For the study of building in a convenient manner the building structural systems can be classified into two types based on the load transmission mechanisms. These are the:
- Gravity Load Resisting Systems
- Lateral Load Resisting Systems
Both the mentioned systems are complementary and interactive to each other. When behaving as n integrated structural system, they must resist and transmit the gravity and the lateral loads coming on it to the foundation than to the ground below.
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Types of Building Structural Systems
The building system is a three - dimensional structure. This system is conceived as a two - dimensional system to facilitate easy analysis and the design. The building can be considered as a two - dimensional subsystems that are lying primarily in horizontal and vertical planes. This means floors, roofs, wall, the plane frames etc. This is explained in the fig-1.
The figure-1 shows the division of the complete structural system into:
- Horizontal or Floor system
- Vertical or Framing system
- Lateral Load Resisting System
Horizontal or Floor Systems in a Building Structure
The floor system in a building is responsible for resisting the gravity loads which will include the dead loads and the live loads that are acting on the building. This is then transmitted to the vertical framing system of the building.
Here the floor system under the load is subjected to flexure and transverse shear. The vertical frame system here due to the same load transmission is subjected to axial compression that is coupled in most cases with the flexure and the shear. This is shown in figure -2.
The floor shown in figure-1 will act as a horizontal diaphragm that will connect and stiffen the various vertical frame elements. The floor diaphragm will act rigidly when subjected to lateral loads. This owes to have a higher in-plane stiffness, which is a highlighting property under lateral load action.
|Fig.1. Horizontal or Floor System - Gravity Load Transmission |
|Fig.2. Vertical Systems - Vertical Load Transmission|
Vertical or Framing SystemsThe vertical framing system is responsible for the resisting the gravity and the lateral loads that are coming from the floor system. This, in turn, is transferred to the foundation and the ground below. The vertical framing system is a three-dimensional arrangement that is made up of beams and columns.
For convenience, the system is divided into plane frames along the transverse and the longitudinal direction. For a cast - in -situ reinforced concrete construction, the vertical system usually will have the following:
- Transfer Girders
The columns are skeleton vertical structural elements that have different cross-sectional shapes like square, rectangular, circular, L-shaped etc. The shape is often specified by the designer or the architect. The column is dictated based on its height and the load acting on it. This is in turn dependent on the type of the floor system, the number of stories, the column spacing etc.
The column is designed so that it will resist the axial compression that is combined with the bi-axial bending moments. These are forces that are induced due to the " Frame action " under the action of gravity and the lateral loads. These loads are more in the lower storeys of the building. This is the reason why high strength concrete is used in the lower columns wit a high reinforcement. As the level goes up, the column size can be optimized. This makes the design economical.
In areas like atriums, the floor height will be very large, mainly greater than one story height. This situation will ask for reducing the unsupported length of the column. This can be provided by employing tie beams. Or else the columns should be properly designed as a slender column.
|Fig.3. Shear Wall Construction|
The thickness of the reinforced concrete bearing walls will vary from 120mm to 200mm. For lower storeys buildings, the thickness of the shear walls can be thicker. Now the walls that are constructed around the lift core will serve as shear walls.
In some of the building construction, it might me required to have a single floor (especially the ground floor) require column free space. This is to meet the requirement of a parking area, or conventional or lobbies.
In such situation, the load bearing vertical element cannot be allowed to continue downwards through the lower floors and then the foundation. This problem is solved by the construction of transfer beams. The transfer beams are very heavy beam whose depth can extend over one full story.
|Fig.4: The Use of Transfer Girders|
The columns in the upper story will be terminated in the transfer girder. The load from the above columns are transferred to the girder. By beam action, the load is transferred to the main columns that are supporting the girder from below.
Suspenders can be called as vertical elements that are used to suspend the floors of a multi-storey building form a central core made of reinforced concrete. The figure-5 is an example a building in Malaysia.
|Fig.5: 30-story high Tun Mustapha Building|
The figure-5 shows Yayasan Sabah Headquarters in Malaysia. The building floors are suspended along with their periphery with tensile rods are 96 in number and are 38mm thick. These rods are hung from the radial steel brackets that are attached to the top of a 14.3m diameter reinforced concrete core.
The structural steel is always a better choice to be used as suspenders or hangers. This is because the majority of the force that is to be taken is direct tension. The steel hangers take up only a little floor space. The suspenders employed may be hung from crossed - braced trusses, large cantilevered beams, vierendeel girders or brackets.
Lateral Load Resisting Systems
The above-described horizontal and vertical sub-systems of a structural system interact together to resist the gravity and the lateral loads.
The lateral load effect is more prominent in tall buildings. This reason will govern the choice of lateral system for high-rise buildings. The lateral load resisting system of the reinforced concrete consist of the following types:
- Shear Walls
|Fig.6.The Comparison of Various System|
Frames are composed of columns and beams as shown in figure - 6(a). Thes ability to resist the lateral loads is due to the rigidities of the beam and column connections and the moment resisting capacities of the individual members. These are called as rigid frames. This name is due to the ends of various framing into the joints is connected rigidly. This will help to undergo rotation under the action of the loads.
In the case of a "flat slab system" the width of the slab mainly along the column walls that will take the place of the beam in order to have the frame action. For buildings that have story from 15 to 20 can employ the sole lateral - resisting system (Figure -6(e))
2. Shear Walls
The shear walls are solid structures which are constructed over the full height of the building. The shear walls take the position of lift or staircase core regions. These are located in the transverse direction like facade walls or the interior walls.
The shear walls are stiff and have great depth in the direction increased lateral loads. Figure-6(b) shows the representation of shear walls.
The walls will restrain the frame deformations in the lower storeys while the frames will restrain the wall deformation in upper storeys. The frame shear wall system is applied for 40 storeys.
In the system of arrangement, closely spaced columns are placed along the periphery of a building. The exterior building will have deep spandrel beams that are connected to these columns. This arrangement form like a perforated box or the framed tube. This arrangement will have a higher rigidity against the lateral loads (Figure-6(d)).
When the outer tube is connected to the inner tube or the internal core system it forms a tube -in the tube. If the sectional plan of the building has a number of perforated cells, then the structural system is called as bundled tube or a multi-cell framed tube. This system is effective up to 80 storeys (Figure-6(e).This system is widely used in big cities of developed countries.
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