Risk Identification for the Construction Phases of the Large Bridge Based on WBS-RBS

: For the construction environment of large bridges is complex and there are too much uncertain factors during the course of the construction, it is difficult for the general method of risk identification to identify the risks wholly and systematically. This study presents a new method of risk identification with modular analysis based on WBS-RBS (Work Breakdown Structure-Risk Breakdown Structure). For the purpose of the better application of this new method in the risk identification of large bridges, a BCICS (Bridge Construction Information Classification System) suitable for construction phase of large bridges was established based on the information classification system of ISO (International Standardization Organization). In order to provide a uniform communication language for the risk analysis personnel, the WBS of the construction of large bridges was established, which was beneficial to the judgment of the matrix elements of the RBM (Risk Breakdown Structure) applying the method of NGT (Nominal Group Technique). The coding system based on BCICS and RBS presented in this study is convenient for the modularized computer storage of the risk information of large bridges in construction phase and has great contribution to establishing the risk database of large bridges in construction phase.


INTRODUCTION
With the development of structural mechanics and computational mechanics and with the development and application of new construction materials, the technology of bridge construction has been improved greatly.The span of bridge becomes larger and larger and the bridge structure becomes more and more complex (Greenstein, 2011;Voo et al., 2011).Bridge construction is a complex and systematic work and there are a variety of risks all the time during the whole phase of bridge construction from construction preparation to construction completion.During the construction phase of a bridge, plenty of work is high above the ground; as a result, the construction of bridges has higher risk compared with the other engineering construction.The occurrence of risk accidents in the phase of bridge construction will lead to great losses to the proprietor and construction enterprises if the construction risk of the bridge has not been paid more attention (Ji and Fu, 2010;Zheng et al., 2008).The risk accidents will has adverse effect on the regular bridge construction and it may interrupt the bridge construction.For the large bridge, the investment of which is so huge, the technology is very complex and the construction period is too long.When the accident of the large bridge in construction phase occurs, the property damage and personal injury is more serious than the ordinary bridge.Therefore, the risk management of large bridges in construction phase has great significance to prevent the occurrence of construction accidents of large bridges.
Quite a lot of research has been done to identify and assess the bridge risk.For example, Stein et al. (1999) developed a risk-based method for assessing the risk associated with scour threat to bridge foundations (Stewart, 2001) presented a broad overview of the concepts, methodology and immediate applications of risk-based assessments of bridges.In particular, two practical applications of reliability-based bridge assessment are considered-risk ranking and life-cycle cost analysis (Higgins et al., 2005) proposed an assessment and risk-ranking methodology that incorporates moment-shear interaction Based on alpha level sets, Wang andElhag (2006, 2007) proposed a fuzzy TOPSIS method and a Fuzzy Group Decision Making (FGDM) approach for bridge risk assessment and presented a Nonlinear Programming (NLP) solution procedure.For the purpose of bridge design against vessel impacts, Geng et al. (2007) proposed a framework of risk assessment system, which consists of five sub-systems: data base for vessel-bridge collision assessment, bridge safety assessment module, risk acceptance criteria, active collision-prevention scheme design and passive structure-protection scheme design.Elhag and Wang (2007) presented an application of artificial neural networks in bridge risk assessment, in which back-propagation neural networks are developed to model bridge risk score and risk categories.Zayed et al. (2007) proposed a Risk index (R) that assessed risk and prioritizes bridges with unknown foundations.
Primary risk parameters and their factors for bridges with unknown foundations were identified and analyzed and a model for calculating R was designed.Mander et al. (2007) applied Incremental Dynamic Analysis (IDA) in a performance-based earthquake engineering context to investigate expected structural response, damage outcomes and financial loss from highway bridges.Wang and Elhag (2008) developed an Adaptive Neural-Fuzzy System (ANFIS) using 506 bridge maintenance projects for bridge risk assessment.Wang et al. (2008) proposed an integrated AHP-DEA methodology to evaluate bridge risks of hundreds or thousands of bridge structures, based on which the maintenance priorities of the bridge structures can be decided.Cho and Kim (2008) dealt with the quantitative risk assessment for the construction phases of the suspension bridge to evaluate the risks in a suspension bridge by considering an ultimate limit state for the fracture of main cable wires and to evaluate the risks for a limit state for the erection control during construction stages.Khan and Datta (2010) presented a fragility analysis of a fan type cable stayed bridge using a Probabilistic Risk Analysis (PRA) to determine its probability of failure under random ground motion.Padgett et al. (2010) presented the results of a seismic risk assessment of the bridge network in Charleston, South Carolina and the surrounding counties to support emergency planning efforts and for prioritization of bridge retrofit.Xiang et al. (2010) proposed the evaluation indexes system of bridge design risk and determined the weightings of risk factors based on Analytical Hierarchy Process (AHP).Deco and Frangopol (2011) provided a rational framework for the quantitative risk assessment of highway bridges under multiple hazards.
The above literature review clearly shows that there are many methods for the risk identification and assessment of the existing bridge.However, little information is presently known on the risk identification of large bridges in construction phase.Therefore, we conducted this study to identify the risk of large bridges in construction phase applying the method of WBS-RBS (Work Breakdown Structure-Risk Breakdown Structure).

WBS-RBS METHOD OF RISK IDENTIFICATION
Basic principles of WBS-RBS: WBS (Work Breakdown Structure) is a grouping mode of project elements facing to the deliverables, which organizes and defines all the working ranges of the project (Gregory, 2005).The sub-grade of WBS represents a more detailed project work.WP (Work Package) is the lowest grade work of WBS, which presents the logical basis to define the activities and specifically assigned works to the individual and organization.In risk management, WP is the elementary unit of risk identification.As a result, for the convenient risk identification, the work should be broken down into the level of WP step by step when WBS is being structured.RBS (Risk Breakdown Structure) is a hierarchical structure of the potential risk sources.Referring to the defining of WBS, RBS can be defined as a grouping mode of project risks facing to the source, which organizes and defines all the risks of the project (David, 2003).The sub-grade of RBS represents a more detailed project risk source.After RBS was presented for the first time, David et al. (2006) innovatively combined WBS and RBS to obtain the RBM (Risk Breakdown Matrix) for the purpose of the risk analysis of project, which is shown in Fig. 1.In brief, the basic principles on WBS-RBS of risk identification is that the risk of each WP in the effective working range defined in WBS will be identified according to the specific risk factors defined in RBS on the basis of establishing reasonable WBS and RBS.

Basic steps of WBS-RBS:
The basic steps of risk identification based on WBS-RBS can be summarized as: • Establishing WBS Advantages of WBS-RBS method for risk identification: From the basic steps and principles, it can be seen that WBS-RBS method has the following advantages compare with the other methods of risk identification.First, by establishing WBS, the project work can be detailed into WP step by step, which is convenient to identify the risk.Then the risk of each WP in WBS can be identified according to RBS.This method can both grasp the global vision for the project and go deep into the details of the project implementation.Second, by establishing WBS, the risks of the project can be classified and divided into levels, which is convenient for the risk analysis personnel to clearly, systematically and effectively identify the risks.As a result, it can prevent missing some risk factor and repeatedly counting the same risk.Furthermore, by establishing WBS, the risks occurred in different stage of the project is clear at a glance, which is convenient for the risk management personnel to distinguish and analyze the risks.Besides, this hierarchic structure of Work-Risk is convenient for the collection, treatment, storage, examination and management of the risk information of the engineering project and it is convenient for the establishment of the dynamic database of the risk source.

CONSTRUCTION RISK IDENTIFICATION BASED ON WBS-RBS
Establishing WBS of the large bridge construction: Generally, the uniform information classification system is used inside each of the participant organization of the project in order to ensure that the internal information can be transmitted smoothly.The different information classification system for different organizations will frequently produce interface barriers.Therefore, the same information classification system must be adopted in order to establish a uniform WBS.Thus, all the participants of the project can understand the WBS and the interface barriers can be avoided.Moreover, the information transmission and risk communication between different organizations can keep smooth.ISO (International Organization for Standardization) technological association has put forward a new information classification system for the construction profession.In this information classification system, the construction information has been divided into 8 facets, which include facility, space, element, work section, construction products, construction aids, management and attributes (ISO, 1994).Based on the information classification system of ISO, a Bridge Construction Information Classification System (BCICS) suitable for construction phase can be established.
The structural system of BCICS adopts 3 facets structure, which include bridge, subproject and work section.Bridge facet can be defined that the bridge types are classified according to the attributes of bridges, such as materials and structure forms.The subproject of the bridge can be classified according to the physical or functional component of the bridge.The subprojects of the bridge can be classified into 3 classes, which include the main stress structures, bridge floor system and accessory structures and each class still can be further classified.The classification of the work sections of the bridge has direct relation to the construction technology process adopted in the construction of the subprojects of the bridge.The work sections of the bridge can be classified into 2 classes, which include substantive work sections and accessory work sections and each class still can be further classified.The classifications of the bridge, subproject and work section are given in Table 1.
The expression of BCICS adopts hybrid coding system, in which the three facets are expressed as A, B and C respectively and the specific classification is expressed based on the decimal system.The coding of BCICS is composed of a letter and a following decimal number code.In order to broaden the expression scope of the coding system and improve the application flexibility of the coding system, the four subsidiary symbols of Uniclass coding system are used in the coding system of this study.The codes from the same facet or from different facets can be grouped together by the four symbols to obtain a new composite definition of the classified information.The four subsidiary symbols and the meaning of each symbol are given in Table 2. Figure 2 shows an application example of BCICS in WBS.From the figure, it can be seen that the WBS based on BCICS can satisfy the requirement that the project work should be broken down easily and advantageously.BCICS has excellent applicability and flexibility for the establishment of WBS and the application of WBS in the risk identification.The fundamental unit suitable for risk identification including the subproject and work section can be defined as RIP (Risk Identification Packages).

Establishing RBS of the large bridge construction:
RBS describes the distribution of the risk resources that the project is to be faced with, which is convenient for the risk analysis personnel to identify and assess the risk.Based on the construction characteristics of large bridges and the related research results, all the risks in the construction phase of large bridges can be classified into two classes: the internal risks and external risks by organizing great deal of risk data in the construction phase of large bridges (Zayed et al., 2007).The internal risks include the personnel risk, the construction    In this study, the expression of RBS adopts the hierarchical structure and the coding system adopts Five Digit System, which include four levels.The first level is expressed as the letter R, whose meaning is the project risk.The second level represents the first grade risk source, in which the internal risk can be expressed as Arabic numeral 1 and the external risk can be expressed as Arabic numeral 2. The third level represents the second grade risk source, which is Fig. 3: Application example of RBS expressed as the decimal coding system.The fourth level represents the risk factor, which is expressed as sequential coding system.Figure 3 shows an application example of RBS.The RBS coding in the figure represents a risk factor of the materials equipments risk derived from the internal of the project, which is that the raw materials and semi-finished or finished products are not qualified with the quality and specifications.

Establishing RBM of the large bridge construction:
Based on BCICS, the managers of the project, the risk analysis experts and the related technical personnel can establish a WBS, in which the project works can be broken down into RIP according to the actual conditions of the project, such as the structure type, the project environment, the project scale, the construction period and so on.When the RBM is being established, RIP should be set as the rows of the matrix and the lowest-level risk factors of RBS should be set as the columns of the matrix.Considering the length of the study, only the RBM of the personnel risk and construction technology risk are given in Fig. 4.
Judging the matrix element of RBM: Judging the matrix element of RBM actually is to determine whether a risk factor exists.If the risk factor exists, the corresponding matrix element value is 1 and otherwise, the corresponding matrix element value is 0. As can be seen from the above-mentioned establishing process of the RBM of bridge construction phase, the main function of the RBM is that it can break down the overall risk in the construction phase of the large bridge into work packages of each construction stage of the bridge.That is to say, each risk factor in the RBS is identified by taking RIP as the basic unit.In light of the above characteristics of RBM, in this study, a modified method of Brain Storming was applied to judge the matrix element of RBM.The modified method of Brain Storming can also be called Intellectual Stimulation Method and Free Thinking Method, which was proposed by Osborn A.F. in 1939(Zheng et al., 2008).With this method in the form of meeting, all the participants present their opinions without scruple to Fig. 4: RBM of the personnel risk and construction technology risk solve the problem creatively in a free and cheerful atmosphere.However, in the form of interactive group discussion face to face, the traditional modified method of Brain Storming is often influenced by some factors in the actual operating process, such as the production blocking, evaluation apprehension and social loafing and so on.Therefore, a modified method of Brain Storming was applied in this study, which was called Nominal Group Technique (NGT) and it is a kind of structured team Brain Storming method (Zheng et al., 2008).In the process of decision of this method, the discussion of the group member and the interpersonal communication will be restricted and each of the group members can think independently.As well as the traditional meeting, all the group members will attend the meeting; however, the group member will first do the individual decision making.
The basic steps of the judgment on the matrix element of RBM using Nominal Group Technique are as follows: • The responsible person of the project risk management is in charge of convening the related managers, designers, risk analysis experts, technical personnel and experienced construction personnel of the project to form the risk analysis group and distributing and introducing the materials concerned risk identification to the group members, such as the technical data on the bridge structure, the construction organization design, the human and social environment of the bridge site, the detailed information of the companies involved, the RBM forms and so on.Then the responsible person of the project risk management explains the establishing process of RBM to the group members and determines the objective of this meeting is to identify the overall risk of the project by judging the values of RBM elements.• The project manager will provide guide to the group members.He should emphasize the necessity of the attendance of each group member and explain the importance of the risk identification.• Before any discussion, each group member has a certain amount of time and space to think independently and fill out the RBM forms.The group member should provide written instructions on his judgment.If the other risk factors below the second grade risk source exist, the names of the risk factors should be written out.• Each group member should submit his RBM forms and written instructions to the project manager and then read out his judgment results and the corresponding reason.The project manager is in charge of write down the other risk factors that the group members proposed on the hanging board.Before all the group members complete reading, no discussion should be done on the recorded risk factors.• Hosted by the project manager, the collective discussion should be conducted inside the group taking RIP as the basic unit, so that each member of the group can make everyone's views on the RBM elements of clear.Meanwhile, the member of the group can compare his views with the others and finally make the evaluation.• The group members modify their judgment and views and take the other risk factors on the hanging broad as the new risk factors to judge.Then the RBM forms should be filled out again and submitted to the project manager.• The project manager collects the judgment values of the RBM elements based on the RBM forms submitted by the group members.The value of one element should be determined as the value that was chosen by more group members.However, if the number of the judgment value 0 is equal to the number of the judgment value 1, the project manager can decide the final value of the element by himself.Finally, the statistical results table of RBM should be obtained to get the final judgment results of RBM elements and the risk identification work based on WBS-RBS is completed.

CONCLUSION
For the construction environment of large bridges is complex and there are too much uncertain factors during the course of the construction, it is difficult for the general method of risk identification to identify the risks wholly and systematically.This study presents a new method of risk identification with modular analysis based on WBS-RBS (Work Breakdown Structure-Risk Breakdown Structure).For the purpose of the better application of this new method in the risk identification of large bridges, a BCICS (Bridge Construction Information Classification System) suitable for construction phase of large bridges was established based on the information classification system of ISO (International Standardization Organization).In order to provide a uniform communication language for the risk analysis personnel, the WBS of the construction of large bridges was established, which was beneficial to the judgment of the matrix elements of the RBM (Risk Breakdown Structure) applying the method of NGT (Nominal Group Technique).The coding system based on BCICS and RBS presented in this study is convenient for the modularized computer storage of the risk information of large bridges in construction phase and has great contribution to establishing the risk database of large bridges in construction phase.

Fig. 2 :
Fig. 2: Application example of BCICS in WBS • Personnel risk: o Quality of the general worker (Including psychological diathesis, moral integrity, operation technique and efficiency) o Quality of the technical personnel (Including psychological diathesis, moral integrity and technology level) o Quality of the management personnel (Including psychological diathesis, moral integrity and management level) o Quality of the supervising personnel o Unstabitily of the staff o Noncooperation of the employer and supervising engineer o Other personnel risks • Construction technology risk: o Backward construction technology o Unreasonable construction technology and scheme o Unsuitable protection measures of construction safety o Failure of the application of new technology and method o Half-baked consideration on the actual condition of the construction site o Unfamiliar with the design drawings and design intention o Construction not according to the drawing o Violating construction standard o Insufficient site information and unforeseeable circumstances underground o Unreasonable personnel organization and arrangement o Unreasonable materials and unreasonable equipment allocation o Other construction technology risk • Design technology risk: o Adverse effect of the grade of the design department the course of the project implementation and the project stage, when the risk exists in, can be definitely known by dealing with the risk values in RBM.

Table 1 :
Bridge Construction Information Classification System (BCICS) technology risk, the design technology risk, the materials and equipments risk and the contract risk, which can be further classified into risk factors.The risk factors of the internal risks are as follows:

Table 2 :
Four subsidiary symbols and the meanings