Compared with the traditional steel composite bracing, prestressed steel composite bracing increases the construction procedure of applying prestress. The steel support of prestressed fish-belly foundation pit adopts special fish-belly beam structure. Under the premise of ensuring the construction quality, it further reduces the number of steel components, provides a more open construction space, makes the construction of earth digging, earth transport and underground structure more convenient, improves the construction period as a whole, and reduces the cost at the same time.

The application and control of prestress is an important part of the support construction. The construction process and requirements should conform to the following provisions:

1, the jacks applying prestress should have reliable and accurate measuring devices;

2The prestress can be applied after the supporting structure is installed and qualified by the quality self-test;

3, on the support and Angle support members to apply prestress, the net force point of the jack pressure should coincide with the support axis, the jack should be placed symmetrically and equidistant on both sides of the support axis, and should be synchronized balanced pressure;

4The "double control" measures should be adopted to control the hydraulic pressure value of tensioner and the elongation of steel strand.

5The pressure of the jack should be applied hierarchically, and the pressure should be kept stable after each level of pressure is applied10Apply the next level of pressure after a minute; After the pre-pressure is added to the design specified value, the pressure shall be stabilized10Minutes later, it can be locked according to the design prepressure value;

6In the process of applying prestress, when there is solder joint cracking, bolt loosening, local buckling and other abnormal conditions, the prestress should be removed, and the weak part of the support can be strengthened before continuing to apply prestress;

7, prefabricated prestressed fish-belly beam composite steel support should be used in the process of support and steel strand tension monitoring construction, when the foundation pit deformation rate reaches the alarm value, the accumulated deformation value reaches the alarm value95%Or the prestress loss of the supporting member reaches the design value20%When the prestress should be added;

8, high outdoor temperature (≥40°), the steel support component is prone to thermal expansion, resulting in large temperature stress. In order to prevent the total support stress from exceeding the alarm value, cooling measures should be taken for the support component, and the pressure can be appropriately relieved when necessary; Low outdoor temperature (≤0°), the steel support is easy to shrink, which will cause large deformation of the envelope structure, thus adversely affecting the surrounding environment. Therefore, prestressing should be added to support members to control the deformation of the envelope structure.

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