A brief discussion on the construction technology and quality control of reverse method for steel columns in complex environment
Abstract The reverse method is an emerging foundation pit support technology developed in recent years. With the domestic With the rapid economic development, high-rise buildings have sprung up like mushrooms after a rain. In the construction of most basements, the traditional formal procedures are adopted, namely: excavation of foundation pit - construction of foundation piles - construction of floor slab - construction of basement vertical components - construction of basement beams and slabs. The time spent on basement construction accounts for a considerable proportion of the entire building. In recent years, new construction technologies such as "reverse method" are gradually being used and becoming mature. This technology is suitable for engineering situations such as urban areas with high building density, adjacent subway lines, buildings and the surrounding environment that are sensitive to settlement deformation, narrow construction sites, tight construction schedules, and thick soft soil layers. It is especially effective for basements with three floors and above. obvious. In the overall construction process of reverse construction, one of the most important processes is: construction of reverse steel columns. The choice of its construction technology determines to a large extent the quality, progress and cost of steel column construction. The structural form of the foundation and basement of the countertop area is generally a steel-concrete composite structure. The steel columns are embedded steel columns. During construction, the steel columns need to be inserted into the piles to a certain depth. Therefore, the hoisting of the steel columns needs to be combined with the concrete of the cast-in-place piles. Pouring is carried out synchronously and crosswise. When installing steel columns, a locator needs to be installed on the top of the pile to serve as a fixed point and support point at the bottom of the steel column. At the same time, a temporary installation bracket should be set up on the ground to serve as a fixed point for the upper part of the steel column and an adjustment device for the positioning and verticality of the steel column axis. . For the reverse construction of steel columns, the industry has always lacked specific and thorough process operations and effective quality assurance measures.
Keywords reverse method; steel column; process flow; quality assurance
Abstract Reversed developed in recent years is a new excavation technology, with the rapid development of the domestic economy, high-rise buildings have mushroomed like filled. Most of the construction in the basement, all positive traditional practices program, namely: excavation pit - Construction of Pile - Construction of the floor - the construction of vertical member basement - the basement slab construction. Basement time-consuming construction accounted for a large proportion of the whole building. In recent years, "Reversed" in construction of the new technology is being gradually applied and mature. The technology is applicable to the urban building density, near metro lines, buildings and surroundings of Settlement sensitive, narrow construction site, the construction period of tension, thick soft soil foundation and other projects, the basement for three and above effect is particularly obvious. In the inverse approach overall construction process, the most important step is: do the reverse construction of steel columns. Selection process of its construction largely determines the quality of the merits of the steel column construction, the speed of progress and how much the cost of inputs like. Form the basis and structure of the basement area is generally inverse made steel-concrete composite structure,
in which steel columns for the embedded steel columns, steel columns need to be inserted into the pile a certain depth construction, thus lifting demand concrete steel columns and piles Pouring synchronous cross conducted. When the steel columns to be installed at the top of the pile installation locator, as the bottom of the steel columns of the fixed point and support points; the same shall be set up temporary mounting bracket at the surface, as a fixed point and positioning and vertical steel columns of the upper portion of the steel column axis While doing construction for the steel columns of the inverse, the industry has been the lack of specific and thorough process operating processes and effective quality assurance measures.
Key words Reversed； steel columns； process； quality assurance
Project Overview
The 4# plot in the middle area of ??the Gangxia River Park area reconstruction project undertaken by our group company is located in the central area of ??Shenzhen. The project site is close to the subway and surrounding areas There are many roads and buildings. This project is part of the renovation project of the entire Gangxia River Park area. About 1/3 of the area on the south side is close to the subway protection zone, so it needs to be reversed, and it is completely reversed. This project is located in the central area of ??Shenzhen. Affected by the surrounding subway, this project adopts the reverse construction method. The foundation is rotary bored piles, the vertical structure of the basement is steel tube concrete columns, and the horizontal structure is steel beams and steel truss slab floors. There are 83 steel tube concrete columns, all of which are reverse steel columns.
1. Characteristics of construction technology
(1) The hoisting of reverse-made steel columns is completed by one-time overall hoisting, without the need for segmented assembly.
(2) When hoisting steel columns in reverse, a locator must be embedded in advance to ensure accurate positioning of the steel columns.
(3) In this construction process, the steel column locator is installed in a "separate" installation, which improves the installation accuracy to a greater extent than the traditional "integral" installation.
(4) In this construction process, the concrete pouring of the steel column is carried out before the sand and gravel backfilling around the steel column, which is opposite to the traditional process, thus avoiding the impact of impurities at the bottom of the column on the steel pipe concrete.
(5) A self-made automatic intermittent water pump is used in this construction process to ensure that there is no water accumulation at the bottom of the steel column.
2. Scope of application
This construction technology is suitable for all reverse method steel column construction, including manual digging pile foundation and rotary digging bored pile foundation.
3. Process principle
3.1 Overall construction principle of reverse-made steel columns
Back-made steel columns are mainly used in reverse construction, and the steel columns are the supports. The upper part is being constructed and connected to the vertical components of the bottom pile foundation. After the construction of the pile foundation (manually dug piles/drilled piles) is completed and the operating holes are formed, the steel columns can be hoisted at one time, and the steel columns can be lifted through the column bottom locator. The bottom of the column is connected and fixed with the pile foundation, then the steel pipe column concrete is poured, and the holes around the steel column are backfilled. Then the upper structure on top of the steel column can be constructed first, and the bottom structure is constructed in reverse order from top to bottom following the earthwork excavation sequence.
3.2 Inverse Steel Column Positioning Principle
Because the position of the steel column is fixed after it is hoisted as a whole and fixedly connected to the pile foundation, and then the connection position with the upper horizontal structure is fixed, so the steel column The accuracy of the fixed position when connecting to the pile foundation is very important. During the construction process, a locator needs to be pre-installed on the top of the pile to ensure the position of the steel column. The installation of the locator requires the construction personnel to go down to the pile hole for positioning and installation. This is the only way Only in this way can the accuracy of the locator installation be ensured, and thus the position accuracy of the steel pile when inserted into the locator.
3.3 Principle of steel pipe concrete pouring
Since the steel pipe columns are all hoisted in one time, the length of the steel pipe column is long, and it can only be poured from the top of the steel pipe column, so it is necessary to adopt High throw earthquake-proof concrete is poured. However, before pouring the steel pipe concrete, it should be ensured that there is no accumulation of water or impurities at the bottom of the steel pipe column. Otherwise, the overall quality of the steel pipe column concrete will be seriously affected. Therefore, corresponding targeted measures need to be taken to ensure the cleanliness of the bottom of the steel pipe column. This process adopts The method is to set up a continuous pumping device on the outside of the bottom of the steel column, first pour concrete for the steel pipe column and then backfill the empty holes around the steel pipe column to prevent mud from entering the inside of the steel pipe column.
4. Construction process flow and operating points
4.1 Construction process flow
4.2 Operation points
4.2.1 Manually dug piles Provide operating space
According to the design requirements, if the design is a manually dug pile, the steel column locator installation operating space can be formed at one time. If the design is a rotary bored pile foundation, rotary excavation must be performed first. For bored pile construction, after the rotary boring pile construction is completed, the empty hole will be backfilled, and then the manual digging hole construction will be carried out again. The diameter of the bored pile must be larger than the diameter of the original bored pile, and the root will be affected by the steel bars on the top of the bored pile. Need to enlarge the hole. The construction of hollowed-out holes is strictly carried out in accordance with manual digging of piles. Pay attention to ventilation in the holes. The arm reinforcement steel and concrete are strictly implemented in accordance with the design. Safety protection is provided in time after the holes are formed. As shown in the figure below:
4.2.2 Breaking the concrete laitance at the pile head
The construction of the engineering piles in the reverse construction area has been completed, and all the empty piles have been excavated. The diameter of the empty holes is larger than that of the columns. The diameter of the pile is 600mm larger, and the protective wall is constructed to 1.7m below the top mark of the pile (the design requires the elevation of the top of the pile for the first time). After the construction of the bored pile is completed, there is 500mm high floating slurry on the top, which needs to be manually chipped because the strength of the pile foundation concrete is relatively high. High, pneumatic picks need to be used for crushing. When the lower crushing operation is performed, large amounts of dust will be generated. Workers must wear gas masks when operating, and ventilation measures should be taken.
4.2.3 Binding of steel bars at the pile head
After the concrete laitance of the pile head is broken, pump water and clean the residue in the hole, and then tie and tie the ten layers of steel mesh at the pile head. The steel mesh here needs to be coordinated with the installation of locator anchors. The steel mesh should be fully tied and the spacing should meet the design requirements.
4.2.4 Install the locator anchor bolt and fixed ring plate
The pile head steel bars are tied, and an additional ￠25 fixed support steel bar is used to assist the locator anchor bolt and fixed ring plate During installation, use a plumb bob hanging wire to position, and lead the positioning cross axis to the bottom pile guard wall. After the top ring plate is installed, guide the positioning cross axis to the surface of the ring plate to elastically waterproof the ink line. Use a spirit level to check the top ring plate to ensure it is installed level. The elevation of the ring plate surface is measured using a level, a 30cm steel ruler and a 7.5kg hammer ball. When using a steel ruler to directly transmit the elevation, change the steel ruler three times until it is correct and then make 4 corresponding points (1m on the ring plate) in the direction of the cross axis of the pile guard wall as the elevation control points for locator installation. As shown in the figure below: 4.2.5 Pour concrete to the bottom of the uppermost fixed ring plate of the locator
After the locator anchor bolts and fixed ring plates are installed, clean the residue in the pile hole and drain away the accumulated water. Ensure that after the concrete at the pile head is poured and vibrated, the elevation is level with the top surface of the uppermost ring plate of the locator. A plastic bucket with a diameter of 300 is pre-buried outside the pile foundation steel bar. After the concrete has initially set, it will be raised as a water pumping hole. After the concrete has finally set, The concrete and laitance above the surface of the uppermost ring plate should be cleaned up in time, and the elevation of the positioning plate should be measured and re-determined accurately, and the positioning axis should be popped up on the positioning plate for the next step of welding the positioning card.
4.2.6 Install and weld the fixed positioning card (locator guide plate)
Before welding the positioning card, the water in the working hole should be pumped out and the positioning should be cleaned again For the floating slurry on the surface of the board, use a plumbometer and a 7.5kg hammer ball to check the positioning cross axis on the ring surface. After it is accurate, weld the positioning card. From this step until the steel column is hoisted and the strength of the mortar water-stop strip reaches the required level, the water pump is in constant working condition. (Special dry water pump: Install a water stop valve at the water pump outlet to prevent water in the water pipe from flowing back into the working hole. It is also equipped with a water level sensor to automatically pump water). As shown in the figure below:
4.2.7 Steel column hoisting
Before hoisting the steel column, a 2mm thick steel cover plate should be added to the hole and fixed by spot welding to the steel column to prevent Foreign objects fell into the steel column after it was installed. The control elevation should be released within the range of 500mm below the side wall of the working hole from the opening, and the same elevation mark should be drawn on the column body with red paint to facilitate the review of the steel column elevation after installation; in addition, positioning cross lines should be released at the opening , control the accurate installation and positioning of steel columns, and review the positioning of steel columns after installation. Make sure that the water in the working hole is drained out before lifting the steel column. During the installation process, it is also necessary to control the verticality of the steel columns within the design requirements. The openings are fixed with channel steel and steel bars welded, and the ends of the channel steel are wrapped and fixed with concrete piers to ensure that the steel columns are firmly fixed. As shown in the figure below:
4.2.8 Apply mortar water-stop strips on column feet
After the steel column is hoisted, the water in the working hole should be drained out, and immediately around the column foot Apply 150mm*200mm1:2 mortar (water mixed with ginseng soup will not leak and will solidify when exposed to water) water-stop strips in the circle. Water pumping must not be interrupted during the construction process. It is strictly forbidden for the water surface to be higher than the surface of the ring plate within 3 hours after the mortar is applied to ensure that stolen objects cannot enter the column. core.
4.2.9 Concrete pouring in the lower conduit
After the construction of the column foot waterstop is completed, the PVC conduit with a diameter of 200mm and a length of 12m is placed along the column core, and the concrete pouring of the column core is completed. After 9m, pull out the conduit and use the pump pipe from the discharge port of the sky pump to extend into the column core for pouring. In this way, it can be ensured that the height of the nozzle at the bottom of the conduit from the concrete surface is between 3 and 9m (the concrete drop height range is determined based on the high-throw earthquake-proof concrete ratio and drop test). Before pouring, an operating platform needs to be set up on the top of the steel pipe column. The steel pipe can be used to set up an independent movable operating platform, which can be used for turnover. During pouring, the conduit needs to be hoisted by a tower crane and raised at any time according to the pouring height.
4.2.10 Steel column verification after the concrete pouring is completed
After the concrete pouring of the column core is completed, the positioning, verticality and elevation of the steel columns should be verified again.
4.2.11 Backfill sand and gravel, and pour steel column fixing rings in the working holes
After the concrete pouring of the steel pipe column is completed and the concrete strength reaches above 75, the surrounding steel piles can be The empty holes are backfilled with stone powder. In addition, the tower columns must be poured with 300 thick C20 micro-expansion concrete fixing rings 500 meters below the floor beams on each floor. When backfilling with stone powder, use a small excavator to evenly backfill around the edge of the column. It is strictly forbidden to offset the steel column due to uneven backfill pressure on one side. The backfilling process should be monitored by surveyors.
5. Quality control
5.1 Quality requirements for key processes
(1) Steel column positioning accuracy requirements: the vertical deviation of the steel column is less than ?3mm, The horizontal deviation of the bottom of the column is less than ?50mm.
(2) Steel pipe concrete quality requirements: Before pouring the concrete in the steel pipe, the height of water accumulation at the bottom of the steel pipe column is ?50mm, and it is strictly forbidden to have mud, debris, etc. at the bottom of the column. (Continued from page 236)
(Continued from page 223) 5.2 Quality issues that should be paid attention to during construction
(1) When the original design is a bored pile and backfilling is required When re-constructing manually dug holes, it should be noted that construction of manually dug hole protection walls is strictly prohibited on backfill and surrounding soft soil. Hole digging and wall protection construction should be carried out on the original solid soil to ensure the safety and stability of the later operation space.
(2) When welding the locator guide plate and the locator bottom plate, due to the humid operating environment, the quality of the weld should be ensured to ensure that the bottom has sufficient shear resistance during the hoisting construction of the steel column to ensure that the steel The overall stability of the column.
(3) When pouring concrete at the bottom of the locator bottom plate, the density of the concrete must be ensured, that is, the completed concrete surface is flush with the top surface of the steel plate, and there must be no gaps to ensure uniform transmission of vertical force on the steel column.
(4) For earthquake-proof concrete pouring in steel pipes, the dropping height of the concrete should strictly comply with the requirements of the concrete manufacturer and experimental tests to avoid discreteness or lack of density caused by the dropping height being too large or too small.
6. Safety measures
(1) Before construction, a construction organization design or construction plan should be prepared to clarify safety technical points and technical measures to ensure safety. Safety technical education and safety technical briefing should be conducted before construction of each process.
(2) All construction workers must wear safety helmets, and workers working at heights must wear safety belts, non-slip shoes, and tool bags.
(3) Common safety hazards of manually dug piles mainly include suffocation and poisoning, electric shock, blind firing, collapse, and falling. During the construction process, safety inspections should be conducted based on the above conditions.
(4) Before starting the hoisting work, the specifications and technical performance of the lifting equipment and the rigging, clasps, clamps, clamps, etc. used should be carefully inspected or tested, and any damage or looseness should be found. , should be replaced or repaired immediately.
(5) All power lines and electrical equipment must be installed by certified electricians, and a dedicated person will be assigned to take charge during the concrete pouring period to ensure the normal supply of electricity and the safety of electrical equipment; other personnel are not allowed to cause chaos without permission. Connecting and pulling wires randomly.
7. Conclusion
This construction technology is applied to the No. 4 block project in the middle area of ??the urban village reconstruction project in Gangxia Heyuan District, Shenzhen City. By using the reverse steel column construction method , effectively guaranteeing the construction quality of steel columns, thereby ensuring the overall quality of the inverse structural system, shortening the construction period and saving construction costs to a great extent. The construction quality of steel columns has been recognized by the construction unit, design unit and supervision unit It has received good reviews and is worthy of reference and promotion in similar construction projects. It has played a very good role in promoting and popularizing the increasing number of reverse construction methods in the current society, making the reverse construction method easier to operate and adopted by more builders and designers. It is also suitable for the construction of highly dense urban agglomerations. has a very positive meaning.
References
[1] "Technical Regulations for Reverse Construction of Underground Construction Engineering" (JGJ165-2010).
[2] "Construction Quality of Concrete Structure Engineering" Specifications for Acceptance of Steel Structure Projects" (GB50204-2011).
[3] "Uniform Standards for Quality Acceptance of Construction Projects" GB50300-2001.
[4] "Specifications for Construction and Acceptance of Steel Structure Projects" 》GB50205-2001.
[5] "Basic forms and dimensions of manual arc welding joints" (GB985.1-2008). ;