– What is top-down or top-down construction? (Top-Down Construction)
rose roofing: Construction from top to bottom or top-down is a relatively new method of pit stabilization that is becoming popular in the construction industry. In pit stabilization using the Top-Down Construction method, it is possible to simultaneously implement the underground floors up to the level under the foundation along with the implementation of the floors on the ground. In the following cases:
High level of underground water
Performance space limitation
Interference with traffic on main streets
It is not possible to obtain consent from the neighboring neighbors
And …
The construction method from top to bottom or top-down is one of the suitable methods for pit stabilization.
Applications of the construction method from top to bottom (top down)
Applications of the construction method from top to bottom (top down)
Stabilization of the pit by top-down construction method
Stabilization of the pit by top-down construction method
History of top-down construction method
For the first time in the 70s, the top-down construction method was used in the construction of metro stations in Paris and Milan and became known as the Cut & Cover method. In this method, first the tunnel structure is built and then the outer environment of the tunnel is filled with embankment materials.
In order to speed up the construction of a building, in 1935 in Tokyo, the top-down construction method was used. In 1950 in Milan, the combination of diaphragm wall and top-down was implemented by ICOS company. Over time, this method has been used in different countries, including Iran.
Regulation of construction method from top to bottom (top down)
Implementation of the top-down (top-down) construction method is provided in BS:8002:1994 of England for guard structures.
Metal top
In top-down construction, the framework of the structure may be in the form of a metal frame, and columns and steel roof systems are used. In metal structures, the roof system may be a block beam type or a mixed roof (a combination of concrete and steel) with a simple slab or steel deck.
Of course, it is not suitable to use the block beam roof system for the underground part of the structure in the top-down construction method, especially if the number of substructure floors is large. The reason for this is that in-plane forces are created in the diaphragm of the roof of the basement floors due to the lateral pressure of the soil.
Therefore, in case of using a metal frame, it is more suitable to use a mixed roof system with a steel deck for the top-down or top-down method. Also, the use of steel columns in the framework of the structure can reduce the weight of the building.
In addition, metal columns can be installed using low-capacity cranes. The main problem of using steel columns is the high cost of materials due to the higher price of steel compared to reinforced concrete. Also, the use of CFT mixed cross-section columns can be a suitable option for the implementation of the metal framework in the top-down or top-down method.
Implementation of metal framework in top-down method – Hakim project – Saman Faraz – Istasazeh company
Implementation of metal framework in the top-down method – Hakim project – Saman Faraz – Istasazeh company
Connecting steel columns with concrete piles under the columns can be done in two ways. In the first method, the end part of the column is armed with a cutter and placed inside the pile. In the second method, a column plate is installed on the pile and then a metal column is implemented on it.
One of the points that should be paid attention to in the design and implementation of the surrounding steel columns is that the retaining wall transfers part of the soil pressure it bears to the surrounding columns.
Therefore, there should be a proper connection between the retaining wall and the surrounding columns. For this reason, cutters are used along the perimeter columns. Cutters may be of stud type or stud section.
Types of pile connection to steel column
Types of pile connection to steel column
Types of cutters
Types of cutters
Top-down construction method
For each pit stabilization project, according to its type and specific conditions, the process of implementing the top-down construction method may be different, but in general, the top-down implementation steps can be described as follows:
Top Dun in projects with large and deep basements
Using the top-down method in projects with large and deep basements
1) Implementation of perimeter retaining walls
Sometimes, at the beginning of the project, the perimeter retaining walls are built to a certain depth below the foundation level. These walls may be implemented in one of the following ways:
In-situ reinforced concrete wall
Diaphragm walls
Candles
Continuous candles
Steel shields
Jet-Grouting columns
Diaphragm retaining wall is more useful than other implementation methods in the construction of perimeter retaining walls.
In order to connect the retaining walls to the roof slab of the lower floors, the necessary equipment for subsequent connections should be provided at specific levels.
Step 1 – Implementation of the surrounding retaining wall
Step 1: Implementation of the surrounding retaining wall
2) Excavating the place of load-bearing piles from below the level of columns, ground floor walls and installing load-bearing columns
One of the main steps of implementing pit stabilization using the top-down construction method is the implementation of the columns of the main structure. For this purpose, piles are first placed below the foundation level of each of the columns, so that after the foundation is placed on these piles, it is possible to install the columns of the main structure.
The columns of the main structure, in concrete and steel structures, are implemented on top of the piles using one of the following methods:
In-situ concrete column with square, rectangular or circular sections based on existing drilling equipment
Precast concrete column
Combining slender columns with concrete jacket during the construction of the main structure
Composite steel column
Step 2 Implementation of piles under the columns, foundations under the columns and columns of the main structure
the level
