How to transfer vertical and lateral loads in the structure depends on the composition and arrangement of vertical load-bearing members of structures such as columns, beams and lateral load-bearing members such as braced frame systems and how these members are connected. This combination is called the structural system.
Types of structural systems:
- Structures that exist in Standard No. 2800
- Structures that do not exist in Standard No. 2800
Standard No. 2800 is a regulation related to the seismic design of buildings in Iran. The structures that exist in Standard No. 2800 include the common structures in design and construction in the country that are relatively easier to execute (they have more executive teams). They have a relatively easier design than other members outside 2800 Standard. But structures outside Standard No. 2800 have many points both in terms of design and execution; and there are stricter rules to execute them. Also, their design is not easy to do and requires experience and information at high and extensive levels.
Types of structural systems from the perspective of Regulation 2800 are classified as follows:
- Bearing wall system
- Building frame system
- Flexural frame system
- Double or combined system
- Consular system
Bearing wall system
In Bearing wall system, the load-bearing task is completely performed by the walls or frames. Cold rolled structures are the most common type and in addition to their lightweight, they also have a very high execution speed. The types of structural systems in this section are mostly used for villa structures and places where the seismic force is not high and are located in low or medium seismic zones and have less than 3 floors. So these are structures that are not very commercially viable.
Construction frame system
Construction frame system is one of the most common and widely used types of structural systems. The reason is the easy implementation with executive teams in our country and also its design does not have special criteria and certain members are designed to resist the side forces. Lateral forces are the strongest forces that enter the structure in a building and are usually caused by an earthquake or wind. They are always in the structure from the sides. Of course, in an earthquake, we also have a case for vertical earthquake loads, which showed itself in Iran for the first time in the Bam earthquake. The simple frame system is divided into several categories from the Standard No. 2800 point of view:
- Reinforced concrete shear walls in three levels of special, medium and normal ductility
- Shear wall with building materials
- Special divergent steel braces
- Swing buckling braces
- Convergent bracing at two levels of medium and normal ductility
In simple building frame systems, the wall or braces are responsible for resisting the lateral forces. These systems are mostly used in steel structures where the beams do not directly contribute to the discussion of earthquake bearing. That is, first, the earthquake load enters the braces or walls, and then it is transferred to the column and then to the foundation of the structure. For this reason, in steel structures with simple bracing, we design beams with articulated connections. This structural system is mostly used in 3- to 7-storey structures in all cities. Its shear wall type is mostly implemented in cities like Mashhad due to more executive teams, but the type of bracing is used in most cities and most areas. Of course, there are limitations to their ductility that can be addressed by changing the type of ductility.
Flexural frame system
Flexural frame system is the most common system in the world and its use is not limited to Iran. The reason why this structural system is widely used is mostly related to architectural issues and the use of many windows and many openings in the building (Especially due to the modern architectural style that requires a lot of light). In this system, lateral forces are first tolerated by the beams and then transmitted to the columns and finally to the foundation. In this structural system, most of the time, the beams have a high height (depth), which is a little annoying for the project architecture. For this reason, in the steel structure system, beams are often used together with stiffeners.In concrete structures, increasing the strength of concrete or changing the stiffness of the structure is on the agenda. These buildings, due to their great beauty and attractiveness, and especially their resemblance to modern and contemporary architectural style, have many windows; they can be used in 1 or 2 story villa buildings or even in buildings up to 200 m high in projects. These types of structures are designed in two types of steel and concrete and three levels of normal, medium and special ductility. The special type has the maximum allowable height and the structure can be designed up to a height of 200 m. One of the negative points of this type of buildings is the possibility of damage to most of the beams after the earthquake, and of course they are not capable of initial operation and need to be repaired or connected parts.
Double or combined Flexural frame system
Double or combined Flexural frame system is one of the most common types of structural systems in the world after the flexural frame. In this structural system, lateral forces are tolerated by the combination of the flexural frame system and the shear wall or braces, and all members in combination with each other resist earthquake forces. Double or combined Flexural frame system is a favorite of many building designers because:
- It has many fuses
- They can be used for buildings up to 200 m high
- It has a high resistance compared to other systems.
The fuses in the building are the same members that are resistant to lateral forces. The fuses are in the double system, shear walls or braces or beams in the flexural frame section, and all together they resist the forces entering the structure.
Due to the fact that these buildings have good resistance to earthquake force and also have many members as fuses, this structural system is used more for:
- Towers
- Sensitive and important structures such as hospitals, police stations, schools, museums, and so on.
Of course, the use of these structures for residential buildings of 5 floors and above is also common in cities such as Tehran, which have a high risk of seismicity, and structures with less ductility can be used in these cases.
In appearance, we will not see any difference between walls that are in double frames and walls that are in simple frames. The same thing applies with braces. Types of double bracing systems are not different in appearance from simple construction frame braces. So what is the difference? The difference is in the ventricles of these systems against earthquakes. The force received by double earthquake systems is very different from the force received by simple frames. How they work is also different from a simple frame system and has its own controls. For example, if the shear wall that is supposed to function as a double frame does not meet the desired controls, according to the regulations, that wall is no longer considered as a double wall and the wall is in the category of simple building frames, and the walls can no longer be expected to withstand lateral loads along with other members.
You can not easily understand what kind of frame or even its ductility by looking at the surface of a skeleton. Because it shows the difference in ductility in places that are completely imperceptible places. For example:
- The number of screws of a screw connection in a steel structure
- In the thickness of the weld in a welded connection in a steel structure
- In the number or thickness of bars used in concrete structures
- In increasing and decreasing the dimensions of sections
So it is relatively difficult to understand what system a structure is using. Whether it is a double frame or a simple frame, it is practically impossible to visually detect ductility.
Console structure system
In Console structuresystem, there are no consoles such as balconies or waterfall houses. In Console structural system, the consoles are in the style of planar columns and the beams must be executed in articulated, and if the structure is to be executed in concrete, for example, steel trusses should be used instead of concrete beams. This structural system has a height limit of 10 m and due to this limitation, it is the best way to use them for spaces similar to warehouses and similar spaces such as poultry farms. The important point is that the lateral forces in this system are supported by the columns and the trusses or articulated beams should not tolerated the lateral forces in this type of structures at all. Because in these structures the total forces are tolerated by the columns, the dimensions of the columns are much larger than usual (at least 2 times).
Types of structural systems outside of Regulation 2800:
- Damper
- Seismic separator
- Super frame
- Steel shear wall
These systems are mostly used in cases:
- Improvement
- High-rise buildings
- Buildings of great importance such as the Central Bank building and so on
- Luxury residential buildings whose builders do not like the earthquake to damage the structure and architecture and expensive equipment that worked inside the building
To design these systems, as mentioned, you either have to have a lot of experience in design or you have done special studies in this field at the M.Sc. level.
Damper
Dampers are a type of structural system designed to withstand lateral forces. The use of damping technology in structures, although not very long-lived (less than 50 years), but in a short time they have made significant progress and have reached all over the world. Dampers, as their name implies, cause damping and extinction of earthquake forces. In fact, the basis of their work is that when the lateral force of wind or earthquake enters a structure, these devices use different mechanisms to resist lateral forces and reduce these forces and reduce their power and strength as much as possible. Depending on the percentage of damping they have, dampers can significantly reduce the lateral displacements of the structure and in a high-rise tower, the displacement of the floors can be reduced to 1/10 of the initial value. Of course, you should note that all materials and all objects have inherent damping, but dampers use special methods and materials, and give the ability to control and increase damping to structural designers.
There are different types of dampers and they are divided into the following categories:
- Viscoelastic dampers
- Friction dampers
- Viscous dampers
- Mass dampers
- Fluid dampers
Dampers are most commonly used in high-rise buildings such as towers, hospitals, schools and sensitive structures. Dampers are also widely used in structures that are being improved. Taipei Tower and Shanghai Tower are examples of towers designed with dampers. Also, among the famous projects that have been designed with dampers in Iran and Tehran, the following can be mentioned:
- Central building of Pasargad Bank
- Mehdi Clinic 800-bed hospital (the largest building equipped with friction dampers in Iran)
- Central building of Tourism Bank
- Seoul 19-storey commercial office tower
- Golden Palace Velenjek commercial building
- Mirdamad office building
Dampers can be used in steel and concrete structures and there is no limit to the type of materials in them. Finally, this system, like all systems outside the Standard No. 2800, needs nonlinear analysis, which is done both static and dynamic, depending on the project conditions that often the dynamic mode prevails in projects.
Seismic separator
Other types of structural systems outside Standard No. 2800 are seismic separators. The main task of scismic separators, as their name suggests, is to separate the building into different parts. This separation is usually done at levels close to the foundation of the structure and the reason is that by dividing the structure in this section, the entering seismic force to the structure is prevented to a great extent and the structure receives less energy from the earthquake and the possibility of damage is greatly reduced. The basis of the use of seismic separators is that by reducing the acceleration that enters the building, the seismic force and the relative displacement in the floors (drift) can be greatly controlled. Separators also play an important role in seismic issues by increasing the alternation time and increasing the damping, and practically reduce the seismic force instead of increasing the bearing capacity of the structure.
Seismic separators are generally classified as follows:
- Friction separator
- Elastomeric separator
- Rubber separator
- Abrasion elastic separator
- Friction pendulum separator
- Lead rubber separator
- Combined use of these separators in series and parallel
Applications of seismic separators, since they are mostly located on the foundation level, are used in the following cases:
- High-rise structures
- The sensitive structures that need to reduce the seismic force
- Structures that need seismic improvement and reducing the seismic force in them are very important.
- Luxury residential buildings that have expensive architectures of building components
Examples of seismic separator projects around the world include the National Museum of New Zealand, the International Hotel in Xichang, China, Suqian Public Hospital No. 1 (one of the best hospitals in China), and the new Beijing Airport (including the main waiting building and watchtower) and projects implemented in the country include the Violet Garden residential tower, Torbat Heydariyeh Crisis Management Building, Palladium Commercial Complex Bridge, Imam Hossein Hospital in Kermanshah, Arefian Hospital in Urmia and Parand residential town.
Separators can not be used only in part of the structure and must be located in the whole structure. Because their presence in one part of the structure practically changes the relative displacement of the floors to a great extent, and in one part of the structure there will be less displacement and in the other part more displacement will occur, which will make the structure worse than without separator.
Regarding the costs, it should be said that often the separators reduce the construction costs and make the sections more efficient. One of the disadvantages of separators is their lack of strength against the vertical force of the earthquake, which can cause problems for the structure.
Super frame
Another type of structural system that has become very popular recently is super frames. super frame structure system, invented by the Japanese, is also known as the open plan system and has numerous advantages. Fortunately, there is an example in our country, and that is the Tabriz World Trade Center. This structural system is designed and built based on 4 special techniques that are its main codes.
- Use of high strength concrete
- Use of precast parts
- Use of pre-stressed or retracted floors
- Use of viscous or mass dampers and so on (in the upper floors of the structure)
These structures were built on the idea of the Japanese, and its initial ideas began after the 1995 Kobe earthquake. In these structures, the design performance is slightly different from conventional building structures. In these structures, the seismic force is applied by a central concrete core, which can be considered as a H-shaped concrete wall with great depth. The forces are then transferred to the super beams (deep and extremely strong beams with special design criteria) and then to the columns above each face and finally to the damper (mostly mass) at the top of the structure, and given that dampers in these structures are mostly in the form of spheres and are placed on top of the structure. The seismic force in the upper floors is greatly reduced and the lateral displacement of the structure is greatly reduced. The main components in this system are:
- Wall with reinforced concrete core
- Strong super beams
- Columns connected to the super beams
- Dampers (mostly on top of the structure and in mass)
Steel shear wall
Steel shear wall is another type of structural system that has recently become very popular in our country. This structural system is similar to a concrete shear wall and is a wall, but instead of its components being made of concrete, they are made of steel sheets, which have different thicknesses depending on the type of building and the number of forces. Steel shear wall system is one of the best systems of steel structures that not only has an extremely strong system in absorbing energy but also is quite economical. This system, like all systems outside Standard No. 2800, needs non-linear analysis, and internal regulations and reference for any of these systems are often not available, and due to the proximity of regulations and soil behaviors and many other parameters, use of US regulations to analyze and design these structures is the main suggestion.
This structural system in some cases with stiffener and some cases without stiffener; in some places it is perforated and in others it without pore and solid. If we consider the behavior of a steel shear wall in a structure to be very simplified, we can say that the walls act as the web of a section and the columns next to it act as wings. One of the advantages of this structural system is its high stiffness and energy absorption rate and it is even harder than reinforced concrete shear wall so that if we want to compare two steel and concrete shear walls on the first floor after the foundation of a tall structure (for example, 40 floors), the thickness of the concrete wall at the lowest level may reach 1.2 m (120 cm). However, if a steel wall is used in the same building, it is likely that a thickness of less than 80 cm will be required (approximately). That is why the use of this system is more practical not only in the structural part but also in the architectural part and creating more spaces, especially in spaces such as parking lots or walls between rooms. But its disadvantages include the possibility of buckling the sheets and the lack of executive expertise in the country. This structural system is used in tall buildings, sensitive structures and residential buildings, and depending on the opinion of the designer and the executive team and the opinion of the employer, it can be used in projects.
Related content:
Leave a Reply