Assembled tower crane foundation is a structural component that combines concrete prefabricated components with tower crane through unbonded prestress technology. Infrastructure is a patented technology of utility model, which aims at convenient disassembly and transportation, repeated use, short installation period, energy saving and remarkable economic benefits.
Precast concrete prefabricated tower crane foundation is suitable for tower cranes with rated lifting torque of trolley luffing horizontal arm not exceeding 400 kn·m, and the shape of concrete precast members is inverted T-shape.
Using assembled tower crane foundation, we should strengthen the control of foundation bearing capacity, positioning and assembly, high-strength bolt fastening, prefabrication quality, prestress application, prestressed reinforcement protection and backfill soil to ensure the safety of tower crane foundation.
1. Foundation bearing capacity The requirements of foundation bearing capacity of assembled tower crane are different according to different types of tower cranes, and the characteristic value of foundation bearing capacity should not be lower than the value specified in the instruction manual. After foundation trench excavation, it should be judged whether the ground durability of tower crane installation parts meets the requirements. The geometric size and shape of the foundation pit should comply with the regulations, and the basement should be smooth, without quicksand and karst caves. The whole foundation should be located on the bearing layer with the same durability and settlement to prevent uneven settlement.
2. Precast concrete prefabricated tower crane foundation (precast concrete) shall ensure that the shape, size, reserved holes, reserved holes and strength grade of precast concrete meet the technical requirements of the product, and must be inspected before use. Those that have not been accepted or failed to pass the acceptance shall not be used.
The acceptance of assembled tower crane foundation (prefabricated parts) mainly includes the following aspects.
(1) According to the Regulations on the Management of Qualification of Construction Enterprises, the production unit of prefabricated components shall have the professional qualification of concrete prefabricated components and engage in production and business activities within the scope of its qualification.
(2) Post-tensioning method is adopted for precast concrete members, and multiple components are assembled together. The strength grade of concrete should not be lower than C35.
(3) The raw materials of prefabricated components shall be provided with the concrete mixture ratio, the certificates of raw materials such as aggregate, cement and steel bars for mixing concrete, the second interview report and the 28-day strength report of concrete.
3. Prefabricated tower crane foundation with concrete cushion shall be installed on the concrete cushion, with the strength grade of C 10-C 15 and the thickness of10-15 mm.. The plane geometry size, horizontal elevation difference and flatness of the cushion should meet the design requirements, especially the surface flatness deviation should be less than 4mm, and the cushion strength should reach more than 80% before installation.
4. The connection between the anchor bolt tower crane and the foundation adopts the special product "Special bolt and nut for fixing the tower crane and the foundation", and its certificate and inspection report should be checked before installation.
5. Prefabrication and assembly of steel strand and tensioning equipment. Tower crane foundation adopts X-knot-free prestress technology, which connects mass concrete precast members into a stressed whole. Prestressed tendons without A-knot usually adopt high-strength and low-relaxation steel strand, the strength of which can reach 1860 MPa, but the diameter of each strand is only 15.24mm, and OVM series clip anchors can be used for its fixed and tensioned ends.
6. Prestressed tensioning unbonded prestressed tensioning is the key process of tower crane foundation construction and must be strictly controlled.
(1) In order to reduce the loss of prestress, the method of overstretching is generally adopted. When tensioning, the elongation value and tensile force of steel strand should be controlled by double indexes, and detailed records should be made.
(2) Prestressed tendons are tensioned. In this process, broken wires or slippage of prestressed tendons should be avoided. The number of broken wires r or slip shall not exceed 3% of the total number of prestressed tendons, and each bundle of steel wires shall not exceed 1.
Seven. Protection of unbonded prestressed tendons
(1) Requirements for wrapping layer Before passing through the steel strand, check whether the wrapping layer of prestressed tendons is damaged. If it is slightly damaged, it should be wrapped and repaired with waterproof tape. If the damage is serious, it should be replaced in time.
(2) Requirements for grease The grease between the prestressed tendons and the wrapping layer should not flow and become brittle in extreme environment to ensure effective protection.
(3) The protection of anchorage zone requires that the unbonded prestressed tendons must be coated with anti-corrosion coating or covered with special plastic caps and metal caps at the exposed parts of anchorage and anchorage clips. Bonding between anchorage and sheath can be sealed by connecting sleeve or waterproof A-knot tape to prevent water from infiltrating into human body.
Eight. The retaining wall is 120 mm thick, and M5 cement mortar is used for backfilling and foundation protection. Drainage holes are left around the retaining wall to prevent rain from eroding the foundation soil. The masonry height of the retaining wall should be flush with the beam surface of the foundation, and the fill or sand should be flush with the beam surface, and compacted twice to ensure the stability of the tower crane foundation and ensure that the total quality is greater than the original foundation quality, otherwise the tower crane may be unstable or overturned. When the foundation is backfilled, the anchor bolts should be set aside to ensure that they are not covered by soil, so that they can be inspected regularly and re-tightened in time if they are found loose. After the nut is tightened, apply butter to the exposed end of the bolt and cover it with a protective cover to prevent corrosion. Drainage of tower foundation should be smooth, and no pit or trench should be dug nearby at will, and there should be no accumulated water within 3m of its outer edge, so as to prevent uneven settlement of foundation caused by soaking the foundation and reduce its bearing capacity.
9. On-site assembly process: play the position line of the cross shaft and the center piece on the concrete cushion → lay 10- 15mm thick medium-fine sand cushion → install the center piece → install prefabricated members and counterweight pieces in turn → wear steel strands → horizontally close the members → tension the steel strands → seal protection → install vertical connection structures such as anchor bolts and column feet and seal protection → backfill and foundation protection.