Structural forms are also known as structural systems are the mechanism that provides structural stability of the building. One or more structural systems could be used for the same high rise building depending on the nature of the structure. The suitability of the lateral load resisting system is to be decided by a structural engineer who has good knowledge of structural analysis and design of tall buildings.
What is a Tall Building
There no specifically mentioned demarking line to separate high rise buildings and low rise buildings. However, as per the methods used in the world, the buildings above 20 stories could be considered as high rise buildings.
The tall buildings shall be designed to withstand the gravitational loads and lateral loads due to the actions of wind, earthquake, etc.
Therefore, there is a requirement of having a good lateral load resisting system for maintaining the lateral stability of the building. Based on the height and other arrangements of the building, the most suitable structural system is selected.
The requirements are considered in the design of tall buildings.
Lateral Deflection of Tall Building
Generally, the lateral deflection of the tall buildings is limited to (height/500). Exceeding this limit could affect the requirements of the services such as the lifts.
Further, excessive deflection could lead to cracking of nonstructural components such as brick walls, cladding, glass curtains, etc. It could lead to the distribution of the loads due to the loss of stiffness.
In addition, causing excessive deflection could felt to the occupant resulting discomfort.
Drift Index
Occupants actually felt the relative deflection between the floors. Drift index is the indicator that provides whether we are within the limit or not.
Further, the drift index is considered as the indicator of the lateral stiffness of the building.
The lateral drift of a building is limited to (1/500) usually.
The drift can be calculated at total drift or inter-floor drift.
Total Drift = The maximum lateral deflection of the building = Δ 1
Total drift Index = Δ 1 / height of the building
Inter floor drift = Difference of Lateral deflection of two slabs (ex: 21 floor and 22 floor) = Δ 2
Drift Index = Δ 2 / floor to floor height
In addition, limitation of the floor vibrations, building accelerations are also a must to maintain the human comport. The building shall be stiff enough to limit its maximum acceleration to a minimum level that felt by humans.
Structural Forms for Tall Buildings
Tall buildings becoming more popular in the word with the development and it has also become a fashion to construct a tall building.
Due to the limited land area in congested environments, the most convenient option is to build a tall building to accommodate all the services. Tall buildings are constructed as mixed developments, residences, office work, etc.
As discussed above, there are key factors to be attended when designing tall buildings. Further, depending on the nature of the structure suitable structural systems shall be selected by the structural engineer to proceed with the design.
Further, these structural forms discussed below can be identified as lateral load resisting systems.
In this article, we are discussing 14 different structural forms.
- Braced frame structures
- Rigid frame structures
- Infilled frame structures
- Shear wall structures
- Coupled shear wall structures
- Wall frame structures
- Framed tube structures
- Tube in Tube or Hull-core structures
- Bundled tube structures
- Braced tube structures
- Outrigger-braced structures
- Suspended structures
- Space structures
- Hybrid structures
Braced Frame Structures
Braced frame structures are mainly constructed in steel buildings. Steel buildings are comparatively weak in lateral stability when compared with the same scale concrete building.
They deform when lateral loads from wind, earthquake, etc are applied without resisting much due to the lack of lateral stiffness. Therefore, frames are braced and convert the structure into a braced frame structure to carry these lateral loads to the foundation.
Members are fixed between the frame to carry the lateral loads in the form of axial tension or compression force. These members are designed after doing the analysis for the lateral loads.
There are different types of lateral bracing systems.
- Single diagonals
- Cross bracings
- K-bracings
- V-bracings
Single Diagonals
Brackings are fixed along the diagonal of the frame. When these frames are fixed, they are arranged in a way that they carry the axial tensile forces. Members can carry higher tensile forces than the compression forces. Therefore, diagonals are fixed in both the direction to carry the lateral forces applied in either direction.
Then, we can design the backings for tensile forces. Further, failure in the compression is minimal.
The bracing in the side of applying the load takes the latera load as a tensile force.
Cross Bracing
Bracings fixed diagonally by crossing each other are done. The following figure indicates the arrangement of the cross bracings.
Cross bracings are fixed in the mainframes in different ways. Instead of having a singe bracing as shown in the above figure, bracings may be fixed between internal frames also.
K-Bracings
The following figure indicates the arrangement of a K-bracing.
Brackings are fixing at mid-height of the column.
V-Bracings
Bracings are fixed in the shape of the letter “V”.
The addition of the bracing to the frame structure reduces lateral deflection.
Rigid Frame Structure
The frame structure provides stability to the building and it is one of the most widely used structural forms. Beams and columns are connected with rigid joints as moment-resisting connections in this structural system.
- Rigid frame structures provide more clear space with the rectangular frame structures at floor levels. It provides more freedom to plan the floor layouts
- Design and construction of rigid frame structures can be done up to 20-25 stories. Beyond these limits, it would be more difficult to control the lateral drift due to the lateral load as they become critical with the increase of height.
- However, steel buildings cannot construct 20-25 stores without lateral bracings. Therefore, these structures are more suitable for concrete structures where concrete columns and beams having sufficient rigidity.
- Column grid could be extended about 6-9m.
- Lateral stability is provided by column, beam and beam-column joint
- Further, sizers of the column and beams are greatly affected by lateral loads in addition to the gravity loads.
- With the increase of the height of the building, element size and spacing of the columns could be adjusted to achieve the required stiffness.
- Increase of the height of the building increases the later load which will act as a shear force to the columns. Column sizing shall be done based on these forces applied to them.
- Further, bending moments due to lateral loads will be increasing at lower levels. Therefore, a deeper beam is required at lower levels. In addition, the same beam height will not be possible on all floors.
Infilled Frame Structures
Masonry infills walls can be used to improve the lateral load resisting capacity of a building. Masonry wall constructed inside the concrete frame makes these types of structures.
Further, the continuation of the infill walls vertically is important to consider for lateral stability. It is not a must to have all the walls to be filled with masonry walls. However, at least one panel could be filled.
In general, these walls are not considered for the lateral stability in medium-rise buildings when they check for earthquake loads.
The quality of the bricks used for these walls is very important to make good solid constructions. Cracks in the wall can be considered as the loss of stiffness of the wall. Having major cracks in the wall does not enable us to consider that wall for lateral stability.
One of the most critical issues is that they tend to remove with time. When plans are changing or the client is changing, they need different internal-external arrangements. Thus, they remove the infill walls. It significantly affects the lateral stability of the structure if it is maintained by the infilled walls.
Therefore, consideration of the lateral stability by the frame could be more useful and safe in these kinds of structural forms.
Shear Wall Structures
Concrete walls constructed vertically fixed at the base with a rigidity to transfer the vertical loads and horizontal loads applied on them can be identified as a shear wall.
Based on the height and floor area of the building, the sufficient number of shear walls having an adequate cross-section area to be built to provide the required stiffness for withstanding the lateral loads.
Shear walls are constructed as lift walls, staircase core walls, partition walls, etc where it can be continued from base to roof.
Since the concrete walls are stiffer than the rigid concrete beam and column structure, shear wall structures could be constructed up to about 34 stories.
The following could be considered importantly related to the structures constructed with shear walls.
- Using the shear walls in construction is more suitable for buildings having repetitive floors. As discussed above, we need to continue the shear walls vertically. Therefore, repetition adds many advantages to the structural design as well as the cost of construction.
- Structures up to 35 stores could be designed for lateral loads by considering the shear walls only. Interaction of the shear wall and frame structure could be considered minimal. In this method, we design shear walls to carry all the lateral loads without transferring to the frame.
- Further, columns can be designed for the vertical loads from the structure and bending moment from the beams based on the different load cases and alternative loads.
- When the layouts are planned, shear walls shall be located in a way that sufficient vertical loads are applied to them. Lateral loads on the walls make tensile stresses if they are not balanced by the compressive stresses applied due to the vertical loads. Further, if the wall is in compression, we could have an economical design.
- In high rise buildings, some times, reduction of wall thicknesses and lengths, discontinuation of walls, etc are done. These actions make a considerable impact on structural behavior. Changes of this nature shall be done with much care and with careful analysis of the structure.
- When the shear walls are not located symmetrically in either direction, the structure will be twisted with the application of lateral loads. These actions shall be considered in the design and computer analysis software shall be used to model the structure to find the behavior.
Coupled Shear Wall Structures
In most of the high rise buildings, the shear walls are constructed around the lift walls. Generally, they are aligned in either direction. Further, there are lobbies between the lift cores.
These lift cores can be connected by concrete beams making the interaction between the wall in the two cores. When two shear walls connected by moment resisting frame is called the coupled shear wall. This connection enhances the lateral load resisting capacity of the structure than the walls are acting separately.
The above figure indicates the arrangement of the coupled shear wall and how it looks likes when modeling. As discussed above, we use to couple the shear walls to enhance their capacities in later load resisting. The following figure clearly indicates the range of enhancement that could be achieved by coupling shear walls.
Wall Frame Structures
Structures that consider the interaction of the wall and frames are considered as wall frame structures. When the number of stories is greater than 15-20 stories, the interaction of the walls and frames could be considered.
Further, consideration of the wall frame interaction improves the lateral stability of the building significantly in these types of structural forms.
The shear wall along with acts as a cantilever and frame shows the shear deformation with the application of the lateral loads. A combination of these two actions reduces the lateral deflection of the building.
Thus, as indicated in the above figure, the lower part of the structures shows the flexural behavior and the upper segment shows the shear behavior.
The following advantages can be highlighted as useful in using wall frame structures.
- Lateral deformation/drift is very lower than that of considering shear wall along.
- Considerable reduction in the bunding moment of the walls/core walls at the basement level.
- Columns can be designed as braced.
Computer analysis could be done the get the exact behavior of the structural elements and their forces.
Framed Tube Structures
The lateral load resisting capacity of the inner concrete walls is limited with the increase of the height of the building when compared to the area of the building.
The length of the shear walls in the direction of lateral loads are applied is the measure of lateral stiffness in that direction. But there are limitations. We cannot continue shear walls throughout the floor.
In such scenarios, consideration of the framed tube action would be useful from other structural forms.
Out frame structure can be used to resist the lateral loads. Depth of the beams and height of the columns are required to increase to take this action.
However, there are limitations of increasing the element sizers at facade as we have to reduce the size of the windows. If we can create a frame around the building as would above figure, it is possible to resist higher load as it acting as a tube structure.
Columns at a spacing of 2-4m with deep beams along the perimeter create a tube structure.
Both the concrete and steel structures can be constructed as tube structures. Further, 40-60 story buildings can be design and constructed with this method.
Though the rectangular shape is more efficient other shapes such as circular and octagonal could also be constructed.
Tube in Tube or Hull-Core Structures
These types of structural forms have good efficiency in lateral load resisting.
Core walls that could be constructed for facilitating lift and staircases could be considered as internal tubes.
This system is one step forward from the frame tube structure that we discussed previously.
In this structural system, the core walls act with the perimeter tube to enhance the lateral load-carrying capacity.
Bundled Tube Structures
This structural form is used in the tallest structures.
This system is a combination of several tubes.
This structural system is used in the tallest building that required a grater stiffeners at the lower levels. Further, this system has a very high lateral load resisting capacity.
Braced Tube Structures
These types of structural forms can be built as steel or concrete structures.
The braced framed fixed around the tube provides very high lateral load resistivity. Further, fixing of bracings of this nature does not affect the internal floor arrangements.
However, it could affect the arrangement of the facade and windows.
The bracings are connected with all the columns makes the distribution of the lateral forces uniformer. Further, due to the connection of the bracings and vertical columns, axial loads on the columns will be distributed to each other.
The columns having higher axial load will transfer the load to the columns having lesser loads.
Outrigger Braced Structures
The structural efficiency of the tall buildings heavily depends on the lateral stiffness and resistance capacity. Out of the available structural systems, outrigger systems are more commonly used especially buildings having repetitive floors.
A deep beam or wall having a height of the floor to floor or steel trusses constructed between two floors can be considered as an outrigger. It connects the core and the perimeter.
The purpose of the outrigger is to copple the internal structures and the perimeter structural system in order to resist the lateral loads. The following factors could affect the performance of the outrigger system as one of the useful structural forms.
- Locations are constructed throughout the height of the building. If we design correctly, the trial and error method could be used to select the best position for the outrigger. Locations that minimize the lateral deflections could be selected by the computer analysis model.
- Number of levels of outriggers are provided
- Their location in the plan
- Presence of belt trusses to engage the adjacent perimeter columns versus stand along with mega columns
- Depth of the outrigger truss
The following figure indicates the reductions that could be achieved by introducing an outrigger system to a high rise building. The connection of the core shear walls enhances the bending moment of the core.
Instead of connecting the perimeter with the core which makes many issues with the building functions, there can be constructed at the perimeter as belt trusses. It allows users to use the floor efficiently. Commonly, outrigger floors are used as service floors.
Providing more number of outriggers reduce the efficiency of additional outriggers. Generally, there could be around 5 maximum outriggers in a building. Further, having two outriggers is more efficient than having one.
Suspended Structures
The key element of these types of structures is the core. The core could be constructed from concrete walls or from the truss elements.
All the floors cantilevered from the core are hanging from the members starting at the roof level. There will be sufficient space ant the ground floor level.
Further, structures such as suspension bridges, etc. could also consider as suspended structures.
Space Structures
Three-dimensional space structure carries the vertical and horizontal loads applied to the structure.
The primary load resisting system is the three-dimensional space structural system.
Structural analysis and design of these structures are comparatively very difficult due to the complex nature of the structure.
A computer analysis model could be used to understand structural behavior such as load paths, etc.
Hybrid Structures
Hybrid structures are the structures of several combinations of structural systems discussed above.
Due to the different combinations, hybrid structures become more complicated structural forms.
In addition, its combination of these structures enables to create very special structural systems. Further, they could be more attentive in shape.
Structural analysis and design are more complicated due to the integration of different systems within one structure. Further, these combinations shall be selected and decided based on the applicability of the nature of the structure.
Due to the complexity of the structural system, a detailed study is required when analyzing and design of these structures. Further, computer-based analysis using suitable software shall be done for knowing the overall behavior of the structure.