Investigating the coefficient of behavior of steel structures with a dual system of bending frame and knee brace

Dissertation for Master's Degree

Civil Engineering – Structure trends

Spring 2014

Chapter One

Generalities of the research

1-1- Introduction:

Mostly, structures are designed for severe earthquakes and accepting levels of damage, and a control in terms of the elastic behavior of structures in the range of moderate earthquakes that have a high probability of occurring annually. They may not be specified. It means that there is no estimate for the elastic behavior of structures in such a case. When severe earthquakes occur, the structure enters the non-linear range, and as a result, a non-linear design is needed for its design, but due to the complexity of the non-linear analysis, it is also time-consuming and costly, and the non-linear programs are not extensive compared to the linear analysis.

One of the important and fundamental parameters in the seismic design of structures is the behavior coefficient. The value of this coefficient has been determined in some regulations from the results of the experiments, and the occurrence of an earthquake is the best laboratory for investigating the behavior of structures. In order to consider the non-linear behavior of the structure with a linear analysis and to determine the amount of energy loss due to Hyster-Zeiss behavior, damping, the effect of added resistance of the structure and the malleability of the structure, a coefficient called the behavior modification coefficient or the behavior coefficient is used. stay within the linear range and without damage. In moderate earthquakes, they will see non-structural damage and during severe and large earthquakes, they will have structural and non-structural damage, but their overall stability will be maintained. It has become bigger and this will make the project uneconomical. Therefore, by considering the non-linear behavior of the structure, it is possible to take advantage of the energy absorption characteristics of the structure and the deformation of its paste and help to make the design economical. It is possible to take advantage of these characteristics of the non-linear behavior of the structure if the structure can tolerate the deformation of the dough. In other words, in the seismic design of the structure, it should be able to dissipate a major part of the incoming energy through inelastic deformations. In order to have a reasonable value for the non-elastic resistance of structures, the correctness of the reduction in elastic resistance is essential.

Regulations   [1] UBC97, by analyzing structures based on their adequacy, considers the effects of non-linear response of the building, additional resistances and formability of different elements. According to the above criteria, the main attention against earthquakes is focused on lateral safety, that is, preventing the destruction of the structure under the effect of the most severe earthquake that is possible during the useful life of the structure. Therefore, the structure that is designed based on such a philosophy (seismic design) enters the non-linear range under the strong forces of the earthquake. As a result, the design of structures for linear behavior under vibrations caused by large earthquakes is basically not economical. Therefore, the structures are designed for a much smaller shear force than the yielding shear force. Reducing the elastic resistance of structures must be done carefully. The manner and amount of this reduction in resistance can be very effective in performing the desired behavior in the structure, therefore, identifying the parameters involved in this field and estimating their relative importance in providing the correct amount of elastic resistance reduction is a very important and necessary category in the design of structures. The reason for this is considering the issues and costs of construction against the risk of an earthquake during the useful life of the structure. Therefore, there will always be an expectation of non-linear behavior for the structure, i.e. the behavior of the structure in deformation beyond the elastic limit caused by forces beyond the elastic limit.Also, the experience of the impact of earthquakes on structures shows that structures behave non-linearly during earthquakes and therefore waste a significant amount of incoming energy in the form of damping and residual. Therefore, the structure can be designed for an earthquake force much lower than the required force in the linear mode.

The behavior coefficient used in the NEHRP, UBC regulations [2] is a constant coefficient that expresses the effect of ductility and extra strength of any structural system. In the interpretation of the regulations,‌ It considers it necessary to apply design engineering judgment in its use. Here, the question arises, what is the basis of engineering judgment based on, and on what principles should the designer consider the value of this coefficient? In this case, there is no article in the ritual. The letters are not mentioned and this itself expresses the complexity of this coefficient. Therefore, obtaining this coefficient for each different structural system is a time-consuming and complex matter for design engineers.

Certainly, only in a non-linear analysis, it is possible to determine the location of the joints by paying attention to the paste behavior of the structures and investigating issues such as strength and formability, and thus determine the weak points of the structures. In order to consider factors such as the malleability of different structural systems and uncertain degrees, additional resistance in structures as well as the ability to absorb and consume energy in the building, various regulations reduce the calculated forces according to the type of structural system and with the help of a coefficient called the behavior coefficient. In this research, the seismic behavior of the windbreak system The knee is examined as a structural system. In this system, at least one end of the diagonal brace is connected to the knee member that is obliquely placed between the beam and the column, instead of being connected to the place where the beam meets the column. The diagonal member provides the stiffness of the system, while the formability under the effect of severe lateral loads is obtained through the flow of the knee member, and the knee member acts as a malleable and replaceable member and prevents the buckling of the diagonal member. Various earthquakes, researchers experimentally and quantitatively and based on their studies, estimated the basic shear force on structures during earthquakes as a fraction of the structure's weight. At that time, for the seismic design of the building, a percentage of the weight of the building was applied to the building as a horizontal load equivalent to the earthquake load, and the building was designed for it, and the total design shear force was obtained as  V=CW.

For the first time in 1933 in Los Angeles, the regulations It was approved that the basic shear factor was 0.1 for special structures and 0.08 for normal structures. In 1943, a relationship was considered for C in which the number of effective floors was determined as a result of the progress of the science of dynamics of structures by examining the effect of the flexibility of structures and the period of the structure.

The idea of ??using limit design for earthquake resistant design of structures was first proposed in 1956 by Hanser[3], in which plastic deformation is used to dissipate the energy entering the structure. 1961 was published in the form of design instructions in one of the PCA publications regarding concrete structures, which became the source of subsequent laboratory and research works in this field. But what is clear is that the first system-dependent behavior coefficient is provided by SEAOC1959 [4] in the form of K coefficient.

The latter concept was used until K gave its place to R behavior coefficient. Everything about R started with the completion of the American Applied Technology Association project (ATC 3-06, 1978). During the early stages of ATC3 [5], proposals were made to create a method of seismic design of structures at two levels of limit design and operational design, which expresses the basic concept and purpose of seismic design of structures.

It should be noted that about three decades ago, no analytical or numerical method for calculating the coefficient of behavior of buildings, which is a symbol of a physical reality, was presented.