Template collapse accident is one of the hazards that easily lead to group casualties in building construction. Especially with the development of urban modernization, there are more and more high-rise buildings, some of which are more than 4.5m high. Formwork projects with fastener-type steel pipe formwork support frame collapse frequently, causing heavy casualties and property losses.
June, 2000 1 65438+1October, 65438+June, the roof platform of the boiler room where a textile company was pouring concrete suddenly collapsed, resulting in 1 1 person's death, 2 serious injuries and1person's minor injuries, with direct economic losses reaching 257. Roof bent adopts φ 48× 3.5 steel pipe. The erection of the framework of thermal power plant is basically based on the main beam axis, and two vertical poles are set on both sides about 0.5m away from the beam axis. The rest of the frame is evenly distributed, and the spacing is not more than1.8m. The actual maximum spacing between vertical poles is about 1.7m, and the vertical spacing between horizontal poles is about1.8m. Most vertical poles are overlapped by butt fasteners, only near the roof formwork, and two rotating fasteners are used for height adjustment. According to the investigation, there is a lack of a horizontal bar every other step in the vertical and horizontal direction, and there is no continuous vertical and horizontal bracing in the bent support. There are no design calculation documents and written technical documents to guide the construction of bent frame installation.
According to the plane analysis of the roof structure, the maximum bearing area of bent frame is about 3.25 square meters (2.5× 1.3m), and the supporting columns at its lower part are two φ 48× 3.5 steel pipes. According to the Technical Specification for Safety of Fastening Steel Pipe Scaffolding in Building Construction (JJ130—2001), the mechanical analysis of scaffolding is as follows:
(A) load analysis
(1) structural static load
Main girder 0.4×1.5×1.3 = 0.78m3.
Secondary beam 0.25×0.9×2.5=0.563 cubic meters.
Floor [(2.5×1.3)-(0.4×1.3)-(2.1.25)] × 0.12 = 0.27 cubic meters.
(0 . 780 . 5630 . 27)×25000 = 40325n………①
(2) Wood formwork load
3.25×300 = 975 Newton ②
(3) Construction live load
3.25× 1000 = 3250 Newton ③
1.2[①②] 1.4③= 54 1 10N
The load transferred to a single vertical pole is 27055n·n N.
(2) Stability analysis
The design of steel pipes and fasteners as bent supports must be analyzed and calculated in detail, which is a mandatory requirement in construction safety management. It is especially important for this kind of high support with a safety support height greater than 4.5m m, and whether the actual erection treatment of formwork support is close to the theoretical calculation assumption is also an extremely important calculation basis. Because of the use of fasteners to connect each other, each node has a series of variable factors, such as eccentricity, displacement, insufficient fastening torque and so on. All these will affect the determination of the slenderness ratio of components in calculation. The slenderness ratio is difficult to determine, which directly affects the stability analysis of components. Therefore, the assumption of calculation mode in practical engineering must be strictly based on the most favorable angle for safety.
Because according to the requirements of the code, the slenderness ratio of axially compressed members should be limited to λ= 150, that is, when the calculated length of steel pipe is l0≤2370mm, the requirements of the code can be met. In order to meet this requirement, it is necessary to form a good hinged state at both ends of the calculation rod (such as installing horizontal scissors to form a state of strong horizontal stiffness). Although the step distance of supporting the horizontal bar in this example is required to be 1800mm, the actual horizontal bar is "missing every other step in the vertical and horizontal direction", so it is checked as "good hinge state" according to the height of 2 steps, λ = (2× 6500 mm
Stability checking: n/(a) = 378.8 n/m m2 > f = 205 n/m m2.
It is not difficult to see from the above analysis that the technical reasons for the accident mainly include:
1. In this case, the design elevation of the roof is relatively large, reaching 2 1m from the ground and 16.5m from the platform on the second floor (elevation 4.50m). So the vertical height of the bracket is relatively high, so? When 48×3.5 steel pipes are used as uprights, the horizontal spacing between 1.0~ 1.7m is obviously too large, and the vertical spacing between horizontal poles is1.8m. There is no continuous vertical and horizontal bracing, which leads to poor integrity of the supporting system, that is, a reliable spatial stress structure is not formed.
2. When the impact force of pumping concrete on formwork support is not included in the construction, the stress on the support stud has reached more than 27kN. Assuming that the calculated length of the steel pipe column is 3600mm, the calculated value in the steel pipe stability checking calculation reaches 378.8 N/m m2, which greatly exceeds the design strength value of 205 N/m m2 and the yield strength value of 235Nm steel. Therefore, the support column is unstable and the collapse accident is inevitable.
In China, fastener-type duct formwork support is a common support method in building construction. However, due to the lack of corresponding professional standards for design and calculation, there are uncertain and unsafe factors in the existing design and calculation, especially the beam-slab formwork support with a support height of more than 4.5m m. Due to the imperfection of safety technology and accident plan, formwork collapse accidents frequently occur. In addition, the construction site lacks the necessary emergency rescue system, and the collapse accident cannot be rescued in time, causing a large number of casualties. In order to prevent formwork collapse accidents, ensure the use safety of fastener-type steel pipe formwork support and the safety of construction personnel, it is necessary to analyze and discuss the preparation content of similar accident plans and the establishment of emergency rescue system.
The first part plan preparation
(a) Structural measures to ensure the stability of the framework
As for the design and calculation of fastener-type steel pipe formwork support, only in the recently issued Technical Specification for Safety of Fastener-type Steel Pipe Scaffolding in Building Construction (JJ130-2001), some calculation provisions are made, but the provisions are based on the calculation method of foreign approximate "geometrically invariant bar system structure" mechanical model. Because the structural requirements of the commonly used fastener-type steel pipe formwork support frame in China's current relevant standards are not as strict as those in foreign standards, and the installation quality of fastener-type steel pipes is greatly influenced by human factors, it is difficult for the fastener-type steel pipe formwork support frame built according to the prescribed traditional habits to meet the mechanical requirements of "geometrically invariant bar system structure". Therefore, if the supporting frame is designed and calculated according to the current code, the overall stiffness of the frame must be improved by structural means to ensure the safety of the frame.
(1) Vertical and horizontal sweeping poles and horizontal poles must be set under the beam. Because according to relevant tests, if these two bars are not set, the ultimate bearing capacity of the vertical pole will be reduced by 1 1. 1%. When setting, it should be noted that the vertical sweeping pole should be fixed on the vertical pole not more than 200mm away from the basic epithelium, and the horizontal sweeping pole should be fixed on the vertical pole directly below the vertical sweeping pole with right-angle fasteners.
In order to ensure the overall stability of the vertical pole, it is necessary to set both longitudinal and transverse horizontal poles when installing the vertical pole.
(2) The spacing of support frames should be 0.9~ 1.5m, and the maximum spacing should not exceed1.8m. Because there is an approximately inverse linear relationship between the spacing of support frames and the ultimate bearing capacity of vertical poles, when the construction load is heavy, properly reducing the spacing of vertical and horizontal horizontal poles to reduce the slenderness ratio of vertical poles can give full play to the strength of steel pipes and make them more economical and reasonable. According to the calculation, if the calculated length of the bar is doubled, the ultimate bearing capacity will be reduced by 50%~70%.
(3) The method of docking and lengthening should be given priority to with formwork support columns. There are two ways to extend the vertical pole: butt joint and lap joint. According to relevant tests, the maximum bearing capacity of butt joint is more than three times that of lap joint.
In addition, it is worth noting that when the top vertical pole is connected by lap, the load on the formwork directly acts on the top cross bar of the support frame, and the force is transmitted to the vertical pole through the indirect friction between the fastener and the steel pipe. Moreover, because the force transmitted by the fastener is small and eccentric, the overall mechanical performance of the support frame is poor. At this time, the structural requirements of lap joint extension are: the spacing between fasteners is 800mm, and the allowable load of each upright is less than 12kN.
When erecting the support frame, it should also be noted that the extension positions of the vertical rod and the horizontal rod should be staggered between adjacent rods, not in the same step.
(4) The spacing between the vertical poles shall not exceed the supporting design regulations, and the maximum spacing shall not exceed 1m, and it shall conform to the current industry standard "Technical Specification for Safety of Fastening Steel Pipe Scaffolding in Building Construction" (JJ130-2001).
The supporting structure at the bottom of the vertical pole must have the ability to support the upper load. When the floor is used as the supporting structure, because the construction load borne by formwork supporting poles is often greater than the design load of the floor, it is necessary to keep two or more poles through calculation. In order to transfer the load reasonably, wooden mats should be set at the bottom of the vertical poles, and the upper and lower vertical poles should be on the same vertical line.
(5) The bracing must be set reasonably. Scissors are beneficial to improve the overall stability of the frame, especially for the supporting frame with a supporting height greater than 4.5m, the reasonable setting of scissors can effectively prevent the impact of pumping concrete on formwork support from causing the overall instability of the frame. According to relevant tests, the ultimate bearing capacity of the reasonably braced support system can be increased by 17%. Therefore, a full formwork support frame should be covered with vertical bracing along the facade around the frame body, and the vertical bracing should be arranged continuously from bottom to top. When the supporting frame is high, or the height-width ratio is greater than or equal to 6, in order to improve the overall rigidity of the frame, sweeping poles must be set at the top and bottom of the frame, and horizontal bracing should be set every 4~6m in the middle, and the bracing must be connected with the vertical poles.
(6) Strictly control the deformation of the support frame to ensure the stability of the frame body. In addition to the deformation of the frame caused by the bearing of the frame, the uneven settlement of the foundation leads to the uneven stress of the vertical pole and local instability. The deformation of the supporting beam at the lower part of the formwork is too large, which will also cause the deformation of the supporting frame.
When the special structure is under construction or the supporting load is heavy, the supporting frame should be unloaded through the adjacent components (walls, columns, etc.). ) as far as possible with a certain strength, and as far as possible with the building reliable connection.
(2) Management measures to ensure construction safety
(1) formwork support works must be designed before construction. The design contents shall include:
A. Strength calculation of supporting system
When calculating, you should consider:
(1) formwork and supporting weight
(2) the weight of concrete and steel bar.
(3) Load of construction personnel and equipment
(4) Concrete dumping and vibrating load.
⑤ Wind load
And calculated according to the most unfavorable state and load combination.
Also check the fastener connection points. The sliding resistance of single fastener is less than 8.5kN, and that of double fastener is less than 12kN.
B. Strength calculation of formwork support system for building and floor support.
C. Structural measures such as selection of supporting materials, specifications and dimensions, connection mode, step distance of horizontal bar, and bracing setting.
D. draw the supporting layout and detailed structural details.
E, concrete pouring methods and procedures, template support installation and removal sequence and other safety technical measures.
Methods and standards for installation and acceptance of supporting systems.
(2) The formwork support project construction will be included in the scope of hazardous operation management. Before issuing the "Concrete Pouring Order", in addition to the formwork system, the supporting system must also be accepted as a whole, and technical designers must participate in the acceptance.
(3) Carefully design the concrete pouring scheme to ensure the stress balance of formwork support, and give priority to the method of pouring from the middle to all sides. In the process of concrete pouring, professional and technical personnel should be sent to observe the stress and deformation of formwork and support system, and stop work immediately if any abnormality is found.
(4) The accident plan must be made for the key prevention parts.
The key parts to prevent the collapse of fastener-type steel pipe formwork support generally include:
(1) Support frame with height greater than 4.5m or height-width ratio greater than or equal to 6.
② Projects with great social impact. Such as urban centers, densely populated areas and major public facilities projects.
(3) Special structural engineering such as large-span and large-section frame beam, large-section cantilever beam slab and large-span and large-area cast-in-place beam slab structure.
(4) Projects with harsh working environment, concentrated construction personnel and difficult rescue.
(3) The basic contents and requirements of the preparation of the collapse accident plan for the fastener-type steel pipe formwork support.
(1) Basic contents of the plan
① General situation of key prevention parts
A. Regional position, surrounding environment and construction access of key prevention parts.
B nature of work, number of workers, tools used, working methods, etc. Key prevention part.
(2) Construction sequence of key prevention parts
List in detail the operating procedures of each operation and the types of work involved.
(3) Focus on hidden dangers in the construction process.
A. The possible adverse consequences of each behavior in the construction process and the types of accidents that may be caused.
B. the possible scope of the accident.
④ Control measures and responsible persons
Conduct safety analysis on hidden dangers, and formulate corresponding control measures to ensure operation safety. In order to ensure the implementation of control measures, it is necessary to clarify the corresponding responsible person.
⑤ Rescue measures
A. According to different construction behaviors, personnel numbers, event types and locations, formulate corresponding rescue methods.
B. formulate measures to prevent the situation from deteriorating after the incident.
C formulate measures to prevent the deterioration of the situation according to various situations that may occur in different stages of the accident.
D determine the methods and routes for rescuing and evacuating people and materials, as well as the methods for emergency contact and communication.
E. Matters needing attention in the rescue process.
(2) the basic requirements of the plan
① Be targeted and practical.
According to different engineering characteristics, different construction methods, different construction machines and tools, different working properties and different construction environments. The plan should run through the whole construction process and strive to be detailed, comprehensive and specific. The measures should be simple and practical, with strong operability and practicality.
② Determine the most unfavorable state and make scientific calculation and analysis.
On the basis of extensive investigation, the key prevention parts are determined, and the strength calculation and stability analysis of the formwork support are carried out under the assumed most unfavorable state. And make a scientific evaluation of the possible hidden dangers of accidents, and provide accurate basis for the formulation of rescue measures.
(3) Draw the plan implementation network diagram.
According to the hypothetical accident situation, the preventive measures are determined, and the plan implementation network diagram is drawn for operation and implementation. After the scheme is formulated, it must be reviewed and qualified before it can be put into use.
(4) It shall be revised in time with the change of construction conditions.
After the scheme is approved and implemented, if the construction situation changes, it should be revised in time to meet the safety needs under the new situation.
The second part is the establishment of emergency rescue system.
After the collapse of formwork support, construction workers often die of respiratory failure due to inhalation of foreign bodies. In this case 1 1 death cases, 8 people died due to foreign body inhalation, accounting for 73%. It can be seen that the failure to carry out effective rescue after the accident is another cause of the tragedy, so establishing an emergency rescue system is an effective measure to reduce casualties. The emergency rescue system for formwork support collapse accident shall include:
(1) Emergency facilities
(1) accident alarm system
An alarm device used for on-site rescue when an accident occurs. The accident alarm system must be connected with the office area, duty room, doorman and other main positions on the construction site.
(2) Support pressure monitoring and automatic early warning system.
It is used to automatically monitor the stress and deformation of formwork support after being stressed, and give an alarm once the stress and deformation exceed the warning value.
(3) Emergency rescue tools
It is mainly a lifting and cutting tool for clearing dam break. Such as jacks, cranes, slings and metal cutters.
(4) Emergency lighting
Used for emergency power failure and rescue lighting at accident site.
(5) Basic emergency medical rescue equipment such as first-aid medicine box and stretcher.
(2) Emergency contact and communication
When an accident requires external rescue, you should call 1 19 alarm and medical rescue 120 in addition to starting the on-site alarm system. At the same time, contact relevant departments according to the communication method specified in the plan.
(3) Emergency evacuation methods
Give priority to the safety of personnel when evacuating, do not rush to rescue property, and evacuate to a safe area in an orderly manner according to the specific situation on site.
(4) Emergency Working Group
The purpose of the emergency working group is to deal with the major dangers that may occur on the construction site. Its purpose is to reduce casualties and pay attention to property losses and environmental pollution. Members of the emergency working group must be trained in emergency rescue knowledge such as emergency medical rescue and accident plan, and can master and use it skillfully. The emergency working group shall regularly organize training and drills.
Composition and responsibilities of the emergency working group:
(1) rescue team: it is mainly responsible for the rescue, evacuation, danger removal and rescue of personnel and materials.
(2) Accident handling team: use all kinds of safe and reliable means according to the accident plan to control the development of the accident quickly. According to the specific situation of the site, the rescue team should be equipped with corresponding rescue methods and necessary rescue tools and conditions.
(3) Liaison Group: responsible for accident alarm and report, as well as on-site rescue liaison and logistics supply, and assisting external professional rescue units to carry out rescue. Command, count and contact all kinds of personnel.
(4) Alert Group: mainly responsible for security alert tasks, maintaining order at the scene of the accident, persuading or evacuating onlookers at the scene, and prohibiting outsiders from breaking into the site protection zone.
(5) General principles of emergency rescue
To ensure the safety of personnel as the first priority, followed by controlling material losses. The most important thing in emergency rescue is speed, because most people who died in collapse died of suffocation, so rescue time is life. In addition, it is necessary to cultivate the awareness of the construction personnel to correctly handle risks. Anyone who finds a dangerous situation should immediately report it with the accident alarm system. Emergency rescue personnel must be members of the emergency working group, and other personnel should be evacuated to a safe area and obey the instructions of the members of the emergency working group.
(six) first aid knowledge and technology
In view of the fact that the injuries caused by the collapse of formwork support are mainly respiratory failure caused by mechanical asphyxia and central nervous system failure caused by craniocerebral injury, members of the emergency working group must master the first aid knowledge and technology such as hemostasis and dressing, fracture fixation, treatment of the wounded, cardiopulmonary resuscitation and so on.
Collapse is one of the "four major injuries" in building construction, and formwork support engineering is one of the main hazards of frequent dam collapse accidents. Therefore, the Ministry of Construction issued the "Notice on Preventing the Collapse of Building Formwork", requiring it to be the main content of the special treatment of building safety. Through the analysis of a recent dam-break accident of formwork support, this paper puts forward some analysis and requirements for the design, structure, management, accident plan and rescue of formwork support engineering in Shanghai, so as to improve the safety technology of formwork support engineering construction.
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