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bracing strut definition: Soil compression braced excavation, or bracing strut is one of the methods of stabilizing narrow excavations, which is often used due to the lack of permission of neighboring neighbors.

In bracing strut method, like the truss retaining structure method, vertical members are installed at regular intervals and then the deformation of the walls is prevented by placing a horizontal member between these vertical members.

In narrow excavations, due to the lack of space to execute the truss inclination member, the truss retaining method is practically eliminated and the strut method is a suitable alternative.

In bracing strut method, compression members (compression strut) are used to transfer soil pressure. The components of this method are as follows:

1) Wall: in addition to resisting soil pressure, walls also act as a support for compression members. There are different types of walls, such as:

.

2) Compression members (Strut or Brace): Compression members or compression strut have the role of transferring soil pressure from one wall to another, which have different types, such as:

  • Double IPE
  • Double IPB
  • Tube
  • Box
  • Concrete section

3) Wale members: As the name implies, these members are responsible for transmitting power from the wall to the compression strut. The wale members take the soil pressure applied from the wall and transfer it to the compression strut. Therefore, strut and their connection are designed based on compressive force, wale members based on shear and flexural force, and wall-based on combined forces.

Earth retention by bracing strut systems in Zafar project - Executed by the managers of Istasazeh company
Earth retention by bracing strut systems in Zafar project – Executed by the managers of Istasazeh company

Design Basics of bracing Strut Method systems

 simulating the bracing Strut systems in 2D or 3D software
simulating the bracing Strut systems in 2D or 3D software

In order to design the excavation by bracing strut method, by simulating the project conditions in 2D or 3D software, the force created in the horizontal and vertical members should be calculated and then by controlling the final limit and operating modes, a suitable member for these members should be designed. Vertical and horizontal members are generally made of steel and are designed according to the regulations of steel structures.

Considering the buckling phenomenon in the design of horizontal members is one of the important issues of the bracing strut method.

 bracing Strut execution method

1) Drilling vertical members’ wells in bracing strut method

As in the case of metal piles in the methods of soil nailing, soil anchorage and truss retaining structure, in the method of bracing strut, the wells of vertical members should be drilled before excavation begins. The location of these wells is done by surveying and according to the executive plans.

To concretize the roots of vertical members, the floor of these wells is lower than the floor level of the foundation.

2) Execution of vertical members in bracing strut method

Often the cross section of vertical members is in the form of a Double IPE. The vertical members are placed vertically in the well and completely connected to the wall, and then the roots of the metal pile are concreted.

The root of the metal pile is about 25% of the depth of the excavation and the diameter of the wells is about 80 to 100 cm.

For better trapping of the vertical member in the root concrete, corner or stud parts are welded to the vertical member as a trapper.

Just as metal piles in the methods of soil nailing, soil anchorage, truss retaining structure, etc. must be completely supported on the excavation wall, for proper operation of the bracing strut method, the vertical member must be completely connected to the wall to minimize deformations. For the vertical member to connect completely, its back is filled with suitable materials that have the minimum soil compressive strength of that part, so that no space is left between the vertical member and the wall.

because that the horizontal members must be connected to the vertical members from both sides, so the vertical members run as far as possible in the two opposite walls facing each other to use their full capacity.

Execution of vertical member by crane and concreting of metal pile roots of Zafar project
Execution of vertical member by crane and concreting of metal pile roots of Zafar project Executed by the managers of Istasazeh company
the root of metal pile
The root of metal pile
Executed by the managers of Istasazeh company
Execution of vertical member and concreting of metal pile roots of Zafar project
Executed by the managers of Istasazeh company

3) Execution of horizontal members in bracing strut method

After the vertical members are executed, the first row of horizontal members must be executed. The horizontal members are in the form of a double IPE or tubes or and so on.

As in the connection of beams to columns in structures, the horizontal members are welded to the vertical members using steel sheets.

To prevent local collapse, wooden beams or steel mesh and concrete shotcrete or other suitable members can be used between the vertical members.

To reduce the effective length of the horizontal members, a cache (a member perpendicular to the horizontal member) can be used. This prevents buckling of the horizontal members.

Connecting horizontal and vertical members to each other in braced excavation in Zafar project
Connecting horizontal and vertical members to each other in braced excavation in Zafar project – Executed by the managers of Istasazeh company
Execution of cache (member perpendicular to horizontal members) to reduce the effective length and thus prevent buckling in Zafar project
Execution of cache (member perpendicular to horizontal members) to reduce the effective length and thus prevent buckling in Zafar project – Executed by the managers of Istasazeh company

4) Completion of horizontal members in bracing strut method

After the execution of the first row of horizontal members, until reaching the level of the second row of horizontal members, excavation is done and steel mesh and concrete shotcrete are also performed. This process continues to the end of the excavation.

Stages of executing the strut method
Stages of executing the strut method

Use of tube members for horizontal members and use of wooden beams to prevent local collapse
Use of tube members for horizontal members and use of wooden beams to prevent local collapse

Stages of executing the bracing strut method
Stages of executing the bracing strut method

Different stabilization components by strut method
Different stabilization components by strut method

Advantages of bracing strut method

  • Limiting deformations
  • Suitable for deep excavations located in urban areas
  • Suitable for low width excavations
  • Ability to recycle part of the material (horizontal members)
  • Do not enter adjacent grounds
  • Ability to adapt to drainage methods
  • Flexibility and the possibility of strengthening the design during and after execution
  • Independence of method efficiency on soil type
  • No need for neighbors’ permission
Advantages of bracing strut method compared to other stabilization & earth retention methods
Advantages of bracing strut method compared to other stabilization & earth retention methods

Disadvantages of bracing strut method

  • Non-economic in large ground areas
  • Occupy the space inside the excavation and the difficulty of excavation and execution of the main structure
  • Relatively low speed of operation
  • Execution restrictions in projects with irregular geometry
  • Impossibility to use to increase the depth of the excavation at the time of execution or after the completion of excavation
  • Requiring special equipment and machinery
  • Requiring experienced people and strict quality control to perform strut, especially connections
Disadvantages of the bracing strut method compared to other stabilization methods
Disadvantages of the bracing strut method compared to other stabilization methods

Video animation of bracing strut method in the deep excavation stabilization

Related Content:

Federal Highway Administration (FHWA)

DeepEX , Braced Excavations/Cross-lot

Highways Today , Strut

Palpile , BRACING AND STRUTTING SYSTEMS

Deep Excavation , Cross-lot

Skyrise Cities , How Tiebacks, Rakers, and Struts Support Shoring Walls

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