1 overview
In China's socialist construction, in order to save investment and time, after investigation and demonstration, the original old steel structure can be reused and put into use after treatment and reinforcement. However, because these old steel structures may be used for a long time, the corrosion damage degree is serious, the chemical composition and mechanical properties of the original steel are not clear, and they are all under load, so their construction is difficult, the technical requirements are high and the risks are great. Therefore, we must master a set of construction techniques and determine feasible construction schemes and measures. From June 5438+0, 2006 to February 2002, I had the honor to preside over the construction and implementation of the reconstruction and expansion project of China-aided Barbados Chipset Market. Through the construction practice of treatment, reinforcement and transformation of the old steel structure in this project, I have studied and summarized the construction techniques and methods in this respect, and introduced them to you through this article for reference by relevant personnel.
2. The original state, design requirements and construction implementation of the old steel structure reconstruction and expansion project
2. 1 Project Introduction
Chipside market reconstruction and expansion project is a low-interest loan project for China to help Barbados. It is a large-scale market construction project using an old steel structure vegetable market built by the British in 1933 and used for nearly 70 years. The construction drawing is designed by Hunan Architectural Design Institute. In addition to the original old steel structure, the original maintenance structure and other parts were all demolished, and reinforced concrete frame mezzanine and electromechanical equipment such as elevators, air conditioners, cold storage and refrigerated display cabinets were added, with a total construction area of 6,200 m2, including 3,600 m2 steel structure, 2,400m2 mezzanine, garbage disposal station and 200m2 cornice.
2.2 The original old steel structure and its corrosion condition
The old steel structure consists of 66 steel columns and 86 steel roof trusses, all of which are connected by ordinary bolts. The original and old section steel 12 pieces were sampled and sent back to China. After inspection and evaluation by Beijing Equipment Installation Company, the chemical composition and mechanical properties are equivalent to those of 20# steel. It is suggested that domestic HPB235 new steel and original section steel should be welded, and its covered electrode is domestic E 4315-J427; According to the observation and preliminary identification of the old steel structure, there are about six kinds of corrosion and damage, which can be summarized as follows: (a) A few steel columns and steel roof trusses are completely covered with oxide layer, but there is almost no corrosion; (b) Most steel columns and steel roof trusses have been corroded, but to a lesser extent; (c) The steel column around the external maintenance wall and the scale covered on the steel roof truss along the southern coast are peeled off or can be peeled off due to corrosion, and there are some deep pitting corrosion; (d) The oxide layer covered on the surface of a few steel columns is completely peeled off or can be peeled off due to corrosion. Generally, there are some deep pitting corrosion, and the flange of the section is seriously thinned and corroded; (e) Some members of the wind-resistant truss under a few steel roof trusses have rusted and need to be replaced with new members and reinforced; (f) Many steel roof trusses are severely deformed and need to be straightened.
2.3 Design drawing requirements
Carry out sand blasting rust removal on the original old steel structure as a whole, remove all oxidation corrosion rust layers covering the steel surface, and clean up rotten holes; Handle, reinforce, add new steel bars and replace 8 new steel columns according to the requirements of the drawings, all by welding method, and the weld seam is 6 mm-8 mm; Steel structure surface painting (5 layers 125μm). The design drawings changed the way that the original roof was only waterproof with iron tiles, but laid a 25mm thick wooden roof panel on the steel roof truss together with waterproof insulation layer, and laid a new colored profiled steel waterproof roof panel on it, which increased a lot of roof load compared with the original building. Therefore, on the original steel roof truss, new supporting purlins are added which are more than 1 times and new reinforced bars which are more than 1/3. According to the damage degree after derusting, the reinforcement scheme of the original old steel column was determined on site, that is, a reinforced steel plate with a thickness of 10mm was welded around the damaged part of the steel column. The steel structure treatment, reinforcement and transformation of the whole project all need to work at height with load.
2.4 Project construction
The project was implemented by Barbados Branch of China State Construction Engineering Corporation. According to the present situation, design requirements and the characteristics of foreign aid projects, I made a detailed construction plan and implementation plan before the steel structure started, and made a detailed disclosure to the technical managers and main operators involved in the construction implementation. Because the whole project team attaches great importance to it, is well prepared, works hard, and has high technical level of technicians and operators, this project has passed the acceptance of the expert acceptance team organized by the Ministry of Foreign Trade and Economic Cooperation of China, won the excellent foreign aid project, passed the acceptance of the professional consulting engineer of the owner, and won the praise of the Barbados government and relevant personnel. According to the materials and models required in the inspection and evaluation report, the new steel, connecting plates and covered electrode used in the whole project are imported from China.
3 old steel structure treatment, reinforcement and reconstruction construction methods and technical points
3. 1 Follow the specifications.
Code for Design of Steel Structures (GBJ17-88);
Technical Specification for Steel Structure Reinforcement (CECS 77: 96);
Code for Welding of Building Steel Structures (JGJ81-91);
Technical specification for inspection, evaluation and reinforcement of steel structures (Yb/T 9257-96);
Code for Construction and Acceptance of Steel Structure Engineering (GB 50205-95);
Rust grade and embroidery grade of steel surface before painting (GB 8923-88);
Technical Specification for Painting of Steel Structures and Pipelines (Yb/T 9256-96);
Standard for Quality Inspection and Evaluation of Steel Structure Engineering (GB5022 1-95), etc.
3.2 Implementation procedures
Inspection, sampling, testing and evaluation of the original and old steel structures → determination of treatment, reinforcement and transformation schemes → construction drawing design → construction preparation → construction implementation operation → inspection, acceptance, evaluation and delivery.
3.3 Construction Steps and Procedures [1]
Preparation before construction (new and old steel materials, covered electrode and connecting bolt types are determined by sampling test) → determination of construction scheme and technical disclosure → construction preparation (drawing confirmation; Check the steel, covered electrode and connecting bolts arriving at the site; Determine the process specification; Determine the production site of prefabricated components; Determine the mode of production organization; Determine the production plan; Prepare construction equipment, machines and tools, protective equipment, etc. ) → prefabrication of steel bars, new steel bars, connecting plates and spare parts. → Assemble new steel bars and joint plates on the ground → Set up operation scaffolding → Sand blasting for derusting, and remove all corrosive oxide layers and slag covered on the surface of the old steel structure → Measure various size data and correct the deformation of the old steel structure → Set unloading supporting steel frame facilities and unload the load on the steel structure at the working face → Connect (or bolt) the reinforced members and new steel bars with the old steel structure at high altitude → Correct the connecting rods → Remove unloading supporting steel frame facilities → Check and measure relevant data → Repair painting and painting operations → Check and accept comprehensively.
3.4 technical points and construction methods that should be mastered in construction implementation
1) The investigation, sampling, test and evaluation of the original and old steel structures must be done well. The original old steel structure and new steel for reinforcement must be sampled and tested by an authoritative professional laboratory, and an evaluation report should be issued to determine the material of the new steel to be purchased, covered electrode specifications (or bolts) and technical requirements (or bolts) of welding connection process.
2) According to the present situation of the original old steel structure after sandblasting and removing all the corrosion oxide layer and slag covered on the surface, discuss with the design to determine the practical and reliable treatment, reinforcement and transformation scheme.
3) Check the material and covered electrode specifications (or bolts) of the new steel delivered to the construction site, which must be consistent with the requirements of the evaluation report issued by the laboratory and shall not be changed at will; When changes are really needed, the laboratory must issue a new evaluation report. Technical managers and professional workers must carefully check whether the new steel and covered electrode specifications (or bolts) used in the specific construction site are consistent with the requirements in the evaluation report (there are three types of covered electrode used in Chipside market expansion project, and each type of covered electrode is used in different parts: E43 15- J427 covered electrode is used to connect the new steel HPB235 imported from China with the old steel structure; E4303-J422 covered electrode for connecting new steel imported from China; The connection between locally purchased British standard steels shall be E70 18 (American standard covered electrode), which shall be absolutely accurate, and all nonconformities shall be eliminated. Any mistake will lead to unpredictable consequences.
4) The rust removal of the original old steel structure and the removal of the corrosion oxide layer and slag covered on the surface should be thorough, and it must be carried out by connecting a special sand blasting rust remover with an air compressor. Individual parts with serious corrosion should be chiseled off manually or polished with steel chisel, hammer and grinder. Sand for sand blasting and rust removal adopts medium-fine yellow sand, which needs to be dried. Due to the heavy pollution caused by aerial work, protective facilities must be set up for the surrounding environment to protect the operators.
5) It is essential to measure various dimensional data and correct the deformation of the original old steel structure. The method of correction should be physical, not heating and baking.
6) It is very important to set up facilities to unload the supporting steel frame and unload the load on the steel structure of the working face, which must be done well. Unloading the supporting steel frame and unloading the load scheme on the steel structure of the working face must be reliable and practical, and attention must be paid to it. Any mistake will lead to unimaginable consequences.
7) The construction steps and operation sequence cannot be reversed at will. If it is really necessary to reverse, it must be confirmed by the competent technical personnel that it will not affect the safety and construction quality, and it can only be carried out with the consent of the chief engineer, otherwise it will lead to unimaginable consequences.
8) The construction of the model must be done well, and it can only be extended to the whole project after the evaluation of technicians and experts from relevant parties.
9) Attention must be paid to the safety and construction quality of high-altitude welding connection. Under the condition of ensuring the safety of aerial work, senior professional welders with high technical level and rich experience should be selected for operation, and the weld quality must be guaranteed to be defect-free. Operators must wear safety belts, and welding tools must be tied in a safe place at high altitude.
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