Judging from the development status of most super-high-rise building industries in China, “outer frame + core tube” is basically adopted. The appearance of this core tube structure design concept puts forward higher requirements for the construction safety of the main structure. Combined with practical engineering projects, we analyze the safety risks and countermeasures of the outer frame column of the super high-rise steel structure, the safety risks and countermeasures of the cantilever steel beam, the safety risks and countermeasures of the waist truss, the safety risks and countermeasures of the waist truss, the safety risks and countermeasures of the outrigger truss, the safety risks and countermeasures of the tower crown structure are discussed one by one. It ensures the organic combination of engineering safety measures and engineering construction technology. The proposed safety measures are highly targeted and feasible, ensuring the safe construction of the project. The comprehensive application of this project confirms the feasibility of relevant countermeasures, so as to provide reference and help for subsequent similar projects.
1 Introduction
The steel structure of this project is mainly distributed in the second phase tower, the surrounding podium and the main structure of the third phase tower. Total steel consumption is about 73340 t. The second phase tower has ninety-nine floors on the ground, with a total height of 468 m, and the height of the main structure (i.e. frame structure) on the ground is 440 m, which is an outer frame with waist truss, core tube (steel support) and outrigger truss structure Among them, the number of waist trusses is 6, which are located on floors 15 to 17, 31 to 33, 47 to 49, 63 to 65, 79 to 81 and 98 to 99, respectively. The steel structures around the skirt building are mainly rigid steel reinforced concrete beams and columns, steel beams, steel columns and trusses structure. The engineering schematic diagram is shown in Fig. 1.
Fig. 1
Schematic diagram of steel structure engineering
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2 Analysis of Key and Difficult Points in Overall Safety Protection of the Project
The main safety injury forms in the steel structure construction of this project [1] are falling from a high altitude, object impact, electrical injury, fire, mechanical injury and poisoning. According to the statistical data of safety accidents in the construction industry, the accidents of falling from a high altitude and objects hitting account for 90% of the total safety accidents, which shows that the risk level of falling from a high altitude and objects hitting is extremely high.
(1)
Safety risks of falling and falling objects
a.
When working at heights, you must wear your seat belt and safety helmet as required.
b.
When working at heights, tools should be loaded into tool bags, which can be taken with you.
c.
When working at heights, the tool handle must wear a rope and put it on the safety belt or wrist.
d.
When working at heights, the removed connecting plates and other wastes shall be cleaned up to the ground in time, and shall not be thrown at will.
(2)
Safety risks of using electrical equipment
a.
Workers use electricity illegally.
b.
Illegal use of electric welding machines or other electrical equipment.
c.
When the electric welding machine is used, it is required that the welding handle wire and the ground wire should be in place, and the welding handle wire [2] should not exceed 30 m. The length of the primary connection between the welding machine and the welding machine is not more than 5 m. If the welding wire is broken, it must be wrapped in insulating tape for three times.
(3)
Safety risks of hot work
a.
Before welding and cutting operations, clean up flammable and explosive materials around and below the operations.
b.
For welding operation at high places, special personnel should be assigned to monitor the welding operation below, and fire connection or isolation measures should be taken.
c.
It is forbidden to smoke in the operation area of the construction site, especially around inflammable and explosive materials.
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The hot work [3] can only be started after the hot work application is allowed. The caretaker and fire control measures should be implemented, the surrounding conditions of hot work should be checked, and inflammable and explosive articles should be cleaned up. Smoking is strictly prohibited when entering the operation area of the construction site, especially around inflammable and explosive materials. Welding and cutting operations are not allowed above flammable and explosive materials such as paint and thinner. When welding at high places, special personnel should be assigned to supervise the welding below, and fire connection or isolation measures should be taken.
(4)
Safety risks of lifting operations
a.
Workers operate illegally, command illegally or violate operating rules.
b.
Safety risks of hoisting dangerous areas. In hoisting dangerous areas, there must be a special person to supervise them, and non-construction personnel are not allowed to enter the dangerous areas. Dangerous areas for hoisting should be designated as warning areas and protected by warning ropes. No one can stand under the lifting object.
c.
Safety risks of operating cranes in violation of regulations. About lifting heavy objects, the hook should be 90° with the ground, and it is forbidden to hang obliquely; Horizontal lifting is strictly prohibited.
d.
Safety risks of using hoisting equipment. At the crane station, ensure that the foundation has sufficient bearing capacity; The rotating part of the crane should have a space distance of not less than 1 m from the surrounding fixtures.
(5)
Poisoning safety risks
a.
When welding the steel reinforced column in the outer frame column, set a fume extractor on the top of the column or open a fume window on the outer frame column to remove toxic gas.
b.
Welders must wear masks to enter the column for welding construction, and assign special personnel to take care of them. In case of poisoning, stop the construction immediately.
c.
When the welder is working in the column, if there is dizziness, vomiting and other phenomena, immediately stop the operation and have a physical examination.
3 Safety Risks of Columns and Countermeasures
Security risks:
Large-scale component hoisting, falling prevention from high altitude, welding poisoning and welding fire prevention.
Response measures:
a.
The outer frame column component is large in volume, with the maximum weight of 53 t. Before hoisting, the hoisting lug plate must be tested for damage, and if it is damaged, the hoisting lug plate must be welded again.
b.
When hoisting the hook [4], check whether the wire rope of the hook is worn or damaged, and check and calculate whether it meets the hoisting requirements according to the hoisting manual. If it does not meet the requirements, the wire rope must be replaced.
c.
Before the outer frame column is hoisted, the steel column must be equipped with an installation operation platform, a ladder and safety protection measures, and the anti-falling rope must be welded, and the ladder must be tied up and dirt cleaned.
d.
In the process of hoisting, the lifting of components must be smooth, and the components must not be dragged on the ground, and stay for one minute immediately after they leave the ground; When turning, you need to have a certain height. Hooking, rotating and moving are carried out alternately and slowly, and when in place, they will fall slowly to prevent the components from swinging and oscillating greatly.
e.
When the outer frame is installed at high altitude, the temporary bolts must be fastened and installed, and the double-splint and self-balancing double-splint butt joint technology should be used to ensure the stable connection of steel columns. Operators must hang safety belts on the operating platform to operate, temporarily fix the components with connecting plates, and pull three cable wind ropes. Two cable wind ropes are fixed on the ear plates of the two pillars next to the next floor, and the other cable wind rope is fixed on the embedded parts of the next core tube.
f.
When welding in the outer frame column, set a fume hood on the top of the column or open a fume window on the outer frame column to remove toxic gas and prevent poisoning incidents.
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4 Safety Risk and Countermeasures of Cantilever Steel Beam
Individual H-beam [5] parts of cantilever steel beams are manufactured in the factory and assembled into units on site, and the units are installed on site. During installation, a hanging basket shall be installed at the node of cantilever beam. On the ring beam (main beam) hanging between the steel pipe columns of the outer cylinder, an inverted chain is set at the cantilever section of the cantilever beam to adjust its levelness. Its form is shown in Fig. 2.
Fig. 2
Cantilever steel beam
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Safety risks: falling prevention from high altitude and welding fire prevention.
Response measures:
a.
Cantilever steel beams are hoisted with snap rings, and the snap rings must be locked when tying hooks.
b.
During hoisting and installation, the installer must hang up the seat belt according to the regulations to prevent falling from the building.
c.
The bottom and periphery of the hanging basket are covered with dense mesh steel wire mesh, and asbestos cloth is laid at the bottom to seal.
5 Safety Risk of Waist Truss and Countermeasures
Waist trusses are located in towers 15F-17F, 31F -33F, 47F -49F, 63F-65F, 79F-81F and 98F-99F. Except for floors 98F-99F, other waist trusses are of two structural floors, which are connected with outer frame columns to form an outer frame. Typical waist truss installation is carried out by sections according to parts and K-shaped units. The schematic diagram of waist truss is shown in Fig. 3.
Fig. 3
Schematic diagram of waist truss
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Safety risks: hoisting of large components, falling prevention at high altitude, welding at high altitude.
Response measures:
a.
Some nodes of the waist truss are assembled for K-byte point hoisting, which is large in size, high in installation precision and difficult to tie hooks. Before hoisting, the hoisting lug plate must be tested for damage, and if it is damaged, the hoisting lug plate must be welded again [6].
b.
When hoisting the hook, check whether the wire rope of the hook is worn or damaged, and check and calculate whether it meets the hoisting requirements according to the hoisting manual. If it does not meet the requirements, the wire rope must be replaced.
c.
In the process of hoisting, tower crane commanders must operate in accordance with relevant hoisting regulations.
d.
When the waist truss diagonal web bar is installed, the upper operating surface is below the operating platform, and the operator must hang the safety belt. Use scaffolding operation platform, and use three wooden springboard as walking road. During welding construction, lay fire-proof asbestos cloth on the wooden springboard and cover it with wind-proof cloth.
e.
During the installation of diagonal web members of waist truss, stirrups must be set up for support due to the long members and heavy weight. Stirrups are set up on the upper surface of the outer frame steel beam on the floor level, below the center point of the diagonal web member. When the diagonal web member is welded into a stable structure, the stirrup is cut off and transported to the ground for the installation of the next waist truss.
6 Safety Risks and Countermeasures of Outrigger Truss
The outrigger truss connects the outer tube column with the core tube, and the chord runs through the web wall of the core tube. The cross sections of the upper and lower chords and web members are H-shaped, box-shaped and steel plate composite cross sections. There are four types of outrigger trusses: OT1, OT2, OT3 and OT4. The outrigger trusses are distributed at 31F -33F, 47F -49F, 63F-65F, 79F-81F. The installation diagram of outrigger truss is shown in Fig. 4.
Fig. 4
Installation diagram of outrigger truss
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Security risks: hoisting of large components, assembling of jig and assembling of parts.
Response measures:
When the outrigger truss is a loose part, there are many hoisting times. Before hoisting, it is necessary to detect whether the hoisting lug plate is damaged. If it is damaged, it is necessary to weld the hoisting lug plate again.
a.
When hoisting the hook, check whether the wire rope of the hook is worn or damaged, and check and calculate whether it meets the hoisting requirements according to the hoisting manual. If it does not meet the requirements, the wire rope must be replaced.
b.
In the process of hoisting, tower crane commanders must operate in accordance with relevant hoisting regulations.
c.
Because the outrigger truss is installed in pieces, the lower suspension rod of the truss must be supported by a jig.
7 Safety Risk of Tower Crown Structure and Countermeasures
The top structure of the tower is composed of steel frame and peripheral structure. The outer structure is parabola-shaped, which is composed of thirteen vertical trusses and connected steel beams between trusses. The total steel consumption of the top structure is about 300 t, and the main sections of members are H-shaped steel and steel pipes, with the largest sections of H800 × 400 × 25 × 40 and CHS300 × 15. The floor top structure has a bottom elevation of 439.750 m, a top elevation of 468.000 m and a total height of 28.250 m. The schematic diagram of steel structure at the top of the tower is shown in Fig. 5.
Fig. 5
Schematic diagram of steel structure at the top of the tower
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Safety risks: falling prevention from high altitude and welding fire prevention.
Response measures:
a.
The horizontal network of the floor [7]. During the construction of the steel frame inside the roof steel structure, horizontal protection is carried out by laying horizontal nets layer by layer. The steel frame inside the roof is composed of four interlayers. During the construction of each layer of steel frame, the steel beams of each part are basically installed in place and then closed with horizontal nets. Then, the installation progress is followed gradually until the whole floor is closed.
b.
Outside the edge of the net. When the roof structure is hoisted, an external hook net is set at the periphery of the bottom of the roof structure. A total of two floors are set up at the outside of the safety net for 3 m, a horizontal net is laid at the bottom of the outside net, a dense mesh net is laid on the horizontal net, and a steel wire net is laid under the horizontal net to form three layers of protection. The external net support frame is made of φ 48 × 3.5 scaffold steel pipe, which is fixed on the peripheral steel column by welding and wire rope.
c.
Operating platform. The peripheral structure is installed in two layers, and the channel operation platform is set up at the top of the first layer structure. The operation platform is made of steel pipes, safety nets are hung on both sides of the platform, and scaffolding boards are laid at the bottom of the platform. The platform is connected with the steel beam through a snap ring welded on the surface of the steel beam, which is mainly used for welding the upper and lower peripheral structures, and also serves as a high-altitude passage.
8 Conclusions
We studied the overall safety risks and main structural safety risks of super-high-rise steel structure construction, put forward specific safety measures, and applied them to engineering practice, and achieved good results.
Acknowledgements
This paper supported by the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No.KJQN202204303).
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