Structural forms, also called structural systems, are mechanisms that guarantee the structural stability of a building. Depending on the type of structure, one or more structural systems may be used for the same tall building. The suitability of the system to support lateral loads is decided by a civil engineer with experience in structural analysis and design of tall buildings.
What is a tall building
There is no specifically mentioned dividing line between high-rise buildings and low-rise buildings. However, according to standard methods around the world, buildings taller than 20 floors can be considered tall buildings.
Tall buildings must be designed to withstand gravity and lateral loads caused by wind, earthquakes, etc.
Therefore, a good lateral load support system is necessary to maintain the lateral stability of the building. Based on the height and other layouts of the building, the most suitable structural system is selected.
The requirements are taken into account when planning tall buildings.
Lateral deflection of a tall building
In general, the lateral deflection of tall buildings is limited to (height/500). If this limit is exceeded, there may be restrictions on the use of service facilities, such as elevators.
Furthermore, excessive deflection can result in cracking of non-structural components such as brick walls, cladding, glass curtains, etc. Due to the loss of rigidity, loads can be distributed.
Additionally, excessive deflection can be uncomfortable for the occupant.
Deviation Index
Occupants actually felt the relative deflection between floors. The drift index is the indicator that shows whether or not we are within the limits.
Furthermore, the drift index is considered an indicator of the lateral stiffness of the building.
The lateral displacement of a building is limited to ( 1/500 ) normally.
The deviation can be calculated as total deviation or as deviation between floors.
Total drift = The maximum lateral deflection of the building = Δ 1
Total drift index = Δ1 / building height
Displacement between floors = difference in lateral deflection of two slabs (ex. 21st floor and 22nd floor) = Δ 2
Drift index = Δ2 / height from floor to floor
Furthermore, limiting ground vibrations and building accelerations is critical to maintaining human well-being. The building must be rigid enough to limit its maximum acceleration to a minimum level that humans can feel.
Structural forms for tall buildings
Tall buildings are becoming more and more popular with development around the world, and it is also fashionable to build tall buildings.
Due to limited land area in densely populated areas, it is more practical to construct a tall building to accommodate all services. High-rise buildings are constructed as mixed buildings, residential buildings, office buildings, etc.
As explained above, there are important factors to consider when designing tall buildings. Furthermore, depending on the type of structure, the civil engineer must select suitable structural systems to proceed with the project.
Additionally, the structural forms discussed below may be referred to as lateral load-bearing systems.
In this article we discuss 14 different structural forms.
- Reinforced frame constructions
- Rigid frame constructions
- Filled Frame Constructions
- Shear wall structures
- Coupled Shear Wall Structures
- Wall Frame Structures
- Framed tubular structures
- Tube-in-tube or shell-core structures
- Bundled tube structures
- Reinforced tube structures
- Cantilever supported structures
- Suspended structures
- spatial structures
- Hybrid structures
Reinforced frame constructions
Reinforced structures are mainly constructed in steel buildings. Steel buildings are comparatively weak in lateral stability compared to those of the same scale concrete building.
They deform under lateral wind loads, earthquakes, etc. without offering much resistance because they lack lateral rigidity. Therefore, the frames are strengthened and the structure is converted into a braced frame to transfer these lateral loads to the foundation.
Elements are fixed between the structure that absorb lateral loads in the form of axial tension or compression force. These elements are designed after analyzing lateral loads.
There are different types of bracing systems.
- Individual diagonals
- Cross braces
- K Keys
- V keys
Individual diagonals
The supports are fixed along the diagonals of the frame. When these structures are attached, they are arranged to absorb axial tensile forces. Elements can absorb higher tensile forces than compressive forces. Therefore, the diagonals are fixed in both directions to absorb the transverse forces acting in both directions.
Then we can design the beams for tensile forces. Furthermore, compression failure is minimal.
The bracing on the load application side absorbs the lateral load as a tensile force.
Cross braces
Diagonal and crossed bracing is carried out. The following figure shows the arrangement of the crossbars.
The cross members are attached to the main structure in several ways. Instead of a single bracket as in the image above, brackets can also be attached between the internal frames.
K Keys
The following figure shows the structure of a K support.
The brackets are attached halfway up the column.
V keys
The supports are fixed in a V shape.
Additional reinforcement of the frame structure reduces lateral deflection.
Rigid frame structure
Frame construction provides stability to the building and is one of the most commonly used forms of construction. In this construction system, beams and columns are connected with rigid connections such as moment connections.
- Rigid frame structures provide more free space with rectangular frame structures at ground level. They give you more freedom when planning floor plans
- Design and construction of rigid structures can be 20 to 25 stories tall. Beyond these limits, it would be more difficult to control lateral displacement due to lateral loading, as this becomes critical as altitude increases.
- However, 20-25 stores cannot be built in steel buildings without side supports. Therefore, these structures are more suitable for concrete structures where the concrete columns and beams have sufficient stiffness.
- The speaker grid can be expanded by approx. 6-9 m.
- Lateral stability is guaranteed by columns, beams and beam-column connections
- Furthermore, the dimensions of columns and beams are greatly influenced by lateral loads in addition to gravitational loads.
- As the height of the building increases, the size of the elements and the spacing of the columns can be adjusted to achieve the required rigidity.
- Increasing the height of the building increases the subsequent load, which acts as a shear force on the supports. The sizing of the column must be based on these forces acting on it.
- Furthermore, bending moments due to lateral loads will increase at lower levels. Therefore, deeper support at lower levels is needed. Furthermore, it is not possible to obtain the same beam height on all floors.
Filled Frame Constructions
Masonry infill walls can be used to improve the lateral load capacity of a building. These types of structures are formed by masonry within the concrete frame.
Furthermore, the vertical continuation of the infill walls is important to ensure lateral stability. It is not absolutely necessary for all walls to be filled with masonry. However, at least one dish could be filled.
Generally, these walls are not taken into account in seismic load tests on the lateral stability of medium-sized buildings.
The quality of the bricks used in these walls is very important to build good and solid structures. Cracks in the wall can be seen as a loss of wall rigidity. Large cracks in the wall do not allow considering the lateral stability of this wall.
One of the most critical problems is that they tend to loosen over time. If plans change or the client changes, other internal and external measures will be necessary. Therefore the infill walls are removed. If these are maintained by the infill walls, the lateral stability of the structure will be significantly compromised.
Therefore, taking into account lateral stability through the structure could be more sensible and safe for such structural forms.
Shear wall structures
Shear walls are concrete walls fixed vertically to the base and have the necessary rigidity to transfer the vertical and horizontal loads that act on them.
Based on the height and floor area of the building, a sufficient number of shear walls with an adequate cross-sectional area must be constructed to provide the necessary stiffness to accommodate lateral loads.
Shear walls are manufactured as elevator walls, stair center walls, partition walls, etc. and can be continued from floor to roof.
Because concrete walls are more rigid than the rigid structure of concrete beams and columns, shear wall structures up to 34 stories high can be built.
In the context of structures constructed from load-bearing walls, the following may be important.
- The use of shear walls in construction is best suited for buildings with repeated floors. As explained above, we need to continue the shear walls vertically. Therefore, repetition offers many advantages for structural design as well as construction costs.
- Buildings with up to 35 branches can be designed for lateral loads considering only the shear walls. The interaction between the shear wall and the frame structure can be considered minimal. In this method, we design shear walls so that they can accommodate all lateral loads without transferring them to the structure.
- Additionally, supports can account for the vertical loads of the structure and the bending moment of the beams based on different load cases and alternatives. Loads .
- When planning floor plans, shear walls should be placed so that they are exposed to sufficient vertical loads. Lateral loads on walls result in tensile stresses if they are not balanced by the compressive stresses created by vertical loads. Furthermore, if the wall is under pressure, we can achieve an economical design.
- In tall buildings, the thickness and length of walls are sometimes reduced, walls are demolished, etc. These measures have a significant impact on structural behavior. Changes of this type must be made very carefully and with careful analysis of the structure.
- If the shear walls are not arranged symmetrically in any direction, the structure will twist under lateral load. These actions must be taken into account in the design and computational analysis software must be used to model the structure to determine the behavior.
Coupled Shear Wall Structures
In most tall buildings, shear walls are built around elevator walls. Generally they are oriented in both directions. There are also lobbies between the elevator cores.
These lifting cores can be connected by concrete beams that allow interaction between the walls of the two cores. When two shear walls are connected by a moment resisting structure, they are called coupled shear walls. This connection increases the lateral load capacity of the structure as if the walls acted individually.
The image above shows the arrangement of the coupled shear wall and its appearance when modeled. As explained above, we use the coupling of shear walls to improve their load transport capabilities later. The figure below clearly shows the area of improvement that can be achieved by coupling shear walls.
Wall Frame Structures
Structures that take into account the interaction between walls and frames are considered wall frame structures. If the number of floors is more than 15-20 floors, the interaction between the walls and the frame can be taken into account.
Furthermore, with such structural forms, the lateral stability of the building is significantly improved, taking into account the wall-structure interaction.
The shear wall acts as a cantilever beam and a frame at the same time and shows shear deformation when lateral loads are applied. The combination of these two actions reduces the lateral deflection of the building.
As shown in the figure above, the lower part of the structures presents flexural behavior and the upper part presents shear behavior.
The following advantages can be highlighted as useful when using wall frame structures.
- The lateral deformation/drift is much less than when seen along the shear wall.
- Significant reduction in the storage time of basement walls/cores.
- Supports can be designed as supports.
Computational analysis could be used to determine the exact behavior of structural elements and their forces.
Framed tubular structures
The lateral load capacity of internal concrete walls is limited by the increase in the height of the building in relation to the built area.
The length of the shear walls in the direction in which the lateral loads act is the measure of the lateral stiffness in that direction. However, there are limitations. We cannot continue the shear walls across the entire floor.
In these scenarios, it would be useful to consider the effectiveness of framed tubes compared to other forms of structure.
Our structure can be used to support lateral loads. To do this, the depth of the beams and the height of the supports must be increased.
However, there are limitations to increasing the size of the facade elements, as we need to reduce the size of the windows. If we can create a frame around the building as in the figure above, it is possible to support greater loads, as it works like a tubular structure.
Columns spaced 2 to 4 m apart with deep beams along the perimeter form a tubular structure.
Both concrete and steel structures can be constructed as tubular structures. Additionally, buildings of 40 to 60 stories can be designed and constructed using this method.
Although the rectangular shape is more efficient, other shapes such as circular and octagonal could also be constructed.
Tube-in-tube or shell-core structures
These types of structural shapes have good resistance to lateral loads.
The central walls, which could be built to build elevators and stairs, could be considered as interior tubes.
This system represents an improvement over the tubular structure we discussed previously.
In this structural system, the central walls cooperate with the perimeter tube to improve the lateral load capacity.
Bundled tube structures
This form of construction is used in taller buildings.
This system is a combination of several tubes.
This structural system is used in the tallest building, which requires greater reinforcements on the lower floors. Furthermore, this system has very high resistance to lateral loads.
Reinforced tube structures
Such projects can be constructed as steel or concrete structures.
The support structure fixed to the tube offers very high resistance to lateral loads. Furthermore, the attachment of supports of this type does not affect the internal layout of the floor.
However, this can have an impact on the layout of the facade and windows.
The struts attached to all supports ensure a more uniform distribution of lateral forces. Furthermore, by connecting the struts to the vertical supports, the axial loads on the supports are distributed among themselves.
Columns with greater axial load transfer the load to columns with lower loads .
Cantilever supported structures
The structural efficiency of tall buildings strongly depends on lateral stiffness and strength. Of the structural systems available, cantilever systems are the most used, especially in buildings with repeated floors.
A deep beam or wall that is floor to floor high or a steel beam erected between two floors can be considered a cantilever. It connects the core and the scope.
The purpose of the boom is to connect the internal structures and the perimeter structural system to support lateral loads. The following factors may affect the performance of the boom system as one of the useful structural forms.
- The sites are built over the entire height of the building. With proper planning, the best boom position can be selected using trial and error. Locations that minimize lateral deflections can be selected by the computational analysis model.
- Number of boom levels available
- Your position in the plane
- Presence of brace beams to integrate adjacent edge columns, as opposed to those that sit together with mega columns
- Boom beam depth
The figure below shows the reductions that could be achieved by installing a cantilever system in a tall building. Linking the shear walls of the core increases the bending moment of the core.
Instead of connecting the perimeter to the core, which brings many problems with building functions, strap ties can be built into the perimeter. This allows users to use the floor efficiently. Generally cantilever floors are used as service floors.
Providing a greater number of barriers reduces the effectiveness of additional barriers. In general, there can be a maximum of about 5 cantilevers in a building. Furthermore, two spears are more efficient than one.
Suspended structures
The key element of this type of structures is the core. The core may consist of concrete walls or lattice elements.
All floors that project from the core hang from the beams that start at roof level. There will be enough space on the ground floor.
Additionally, structures such as suspension bridges, etc. can also be considered suspended structures.
spatial structures
The three-dimensional spatial structure supports the vertical and horizontal loads acting on the structure.
The primary load-bearing system is the three-dimensional space frame system.
Due to the complexity of the structure, structural analysis and design of these structures are comparatively difficult.
To understand structural behavior such as load paths, etc., a computational analysis model could be used.
Hybrid structures
Hybrid structures are structures composed of multiple combinations of the structural systems discussed above.
Hybrid structures become more complicated designs due to the different combinations.
Furthermore, the combination of these structures allows the creation of very special structural systems, which also allow for even more careful modeling.
Structural analysis and design are more complicated due to the integration of different systems into one structure. Furthermore, these combinations need to be selected and decided based on the applicability of the structure type.
Due to the complexity of the structural system, a detailed study is necessary in the analysis and design of these structures. Furthermore, a computer-assisted analysis must be performed using appropriate software to determine the overall behavior of the structure.
