What is the truss guard structure and how is it implemented?

rose roofing: Truss Retaining Structure is a common method for stabilizing medium and shallow excavation soil for building construction. This method is especially useful in urban areas. High execution speed and affordability are the advantages of this support system. The impossibility of implementation for deep pits is also one of its limitations. There is no need to get permission from the neighbors for the truss method. The first step in the implementation of the truss guard structure is to dig the wells of the upright members; This step should be done before excavation. Placing the upright members in the well, implementing the inclined members of the guard structure and connecting the diagonal members to the upright members are other stages of setting up the guard structure. Finally, the wall of the pit should be meshed and concreted and the trusses should be connected to each other. The design and implementation of the truss guard structure should be done by experts and engineers and under their supervision.

What is a truss guard structure?

Controlling the deformation and stability of excavation walls under seismic and static loads is one of the most important issues in geotechnical engineering. Choosing an appropriate support system in urban areas that can increase the stability of excavations has also been one of the long-term challenges of design engineers. Among the various pit stabilization methods, the truss method stands out. Truss is a triangular vertical support system that is used for safety and improving the bearing capacity and resistance of the pit wall of buildings under construction against lateral stresses.

When a part of the soil is excavated, changes occur in it. These changes cause tension and strain around the soil. These forces can also be transferred to nearby structures and infrastructures and affect them. Due to the expansion of urban spaces and the increase in the amount of construction in residential areas, there is an increasing demand for the use of underground spaces. Therefore, the safety of surrounding buildings in urban areas with the design of braced excavations has become a serious concern for nearby residents. The pit stabilization system using the truss method, if the principles are implemented, solves these concerns to a large extent.

The main members or elements in the truss guard structure
Standing members
Horizontal members
willing members
The root or base of the inclined member
The root or base of the horizontal member

The vertical members are placed vertically in the well and completely attached to the wall. Then the root of the metal pile is concreted.

Truss guard structure design methods

One of the most important things for designing and building a support system for drilling structures is to predict their performance before starting construction. There are two methods for designing the truss guard structure system:

Design using finite element software such as Plaxis
Design using standard tables

The use of software such as Plaxsis provides you with two-dimensional and three-dimensional models of the design of the truss guard structure. In this software, simulations are performed based on bending and axial stiffness of vertical, inclined and truss members. Deformation of the complications adjacent to the pit is also calculated on the same basis.

Sometimes the design of the truss system is done from special standard tables. In these tables, the dimensions of the foundation and the details of the implementation of the truss guard structure based on the characteristics of the soil (such as adhesion and the angle of internal friction of the soil) are presented in the form of lines and diagrams.

Retaining wall system behavior and performance analysis according to two-dimensional analysis can be done using static and semi-static methods. Semi-static method is one of the methods of seismic analysis of structures.

What are the steps of implementing the truss guard structure?
Digging wells of upright members
Placing upright members in the well
Implementation of inclined members of the guard structure
Connecting the horizontal and diagonal members of the truss to the main vertical and inclined members (completion of the truss guard structure system)
Implementation of meshing and concrete spraying (shotcrete)
Connecting adjacent trusses to each other by bracing members

1- Drilling holes for vertical members of the first stage in the implementation of the truss

The first step in stabilizing the building pit using the truss method is to dig a well for vertical members. Well drilling is done before excavation or excavation. The location of wells is determined through mapping. The diameter of the wells is considered to be 80-100 cm. The depth of the wells should be below the level of the foundation floor. This additional depth is for concreting at the root of the vertical members (next stage).

2- Placing upright members in the well

By digging the well, the vertical members are placed in the wells and they are completely attached to the wall of the well. The complete adhesion of the vertical members to the wall helps to minimize the deformations and in this way the performance of the truss guard structure is at its best. Then concrete pouring is done at the root or base of these vertical elements. Concreting should be done in such a way that no empty space remains between the vertical member and the wall. Vertical members can be IPE or angle beams.

3- Implementation of inclined members of the guard structure

For the operation of inclined members, it is necessary to create a stable slope in front of the vertical members of the truss system. This slope is created through excavation. The inclined member is placed in the slope. Then one side is welded to the vertical member and one end is also placed on the foundation. After the inclined members are all in place, they are connected to each other by shims. This work makes the function of the truss system to stabilize the soil and stabilize the pit, integrated and coordinated.

4- Completion stage of guarding structure with truss and meshing and concrete spraying

The guard structure of the truss will be completed once