Analysis on prevention and rescue measures of the

Analysis on prevention and rescue measures of formwork collapse accident

formwork collapse accident is one of the risk sources that are very easy to cause mass casualties in construction. Especially with the development of urban modernization, more and more workpieces with large storey height can be conveniently placed on the workbench, and some of them are more than 4.5m high, and the formwork engineering with fastener type steel pipe formwork support frame has frequent collapse accidents, Causing heavy casualties and property losses

on November 16, 2000, a dam collapse accident occurred suddenly on the boiler building surface platform of a textile company that was pouring concrete, causing 11 deaths, 2 serious injuries and 1 minor injury, and the direct economic loss reached 2.575 million yuan. The roof bent support adopts Φ forty-eight × 3.5 steel pipe erection. The erection of the thermal power plant is basically based on the axis of the main beam. Two vertical poles are set on both sides about 0.5m away from the axis of the beam. The spacing of the remaining parts is evenly distributed based on the principle that the spacing does not exceed 1.8m. The actual maximum spacing of the vertical poles is about 1.7m, and the vertical spacing of the horizontal poles is about 1.8m. Most of the vertical poles are overlapped up and down with butt fasteners. Only at the position close to the roof panel formwork, two rotating fasteners are used for overlapping in order to adjust the height. According to the survey, the horizontal bar is missing at every other step in the vertical and horizontal direction, and there is no continuous vertical and horizontal diagonal bracing in the bent support. There are no design calculation documents and written technical documents guiding construction for bent erection

according to the plane analysis of roof structure, the maximum load-bearing area of bent support is generally 3.25m2 (2.5 × 1.3m), with 2 lower supporting poles Φ forty-eight × 3.5 steel pipe. The following is the mechanical analysis of the scaffold body according to the technical code for safety of steel tubular scaffold with couplers in construction (jgj130-2001):

(I) load analysis

(1) structural static load

main beam 0.4 × one point five × 1.3=0.78m3

secondary beam 0.25 × zero point nine × 2.5=0.563m3

floor [(2.5 × 1.3） ­ （0.4 × 1.3） ­ （2.1 × 0.25）] × 0.12=0.27m3

（0.78+0.563+0.27） × 25000=40325n............... ①

(2) wood formwork load

3.25 × 300=975n................................. ②

(3) construction live load

3.25 × 1000=3250n................................. ③

1.2 [① + ②] +1.4 ③ =54110n

the load transmitted to a single pole is 27055n

(II) stability analysis

detailed stress analysis and calculation must be carried out for the support design with steel pipes and fasteners as bents, which is a mandatory requirement in construction safety management, which is particularly important for the high support with the safety support height greater than 4.5m. While the actual construction of template support, the new generation of information technology industry should focus on strategic and leading industries such as software, integrated circuit, new display, cloud computing, big data, virtual display, green computing, artificial intelligence and intelligent hardware; The new material industry should strive to break through a batch of advanced basic materials urgently needed in key utilization fields, such as advanced steel materials and petrochemical materials. Whether the theory and theoretical calculation assumptions are similar is an extremely important calculation basis. Due to the mutual connection of fasteners, there are a series of variable factors in each node, such as eccentricity, displacement, insufficient solid torque, etc., which affect the determination of the slenderness ratio of the member in the calculation. It is difficult to determine the slenderness ratio, which directly affects the stability analysis of members. Therefore, the assumption of calculation mode in practical engineering must be strictly based on the perspective that is most conducive to safety

as the slenderness ratio of axial compression members shall be limited to λ= Within 150, that is, when the calculated length of steel pipe l0 ≤ 2370mm is considered, it can meet the specification requirements. To meet this requirement, it is necessary to form a good hinged state at both ends of the calculated rod (such as the installation of horizontal diagonal bracing, forming a state of strong horizontal stiffness). Although the step distance of the supporting horizontal bar in this case is required to be 1800mm, the actual horizontal bar "is missing every other step distance in the vertical and horizontal direction", so the two step height is used as the "good articulated state" for checking calculation, then λ= （2 × 1800)/15.78=228, check the table, =0.146

stability checking calculation: n/(· a) =378.8n/mm2 f=205n/mm2

it is not difficult to see from the above analysis that the main technical reasons for the accident are:

1. The design elevation of the roof in this case is large, up to 21m from the ground, and the distance from the second floor platform (elevation +4.50m) is 16.5m. Therefore, the vertical height of the support is large, so the use of ø 48 × 3.5 the steel pipe is used as a vertical pole, the horizontal spacing of 1.0~1.7m is obviously too large, the spacing between the upper and lower horizontal poles is 1.8m, and there is no continuous vertical and horizontal diagonal bracing, resulting in the poor integrity of the support system, that is, there is no reliable spatial stress structure

2. When the impact force of pumping concrete directly on the formwork support during construction is not included, the force on the support pole has reached more than 27kn. Assuming that the calculated length of the steel pipe upright is 3600mm, the calculated value in the steel pipe stability checking calculation has reached 378.8n/mm2, which has greatly exceeded the design strength of 205n/mm2 and the yield strength of 235nmm2 of ~235 steel. Therefore, the support pole is unstable, and the collapse accident is inevitable

in China, the fastener type air duct formwork support frame is a common support method in building construction, but 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 for the beam and slab formwork support frame with a support height greater than 4.5m, due to the imperfection of safety technology and accident plan, formwork collapse accidents occur frequently. In addition, the construction site lacks the necessary emergency rescue system, which cannot be rescued in time after the collapse accident, resulting in a large number of casualties. In order to prevent the occurrence of the formwork collapse accident and ensure the use safety of the fastener type steel pipe formwork support frame and the safety of the construction personnel, it is necessary to analyze and discuss the preparation content of the plan for similar accidents and the establishment of the emergency rescue system

part I preparation of the plan

(I) structural measures to ensure the stability of the scaffold

about the buckle continuous experiment: the design calculation of the one-piece steel pipe formwork support frame after the parameter setting of a batch of samples is completed, only some calculation provisions are made in article 5.6 of the recently issued and implemented technical code for safety of steel pipe scaffold with couplers in construction (JGJ), However, the regulations refer to the calculation method of the foreign mechanical model of approximate "geometrically immutable frame structure". Because the current relevant standards in China have less stringent requirements on the structure of the commonly used fastener steel pipe formwork support frame than foreign standards, and the installation quality of fastener steel pipe is greatly affected by human factors, It makes it difficult for the fastener type steel pipe formwork support frame erected according to the traditional custom to meet the mechanical requirements of "geometrically immutable bar structure". Therefore, if the support frame is designed and calculated according to the current specifications, the overall stiffness of the frame must also be improved by structural means to ensure the safety of the frame

(1) vertical and horizontal sweeping rods and horizontal rods under the beam must be set. Because according to relevant tests, if these two members are not set, the ultimate bearing capacity of the upright will be reduced by 11.1%. Attention should be paid to when setting: the longitudinal sweeping rod should be fixed on the vertical rod not more than 200mm away from the upper surface of the base with right angle fasteners, and the horizontal sweeping rod should be fixed on the vertical rod immediately below the longitudinal sweeping rod with right angle fasteners

in order to ensure the overall stability of the upright, it is also necessary to set vertical and horizontal horizontal poles while installing the upright

(2) the step distance of the support frame should be 0.9~1.5m, and the maximum should not exceed 1.8m. Because there is an approximately inverse linear relationship between the step distance of the support frame and the ultimate bearing capacity of the pole, when the construction load is large, appropriately reduce the step distance of the longitudinal and transverse horizontal pole to reduce the slenderness ratio of the pole, which can give full play to the strength of the steel pipe and make it more economical and reasonable. According to the calculation, if the calculated length of the member is doubled, its ultimate bearing capacity will be reduced by 50% - 70%

(3) the method of butt joint and extension shall be preferred for the upright rod of the formwork support frame. There are two ways of pole extension, butt joint and lap joint. According to relevant tests, the maximum bearing capacity of butt joint is more than 3 times that of lap joint

in addition, it is worth noting that when the top upright is connected by lap joint, the load on the formwork acts directly on the cross bar on the top of the support frame, and the force is transmitted to the upright through the indirect friction between the fastener and the steel pipe, and the overall stress performance of the support frame is poor because the fastener can transmit a small force and has a certain eccentricity. At this time, the structural requirements for lap joint and extension are: the fastener spacing should be 800mm, and the allowable load of each upright should be less than 12kn

when erecting the support frame, it should also be noted that the extension position of the vertical rod and the horizontal rod should be such that the adjacent rods are staggered and not in the same step

(4) the spacing of upright rods shall not exceed the provisions of support design, and the maximum shall not exceed 1m, and shall comply with the provisions of the current industry standard "technical code for safety of steel tubular scaffold with couplers in construction" (JGJ)

the supporting structure at the bottom of the 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 the formwork supporting pole is often greater than the design load of the floor, it is necessary to determine to maintain two or more layers of poles by calculation. In order to transmit the load reasonably, the wooden base plate should be set at the bottom of the pole, and the upper and lower poles should be on the same vertical line

(5) diagonal bracing must be set reasonably. Bridging is conducive to improving the overall stability of the frame, especially for the support frame with a support height greater than 4.5m. Reasonable setting of bridging can effectively prevent the overall instability of the frame caused by the impact of pumped concrete on the formwork support. According to relevant tests, the ultimate bearing capacity of the bracing system with reasonable cross bracing can be increased by 17%. Therefore, the full formwork support frame should be fully equipped with vertical diagonal bracing along the outer facade around the frame body, and the vertical diagonal bracing should be set continuously from bottom to top. When the support frame is high, or when the height width ratio is ≥ 6, in order to improve the overall stiffness of the frame, full horizontal diagonal braces must be set at the top and bottom of the frame, and every 4~6m in the middle, and the diagonal braces must be connected with the upright

(6) strictly control the deformation of the support frame to ensure the stability of the frame. In addition to the deformation of the frame caused by the load-bearing of the frame, the uneven settlement of the foundation leads to the uneven stress of the pole and local instability. The deformation of the support beam at the lower part of the formwork is too large, which will also cause the deformation of the support frame

when special structure construction or support load is large, the support frame should be unloaded through adjacent members (walls, columns, etc.) with certain strength as far as possible, and try to realize reliable connection with the building

(II) management measures to ensure construction safety

(1) formwork support works must be designed before construction. The design content should include:

A. calculation of support system strength

calculation should consider:

① formwork and support weight

② self weight of concrete and reinforcement

③ loads of construction personnel and equipment

④ loads caused by concrete dumping and vibration

⑤ wind load

and calculate according to the most unfavorable state and combination of loads

the anti sliding force of single fastener must be less than 8.5kn, and the anti sliding force of double fastener must be less than 12kn

b. strength calculation of the floor and ground supporting the formwork support system

c. structural measures such as the selection of support materials, specifications and sizes, joint methods, horizontal bar step distance and diagonal bracing settings

d. draw the support layout and detail structure

e. concrete pouring method and procedure, installation and removal sequence of formwork support and its