Safety decision analysis of collapse accident based on “accident tree–analytic hierarchy process”

2.1 Analysis of formwork and scaffold collapse

Through the analysis of the safety accident notifications in the past 10 years, it can be seen that the formwork and scaffold collapse accidents generally occurred in the formwork support stage, the use stage, and the demolding and dismantling stage.

1. Formwork support stage: the causes of the collapse at this stage are no construction plan or no formwork support according to the construction plan, unlicensed personnel for formwork support, no approval and disclosure, and excessive personnel or local concentrated accumulation of materials in the process of formwork support.

2. Use stage: the collapse at this stage is mainly caused by unqualified materials, structural errors, or improper formwork technology. The structural problems generally include insufficient bearing capacity due to the large three-dimensional size of the frame, insufficient scissors, no sweeping rod, insufficient support points, excessive spacing between vertical rods, and unstable connection points.

3. Demolding and dismantling stage: the collapse at this stage is mainly due to concrete that has not yet reached the design strength, demolding and dismantling sequence errors, and excessive local stress or excessive load on the frame.

2.2 Analysis of foundation pit earthwork collapse

3.1 Qualitative analysis of accident tree

3.1.1 Accident tree of formwork and scaffold collapse

According to the analysis of the causes of many collapse accidents, it can be seen that the collapse of the formwork and scaffold [40] may occur at the time of erection, use, and demolition. The basic events mainly affecting the collapse include 24 events, such as not setting up according to the construction plan, excessive load, and unqualified materials. The meaning of each symbol for the formwork and scaffold collapse accident is shown in Table 1. An FTA model was established for building collapse accidents, as shown in Figure 1.

Table 1

Sign meaning of formwork and scaffold collapse fault tree

Symbol	Meaning	Symbol	Meaning
T	Formwork and scaffold collapse	X 8	Unqualified materials for the backing plate
M 1	Collapse when supporting	X 9	Insufficient bearing capacity of wooden supports
M 2	Collapse in use	X 10	The three-dimensional dimension of the scaffold is too large
M 3	Collapse in dismantling	X 11	Inadequate diagonal bridging
M 4	False construction	X 12	Inadequate bottom horizontal tube
M 5	Heavy load	X 13	No plate
M 6	Unqualified material	X 14	Insufficient braced point
M 7	Structural error	X 15	Overlong hanging height of the top pole
M 8	Inferior process	X 16	False pouring order
M 9	False order of dismantling	X 17	Pump line vibration shaking the support frame
M 10	Weak connection point	X 18	Pole joint in the same horizontal plane
X 1	Weak concrete	X 19	No installment in the horizontal connection rod
X 2	Overloading due to excessive constructors	X 20	Insufficient pre-tightening torque of the fastener
X 3	Construction by unlicensed personnel	X 21	The pole is not vertical
X 4	Lax regulation	X 22	Adopting lap joint in the vertical pole
X 5	Disapproval and disclosure	X 23	A double fastener is not used for the highest node of a vertical bar
X 6	Overloading by centralized stacking of materials	X 24	Excessive spacing between vertical poles
X 7	Adopting inferior steel pipes and fasteners

Figure 1

Accident tree of formwork and scaffold collapse.

To determine the basic events that cause collapse accidents, the fault tree was decomposed by the Boolean algebra method to determine the minimum cut set of the fault tree:

T = M 1 + M 2 + M 3 = M 4 M 5 + M 6 + M 7 + M 8 + M 9 X 1 X 2 = X 2 X 3 + X 2 X 4 + X 2 X 5 + X 3 X 6 + X 4 X 6 + X 5 X 6 + X 7 + X 8 + X 9 + X 10 + X 11 + X 12 + X 13 + X 14 + X 15 + X 16 + X 17 + X 18 + X 19 + X 20 + X 21 + X 22 + X 23 + X 24 .

From the above formula, it can be concluded that there are 24 minimum cut sets of the formwork and scaffold collapse accident tree; that is, there are 24 basic events that cause the collapse of the formwork and scaffold. The formula also shows that there are 24 possible ways for the system to cause collapse accidents:

{ X 2 , X 3 } ; { X 2 , X 4 } ; { X 2 , X 5 } ; { X 3 , X 6 } ; { X 4 , X 6 } ; { X 5 , X 6 } ; { X 7 } ; { X 8 } { X 9 } ; { X 10 } ; { X 11 } ; { X 12 } ; { X 13 } ; { X 14 } ; { X 15 } ; { X 16 } ; { X 17 } ; { X 18 } ; { X 19 } ; { X 20 } ; { X 21 } ; { X 22 } ; { X 23 } ; { X 24 } .

After determining the minimum cut set of the fault tree, the structural importance analysis, which is the other part of the qualitative analysis of the fault tree, was conducted. This analysis involves the calculation of the contribution of each basic event to the top event when the probability of occurrence of each basic event is the same; thus, it belongs to the qualitative importance analysis. In the minimal cut set of the fault tree, the cut set of only one basic event is defined as a first-order minimal cut set; the cut set of two basic events is defined as a second-order minimal cut set; etc. Each minimum cut set represents a possibility of the occurrence of the top event and thus provides a basis for accident investigation and accident prevention. According to the minimum cut set of the fault tree calculated above, the basic event importance of the first-order minimum cut set is the largest, and the others can be calculated according to the approximate discriminant:

(1) I ϕ ( X i ) = ∑ K j ∈ x j 1 2 n j − 1 ,

where I ϕ ( X i ) is the importance coefficient of the basic event structure X _i ; K _j is the jth minimal cut set; and n j is the number of basic events in the minimal cut set K _j of the basic events X _i .

The following is the structural importance order of each basic event after calculation:

X 7 = X 8 = X 9 = X 10 = X 11 = X 12 = X 13 = X 14 = X 15 = X 16 = X 17 = X 18 = X 19 = X 20 = X 21 = X 22 = X 23 = X 24 > X 2 = X 6 > X 3 = X 4 = X 5 > X 1 .

The structural importance of each of the basic events X ₇ to X ₂₄ is equal and is the highest because each of their minimum cut sets has only one basic event. As their structural importance is the highest, it means that they have the greatest impact on top events, which is also where there are vulnerabilities in the security system. The corresponding measures derived from these aspects can effectively control the occurrence of accidents and improve the safety of the system.

3.1.2 Foundation pit earthwork collapse

The meaning of each symbol for the fault tree of the foundation pit soil collapse is shown in Table 2.

Table 2

Meanings of symbols in the accident tree of foundation pit earthwork collapse

Symbol	Meaning	Symbol	Meaning
T	Foundation pit earthwork collapse	X 4	No safe disclosure
M 1	Unsafe human behaviors	X 5	No monitoring of the slope and surrounding environment
M 2	Unsafe conditions of objects	X 6	The slope is not set according to the rules
M 3	Three violations	X 7	Poor soil
M 4	Disordered management	X 8	Penetration of surface or groundwater
X 1	No slope is set	X 9	No effective drainage measures
X 2	False operation	X 10	No derailment fences and warning signs
X 3	Heavy objects are stacked in pits or pile holes	X 11	Environmental deterioration after winter and rainy seasons

Similarly, using the Boolean algebra method to simplify the fault tree (Figure 2), a total of 40 minimum cut sets of the foundation pit earthwork collapse fault tree could be obtained, indicating that there are 40 possible ways for the system to cause accidents.

Figure 2

Accident tree of the foundation pit earthwork collapse.

After calculation, the order of importance of each basic event structure was

X 5 = X 6 > X 1 = X 2 = X 3 = X 4 > X 7 = X 8 = X 9 = X 10 = X 11 .

By analyzing the importance of the structure, the order of structural importance of the various basic events was revealed. The structural importance of not monitoring the slope and surrounding environment and not supporting the slope according to the regulations makes them the most important basic events; they have the greatest impact on the occurrence of the top event and are more likely to lead to the occurrence of the top event. To improve the safety and reliability of the foundation pit trench earthwork system, all measures must be taken to monitor the slope and the surrounding environment before construction, and the slope support must be carried out in strict accordance with the regulations to prevent the occurrence of collapse accidents to the greatest extent.

4.1 Quantitative analysis of the AHP

The AHP is a qualitative and quantitative decision-making method of multi-objective analysis proposed by Seaty [41]. The main principle is as follows. First, the relevant factors are classified into different levels, including the target layer, the criterion layer, and the indicator layer. Afterward, the relative importance of each layer is determined. Finally, the relative importance of each measure in the indicator layer is calculated, which means that the measures are ordered.

The model of hierarchical analysis consists of a target layer U, several criteria layers U _i , and an indicator layer U _ij . Based on the accident tree analysis mentioned above, the target layer is to prevent collapse accidents during construction. The criteria layer is obtained by neutralizing the intermediate event description of the accident tree. For example, unqualified materials and structural errors are attributed to the lack of regular maintenance and inspection of equipment. Similarly, the indicator layer is to neutralize the basic event description of the accident tree. For instance, the qualification rate of the materials is related to the inferior steel pipe fasteners, unqualified plates, and insufficient bearing capacity of timber struts.

Based on the above fault tree and analysis, a comprehensive index system of the influencing factors of collapse accidents can be constructed, as shown in Figure 3; the analytic hierarchy model is shown in Table 3.

Figure 3

Synthetic index system of collapse accident causes.

4.2 Constructing a judgment matrix, calculating the weight of each index, and checking consistency

To construct the judgment matrix, seven experts were invited to independently assign values to each indicator. Then, a discussion was held to determine the average; finally, the judgment matrix was obtained. The following is an example of the U − U _i judgment matrix (Table 4).

The U _i − U _ij judgment matrix after the calculation is as follows:

W U − U i = ( 0.3114 , 0.0861 , 0.4731 , 0.1294 ) T ; A W U − U i = ( 2.1296 , 0.6024 , 0.7754 , 0.7473 ) ; λ max 1 = 4.0847 ; C .I . U − U i = 0.0306 .

The judgment matrix in the work is entirely of the fourth order. The average random consistency of the indicator table was checked to obtain R.I. = 0.89; the same check is carried out as follows:

C.R. _{U−U i} = 0.343 < 0.1, which means that the judgment matrix U − U _i passed the consistency check.

4.3 Comprehensive weight comparison of each index

The results show that W _{U−U i} = (0.3114, 0.0861, 0.4731, 0.1294) ^T . Similarly, the following are the weights of the other indicators according to the calculation.

In Tables 5 and 6 W ₁ = 0.3114 is the comprehensive weight of the index of U ₁; W ₂ = 0.0861 is the comprehensive weight of the index of U ₂; W ₃ = 0.4731 is the comprehensive weight of the index of U ₃; and W ₄ = 0.1294 is the comprehensive weight of the index of U ₄.

The results show that the maintenance and inspection of equipment have the highest weight among the various indexes, followed by supervision and management. Safety management measures should be formulated with these two as the focus, followed by safety education and protection measures and environmental status inspection and monitoring.

5.1 Cause by objects – regular maintenance and inspection of equipment

The following measures can be taken to prevent collapses caused by equipment on the construction site. First, the equipment should be maintained and inspected, and the machines with problems or overloaded operations should be strictly prohibited. The mechanical equipment must be checked before use, and the examination must be recorded. In addition, the construction equipment should be used according to the specifications, with the prohibition of invalid mechanical equipment. Moreover, the effectiveness and completion of tower crane safety devices should be guaranteed, based on regular inspection, repair, and maintenance. The construction elevator safety devices should be effective and complete to ensure that there is no overloaded use. In brief, all the safety devices should be tested every day before utilization.

5.2 Cause by human beings – establishment of supervision and management responsibility system

A safety management organization should be established; it should consist of a project manager, site manager, project deputy chief engineer, safety director, full-time security officer, and subcontractor. The project manager is the main person responsible for safe production. The safety director takes charge of the supervision and management of the supervision unit and is answerable to the owner. The main contractor is in charge of the project, and the subcontractor is answerable to the main contractor. In addition, the main contractor takes responsibility for the subcontractors designated by the owner with regard to the pile foundation, earthwork, ejector anchor slope protection, integrated wiring, fire alarm equipment, installation, high-voltage power supply, and elevators. In a word, everyone must guarantee safe production.

Responsibility and management systems are required for safe production. The security duty of all personnel should be clarified to set up the management system for different inspections. A safe disclosure system is needed. The engineering and technical personnel should ensure the safety of the operation team before construction [42], which must be implemented by the on-site project department, with a signature for verification. In addition, the personnel is required to go into the site and conduct the supervision, management, inspection, and guidance.