Inelastic analysis of steel buildings with gantry brace system under the influence of earthquake force and investigation of the geometry of gantry brace system

Dissertation for M.Sc degree

Civil Engineering, Structural Orientation

Abstract:

One of the ways to strengthen the building against lateral loads is to use the wind system (convergent or divergent). The increasing use of steel braces to deal with earthquake forces requires that the seismic performance of these types of systems be given more attention. The common form of converging windbreaks causes many problems in providing space for building openings. In order to provide enough space for the openings in the building and to solve the existing problem, the architects have started inventing a new type of braces that are similar to the 7 or 8 (chevron) braces, with the difference that in order to provide space for the openings, its members are not straight and are connected from a point with different slopes and at the other end, they are connected to the joint of the beam and the column. One of the most common types of braces in recent years is gate braces. There is no discussion about these braces in the 2800 standard, but their use is increasing and most designers assume this type of braces as concentric braces.

In this thesis, by placing gate braces and concentric cross and off-axis braces in different two-dimensional steel building frames, nonlinear static and dynamic analyzes have been performed and their performance has been compared. With the help of analytical results, the performance of the frames, including pushover and hysteresis curves, ductility coefficient, initial hardness, additional resistance coefficient of frames, energy absorption and the way plastic joints are formed have been investigated. The results show that the behavior of the gate windlass largely depends on the position of the middle node. Its hardness is lower than the cross brace and its ductility is lower than the EBF brace. Of course, many engineers do not pay attention to the problem of its instability outside of the plane and the joint leaving the frame plane during an earthquake due to the two-dimensional concept of this type of bracing, which causes irreparable damage during an earthquake. Therefore, the need to investigate the issue of buckling and stability in these wind braces is determined, which is also addressed in this treatise. Keywords: inelastic analysis, steel frames, gate brace, earthquake force. Symptoms: A. Acceleration based on design. B. Building reflection coefficient. C. Earthquake coefficient. Cd coefficient.

Key words: inelastic analysis, steel frames, gate brace, earthquake force

Symptoms

A acceleration based on design

B building reflection coefficient

C earthquake coefficient

Cd plastic deformation coefficient

Ci stiffness coefficient

E elastic coefficient

e eccentricity

ei eccentricity of the middle node of the gate brace

Fi lateral force acting on floor i

g acceleration of gravity

hi height of floor i from the base level

I factor of importance of the building

Ix surface moment of inertia relative to the x axis

Iout moment of inertia out of plane

Iin moment In-plane inertia

K coefficient of effective length

Ke with initial slope of effective elastic stiffness

Lc restrained length

Li length of member i

m mass

Molangor force relative to point O

MP  plastic anchor capacity of the cross section of the beam connection

Pi axial force

R behavior coefficient For limit state design

Rw design behavior factor by allowable stress method

Pcr critical buckling load

Pe Euler load

Pei Euler load of the i-th element

T rotation time of the structure in the desired direction

T0 a number determined by the type of soil

Ts a number determined by the type of soil

ti stability function Unsheared column with hinged ends

V base shear caused by earthquake excitation

Vy overall yield limit of the structure

Vs force corresponding to the formation of the first hinge hinge in the structure

VP plastic shear capacity of the cross-section of the connecting beam

Vw force within the limit of allowable stresses

Vu final shear of the structure base in elastic state

w weight

Wi weight of floor i

Y allowable stress factor

? degree of reduction of stiffness

? structural plasticity capacity

Angular frequency of the structure

Driving force frequency

Frequency with depreciation

Critical depreciation coefficient

? additional resistance coefficient

?max maximum relative lateral displacement

?y change of relative lateral displacement

?u final displacement of the structure in the elastic state

Introduction

In principle, from the point of view of engineering, it is considered suitable and acceptable design that can meet the favorable conditions in terms of economy, efficiency, resistance and so on. Bring it to a reasonable and acceptable level. Although sometimes economic and architectural issues cause the loss of resistance and proper performance of the building against the incoming loads, but at the same time, while providing sufficient resistance and stability, it is necessary to try to make the building as efficient as possible and economically optimal. According to the above explanation, currently the best solution is to find ways to improve the current construction process. That is, with a few changes in the implementation methods and of course by doing the work according to the regulations from the beginning to the completion of the implementation of the projects, much better results can be achieved.

The resistance of any structure against an earthquake depends on two factors: one is the type of construction and the application of engineering principles and rules in its design and implementation, and the other is the magnitude and power of earthquakes in recent years through the mass media. resistant to earthquakes is heard; Methods such as placing buildings on sliding blocks, digging very large channels around the foundations, suspending the building from chains! , hanging large pendulums from the ceiling and. The interesting thing about these solutions is that they are almost non-scientific considering the construction situation in a country like Iran, which is also on a large scale. Of course, not only in Iran, but in most countries, this work is largely impossible, and even if they have the ability to be implemented, they are very expensive, and they are not able to be implemented for all buildings. In addition to these methods, things like using separators, dampers and energy absorbers (placing special plastic springs of one or more layers behind the building) to reduce damages and losses seem more practical. Sometimes, architectural considerations such as creating space or creating a suitable facade in the building cause structural engineers to start new structural innovations. Of course, for better utilization and proper performance, it is necessary to check the behavior of the structure in an earthquake in order to prevent its possible damages. From this point of view, buildings are generally divided into four categories: steel buildings, concrete buildings, buildings with building materials (bricks) and wooden buildings.

Due to the greater use and novelty of the construction of concrete and steel structures in the present era, we discuss and review the existing laws in the field of construction of these two types of structures. Concrete and steel structures, if they are built based on the principles of engineering and the existing rules and regulations, do not differ so much in terms of resistance. Remembering that steel is less resistant to heat and chemicals than concrete (fire and melting, rust, decay, etc.) in an earthquake, the more malleable and flexible the structural members are, the less financial and life losses will be incurred. For this purpose, it is better to use low carbon steel, weldable and highly malleable. Of course, the fact that a structure is made of steel is not a guarantee of its resistance against earthquakes.

In steel buildings, braces after beams and columns and during earthquakes and wind are even more important and are a very important factor for resistance to earthquakes and other lateral loads. Steel braces are among the systems that resist lateral forces, by placing braces in a number of building frames in each stretch and with the help of the rigid diaphragm function of the structure floor, that direction can be considered restrained. Wind braces are divided into two types: convergent and divergent. The design and implementation of braces must be done with the utmost precision and based on engineering principles, especially regarding the location of the braces themselves, the type and size of the profile used, the amount and type and length of welds, the type of weld seam, and so on.

Hardness, ductility and resistance are important parameters in the seismic response of earthquake-resistant structures. While bending frames offer very high plasticity, they have very low stiffness, so that in these frames, displacement controls usually govern the design, and as a result, all the capacity of the structure is not used. Coaxial frames, unlike bending frames, have high lateral stiffness and very low plasticity, which results in a very low coefficient of behavior for these frames.