, models of five-story structures were used to study the impacts of soil-structure interaction (SSI) on three different types of soil: soft, medium, and hard. The results of the study demonstrated that the number of floors and the structure's irregularity have a significant impact on the seismic response of the structure in terms of displacement and acceleration. As the number of floors and irregularity increase, the maximum displacement response of the structure's floors decreases. The study also highlighted the substantial influence of soil type on the seismic response of isolated structures when considering the effects of SSI. Furthermore, the findings indicated that the presence of isolators on rigid foundations had a negligible effect on the dynamic response of the structures, compared to structures without isolators.

Concrete-filled columns, if properly designed, can withstand fire for a duration of 2 hours or more. The Iranian National Building Code, Part 10, provides guidelines for calculating the fire resistance time of individual CFT columns. However, the evaluation of fire performance for CFT columns should consider the overall structure and factors such as beam moments and column effective length. The absence of specific methods to assess the fire performance of CFT columns in real structures limits their application in common projects. This study introduces a systematic approach to evaluate the fire performance of CFT columns in a structure. For this purpose, 10-story buildings in Tehran were designed with different lateral load-resisting systems: 1- special moment frame, 2- special shear wall, and 3- dual system. The buildings were designed with varying story heights ranging from 3 to 4 meters and two types of concrete aggregates: 1- carbonate and 2- silica. The results indicate that, for a story height of 3.2 meters and using silica aggregate, approximately 7%, 19%, and 48% of the columns in the dual system, shear wall system, and moment frame system respectively, failed during the fire. The analysis revealed that an increase in story heights had a significant impact on the fire behavior of moment frames, while it had no notable effect on other systems. Sensitivity analyses demonstrated that a reduction factor of 0.1 for the rotational stiffness of the columns in structural analysis is appropriate, as it considers moment redistribution and ensures model convergence.

The linked column frame (LCF) system is a new steel lateral load resisting system that is developed with the aim of creating the capability of quick and simple repair of buildings after earthquakes. The LCF system is a combination of a primary linked column (LC) system and a secondary moment frame (MF) system that together resist lateral loads. On the other hand, more recent studies have shown that the linked column (LC) system individually has the ability to resist lateral loads and provides sufficient seismic capacity. Therefore, due to the importance of presenting the seismic performance factors of response modification coefficient (R), over strength factor (Ω0 ) and deflection amplification factor (Cd ) in seismic codes and the need for these factors for the seismic analysis and structural design using a linear analysis method, in this research the quantification of the values of these seismic factors for the linked column system (LCS) is discussed. Moreover, a comparison of the value of materials used in the skeleton of models designed with different structural systems is made, which examines the weight of elements in each system separately, and it compares the LCF and LCS system with other common structural systems in terms of steel used in the structure. The results show that the response modification coefficient of the linked column system (LCS) is equal to 8, similar to the linked column frame (LCF) and moment resisting frame (MRF) systems. But the over strength factor of the LCS system is slightly lower than the LCF and MRF systems, which is equal to 2.7. Also, the deflection amplification factor of the LCF system is the same as the moment resisting frame system, but in the design of the LCS system, using a linear analysis, a larger interstory drift can be allowed in the upper half of the building, because for the design of linked column (LC), the rotation of links is usually a critical criterion for design.