*2.1. Major Delivery Methods in Practice*

Design-Bid-Build (DBB) refers to the sequential and phased project delivery method involving three key players: the owner, the designer (architect), and the general contractor (builder). In this contractual structure, the owner contracts with the designer and the contractor, respectively, monitoring the activities of the designer and the contractor to ensure compliance with the contract requirements [13,17]. The owner signs a contract with the designer first and then signs an agreement with the contractor through bidding after the design contract is completed. There is no direct connection between the designer and the contractor and all information needs to be transmitted after the owner's decision.

Construction management at risk (CMR) might be the preferred project delivery method when owners need a defined completion date and price. The CMR manager is responsible for providing consultation on architectural services in evaluating costs, schedule, materials, and the like, and advising on optimizations and design alternatives, playing the role of a general contractor during the construction phase. A CMR manager is also responsible for monitoring and controlling the construction process in terms of costs, time, and other requirements to ensure a guaranteed maximum price (GMP) for the project [7,17]. Similar to DBB, the owner contracts with both the designer and the CMR manager.

In the design-build (DB) method, the design and construction are carried out by one entity. The owner only needs to sign one contract covering architecture, engineering, and construction and contracts with a single enterprise responsible for design and construction [6]. The owner will give priority to the DB method when he cannot bear too much risk and responsibility. Because it is a single entity responsible for design and construction, it avoids the possible opposition in DBB [7]. Since the contractor is liable for all coordination efforts, the owner's contract administration and site representative risks and costs are reduced.

Integrated Project Delivery (IPD) is defined as "a method of project delivery characterized by a contractual arrangement among a minimum of the owner, constructor, and designer that aligns the commercial interests of all participants" [18]. IPD integrates all elements of the system into a single process that synergistically utilizes the talents and abilities of all participants through all stages of design, fabrication, and construction to optimize project outcomes, increase value, reduce waste, and maximize efficiency [19]. IPD method includes some contract principles and behavior principles that promote participants' early cooperation, increase mutual trust, and integrate multiple participants under one contract.

#### *2.2. Selection of Major Delivery Methods*

Project delivery methods have evolved from traditional DBB to IPD, but not all projects are suitable for newly developed delivery methods. The same type of project even may be suitable for different delivery methods. Each project should develop a project delivery method adapted to its characteristics. It is not so much that the project delivery is selected, it is better designed [20,21]. Some researchers hope to summarize the experience of selecting project delivery methods through existing project cases. Alleman et al. [21] investigated 291 US highway projects and believed that the alternative contracting methods (DB and CM) have better cost and schedule benefits and are therefore more suitable for

highway construction. Demetracopoulou et al. [11] tested 57 lessons learned from Texas highway projects to help clarify the difficulty of choosing PDM. Franz et al. [22] verified the data of 212 projects to compare the cost and schedule performance of different delivery methods. Performance-based research also includes that these provide useful references for encouraging researchers to fully understand PDM and choices [23–25].

Another part of the researchers' hope is to develop a model of delivery method selection based on summarizing the selection criteria. This in turn includes appropriate determination of selection factors or criteria and reasonable methods. The factors for choosing a PDM are constantly enriched [26]. Decision makers focus early on the specific goals of the project [27,28], and as project complexity increases, factors expand to collaboration, integration, sustainability, corruption prevention, etc., [19,29,30]. The corresponding selection methods and models become more and more complex. By calculating the relative importance of different factors in the project goal hierarchy to choose the most appropriate delivery method, the AHP method has become the most commonly used method [4,13]. The artificial neural network method developed by Chen et al. identified similar projects between the target projects in the database and reduced the dependence on an expert's judgment [3]. Many researchers have to work on fuzzy methods in choosing the appropriate PDM to improve the reliability of decision making [2,14,15,31,32]. Additionally, many researchers developed multi-attribute decision making support tools [16,33–37].

However, these efforts may face some difficulties. The complexity of the project makes the choice of delivery method often inconsistent. When researchers try to use project performance indicators (such as cost, schedule, production efficiency, etc.) to select delivery methods, they often draw inconsistent conclusions. Feghaly et al. [38] concluded that DB was statistically superior to DBB in terms of project speed and intensity. Carpenter and Bausman [39] compared the performance of DBB and CM at Risk in public school construction, but the results showed that no one delivery method could meet all performance requirements. Project delivery methods should meet the requirements of the project characteristics. However, the evolution of projects and environmental changes constantly create new features and requirements which weaken the effectiveness of the model.
